iPTF16axa: A Tidal Disruption Event at z=0.108 [HEAP]


We report the discovery by the intermediate Palomar Transient Factory (iPTF) of tidal disruption event (TDE) candidate iPTF16axa at $z=0.108$, and present its broadband photometric and spectroscopic evolution from 3 months of follow-up observations with ground-based telescopes and \textsl{Swift}. The light curve is well fitted with a $t^{-5/3}$ decay, and we constrain the rise-time to peak to be $<$49 rest-frame days after disruption, a factor of $>3$ shorter than the fallback timescale expected for the $\sim 4\times$10$^{7}$ M$_\odot$ black hole inferred from the host galaxy luminosity. The UV and optical spectral energy distribution (SED) is well described by a constant blackbody temperature of T$\sim$ 3$\times$10$^4$ K over the monitoring period, with an observed peak luminosity of 1.1$\times$10$^{44}$ ergs$^{-1}$. The optical spectra are characterized by a strong blue continuum and broad \HeII \ and H$\alpha$ lines characteristic of TDEs. We compare the photometric and spectroscopic signatures of iPTF16axa with 11 TDEs in the literature with well-sampled optical light curves. Based on a single-temperature fit to the optical and near-UV photometry, most of these TDEs have peak luminosities confined between log (L [erg s$^{-1}$]) = 43.2-44.2, with constant temperatures of a few $\times 10^{4}$ K during their power-law declines, implying blackbody radii on the order of ten times the tidal disruption radius, that decrease monotonically with time. For TDEs with hydrogen and helium emission, the high helium-to-hydrogen ratios suggest that the emission arises from high-density gas, where nebular arguments break down. We find no correlation between the peak luminosity and the black hole mass, contrary to the expectations for TDEs to have $\dot{M} \propto M_{\rm BH}^{1/2}$.

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T. Hung, S. Gezari, N. Blagorodnova, et. al.
Tue, 7 Mar 17

Comments: 13 pages, 14 figures, submitted to ApJ