Ion beam test results of the Plastic Scintillator Detector of DAMPE [IMA]

The DArk Matter Particle Explorer (DAMPE) is one of the four satellites within Strategic Pioneer Research Program in Space Science of the Chinese Academy of Science (CAS). DAMPE can detect electrons, photons and ions in a wide energy range (5 GeV to 10 TeV) and ions up to iron (100GeV to 100 TeV). Plastic Scintillator Detector (PSD) is one of the four payloads in DAMPE, providing e/{\gamma} separation and charge identification up to Iron. An ion beam test was carried out for the Qualification Model of PSD in CERN with 40GeV/u Argon primary beams. The Birk’s saturation and charge resolution of PSD were investigated.

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R. Qiao, W. Peng, D. Guo, et. al.
Thu, 19 Jan 17

Comments: 8 pages, 11 figures

Gamma rays from clumpy wind-jet interactions in high-mass microquasars [HEAP]

The stellar winds of the massive stars in high-mass microquasars are thought to be inhomogeneous. The interaction of these inhomogeneities, or clumps, with the jets of these objects may be a major factor in gamma-ray production. Our goal is to characterize a typical scenario of clump-jet interaction, and calculate the contribution of these interactions to the gamma-ray emission from these systems. We use axisymmetric, RHD simulations to model the emitting flow in a typical clump-jet interaction. Using the simulation results we perform a numerical calculation of the high-energy emission from one of these interactions. The radiative calculations are done for relativistic electrons locally accelerated at the jet shock, and the synchrotron and IC radiation spectra are computed for different stages of the shocked clump evolution. We also explore different parameter values, such as viewing angle and magnetic field strength. We generalize phenomenologically the results derived from one clump-jet interaction to multiple interactions under different wind models, estimating the clump-jet interaction rates, and the resulting luminosities in the GeV range. If particles are efficiently accelerated in clump-jet interactions, the apparent gamma-ray luminosity through IC scattering with the stellar photons can be significant even for rather strong magnetic fields and thus efficient synchrotron cooling. Moreover, despite the standing nature or slow motion of the jet shocks for most of the interaction stage, Doppler boosting in the postshock flow is relevant even for mildly relativistic jets. For clump-to-average wind density contrasts $\gtrsim 10$, clump-jet interactions could be bright enough to match the observed GeV luminosity in Cyg~X-1 and Cyg~X-3 when a jet is present in these sources, with required non-thermal-to-total available power fractions $\sim 0.01$ and $0.1$, respectively.

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V. Cita, S. Palacio, V. Bosch-Ramon, et. al.
Thu, 19 Jan 17

Comments: 9 pages, 7 figures, accepted for publication in A&A

OpenCluster: A Flexible Distributed Computing Framework for Astronomical Data Processing [IMA]

The volume of data generated by modern astronomical telescopes is extremely large and rapidly growing. However, current high-performance data processing architectures/frameworks are not well suited for astronomers because of their limitations and programming difficulties. In this paper, we therefore present OpenCluster, an open-source distributed computing framework to support rapidly developing high-performance processing pipelines of astronomical big data. We first detail the OpenCluster design principles and implementations and present the APIs facilitated by the framework. We then demonstrate a case in which OpenCluster is used to resolve complex data processing problems for developing a pipeline for the Mingantu Ultrawide Spectral Radioheliograph. Finally, we present our OpenCluster performance evaluation. Overall, OpenCluster provides not only high fault tolerance and simple programming interfaces, but also a flexible means of scaling up the number of interacting entities. OpenCluster thereby provides an easily integrated distributed computing framework for quickly developing a high-performance data processing system of astronomical telescopes and for significantly reducing software development expenses.

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S. Wei, F. Wang, H. Deng, et. al.
Thu, 19 Jan 17

Comments: N/A

A global study of Type B quasi-periodic oscillation in black hole X-ray binaries [HEAP]

We performed a global study on the timing and spectral properties of type-B quasi-periodic oscillations (QPOs) in the outbursts of black hole X-ray binaries. The sample is built based on the observations of {\it Rossi X-ray Timing Explorer}, via searching in the literature in RXTE era for all the identified type-B QPOs. To enlarge the sample, we also investigated some type-B QPOs that are reported but not yet fully identified. Regarding to the time lag and hard/soft flux ratio, we found that the sources with type-B QPOs behave in two subgroups. In one subgroup, type-B QPO shows a hard time lag that firstly decreases and then reverse into a soft time lag along with softening of the energy spectrum. In the other subgroup, type-B QPOs distribute only in a small region with hard time lag and relatively soft hardness. These findings may be understood with a diversity of the homogeneity showing up for the hot inner flow of different sources. We confirm the universality of a positive relation between the type-B QPO frequency and the hard component luminosity in different sources. We explain the results by considering that the type-B QPO photons are produced in the inner accretion flow around the central black hole, under a local Eddington limit. Using this relationship, we derived an mass estimation of $9.3-27.1$ $\rm{M_{\odot}}$ for the black hole in H 1743-322.

