LAGO: the Latin American Giant Observatory [IMA]

The Latin American Giant Observatory (LAGO) is an extended cosmic ray observatory composed of a network of water-Cherenkov detectors (WCD) spanning over different sites located at significantly different altitudes (from sea level up to more than $5000$\,m a.s.l.) and latitudes across Latin America, covering a wide range of geomagnetic rigidity cut-offs and atmospheric absorption/reaction levels. The LAGO WCD is simple and robust, and incorporates several integrated devices to allow time synchronization, autonomous operation, on board data analysis, as well as remote control and automated data transfer.
This detection network is designed to make detailed measurements of the temporal evolution of the radiation flux coming from outer space at ground level. LAGO is mainly oriented to perform basic research in three areas: high energy phenomena, space weather and atmospheric radiation at ground level. It is an observatory designed, built and operated by the LAGO Collaboration, a non-centralized collaborative union of more than 30 institutions from ten countries.
In this paper we describe the scientific and academic goals of the LAGO project – illustrating its present status with some recent results – and outline its future perspectives.

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I. Sidelnik, H. Asorey and LAGO. Collaboration
Fri, 17 Mar 17

Comments: 4 pages, 2 figures, Proceedings of the 9th International Workshop on Ring Imaging Cherenkov Detectors (RICH 2016), Lake Bled, Slovenia

Quasars as standard candles I: The physical relation between disc and coronal emission [HEAP]

A tight non-linear relation exists between the X-ray and UV emission in quasars (i.e. $L_{\rm X}\propto L_{\rm UV}^{\gamma}$), with a dispersion of $\sim$0.2~dex over \rev{$\sim$3~orders of magnitude in luminosity}. Such observational evidence has two relevant consequences: (1) an ubiquitous physical mechanism must regulate the energy transfer from the accretion disc to the X-ray emitting {\it corona}, and (2) the non-linearity of the relation provides a new, powerful way to estimate the absolute luminosity, turning quasars into a new class of {\it standard candles}.
Here we propose a modified version of this relation which involves the emission line full-width half maximum, $L_{\rm X}\propto L_{\rm UV}^{\hat\gamma}\upsilon_{\rm fwhm}^{\hat\beta}$. We interpret this new relation through a simple, {\it ad-hoc} model of accretion disc corona, derived from the works of Svensson \& Zdziarski (1994) and Merloni \& Fabian (2002), where it is assumed that reconnection and magnetic loops above the accretion disc can account for the production of the primary X-ray radiation.
We find that the monochromatic optical-UV (2500 \AA) and X–ray (2 keV) luminosities depend on the black hole mass and accretion rate as $L_{\rm UV}\propto M_{\rm BH}^{4/3} (\dot{M}/\dot{M}_{\rm Edd})^{2/3}$ and $L_{\rm X}\propto M_{\rm BH}^{19/21} (\dot{M}/\dot{M}_{\rm Edd})^{5/21}$, respectively. Assuming a broad line region size function of the disc luminosity $R_{\rm blr}\propto L_{\rm disc}^{0.5}$ we finally have that $L_{\rm X}\propto L_{\rm UV}^{4/7} \upsilon_{\rm fwhm}^{4/7}$. Such relation is remarkably consistent with the slopes and the normalization obtained from a fit of a sample of 545 optically selected quasars from SDSS DR7 cross matched with the latest XMM–{\it Newton} catalogue 3XMM-DR6.
The homogeneous sample used here has a dispersion of 0.21 dex, which is much lower than previous works in the literature and suggests a tight physical relation between the accretion disc and the X-ray emitting corona. We also obtained a possible physical interpretation of the $L_{\rm X}-L_{\rm UV}$ relation (considering also the effect of $\upsilon_{\rm fwhm}$), which puts the determination of distances based on this relation on a sounder physical grounds. The proposed new relation does not evolve with time, and thus it can be employed as a cosmological indicator to robustly estimate cosmological parameters.

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E. Lusso and G. Risaliti
Fri, 17 Mar 17

Comments: 15 pages, 9 figures, accepted for publication in Astronomy & Astrophysics

Charged massive scalar field configurations supported by a spherically symmetric charged reflecting shell [CL]

The physical properties of bound-state charged massive scalar field configurations linearly coupled to a spherically symmetric charged reflecting shell are studied {\it analytically}. To that end, we solve the Klein-Gordon wave equation for a static scalar field of proper mass $\mu$, charge coupling constant $q$, and spherical harmonic index $l$ in the background of a charged shell of radius $R$ and electric charge $Q$. It is proved that the dimensionless inequality $\mu R<\sqrt{(qQ)^2-(l+1/2)^2}$ provides an upper bound on the regime of existence of the composed charged-spherical-shell-charged-massive-scalar-field configurations. Interestingly, we explicitly show that the {\it discrete} spectrum of shell radii $\{R_n(\mu,qQ,l)\}_{n=0}^{n=\infty}$ which can support the static bound-state charged massive scalar field configurations can be determined analytically. We confirm our analytical results by numerical computations.

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S. Hod
Fri, 17 Mar 17

Comments: 8 pages

Narrow phase-dependent features in X-ray Dim Isolated Neutron Stars: a new detection and upper limits [HEAP]

We report on the results of a detailed phase-resolved spectroscopy of archival XMM–Newton observations of X-ray Dim Isolated Neutron Stars (XDINSs). Our analysis revealed a narrow and phase-variable absorption feature in the X-ray spectrum of RX J1308.6+2127. The feature has an energy of $\sim$740 eV and an equivalent width of $\sim$15 eV. It is detected only in $\sim$ 1/5 of the phase cycle, and appears to be present for the entire timespan covered by the observations (2001 December – 2007 June). The strong dependence on the pulsar rotation and the narrow width suggest that the feature is likely due to resonant cyclotron absorption/scattering in a confined high-B structure close to the stellar surface. Assuming a proton cyclotron line, the magnetic field strength in the loop is B$_{loop} \sim 1.7 \times 10^{14}$ G, about a factor of $\sim$5 higher than the surface dipolar magnetic field (B$_{surf} \sim 3.4 \times 10^{13}$ G). This feature is similar to that recently detected in another XDINS, RX J0720.4-3125, showing (as expected by theoretical simulations) that small scale magnetic loops close to the surface might be common to many highly magnetic neutron stars (although difficult to detect with current X-ray instruments). Furthermore, we investigated the available XMM–Newton, data of all XDINSs in search for similar narrow phase-dependent features, but could derive only upper limits for all the other sources.

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A. Borghese, N. Rea, F. Zelati, et. al.
Fri, 17 Mar 17

Comments: 10 pages, 5 figures, 4 tables. Accepted for publication in MNRAS

New constraints on binary evolution enhance the supernova type Ia rate [HEAP]

Even though Type Ia supernovae (SNIa) play an important role in many fields in astronomy, the nature of the progenitors of SNIa remain a mystery. One of the classical evolutionary pathways towards a SNIa explosion is the single degenerate (SD) channel, in which a carbon-oxygen white dwarf accretes matter from its non-degenerate companion until it reaches the Chandrasekhar mass. Constraints on the contribution from the SD channel to the overall SNIa rate come from a variety of methods, e.g. from abundances, from signatures of the companion star in the light curve or near the SNIa remnant, and from synthetic SNIa rates. In this proceedings, I show that when incorporating our newest understandings of binary evolution, the SNIa rate from the single degenerate channel is enhanced. I also discuss the applicability of these constraints on the evolution of SNIa progenitors.

