Abstract
We present the detection of persistent soft X-ray radiation with Lx ~ 1041-1042 erg s-1 at the location of the extremely luminous, double-humped transient ASASSN-15lh as revealed by Chandra and Swift. We interpret this finding in the context of observations from our multiwavelength campaign, which revealed the presence of weak narrow nebular emission features from the host-galaxy nucleus and clear differences with respect to superluminous supernova optical spectra. Significant UV flux variability on short timescales detected at the time of the rebrightening disfavors the shock interaction scenario as the source of energy powering the long-lived UV emission, while deep radio limits exclude the presence of relativistic jets propagating into a low-density environment. We propose a model where the extreme luminosity and double-peaked temporal structure of ASASSN-15lh is powered by a central source of ionizing radiation that produces a sudden change in the ejecta opacity at later times. As a result, UV radiation can more easily escape, producing the second bump in the light curve. We discuss different interpretations for the intrinsic nature of the ionizing source. We conclude that, if the X-ray source is physically associated with the optical-UV transient, then ASASSN-15lh most likely represents the tidal disruption of a main-sequence star by the most massive spinning black hole detected to date. In this case, ASASSN-15lh and similar events discovered in the future would constitute the most direct probes of very massive, dormant, spinning, supermassive black holes in galaxies. Future monitoring of the X-rays may allow us to distinguish between the supernova hypothesis and the hypothesis of a tidal disruption event.
Highlights
Optical surveys sampling the sky over timescales of a few days significantly advanced our knowledge of astronomical transients of different origins, including superluminous supernovae (SLSNe; Chomiuk et al 2011; Quimby et al 2011; GalYam 2012), very fast-rising stellar explosions (e.g., Drout et al 2014), and stellar tidal disruption events (TDEs) caused by supermassive black holes (Rees 1988; Komossa 2015)
We obtained four epochs of deep X-ray observations of ASASSN-15lh with the Chandra X-ray Observatory (CXO) on 2015 November 12, 2015 December 13 (10 ks), 2016 February 20 (40 ks), and 2016 August 19 (30 ks, PI Margutti), corresponding to dt = 129.4 days, dt = 154.6, dt = 210.5 days, and dt = 357.8 days rest-frame since optical maximum light, which occurred on 2015 June 5 (Dong et al 2016)
Our reanalysis of early-time spectra does not confirm the robust association of ASASSN-15lh with SLSNe claimed by previous studies, and invites us to be open-minded about the nature of ASASSN-15lh
Summary
Optical surveys sampling the sky over timescales of a few days significantly advanced our knowledge of astronomical transients of different origins, including superluminous supernovae (SLSNe; Chomiuk et al 2011; Quimby et al 2011; GalYam 2012), very fast-rising stellar explosions (e.g., Drout et al 2014), and stellar tidal disruption events (TDEs) caused by supermassive black holes (Rees 1988; Komossa 2015). The very large energy radiated by ASSASN-15lh (Erad ~ (1.5–2 × 1052 erg, Godoy-Rivera et al 2016) requires extreme properties of the progenitor star and sources of energy that are different from the standard radioactive decay of 56Ni that powers normal H-stripped SNe in the local universe (Chatzopoulos et al 2016; Dong et al 2016; Kozyreva et al 2016; van Putten & Della Valle 2017) In this context, the double-humped light curve of ASASSN-15lh has been interpreted by Chatzopoulos et al (2016) as a signature of the interaction of massive SN ejecta Mej ~ 36 M☉ with an H-poor circumstellar shell of MCSM ~ 20 M☉, possibly supplemented by radiation from a newly born rapidly rotating magnetar (Metzger et al 2015; Bersten et al 2016; Dai et al 2016; Sukhbold & Woosley 2016).
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