A multiwavelength study of the relativistic tidal disruption canidate Sw J2058+05 at late times

Swift J2058.4+0516 (Sw J2058+05, hereafter) has been suggested as the second member (after Sw J1644+57) of the rare class of tidal disruption events accompanied by relativistic ejecta. Here we report a multiwavelength (X-ray, ultraviolet/optical/infrared, radio) analysis of 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 ≳160 within a span of Δt/t ≤ 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 104−6 M⊙. (2) We detect rapid (≲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 ≲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±4.5 AU (∼1015 cm) at all times during the outburst, the blackbody temperature drops significantly from ∼ 30,000 K at early times to a value of ∼ 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. For such systems, we suggest the rapid X-ray dropoff as a diagnostic for black hole mass.