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S. Toonen
Fri, 17 Mar 17

Comments: 3 figures, 6 pages, Proceedings of the workshop: “The Golden Age of Cataclysmic Variables and Related Objects III”, Palermo, Italy, Sep 7-12, 2015

Lectures on the Infrared Structure of Gravity and Gauge Theory [CL]

This is a redacted transcript of a course given by the author at Harvard in spring semester 2016. It contains a pedagogical overview of recent developments connecting the subjects of soft theorems, the memory effect and asymptotic symmetries in four-dimensional QED, nonabelian gauge theory and gravity with applications to black holes. The lectures may be viewed online at Please send typos or corrections to

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A. Strominger
Fri, 17 Mar 17

Comments: 154 pages, 21 figures

An investigation of pulsar searching techniques with the Fast Folding Algorithm [IMA]

Here we present an in-depth study of the behaviour of the Fast Folding Algorithm, an alternative pulsar searching technique to the Fast Fourier Transform. Weaknesses in the Fast Fourier Transform, including a susceptibility to red noise, leave it insensitive to pulsars with long rotational periods (P > 1 s). This sensitivity gap has the potential to bias our understanding of the period distribution of the pulsar population. The Fast Folding Algorithm, a time-domain based pulsar searching technique, has the potential to overcome some of these biases. Modern distributed-computing frameworks now allow for the application of this algorithm to all-sky blind pulsar surveys for the first time. However, many aspects of the behaviour of this search technique remain poorly understood, including its responsiveness to variations in pulse shape and the presence of red noise. Using a custom CPU-based implementation of the Fast Folding Algorithm, ffancy, we have conducted an in-depth study into the behaviour of the Fast Folding Algorithm in both an ideal, white noise regime as well as a trial on observational data from the HTRU-S Low Latitude pulsar survey, including a comparison to the behaviour of the Fast Fourier Transform. We are able to both confirm and expand upon earlier studies that demonstrate the ability of the Fast Folding Algorithm to outperform the Fast Fourier Transform under ideal white noise conditions, and demonstrate a significant improvement in sensitivity to long-period pulsars in real observational data through the use of the Fast Folding Algorithm.

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A. Cameron, E. Barr, D. Champion, et. al.
Fri, 17 Mar 17

Comments: 19 pages, 15 figures, 3 tables

The reflection spectrum of the low-mass X-ray binary 4U 1636-53 [HEAP]

We present 3-79 keV NuSTAR observations of the neutron star low-mass X-ray binary 4U 1636-53 in the soft, transitional and hard state. The spectra display a broad emission line at 5-10 keV. We applied several models to fit this line: A GAUSSIAN line, a relativistically broadened emission line model, KYRLINE, and two models including relativistically smeared and ionized reflection off the accretion disc with different coronal heights, RELXILL and RELXILLLP. All models fit the spectra well, however, the KYRLINE and RELXILL models yield an inclination of the accretion disc of $\sim88\degree$ with respect to the line of sight, which is at odds with the fact that this source shows no dips or eclipses. The RELXILLLP model, on the other hand, gives a reasonable inclination of $\sim56\degree$. We discuss our results for these models in this source and the possible primary source of the hard X-rays.

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Y. Wang, M. Mendez, A. Sanna, et. al.
Fri, 17 Mar 17

Comments: 9 pages, 8 figures

Early UV emission from disk-originated matter (DOM) in type Ia supernovae in the double degenerate scenario [HEAP]

We show that the blue and UV excess emission at the first few days of some type Ia supernovae (SNe Ia) can be accounted for in the double degenerate (DD) scenario by the collision of the SN ejecta with circumstellar matter that was blown by the accretion disk formed during the merger process of the two white dwarfs (WDs). We assume that in cases of excess early light the disk blows the circumstellar matter, that we term disk-originated matter (DOM), hours to days before explosion. To perform our analysis we first provide a model-based definition for early excess light, replacing the definition of excess light relative to a power-law fit to the rising luminosity. We then examine the light curves of the SNe Ia iPTF14atg and SN 2012cg, and find that the collision of the ejecta with a DOM in the frame of the DD scenario can account for their early excess emission. Thus, early excess light does not necessarily imply the presence of a stellar companion in the frame of the single-degenerate scenario. Our findings further increase the variety of phenomena that the DD scenario can account for, and emphasize the need to consider all different SN Ia scenarios when interpreting observations.

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N. Levanon and N. Soker
Fri, 17 Mar 17

Comments: 7 pages, 5 figures. Will be submitted in two days to allow comments by readers

Secluded and Flipped Dark Matter and Stueckelberg Extensions of the Standard Model [CL]

We consider here three dark matter models with the gauge symmetry of the standard model plus an additional local $U(1)_D$ factor. One model is secluded and two models are flipped. All of these models include one dark fermion and one vector boson that attains mass through the Stueckelberg mechanism. We show that the flipped models provide examples dark matter composed of “least interacting particles” (LIPs). Such particles are therefore compatible with the constraints obtained from both laboratory measurements and astrophysical observations.

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E. Fortes, V. Pleitez and F. Stecker
Fri, 17 Mar 17

Comments: 6 pages, no figures

Accretion Flow Properties of Swift J1753.5-0127 during its 2005 outburst [HEAP]

Galactic X-ray binary black hole candidate Swift~J1753.5-0127 was discovered on June 30 2005 by Swift/BAT instrument. In this paper, we make detailed analysis of spectral and timing properties of its 2005 outburst using RXTE/PCA archival data. We study evolution of spectral properties of the source from spectral analysis with the additive table model {\it fits} file of the Chakrabarti-Titarchuk two-components advective flow (TCAF) solution. From spectral fit, we extract physical flow parameters, such as, Keplerian disk accretion rate, sub-Keplerian halo rate, shock location and shock compression ratio, etc. We also study the evolution of temporal properties, such as observation of low frequency quasi-periodic oscillations (QPOs), variation of X-ray intensity throughout the outburst. From the nature of the variation of QPOs, and accretion rate ratios (ARRs=ratio of halo to disk rates), we classify entire 2005 outburst into two harder (hard-intermediate and hard) spectral states. No signature of softer (soft-intermediate and soft) spectral states are seen. This may be because of significant halo rate throughout the outburst. This behavior is similar to a class of other short orbital period sources, such as, MAXI~J1836-194, MAXI~J1659-152 and XTE~J1118+480. Here, we also estimate probable mass range of the source, to be in between $4.75 M_\odot$ to $5.90 M_\odot$ based on our spectral analysis.

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D. Debnath, A. Jana, S. Chakrabarti, et. al.
Fri, 17 Mar 17

Comments: 14 pages, 5 Figures, ApJ (communicated)

Study of statistical properties of hybrid statistic in coherent multi-detector compact binary coalescences Search [CL]

In this article, we revisit the problem of coherent multi-detector search of gravitational wave from compact binary coalescence with Neutron stars and Black Holes using advanced interferometers like LIGO-Virgo. Based on the loss of optimal multi-detector signal-to-noise ratio (SNR), we construct a hybrid statistic as a best of maximum-likelihood-ratio(MLR) statistic tuned for face-on and face-off binaries. The statistical properties of the hybrid statistic is studied. The performance of this hybrid statistic is compared with that of the coherent MLR statistic for generic inclination angles. Owing to the single synthetic data stream, the hybrid statistic gives low false alarms compared to the multi-detector MLR statistic and small fractional loss in the optimum SNR for a large range of binary inclinations. We have demonstrated that for a LIGO-Virgo network and binary inclination, \epsilon < 70 deg. and \epsilon > 110 deg., the hybrid statistic captures more than 98% of network optimum matched filter SNR with low false alarm rate. The Monte-Carlo exercise with two distributions of incoming inclination angles namely, U[cos(\epsilon)] and more realistic distribution proposed by B. F. Schutz are performed with hybrid statistic and gave ~5% and ~7% higher detection probability respectively compared to the two stream multi-detector MLR statistic for a fixed false alarm probability of 10^-5.

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K. Haris and A. Pai
Fri, 17 Mar 17

Comments: Published in Phys. Rev. D

Clustering of Gamma-Ray bursts through kernel principal component analysis [CL]

We consider the problem related to clustering of gamma-ray bursts (from “BATSE” catalogue) through kernel principal component analysis in which our proposed kernel outperforms results of other competent kernels in terms of clustering accuracy and we obtain three physically interpretable groups of gamma-ray bursts. The effectivity of the suggested kernel in combination with kernel principal component analysis in revealing natural clusters in noisy and nonlinear data while reducing the dimension of the data is also explored in two simulated data sets.

