Continuum-Fitting the X-ray Spectra of Tidal Disruption Events

Sixiang Wen; Peter G. Jonker; Nicholas C. Stone; Ann I. Zabludoff; Dimitrios Psaltis

doi:10.3847/1538-4357/ab9817

paper

Continuum-Fitting the X-ray Spectra of Tidal Disruption Events

arXiv:2003.12583 · doi:10.3847/1538-4357/ab9817

Abstract

We develop a new model for X-ray emission from tidal disruption events (TDEs), applying stationary general relativistic ``slim disk'' accretion solutions to supermassive black holes (SMBHs) and then ray-tracing the photon trajectories from the image plane to the disk surface, including gravitational redshift, Doppler, and lensing effects self-consistently. We simultaneously and successfully fit the multi-epoch XMM-Newton X-ray spectra for two TDEs: ASASSN-14li and ASASSN-15oi. We test explanations for the observed, unexpectedly slow X-ray brightening of ASASSN-15oi, including delayed disk formation and variable obscuration by a reprocessing layer. We propose a new mechanism that better fits the data: a ``Slimming Disk'' scenario in which accretion onto an edge-on disk slows, reducing the disk height and exposing more X-rays from the inner disk to the sightline over time.For ASASSN-15oi, we constrain the SMBH mass to $4. 0_{- 3.1}^{+ 2.5} \times 1 0^{6} M_{⊙}$ . For ASASSN-14li, the SMBH mass is $1 0_{- 7}^{+ 1} \times 1 0^{6} M_{⊙}$ and the spin is $> 0.3$ . For both TDEs, our fitted masses are consistent with independent estimates; for ASASSN-14li, application of the external mass constraint narrows our spin constraint to $> 0.85$ . The mass accretion rate of ASASSN-14li decays slowly, as $\propto t^{- 1.1}$ , perhaps due to inefficient debris circularization. Over $\approx$ 1100 days, its SMBH has accreted $Δ M \approx 0.17 M_{⊙}$ , implying a progenitor star mass of $> 0.34 M_{⊙}$ , i.e., no ``missing energy problem.'' For both TDEs, the hydrogen column density declines to the host galaxy plus Milky Way value after a few hundred days, suggesting a characteristic timescale for the depletion or removal of obscuring gas.

arXiv commentAccepted for publication in ApJ

Continuum-Fitting the X-ray Spectra of Tidal Disruption Events

Abstract

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