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H. Gao, L. Zhang, Y. Chen, et. al.
Thu, 19 Jan 17

Comments: 12 pages, 9 figures

Mercury's magnetic field in the MESSENGER era [EPA]

MESSENGER magnetometer data show that Mercury’s magnetic field is not only exceptionally weak but also has a unique geometry. The internal field resembles an axial dipole that is offset to the North by 20% of the planetary radius. This implies that the axial quadrupol is particularly strong while the dipole tilt is likely below 0.8 degree. The close proximity to the sun in combination with the weak internal field results in a very small and highly dynamic Hermean magnetosphere. We review the current understanding of Mercury’s internal and external magnetic field and discuss possible explanations. Classical convection driven core dynamos have a hard time to reproduce the observations. Strong quadrupol contributions can be promoted by different measures, but they always go along with a large dipole tilt and generally rather small scale fields. A stably stratified outer core region seems required to explain not only the particular geometry but also the weakness of the Hermean magnetic field. New interior models suggest that Mercury’s core likely hosts an iron snow zone underneath the core-mantle boundary. The positive radial sulfur gradient likely to develop in such a zone would indeed promote stable stratification. However, even dynamo models that include the stable layer show Mercury-like magnetic fields only for a fraction of the total simulation time. Large scale variations in the core-mantle boundary heat flux promise to yield more persistent results but are not compatible with the current understanding of Mercury’s lower mantle.

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J. Wicht and D. Heyner
Thu, 19 Jan 17

Comments: 40 pages, 14 figures

How to break the density-anisotropy degeneracy in spherical stellar systems [GA]

We present a new non-parametric Jeans code, GravSphere, that recovers the density $\rho(r)$ and velocity anisotropy $\beta(r)$ of spherical stellar systems, assuming only that they are in a steady-state. Using a large suite of mock data, we confirm that with only line-of-sight velocity data, GravSphere provides a good estimate of the density at the projected stellar half mass radius, $\rho(R_{1/2})$, but is not able to measure $\rho(r)$ or $\beta(r)$, even with 10,000 tracer stars. We then test three popular methods for breaking this $\rho-\beta$ degeneracy: using multiple populations with different $R_{1/2}$; using higher order `Virial Shape Parameters’ (VSPs); and including proper motion data.
We find that two populations provide an excellent recovery of $\rho(r)$ in-between their respective $R_{1/2}$. However, even with a total of $\sim 7,000$ tracers, we are not able to well-constrain $\beta(r)$ for either population. By contrast, using 1000 tracers with higher order VSPs we are able to measure $\rho(r)$ over the range $0.5 < r/R_{1/2} < 2$ and broadly constrain $\beta(r)$. Including proper motion data for all stars gives an even better performance, with $\rho$ and $\beta$ well-measured over the range $0.25 < r/R_{1/2} < 4$.
Finally, we test GravSphere on a triaxial mock galaxy that has axis ratios typical of a merger remnant, $[1:0.8:0.6]$. In this case, GravSphere can become slightly biased. However, we find that when this occurs the data are poorly fit, allowing us to detect when such departures from spherical symmetry become problematic.

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J. Read and P. Steger
Thu, 19 Jan 17

Comments: 18 pages; 1 table; 10 Figures. Submitted to MNRAS. Comments welcome!

Astrometry of OH/IR stars using 1612 MHz hydroxyl masers. I. Annual parallaxes of WX Psc and OH138.0+7.2 [SSA]

We report on the measurement of the trigonometric parallaxes of 1612 MHz hydroxyl masers around two asymptotic giant branch stars, WX Psc and OH138.0+7.2, using the NRAO Very Long Baseline Array with in-beam phase referencing calibration. We obtained a 3-sigma upper limit of <=5.3 mas on the parallax of WX Psc, corresponding to a lower limit distance estimate of >~190 pc. The obtained parallax of OH138.0+7.2 is 0.52+/-0.09 mas (+/-18%), corresponding to a distance of 1.9(+0.4,-0.3) kpc, making this the first hydroxyl maser parallax below one milliarcsecond. We also introduce a new method of error analysis for detecting systematic errors in the astrometry. Finally, we compare our trigonometric distances to published phase-lag distances toward these stars and find a good agreement between the two methods.

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G. Orosz, H. Imai, R. Dodson, et. al.
Thu, 19 Jan 17

Comments: Preprint, accepted for publication in The Astronomical Journal (January 17, 2017)