A Multiwavelength Study of the Relativistic Tidal Disruption Candidate Sw J2058+05 at Late Times

Abstract

We report a multiwavelength (X-ray, ultraviolet/optical/infrared, radio) analysis of the relativistic tidal disruption event candidate Sw J2058+05 from 3 months to 3 yr post-discovery in order to study its properties and compare its behavior with that of Sw J1644+57. Our main results are as follows. (1) The long-term X-ray light curve of Sw J2058+05 shows a remarkably similar trend to that of Sw J1644+57. After a prolonged power-law decay, the X-ray flux drops off rapidly by a factor of \(\gtrsim 160\) within a span of \(\Delta\)$t\(/\)t\( \)\le\( 0.95. Associating this sudden decline with the transition from super-Eddington to sub-Eddington accretion, we estimate the black hole mass to be in the range of \)10^{4-6}\( M\)_{\odot}\(. (2) We detect rapid (\)\lesssim 500\( s) X-ray variability before the dropoff, suggesting that, even at late times, the X-rays originate from close to the black hole (ruling out a forward-shock origin). (3) We confirm using HST and VLBA astrometry that the location of the source coincides with the galaxy's center to within \)\lesssim 400\( pc (in projection). (4) We modeled Sw J2058+05's ultraviolet/optical/infrared spectral energy distribution with a single-temperature blackbody and find that while the radius remains more or less constant at a value of \)63.4 \pm 4.5\( AU (\)\sim 10^{15}\( cm) at all times during the outburst, the blackbody temperature drops significantly from \)\sim\( 30,000 K at early times to a value of \)\sim$ 15,000 K at late times (before the X-ray dropoff). Our results strengthen Sw J2058+05's interpretation as a tidal disruption event similar to Sw J1644+57.