A Mildly Relativistic Outflow from the Energetic, Fast-rising Blue Optical Transient CSS161010 in a Dwarf Galaxy

Abstract

We present X-ray and radio observations of the Fast Blue Optical Transient CRTS-CSS161010 J045834 −081803 (CSS161010 hereafter) at t=69-531 days. CSS161010 shows luminous X-ray (L x ∼5× 10 39 erg s −1 ) and radio (L ν ∼10 29 erg s −1 Hz −1 ) emission. The radio emission peaked at ∼100 days posttransient explosion and rapidly decayed. We interpret these observations in the context of synchrotron emission from an expanding blast wave. CSS161010 launched a mildly relativistic outflow with velocity Γβc0.55c at ∼100 days. This is faster than the non-relativistic AT 2018cow (Γβc∼0.1c) and closer to ZTF18abvkwla (Γβc0.3c at 63 days). The inferred initial kinetic energy of CSS161010 (E k 10 51 erg) is comparable to that of long gamma-ray bursts, but the ejecta mass that is coupled to the mildly relativistic outflow is significantly larger ( -). This is consistent with the lack of observed γ-rays. The luminous X-rays were produced by a different emission component to the synchrotron radio emission. CSS161010 is located at ∼150 Mpc in a dwarf galaxy with stellar mass M * ∼10 7 M e and specific star formation rate sSFR∼0.3 Gyr −1 . This mass is among the lowest inferred for host galaxies of explosive transients from massive stars. Our observations of CSS161010 are consistent with an engine-driven aspherical explosion from a rare evolutionary path of a H-rich stellar progenitor, but we cannot rule out a stellar tidal disruption event on a centrally located intermediate-mass black hole. Regardless of the physical mechanism, CSS161010 establishes the existence of a new class of rare

Fast Optical Transients from Stellar-mass Black Hole Tidal Disruption Events in Young Star Clusters

“…Although it remains to be determined whether bright radio emission is a defining feature of the FBOT class broadly, radio emission (consistent with self-absorbed synchrotron radiation) is observed for the AT2018cow, ZTF18abvkwla, and CSS161010 events. This radio emission suggests the presence of circumstellar medium with densities ranging from roughly 10-10 6 cm −3 , for various observation epochs and for a range of microphysics assumptions (Margutti et al 2019;Coppejans et al 2020;Ho et al 2020). If prior to its tidal disruption, a main-sequence star is tidally captured by a BH, such that a small amount of orbital energy is deposited into the star (e.g., Fabian et al 1975, see left-most panel of Figure 5 for illustration), a small amount of debris may be unbound from the star.…”

“…The majority of FBOTs have been identified via archival searches of various optical surveys, including the Pan-STARRS1 Medium Deep Survey (Drout et al 2014), the Dark Energy Survey (Pursiainen et al 2018), Kepler (Rest et al 2018), the Supernova Legacy Survey (Arcavi et al 2016), and the Zwicky Transient Facility (ZTF; Ho et al 2020). In addition to the archival searches, a handful of FBOTs have been discovered while still active, notably AT2018cow (also ATLAS18qqn; Prentice et al 2018;Rivera Sandoval et al 2018;Smartt et al 2018;Margutti et al 2019), ZTF18abvkwla (Ho et al 2020), and CSS161010 (Coppejans et al 2020), enabling X-ray and/or radio follow-up.…”

“…Various analyses have computed volumetric rates of transients similar to FBOTs, with estimates ranging from roughly 100 Gpc −3 yr −1 to more than 1000 Gpc −3 yr −1 in the local universe (e.g., Drout et al 2014;Pursiainen et al 2018;Coppejans et al 2020;Ho et al 2020). Although the precise rate remains uncertain, the general consensus appears to be that FBOTs are roughly two to three orders of magnitude rarer than standard core-collapse supernovae (e.g., Botticella et al 2008).…”

“…Thus, the catalog of observed transients of both known and unknown origin is growing and will continue to grow rapidly. We conclude this study by examining the electromagnetic (EM) features of stellar-mass BH TDEs and compare these events specifically with the emerging class of Fast Blue Optical Transients (FBOTs; e.g., Drout et al 2014;Arcavi et al 2016;Pursiainen et al 2018;Rest et al 2018;Margutti et al 2019;Coppejans et al 2020;Ho et al 2020). On the basis of event rates, host-galaxy properties, and overall transient features such as rise times and peak luminosities, we demonstrate that stellarmass BH TDEs may indeed be a viable mechanism for FBOTlike events.…”

The Zwicky Transient Facility Bright Transient Survey. II. A Public Statistical Sample for Exploring Supernova Demographics*

“…As most of these evolved into relatively ordinary SN classes later in their evolution, it is not obvious that any additional special conditions are required beyond dense, extended CSM in most cases to produce a rapidly evolving transient, and we infer that this is also the case for most short-timescale events in the Dark Energy Survey, Pan-STARRS, and other earlier surveys. AT2018cow-like events, with their luminous multiwavelength emission (Ho et al 2019a(Ho et al , 2020bMargutti et al 2019;Coppejans et al 2020), represent a dramatic but rare exception, and certain SNe may also require an engine-driven jet to power the fast, energetic shock inferred from radio and X-ray observations (Ho et al 2020c).…”