Continuum-fitting the X-Ray Spectra of Tidal Disruption Events

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 edgeon 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) x 10(6)M(circle dot). For ASASSN-14li, the SMBH mass is 10(-7)(+1) x 10(6) M-circle dot, 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 proportional to t(-1.1), perhaps due to inefficient debris circularization. Over approximate to 1100 days, its SMBH has accreted Delta M approximate to 0.17M(circle dot), implying a progenitor star mass of >0.34M(circle dot), 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.