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S. Modak, A. Chattopadhyay and T. Chattopadhyay
Fri, 17 Mar 17

Comments: 30 pages, 10 figures

Yet another introduction to relativistic astrophysics [HEAP]

Late Winter Lecture Notes, Short Course (10 hours) of Relativistic Astrophysics held at the Department of Physics and Astronomy of the University of Padova, March 13-17, 2017.

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L. Foschini
Fri, 17 Mar 17

Comments: 132 pages

Searching For Pulsars Associated With the Fermi GeV Excess [HEAP]

The Fermi Large Area Telescope has detected an extended region of GeV emission toward the Galactic Center that is currently thought to be powered by dark matter annihilation or a population of young and/or millisecond pulsars. In a test of the pulsar hypothesis, we have carried out an initial search of a 20 deg**2 area centered on the peak of the galactic center GeV excess. Candidate pulsars were identified as a compact, steep spectrum continuum radio source on interferometric images and followed with targeted single-dish pulsation searches. We report the discovery of the recycled pulsar PSR 1751-2737 with a spin period of 2.23 ms. PSR 1751-2737 appears to be an isolated recycled pulsar located within the disk of our Galaxy, and it is not part of the putative bulge population of pulsars that are thought to be responsible for the excess GeV emission. However, our initial success in this small pilot survey suggests that this hybrid method (i.e. wide-field interferometric imaging followed up with single dish pulsation searches) may be an efficient alternative strategy for testing whether a putative bulge population of pulsars is responsible for the GeV excess.

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D. Bhakta, J. Deneva, D. Frail, et. al.
Thu, 16 Mar 17

Comments: MNRAS, in press

A Comprehensive Library of X-ray Pulsars in the Small Magellanic Cloud: Time Evolution of their Luminosities and Spin Periods [HEAP]

We have collected and analyzed the complete archive of {\itshape XMM-Newton\} (116), {\itshape Chandra\} (151), and {\itshape RXTE\} (952) observations of the Small Magellanic Cloud (SMC), spanning 1997-2014. The resulting observational library provides a comprehensive view of the physical, temporal and statistical properties of the SMC pulsar population across the luminosity range of $L_X= 10^{31.2}$–$10^{38}$~erg~s$^{-1}$. From a sample of 67 pulsars we report $\sim$1654 individual pulsar detections, yielding $\sim$1260 pulse period measurements. Our pipeline generates a suite of products for each pulsar detection: spin period, flux, event list, high time-resolution light-curve, pulse-profile, periodogram, and spectrum. Combining all three satellites, we generated complete histories of the spin periods, pulse amplitudes, pulsed fractions and X-ray luminosities. Some pulsars show variations in pulse period due to the combination of orbital motion and accretion torques. Long-term spin-up/down trends are seen in 12/11 pulsars respectively, pointing to sustained transfer of mass and angular momentum to the neutron star on decadal timescales. Of the sample 30 pulsars have relatively very small spin period derivative and may be close to equilibrium spin. The distributions of pulse-detection and flux as functions of spin-period provide interesting findings: mapping boundaries of accretion-driven X-ray luminosity, and showing that fast pulsars ($P<$10 s) are rarely detected, which yet are more prone to giant outbursts. Accompanying this paper is an initial public release of the library so that it can be used by other researchers. We intend the library to be useful in driving improved models of neutron star magnetospheres and accretion physics.

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J. Yang, S. Laycock, D. Christodoulou, et. al.
Thu, 16 Mar 17

Comments: 17 pages, 11 + 58 (appendix) figures. To appear in the Astrophysical Journal Supplement

Relativistic Turbulence with Strong Synchrotron and Synchrotron-Self-Compton Cooling [HEAP]

Many relativistic plasma environments in high-energy astrophysics, including pulsar wind nebulae, hot accretion flows onto black holes, relativistic jets in active galactic nuclei and gamma-ray bursts, and giant radio lobes, are naturally turbulent. The plasma in these environments is often so hot that synchrotron and inverse-Compton (IC) radiative cooling becomes important. In this paper we investigate the general thermodynamic and radiative properties (and hence the observational appearance) of an optically thin relativistically hot plasma stirred by driven magnetohydrodynamic (MHD) turbulence and cooled by radiation. We find that if the system reaches a statistical equilibrium where turbulent heating is balanced by radiative cooling, the effective electron temperature tends to attain a universal value $\theta = kT_e/m_e c^2 \sim 1/\sqrt{\tau_T}$, where $\tau_T=n_e\sigma_T L \ll 1$ is the system’s Thomson optical depth, essentially independent of the strength of turbulent driving or magnetic field. This is because both MHD turbulent dissipation and synchrotron cooling are proportional to the magnetic energy density. We also find that synchrotron self-Compton (SSC) cooling and perhaps a few higher-order IC components are automatically comparable to synchrotron in this regime. The overall broadband radiation spectrum then consists of several distinct components (synchrotron, SSC, etc.), well separated in photon energy (by a factor $\sim \tau_T^{-1}$) and roughly equal in power. The number of IC peaks is checked by Klein-Nishina effects and depends logarithmically on $\tau_T$ and the magnetic field. We also examine the limitations due to synchrotron self-absorption, explore applications to Crab PWN and blazar jets, and discuss links to radiative magnetic reconnection.

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D. Uzdensky
Thu, 16 Mar 17

Comments: 12 pages, 1 figure; submitted for publication. Comments welcome!

The early B-type star Rho Oph A is an X-ray lighthouse [SSA]

We present the results of a 140 ks XMM-Newton observation of the B2 star $\rho$ Ophiuchi A. The star exhibited strong X-ray variability: a cusp-shaped increase of rate, similar to the one we partially observed in 2013, and a bright flare. These events are separated in time by about 104 ks, which likely correspond to the rotational period of the star (1.2 days). Time resolved spectroscopy of the X-ray spectra shows that the first event is almost only due to an increase of the plasma emission measure, while the second increase of rate is mainly due is a major flare, with temperatures in excess of 60 MK ($kT\sim5$ keV). From the analysis of its rise we infer a magnetic field of $\ge300$ G and a size of the flaring region of $\sim1.4-1.9\times10^{11}$ cm, which corresponds to $\sim25\%-30\%$ of the stellar radius. We speculate that either an intrinsic magnetism that produces a hot spot on its surface, or an unknown low mass companion are the source of such X-rays and variability. A hot spot of magnetic origin should be a stable structure over a time span of $\ge$2.5 years, and suggests an overall large scale dipolar magnetic field that produce an extended feature on the stellar surface. In the second scenario, a low mass unknown companion is the emitter of X-rays and it should orbit extremely close to the surface of the primary in a locked spin-orbit configuration, almost on the verge of collapsing onto the primary. As such, the X-ray activity of the secondary star would be enhanced by both its young age and the tight orbit like in RS Cvn systems and $\rho$ Ophiuchi would constitute an extreme system worth of further investigation.

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I. Pillitteri, S. Wolk, F. Reale, et. al.
Thu, 16 Mar 17

Comments: 10 pages, 7 figures, 2 tables, A&A accepted

What can we learn about GRB from the variability timescale related correlations? [HEAP]

Recently, two empirical correlations related to the minimum variability timescale ($\rm MTS$) of the lightcures are discovered in gamma-ray bursts (GRBs). One is the anti-correlation between $\rm MTS$ and Lorentz factor $\Gamma$, the other is the anti-correlation between the $\rm MTS$ and gamma-ray luminosity $L_\gamma$. Both the two correlations might be used to explore the activity of the central engine of GRBs. In this paper we try to understand these empirical correlations by combining two popular black hole (BH) central engine models (namely, Blandford \& Znajek mechanism and neutrino-dominated accretion flow). By taking the $\rm MTS$ as the timescale of viscous instability of the neutrino-dominated accretion flow (NDAF), we find that these correlations favor the scenario in which the jet is driven by Blandford-Znajek (BZ) mechanism.

