Inspirals from the innermost stable circular orbit of Kerr black holes: Exact solutions and universal radial flow
(2022)
AT2019azh: an unusually long-lived, radio-bright thermal tidal disruption event
Monthly Notices of the Royal Astronomical Society Oxford University Press 511:4 (2022) 5328-5345
Abstract:
Tidal disruption events (TDEs) occur when a star is destroyed by a supermassive black hole at the centre of a galaxy, temporarily increasing the accretion rate on to the black hole and producing a bright flare across the electromagnetic spectrum. Radio observations of TDEs trace outflows and jets that may be produced. Radio detections of the outflows from TDEs are uncommon, with only about one-third of TDEs discovered to date having published radio detections. Here, we present over 2 yr of comprehensive, multiradio frequency monitoring observations of the TDE AT2019azh taken with the Very Large Array and MeerKAT radio telescopes from approximately 10 d pre-optical peak to 810 d post-optical peak. AT2019azh shows unusual radio emission for a thermal TDE, as it brightened very slowly over 2 yr, and showed fluctuations in the synchrotron energy index of the optically thin synchrotron emission from 450 d post-disruption. Based on the radio properties, we deduce that the outflow in this event is likely non-relativistic and could be explained by a spherical outflow arising from self-stream intersections or a mildly collimated outflow from accretion on to the supermassive black hole. This data set provides a significant contribution to the observational data base of outflows from TDEs, including the earliest radio detection of a non-relativistic TDE to date, relative to the optical discovery.Tidal disruption event discs are larger than they seem: removing systematic biases in TDE X-ray spectral modelling
Monthly Notices of the Royal Astronomical Society: Letters Oxford University Press 507:1 (2021) L24-L28
Abstract:
The physical sizes of tidal disruption event (TDE) accretion discs are regularly inferred, from modelling of the TDEs X-ray spectrum as a single-temperature blackbody, to be smaller than the plausible event horizons of the black holes which they occur around – a clearly unphysical result. In this Lltter, we demonstrate that the use of single-temperature blackbody functions results in the systematic underestimation of TDE accretion disc sizes by as much as an order of magnitude. In fact, the radial ‘size’ inferred from fitting a single-temperature blackbody to an observed accretion disc X-ray spectrum does not even positively correlate with the physical size of that accretion disc. We further demonstrate that the disc-observer inclination angle and absorption of X-ray photons may both lead to additional underestimation of the radial sizes of TDE discs, but by smaller factors. To rectify these issues, we present a new fitting function which accurately reproduces the size of an accretion disc from its 0.3−10 keV X-ray spectrum. Unlike traditional approaches, this new fitting function does not assume that the accretion disc has reached a steady-state configuration, an assumption which is unlikely to be satisfied by most TDEs.An upper observable black hole mass scale for tidal destruction events with thermal X-ray spectra
Monthly Notices of the Royal Astronomical Society Oxford University Press 505:2 (2021) 1629-1644
Abstract:
We comprehensively model the X-ray luminosity emergent from time-dependent relativistic accretion discs, developing analytical models of the X-ray luminosity of thermal disc systems as a function of black hole mass M, disc mass Md, and disc α-parameter. The X-ray properties of these solutions will be directly relevant for understanding tidal disruption event (TDE) observations. We demonstrate an extremely strong suppression of thermal X-ray luminosity from large mass black holes, LX ∼ exp (− m7/6), where m is a dimensionless mass, roughly the black hole mass in unity of 106M⊙. This strong suppression results in upper observable black hole mass limits, which we demonstrate to be of order Mlim ≃ 3 × 107M⊙, above which thermal X-ray emission will not be observable. This upper observable black hole mass limit is a function of the remaining disc parameters, and the full dependence can be described analytically (equation 82). We demonstrate that the current population of observed X-ray TDEs is indeed consistent with an upper black hole mass limit of order M ∼ 107M⊙, consistent with our analysis.A maximum X-ray luminosity scale of disc-dominated tidal destruction events
Monthly Notices of the Royal Astronomical Society Oxford University Press 504:4 (2021) 5144-5154