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W. Xie, W. Lei and D. Wang
Thu, 16 Mar 17

Comments: 6 pages, 3 figures, accepted for publication in ApJ

Star formation, supernovae, iron, and $α$: consistent cosmic and Galactic histories [HEAP]

Recent versions of the observed cosmic star-formation history (SFH) have resolved an inconsistency between the SFH and the observed cosmic stellar mass density history. Here, we show that the same SFH revision scales up by a factor $\sim 2$ the delay-time distribution (DTD) of Type Ia supernovae (SNe Ia), as determined from the observed volumetric SN Ia rate history, and thus brings it into line with other field-galaxy SN Ia DTD measurements. The revised-SFH-based DTD has a $t^{-1.1 \pm 0.1}$ form and a Hubble-time-integrated SN Ia production efficiency of $N/M_\star=1.25\pm 0.10$ SNe Ia per $1000~{\rm M_\odot}$ of formed stellar mass. Using these revised histories and updated, purely empirical, iron yields of the various SN types, we rederive the cosmic iron accumulation history. Core-collapse SNe and SNe Ia have contributed about equally to the total mass of iron in the Universe today, as deduced also for the Sun. We find the track of the average cosmic gas element in the [$\alpha$/Fe] vs. [Fe/H] abundance-ratio plane, as well as the track for gas in galaxy clusters, which have a higher DTD and have had a distinct, burst-like, SFH. Our cosmic $[\alpha$/Fe] vs. [Fe/H] track is broadly similar to the observed main locus of Galactic stars in this plane, indicating a Milky Way (MW) SFH similar in form to the cosmic one, and we find a MW SFH that makes the track closely match the stellar locus. The cluster DTD with a short-burst SFH at $z=3$ produces a track that matches well the observed `high-$\alpha$’ locus of MW stars, suggesting the halo/thick-disk population has had a galaxy-cluster-like formation history.

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D. Maoz and O. Graur
Thu, 16 Mar 17

Comments: Submitted to MNRAS, comments welcome

The quest for blue supergiants: binary merger models for the evolution of the progenitor of SN 1987A [SSA]

We present the results of a detailed, systematic stellar evolution study of binary mergers for blue supergiant (BSG) progenitors of Type II supernovae. In particular, these are the first evolutionary models that can simultaneously reproduce nearly all observational aspects of the progenitor of SN 1987A, $\text{Sk}-69\,^{\circ}202$, such as its position in the HR diagram, the enrichment of helium and nitrogen in the triple-ring nebula, and its lifetime before its explosion. The merger model, based on the one proposed by Podsiadlowski 1992 et al. and Podsiadlowski 2007 et al., consists of a main sequence secondary star that dissolves completely in the common envelope of the primary red supergiant at the end of their merger. We empirically explore a large initial parameter space, such as primary masses ($15\,\text{M}_{\odot}$, $16\,\text{M}_{\odot}$, and $17\,\text{M}_{\odot}$), secondary masses ($2\,\text{M}_{\odot}$, $3\,\text{M}_{\odot}$, …, $8\,\text{M}_{\odot}$) and different depths up to which the secondary penetrates the He core of the primary during the merger. The evolution of the merged star is continued until just before iron-core collapse and the surface properties of the 84 pre-supernova models ($16\,\text{M}_{\odot}-23\,\mathrm{M}_{\odot}$) computed have been made available in this work. Within the parameter space studied, the majority of the pre-supernova models are compact, hot BSGs with effective temperature $>12\,\text{kK}$ and radii of $30\,\text{R}_{\odot}-70\,\mathrm{R}_{\odot}$ of which six match nearly all the observational properties of $\text{Sk}-69\,^{\circ}202$.

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A. Menon and A. Heger
Thu, 16 Mar 17

Comments: Submitted to MNRAS. 21 pages, 11 figures, 7 tables

Dirac states of an electron in a circular intense magnetic field [HEAP]

Neutron-star magnetospheres are structured by very intense magnetic fields extending from 100 to 10 5 km traveled by very energetic electrons and positrons with Lorentz factors up to $\sim$ 10 7. In this context, particles are forced to travel almost along the magnetic field with very small gyro-motion, potentially reaching the quantified regime. We describe the state of Dirac particles in a locally uniform, constant and curved magnetic field in the approximation that the Larmor radius is very small compared to the radius of curvature of the magnetic field lines. We obtain a result that admits the usual relativistic Landau states as a limit of null curvature. We will describe the radiation of these states, that we call quantum curvature or synchro-curvature radiation, in an upcoming paper.

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G. Voisin, S. Bonazzola and F. Mottez
Thu, 16 Mar 17

Comments: N/A

A possible solution of the puzzling variation of the orbital period of MXB 1659-298 [HEAP]

MXB 1659-298 is a transient neutron star Low-Mass X-ray binary system that shows eclipses in the light curve with a peiodicity of 7.1 hr. MXB 1659-298 on outburst in August 2015 after 14 years of quiescence. We span a baseline of 40 years using the eight eclipse arrival times present in literature and adding 51 eclipse arrival times collected during the last two outbursts. We find that the companion star mass is $0.76 $ M$_{\odot}$, the inclination angle of the system is $72^{\circ}\!.4$ and the corona surrounding the neutron star has a size of $R_c \simeq 3.5 \times 10^8$ cm. A simple quadratic ephemeris do not fit the delays associated with the eclipse arrival times, the addition of a sinusoidal term is needed. We infer a binary orbital period of $P=7.1161099(3)$ hr and an orbital period derivative of $\dot{P}=-8.5(1.2) \times 10^{-12}$ s s$^{-1}$; the sinusoidal modulation has a period of $2.31 \pm 0.02$ yr. These results are consistent with a conservative mass transfer scenario during the outbursts and with a totally non-conservative mass transfer scenario during X-ray quiescence with the same mass transfer rate. The periodic modulation can be explained by either a gravitational quadrupole coupling due to variations of the oblateness of the companion star or with a presence of celestial body by orbiting around the binary system; in the latter case the mass of a third body is M$_3 = 21 \pm 2$ M$_J$.

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R. Iaria, A. Gambino, T. Salvo, et. al.
Thu, 16 Mar 17

Comments: 9 pages, 6 figures. Submitted to MNRAS on 2016 November 21, revised version after referee report

MHD simulations of oscillating cusp-filling tori around neutron stars — missing upper kHz QPO [HEAP]

We performed axisymmetric, grid-based, ideal magnetohydrodynamic (MHD) simulations of oscillating cusp-filling tori orbiting a non-rotating neutron star. A pseudo-Newtonian potential was used to construct the constant angular momentum tori in equilibrium. The inner edge of the torus is terminated by a “cusp” in the effective potential. The initial motion of the model tori were perturbed with uniform sub-sonic vertical and diagonal velocity fields. As the configuration evolved in time, we measured the mass accretion rate on the neutron star surface and obtained the power spectrum. The prominent mode of oscillation in the cusp torus is the radial epicyclic mode. From our analysis it follows that the mass accretion rate carries a modulation imprint of the oscillating torus, and hence so does the boundary layer luminosity.

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V. Parthasarathy, W. Kluzniak and M. Cemeljic
Thu, 16 Mar 17

Comments: Submitted as a Letter to MNRAS. 5 pages, 3 figures, 1 table. Comments are welcome

A multi-observatory database of X-ray pulsars in the Magellanic Clouds [HEAP]

Using hundreds of XMM-Newton and Chandra archival observations and nearly a thousand RXTE observations, we have generated a comprehensive library of the known pulsars in the Small and Large Magellanic Clouds (SMC, LMC). The pulsars are detected multiple times across the full parameter spaces of X-ray luminosity ($L_X= 10^{31-38}$~erg/s) and spin period ( P$<$1s — P$>$1000s) and the library enables time-domain studies at a range of energy scales. The high time-resolution and sensitivity of the EPIC cameras are complemented by the angular resolution of Chandra and the regular monitoring of RXTE. Our processing %$\sim$15 year pipeline uses the latest calibration files and software to generate a suite of useful products for each pulsar detection: event lists, high time-resolution light curves, periodograms, spectra, and complete histories of $\dot{P}$, the pulsed fraction, etc., in the broad (0.2-12 keV), soft (0.2-2 keV), and hard (2-12 keV) energy bands. After combining the observations from these telescopes, we found that 28 pulsars show long-term spin up and 25 long-term spin down. We also used the faintest and brightest sources to map out the lower and upper boundaries of accretion-powered X-ray emission: the propeller line and the Eddington line, respectively. We are in the process of comparing the observed pulse profiles to geometric models of X-ray emission in order to constrain the physical parameters of the pulsars. Finally we are preparing a public release of the library so that it can be used by others in the astronomical community.

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J. Yang, S. Laycock, J. Drake, et. al.
Thu, 16 Mar 17

Comments: 6 pages, 6 figures

Dark Matter Constraints from a Joint Analysis of Dwarf Spheroidal Galaxy Observations with VERITAS [HEAP]

We present constraints on the annihilation cross section of WIMP dark matter based on the joint statistical analysis of four dwarf galaxies with VERITAS. These results are derived from an optimized photon weighting statistical technique that improves on standard imaging atmospheric Cherenkov telescope (IACT) analyses by utilizing the spectral and spatial properties of individual photon events. We report on the results of $\sim$230 hours of observations of five dwarf galaxies and the joint statistical analysis of four of the dwarf galaxies. We find no evidence of gamma-ray emission from any individual dwarf nor in the joint analysis. The derived upper limit on the dark matter annihilation cross section from the joint analysis is $1.35\times 10^{-23} {\mathrm{ cm^3s^{-1}}}$ at 1 TeV for the bottom quark ($b\bar{b}$) final state, $2.85\times 10^{-24}{\mathrm{ cm^3s^{-1}}}$ at 1 TeV for the tau lepton ($\tau^{+}\tau^{-}$) final state and $1.32\times 10^{-25}{\mathrm{ cm^3s^{-1}}}$ at 1 TeV for the gauge boson ($\gamma\gamma$) final state.

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VERITAS. Collaboration, S. Archambault, A. Archer, et. al.
Thu, 16 Mar 17

Comments: 14 pages, 9 figures, accepted for publication in PRD

Gravitational Waves from Core-Collapse Supernovae [HEAP]

Gravitational waves are a potential direct probe for the multi-dimensional flow during the first second of core-collapse supernova explosions. Here we outline the structure of the predicted gravitational wave signal from neutrino-driven supernovae of non-rotating progenitors from recent 2D and 3D simulations. We sketch some quantitative dependencies that govern the amplitudes of this signal and its evolution in the time-frequency domain.

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B. Muller
Thu, 16 Mar 17

Comments: 4 pages, invited contribution prepared for the minisymposium “Gravitational Waves: Sources and Detection” at the 13th International Conference on Mathematical and Numerical Aspects of Wave Propagation, Minnesota, 2017

Modelling Jets, Tori and Flares in Pulsar Wind Nebulae [HEAP]

In this contribution we review the recent progress in the modeling of Pulsar Wind Nebulae (PWN). We start with a brief overview of the relevant physical processes in the magnetosphere, the wind-zone and the inflated nebula bubble. Radiative signatures and particle transport processes obtained from 3D simulations of PWN are discussed in the context of optical and X-ray observations. We then proceed to consider particle acceleration in PWN and elaborate on what can be learned about the particle acceleration from the dynamical structures called “wisps” observed in the Crab nebula. We also discuss recent observational and theoretical results of gamma-ray flares and the inner knot of the Crab nebula, which had been proposed as the emission site of the flares. We extend the discussion to GeV flares from binary systems in which the pulsar wind interacts with the stellar wind from a companion star. The chapter concludes with a discussion of solved and unsolved problems posed by PWN.

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O. Porth, R. Buehler, B. Olmi, et. al.
Thu, 16 Mar 17

Comments: To appear in “Jets and Winds in Pulsar Wind Nebulae, Gamma-ray Bursts and Blazars: Physics of Extreme Energy Release” of the Space Science Reviews series. The final publication is available at Springer via this http URL

The X-ray properties of Be/X-ray pulsars in quiescence [HEAP]

Observations of accreting neutron stars (NS) with strong magnetic fields can be used not only for studying the accretion flow interaction with NS magnetospheres, but also for understanding the physical processes inside NSs and for estimating their fundamental parameters. Of particular interest are (i) the interaction of a rotating neutron star (magnetosphere) with the in-falling matter at different accretion rates, and (ii) the theory of deep crustal heating and the influence of a strong magnetic field on this process. Here, we present results of the first systematic investigation of 16 X-ray pulsars with Be optical companions during their quiescent states, based on data from the Chandra, XMM-Newton and Swift observatories. The whole sample of sources can be roughly divided into two distinct groups: i) relatively bright objects with a luminosity around ~10^34 erg/s and (hard) power-law spectra, and ii) fainter ones showing thermal spectra. X-ray pulsations were detected from five objects in group i) with quite a large pulse fraction of 50-70 per cent. The obtained results are discussed within the framework of the models describing the interaction of the in-falling matter with the neutron star magnetic field and those describing heating and cooling in accreting NSs.

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S. Tsygankov, R. Wijnands, A. Lutovinov, et. al.
Thu, 16 Mar 17

Comments: 17 pages, 3 figures, 3 tables, submitted to MNRAS

The NuSTAR view of the true Type 2 Seyfert NGC3147 [GA]

We present the first NuSTAR observation of a ‘true’ Type 2 Seyfert galaxy. The 3-40 keV X-ray spectrum of NGC3147 is characterised by a simple power-law, with a standard {\Gamma}~1.7 and an iron emission line, with no need for any further component up to ~40 keV. These spectral properties, together with significant variability on time-scales as short as weeks (as shown in a 2014 Swift monitoring campaign), strongly support an unobscured line-of-sight for this source. An alternative scenario in terms of a Compton-thick source is strongly disfavoured, requiring an exceptional geometrical configuration, whereas a large fraction of the solid angle to the source is filled by a highly ionised gas, whose reprocessed emission would dominate the observed luminosity. Moreover, in this scenario the implied intrinsic X-ray luminosity of the source would be much larger than the value predicted by other luminosity proxies, like the [OIII]{\lambda}5007 emission line extinction-corrected luminosity. Therefore, we confirm with high confidence that NGC3147 is a true Type 2 Seyfert galaxy, intrinsically characterised by the absence of a BLR.

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S. Bianchi, A. Marinucci, G. Matt, et. al.
Thu, 16 Mar 17

Comments: 5 pages, 5 figures, accepted for publication in MNRAS

Probing the cosmic ray mass composition in the knee region through TeV secondary particle fluxes from solar surroundings [HEAP]

The possibility of estimating the mass composition of primary cosmic rays above the knee of its energy spectrum through the study of high energy gamma rays, muons and neutrinos produced in the interactions of cosmic rays with the solar ambient matter and radiation has been explored. It is found that the theoretical fluxes of TeV gamma rays, muons and neutrinos from a region around $15^{o}$ of the Sun are sensitive to mass composition of cosmic rays in the PeV energy range. The experimental prospects for detection of such TeV gamma rays/neutrinos by future experiments are discussed.

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P. Banik, B. Bijay, S. Sarkar, et. al.
Thu, 16 Mar 17

Comments: 10 pages, 7 figures, to appear in Physical Review D

Descattering of Giant Pulses in PSR B1957+20 [HEAP]

The interstellar medium scatters radio waves which causes pulsars to scintillate. For intrinsically short bursts of emission, the observed signal should be a direct measurement of the impulse response function. We show that this is indeed the case for giant pulses from PSR B1957+20: from baseband observations at 327 MHz, we demonstrate that the observed voltages of a bright pulse allow one to coherently descatter nearby ones. We find that while the scattering timescale is $12.3\,\mu$s, the power in the descattered pulses is concentrated within a span almost two orders of magnitude shorter, of $\lesssim\!200\,$ns. This sets an upper limit to the intrinsic duration of the giant pulses. We verify that the response inferred from the giant pulses is consistent with the scintillation pattern obtained by folding the regular pulsed emission, and that it decorrelates on the same timescale, of~$84\,$s. In principle, with large sets of giant pulses, it should be possible to constrain the structure of the scattering screen much more directly than with other current techniques, such as holography on the dynamic spectrum and cyclic spectroscopy.

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R. Main, M. Kerkwijk, U. Pen, et. al.
Thu, 16 Mar 17

Comments: 7 pages, 5 figures

Powerful Solar Signatures of Long-Lived Dark Mediators [HEAP]

Dark matter capture and annihilation in the Sun can produce detectable high-energy neutrinos, providing a probe of the dark matter-proton scattering cross section. We consider the case when annihilation proceeds via long-lived dark mediators, which allows gamma rays to escape the Sun and reduces the attenuation of neutrinos. For gamma rays, there are exciting new opportunities, due to detailed measurements of GeV solar gamma rays with Fermi, and unprecedented sensitivities in the TeV range with HAWC and LHAASO. For neutrinos, the enhanced flux, particularly at higher energies ($\sim$TeV), allows a more sensitive dark matter search with IceCube and KM3NeT. We show that these search channels can be extremely powerful, potentially improving sensitivity to the dark matter spin-dependent scattering cross section by several orders of magnitude relative to present searches for high-energy solar neutrinos, as well as direct detection experiments.

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R. Leane, K. Ng and J. Beacom
Thu, 16 Mar 17

Comments: 16 pages, 8 figures

Cosmic rays, gas and dust in nearby anticentre clouds : I — CO-to-H2 conversion factors and dust opacities [HEAP]

We aim to explore the capabilities of dust emission and rays for probing the properties of the interstellar medium in the nearby anti-centre region, using gamma-ray observations with the Fermi Large Area Telescope (LAT), and the thermal dust optical depth inferred from Planck and IRAS observations. In particular, we aim at quantifying potential variations in cosmic-ray density and dust properties per gas nucleon across the different gas phases and different clouds, and at measuring the CO-to-H2 conversion factor, X$_{CO}$ , in different environments. We have separated six nearby anti-centre clouds that are coherent in velocities and distances, from the Galactic-disc background in HI 21-cm and $^{12}$CO 2.6-mm line emission. We have jointly modelled the gamma-ray intensity recorded between 0.4 and 100 GeV, and the dust optical depth at 353 GHz as a combination of HI-bright, CO-bright, and ionised gas components. The complementary information from dust emission and gamma rays was used to reveal the gas not seen, or poorly traced, by HI , free-free, and $^{12}$CO emissions, namely (i) the opaque HI and diffuse H$_2$ present in the Dark Neutral Medium at the atomic-molecular transition, and (ii) the dense H$_2$ to be added where $^{12}$CO lines saturate. The measured interstellar gamma-ray spectra support a uniform penetration of the cosmic rays with energies above a few GeV through the clouds. We find a gradual increase in grain opacity as the gas becomes more dense. The increase reaches a factor of four to six in the cold molecular regions that are well shielded from stellar radiation. Consequently, the X$_{CO}$ factor derived from dust is systematically larger by 30% to 130% than the gamma-ray estimate. We also evaluate the average gamma-ray X$_{CO}$ factorfor each cloud, and find that X$_{CO}$ tends to decrease from diffuse to more compact molecular clouds, as expected from theory.

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Q. Remy, I. Grenier, D. Marshall, et. al.
Thu, 16 Mar 17

Comments: N/A

On the Chemistry of the Young Massive Protostellar core NGC 2264 CMM3 [GA]

We present the first gas-grain astrochemical model of the NGC 2264 CMM3 protostellar core. The chemical evolution of the core is affected by changing its physical parameters such as the total density and the amount of gas-depletion onto grain surfaces as well as the cosmic ray ionisation rate, $\zeta$. We estimated $\zeta_{\text {CMM3}}$ = 1.6 $\times$ 10$^{-17}$ s$^{-1}$. This value is 1.3 times higher than the standard CR ionisation rate, $\zeta_{\text {ISM}}$ = 1.3 $\times$ 10$^{-17}$ s$^{-1}$. Species response differently to changes into the core physical conditions, but they are more sensitive to changes in the depletion percentage and CR ionisation rate than to variations in the core density. Gas-phase models highlighted the importance of surface reactions as factories of large molecules and showed that for sulphur bearing species depletion is important to reproduce observations.
Comparing the results of the reference model with the most recent millimeter observations of the NGC 2264 CMM3 core showed that our model is capable of reproducing the observed abundances of most of the species during early stages ($\le$ 3$\times$10$^4$ yrs) of their chemical evolution. Models with variations in the core density between 1 – 20 $\times$ 10$^6$ cm$^{-3}$ are also in good agreement with observations during the early time interval 1 $\times$ 10$^4 <$ t (yr) $<$ 5 $\times$ 10$^4$. In addition, models with higher CR ionisation rates (5 – 10) $\times \zeta_{\text {ISM}}$ are often overestimating the fractional abundances of the species. However, models with $\zeta_{\text {CMM3}}$ = 5 $\zeta_{\text {ISM}}$ may best fit observations at times $\sim$ 2 $\times$ 10$^4$ yrs. Our results suggest that CMM3 is (1 – 5) $\times$ 10$^4$ yrs old. Therefore, the core is chemically young and it may host a Class 0 object as suggested by previous studies.

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Z. Awad and O. Shalabeia
Thu, 16 Mar 17

Comments: 24 pages, 4 figures, 3 Tables. Accepted for publication in Astrophysics and Space Science

Probing the Interstellar Dust towards the Galactic Centre: Dust Scattering Halo around AX J1745.6-2901 [HEAP]

AX J1745.6-2901 is an X-ray binary located at only 1.45 arcmin from Sgr A*, showcasing a strong X-ray dust scattering halo. We combine Chandra and XMM-Newton observations to study the halo around this X-ray binary. Our study shows two major thick dust layers along the line of sight (LOS) towards AX J1745.6-2901. The LOS position and $N_{H}$ of these two layers depend on the dust grain models with different grain size distribution and abundances. But for all the 19 dust grain models considered, dust Layer-1 is consistently found to be within a fractional distance of 0.11 (mean value: 0.05) to AX J1745.6-2901 and contains only (19-34)% (mean value: 26%) of the total LOS dust. The remaining dust is contained in Layer-2, which is distributed from the Earth up to a mean fractional distance of 0.64. A significant separation between the two layers is found for all the dust grain models, with a mean fractional distance of 0.31. Besides, an extended wing component is discovered in the halo, which implies a higher fraction of dust grains with typical sizes $\lesssim$ 590 \AA\ than considered in current dust grain models. Assuming AX J1745.6-2901 is 8 kpc away, dust Layer-2 would be located in the Galactic disk several kpc away from the Galactic Centre (GC). The dust scattering halo biases the observed spectrum of AX J1745.6-2901 severely in both spectral shape and flux, and also introduces a strong dependence on the size of the instrumental point spread function and the source extraction region. We build Xspec models to account for this spectral bias, which allow us to recover the intrinsic spectrum of AX J1745.6-2901 free from dust scattering opacity. If dust Layer-2 also intervenes along the LOS to Sgr A* and other nearby GC sources, a significant spectral correction for the dust scattering opacity would be necessary for all these GC sources.

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C. Jin, G. Ponti, F. Haberl, et. al.
Thu, 16 Mar 17

Comments: 20 pages, 18 figures, 5 tables, accepted for publication in MNRAS

Non-cyclic geometric phases and helicity transitions for neutrino oscillations in magnetic field [CL]

We show that neutrino spin and spin-flavor transitions involve non-vanishing geometric phases. Analytical expressions are derived for non-cyclic geometric phases arising due to neutrino oscillations in magnetic fields and matter. Several calculations are performed for different cases of rotating and non-rotating magnetic fields in the context of solar neutrinos and neutrinos produced inside neutron stars. It is shown that the neutrino eigenstates carry non-vanishing geometric phases even at large distances from their original point of production. Also the effects of critical magnetic fields and cross boundary effects in case of neutrinos emanating out of neutron stars are analyzed.

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S. Joshi and S. Jain
Thu, 16 Mar 17

Comments: 12 pages, 7 figures

XMM-Newton and NuSTAR simultaneous X-ray observations of IGR J11215-5952 [HEAP]

We report the results of an XMM-Newton and NuSTAR coordinated observation of the Supergiant Fast X-ray Transient (SFXT) IGRJ11215-5952, performed on February 14, 2016, during the expected peak of its brief outburst, which repeats every about 165 days. Timing and spectral analysis were performed simultaneously in the energy band 0.4-78 keV. A spin period of 187.0 +/- 0.4 s was measured, consistent with previous observations performed in 2007. The X-ray intensity shows a large variability (more than one order of magnitude) on timescales longer than the spin period, with several luminous X-ray flares which repeat every 2-2.5 ks, some of which simultaneously observed by both satellites. The broad-band (0.4-78 keV) time-averaged spectrum was well deconvolved with a double-component model (a blackbody plus a power-law with a high energy cutoff) together with a weak iron line in emission at 6.4 keV (equivalent width, EW, of 40+/-10 eV). Alternatively, a partial covering model also resulted in an adequate description of the data. The source time-averaged X-ray luminosity was 1E36 erg/s (0.1-100 keV; assuming 7 kpc). We discuss the results of these observations in the framework of the different models proposed to explain SFXTs, supporting a quasi-spherical settling accretion regime, although alternative possibilities (e.g. centrifugal barrier) cannot be ruled out.

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L. Sidoli, A. Tiengo, A. Paizis, et. al.
Thu, 16 Mar 17

Comments: 13 pages, 11 figures, accepted for publication on The Astrophysical Journal

Indication of a massive circumbinary planet orbiting the Low Mass X-ray Binary MXB 1658-298 [HEAP]

We present an X-ray timing analysis of the transient X-ray binary MXB 1658-298, using data obtained from the RXTE and XMM-Newton observatories. We have made 27 new mid eclipse time measurements from observations made during the two outbursts of the source. These new measurements have been combined with the previously known values to study long term changes in orbital period of the binary system. We have found that the mid-eclipse timing record of MXB 1658-298 is quite unusual. The long term evolution of mid-eclipse times indicates an overall orbital period decay with a time scale of — 6.5(7) x 10^7 year. Over and above this orbital period decay, the O-C residual curve also shows a periodic residual on shorter timescales. This sinusoidal variation has an amplitude of ~9 lt-sec and a period of ~760 d. This is indicative of presence of a third body around the compact X-ray binary. The mass and orbital radius of the third body are estimated to lie in the range, 20.5-26.9 Jupiter mass and 750-860 lt-sec, respectively. If true, then it will be the most massive circumbinary planet and also the smallest period binary known to host a planet.

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C. Jain, B. Paul, R. Sharma, et. al.
Tue, 14 Mar 17

Comments: 5 pages, 3 figures, Accepted for publication in Monthly Notices of the Royal Astronomical Society Letters

The X-ray continuum time-lags and intrinsic coherence in AGN [HEAP]

We present the results from a systematic analysis of the X-ray continuum (`hard’) time-lags and intrinsic coherence between the $2-4\,\mathrm{keV}$ and various energy bands in the $0.3-10\,\mathrm{keV}$ range, for ten X-ray bright and highly variable active galactic nuclei (AGN). We used all available archival \textit{XMM-Newton} data, and estimated the time-lags following Epitropakis \& Papadakis (2016). By performing extensive numerical simulations, we arrived at useful guidelines for computing intrinsic coherence estimates that are minimally biased, have known errors, and are (approximately) Gaussian distributed. Owing to the way we estimated the time-lags and intrinsic coherence, we were able to do a proper model fitting to the data. Regarding the continuum time-lags, we are able to demonstrate that they have a power-law dependence on frequency, with a slope of $-1$, and that their amplitude scales with the logarithm of the light-curve mean-energy ratio. We also find that their amplitude increases with the square root of the X-ray Eddington ratio. Regarding the intrinsic coherence, we found that it is approximately constant at low frequencies. It then decreases exponentially at frequencies higher than a characteristic `break frequency.’ Both the low-frequency constant intrinsic-coherence value and the break frequency have a logarithmic dependence on the light-curve mean-energy ratio. Neither the low-frequency constant intrinsic-coherence value, nor the break frequency exhibit a universal scaling with either the central black hole mass, or the the X-ray Eddington ratio. Our results could constrain various theoretical models of AGN X-ray variability.

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A. Epitropakis and I. Papadakis
Tue, 14 Mar 17

Comments: 36 pages, 6 tables, 42 figures (accepted for publication in MNRAS)

Do FRB Mark Dark Core Collapse? [HEAP]

Are some neutron stars produced without a supernova, without ejecting mass in a remnant? Theoretical calculations of core collapse in massive stars often predict this. The observation of the repeating FRB 121102, whose dispersion measure has not changed over several years, suggests that dark core collapses are not just failures of computer codes, but may be real. The existence of one repeating FRB with unchanging dispersion measure is not conclusive, but within a decade hundreds or thousands of FRB are expected to be discovered, likely including scores of repeaters, permitting useful statistical inferences. A na\”{\i}ve supernova remnant model predicts observable decline in dispersion measure for 100 years after its formation. If an upper limit on the decline of 2 pc/cm$^3$-y is set for five repeating FRB, then the na\”{\i}ve model is rejected at the 95\% level of confidence. This may indicate dark neutron star formation without a supernova or supernova remnant. This hypothesis may also be tested with LSST data that would show, if present, a supernova at an interferometric FRB position if it occurred within the LSST epoch.

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J. Katz
Tue, 14 Mar 17

Comments: 4 pp

Determination of the Magnetic Fields of Magellanic X-Ray Pulsars [HEAP]

The 80 high-mass X-ray binary (HMXB) pulsars that are known to reside in the Magellanic Clouds (MCs) have been observed by the XMM-Newton and Chandra X-ray telescopes on a regular basis for 15 years, and the XMM-Newton and Chandra archives contain nearly complete information about the duty cycles of the sources with spin periods P_S < 100 s. We have rerprocessed the archival data from both observatories and we combined the output products with all the published observations of 31 MC pulsars with P_S < 100 s in an attempt to investigate the faintest X-ray emission states of these objects that occur when accretion to the polar caps proceeds at the smallest possible rates. These states determine the so-called propeller lines of the accreting pulsars and yield information about the magnitudes of their surface magnetic fields. We have found that the faintest states of the pulsars segregate into five discrete groups which obey to a high degree of accuracy the theoretical relation between spin period and X-ray luminosity. So the entire population of these pulsars can be described by just five propeller lines and the five corresponding magnetic moments (0.29, 0.53, 1.2, 2.9, and 7.3, in units of 10^30 G cm^3).

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D. Christodoulou, S. Laycock, J. Yang, et. al.
Tue, 14 Mar 17

Comments: To appear in Reserch in Astronomy and Astrophysics

Constructing Gravitational Waves from Generic Spin-Precessing Compact Binary Inspirals [CL]

The coalescence of compact objects is one of the most promising sources of gravitational waves for ground-based interferometric detectors, such as advanced LIGO and Virgo. Generically, com- pact objects in binaries are expected to be spinning with spin angular momenta misaligned with the orbital angular momentum, causing the orbital plane to precess. This precession adds rich structure to the gravitational waves, introducing such complexity that an analytic closed-form description has been unavailable until now. We here construct the first closed-form frequency- domain gravitational waveforms that are valid for generic spin-precessing quasicircular compact binary inspirals. We first construct time-domain gravitational waves by solving the post-Newtonian precession equations of motion with radiation reaction through multiple scale analysis. We then Fourier transform these time-domain waveforms with the method of shifted uniform asymptotics to obtain closed-form expressions for frequency-domain waveforms. We study the accuracy of these analytic, frequency-domain waveforms relative to waveforms obtained by numerically evolving the post-Newtonian equations of motion and find that they are suitable for unbiased parameter estimation for 99.2%(94.6%) of the binary configurations we studied at a signal-to-noise ratio of 10(25). These new frequency-domain waveforms could be used for detection and parameter estimation studies due to their accuracy and low computational cost.

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K. Chatziioannou, A. Klein, N. Yunes, et. al.
Tue, 14 Mar 17

Comments: 21 pages, submitted to Phys. Rev. D

Radial modes of levitating atmospheres around Eddington-luminosity neutron stars [HEAP]

We consider an optically thin radiation-supported levitating atmosphere suspended well above the stellar surface, as discussed recently in the Schwarzschild metric for a star of luminosity close to the Eddington value. Assuming the atmosphere to be geometrically thin and polytropic, we investigate the eigenmodes and calculate the frequencies of the oscillations of the atmosphere in Newtonian formalism. The ratio of the two lowest eigenfrequencies is $\sqrt{\gamma+1}$, i.e., it only depends on the adiabatic index.

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D. Bollimpalli and W. Kluzniak
Tue, 14 Mar 17

Comments: 6 pages, 3 figures, Submitted for publication to MNRAS

First Detection of Mid-Infrared Variability from an Ultraluminous X-Ray Source Holmberg II X-1 [HEAP]

We present mid-infrared (IR) light curves of the Ultraluminous X-ray Source (ULX) Holmberg II X-1 from observations taken between 2014 January 13 and 2017 January 5 with the \textit{Spitzer Space Telescope} at 3.6 and 4.5 $\mu$m in the \textit{Spitzer} Infrared Intensive Transients Survey (SPIRITS). The mid-IR light curves, which reveal the first detection of mid-IR variability from a ULX, is determined to arise primarily from dust emission rather than from a jet or an accretion disk outflow. We derived the evolution of the dust temperature ($T_\mathrm{d}\sim600 – 800$ K), IR luminosity ($L_\mathrm{IR}\sim3\times10^4$ $\mathrm{L}_\odot$), mass ($M_\mathrm{d}\sim1-3\times10^{-6}$ $\mathrm{M}_\odot$), and equilibrium temperature radius ($R_\mathrm{eq}\sim10-20$ AU). A comparison of X-1 with a sample spectroscopically identified massive stars in the Large Magellanic Cloud on a mid-IR color-magnitude diagram suggests that the mass donor in X-1 is a supergiant (sg) B[e]-star. The sgB[e]-interpretation is consistent with the derived dust properties and the presence of the [Fe II] ($\lambda=1.644$ $\mu$m) emission line revealed from previous near-IR studies of X-1. We attribute the mid-IR variability of X-1 to increased heating of dust located in a circumbinary torus. It is unclear what physical processes are responsible for the increased dust heating; however, it does not appear to be associated with the X-ray flux from the ULX given the constant X-ray luminosities provided by serendipitous, near-contemporaneous X-ray observations around the first mid-IR variability event in 2014. Our results highlight the importance of mid-IR observations of luminous X-ray sources traditionally studied at X-ray and radio wavelengths.

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R. Lau, M. Heida, M. Kasliwal, et. al.
Tue, 14 Mar 17

Comments: 9 page, 4 figures, 1 table, Accepted to ApJ Letters

Double O-Ne-Mg white dwarfs merging as the source of the Powerfull Gravitational Waves for LIGO/VIRGO type interferometers [HEAP]

New strong non spiralling gravitational waves (GW) source for LIGO/VIRGO detectors are proposed. It is noted that double O-Ne-Mg white dwarfs mergers can produce strong gravitational waves with frequencies in the 600-1200 Hz range. Such events can be followed by the Super Nova type Ia.

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V. Lipunov
Tue, 14 Mar 17

Comments: 4 pages, submitted to New Astronomy

On the Absence of Non-thermal X-ray emission around Runaway O stars [HEAP]

Theoretical models predict that the compressed interstellar medium around runaway O stars can produce high-energy non-thermal diffuse emission, in particular, non-thermal X-ray and $\gamma$-ray emission. So far, detection of non-thermal X-ray emission was claimed for only one runaway star AE Aur. We present a search for non-thermal diffuse X-ray emission from bow shocks using archived XMM-Newton observations for a clean sample of 6 well-determined runaway O stars. We find that none of these objects present diffuse X-ray emission associated to their bow shocks, similarly to previous X-ray studies toward $\zeta$ Oph and BD$+$43$^{\circ}$3654. We carefully investigated multi-wavelength observations of AE Aur and could not confirm previous findings of non-thermal X-rays. We conclude that so far there is no clear evidence of non-thermal extended emission in bow shocks around runaway O stars.

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J. Toala, L. Oskinova and R. Ignace
Tue, 14 Mar 17

Comments: 6 pages, 2 tables, 3 figures; Accepted to ApJ Letters

Classical collapse to black holes and white hole quantum bounces: A review [CL]

In the last four decades different programs have been carried out aiming at understanding the final fate of gravitational collapse of massive bodies once some prescriptions for the behaviour of gravity in the strong field regime are provided. The general picture arising from most of these scenarios is that the classical singularity at the end of collapse is replaced by a bounce. The most striking consequence of the bounce is that the black hole horizon may live for only a finite time. The possible implications for astrophysics are important since, if these models capture the essence of the collapse of a massive star, an observable signature of quantum gravity may be hiding in astrophysical phenomena. One intriguing idea that is implied by these models is the possible existence of exotic compact objects, of high density and finite size, that may not be covered by an horizon. The present article outlines the main features of these collapse models and some of the most relevant open problems. The aim is to provide a comprehensive (as much as possible) overview of the current status of the field from the point of view of astrophysics. As a little extra, a new toy model for collapse leading to the formation of a quasi static compact object is presented.

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D. Malafarina
Tue, 14 Mar 17

Comments: 29 pages, 8 figures, comments are welcome

Synthetic streams in a Gravitational Wave inspiral search with a multi-detector network [CL]

Gravitational Wave Inspiral search with a global network of interferometers when carried in a phase coherent fashion would mimic an effective multi-detector network with synthetic streams constructed by the linear combination of the data from different detectors. For the first time, we demonstrate that the two synthetic data streams pertaining to the two polarizations of Gravitational Wave can be derived prior to the maximum-likelihood analysis in a most natural way using the technique of singular-value-decomposition applied to the network signal-to-noise ratio vector. We construct the network matched filters in combined network plus spectral space which capture both the synthetic streams. We further show that the network LLR is then sum of the LLR of each synthetic stream. The four extrinsic parameters are mapped to the two amplitudes and two phases. The maximization over these is a straightforward approach closely linked to the single detector approach. Towards the end, we connect all the previous works related to the multi-detector Gravitational Wave inspiral search and express in the same notation in order to bring under the same footing.

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K. Haris and A. Pai
Tue, 14 Mar 17

Comments: LIGO laboratory document number: LIGO-P1300229

Probing the Extragalactic Cosmic Rays origin with gamma-ray and neutrino backgrounds [HEAP]

The GeV-TeV gamma-rays and the PeV-EeV neutrino backgrounds provide a unique window on the nature of the ultra-high-energy cosmic-ray (UHECR). We discuss the implications of the recent Fermi-LAT data regarding the extragalactic gamma-ray background (EGB) and related estimates of the contribution of point sources as well as IceCube neutrino data on the origin of the ultra-high-energy cosmic-ray (UHECR). We calculate the diffuse flux of cosmogenic $\gamma$-rays and neutrinos produced by the UHECRs and derive constraints on the possible cosmological evolution of UHECR sources. In particular, we show that the mixed-composition scenario considered in \citet{Globus2015b}, that is in agreement with both (i) Auger measurements of the energy spectrum and composition up to the highest energies and (ii) the ankle-like feature in the light component detected by KASCADE-Grande, is compatible with both the Fermi-LAT measurements and with current IceCube limits. We also discuss the possibility for future experiments to detect associated cosmogenic neutrinos and further constrain the UHECR models, including possible subdominant UHECR proton sources.

Read this paper on arXiv…

N. Globus, D. Allard, E. Parizot, et. al.
Tue, 14 Mar 17

Comments: 6 pages, 4 figures, submitted to ApJ Letters