A mildly relativistic outflow from the energetic, fast-rising blue optical transient CSS161010 in a dwarf galaxy

• Published in: arXiv.org 2020-05
• Main Authors: Coppejans, D L, Margutti, R, Terreran, G, Nayana, A J, Coughlin, E R, Laskar, T, Alexander, K D, Bietenholz, M, Caprioli, D, Chandra, P, Drout, M, Frederiks, D, Frohmaier, C, Hurley, K, Kochanek, C S, MacLeod, M, Meisner, A, Nugent, P E, Ridnaia, A, Sand, D J, Svinkin, D, Ward, C, Yang, S, Baldeschi, A, Chilingarian, I V, Dong, Y, Esquivia, C, Fong, W, Guidorzi, C, Lundqvist, P, Milisavljevic, D, Paterson, K, Reichart, D E, Shappee, B, Stroh, M C, Valenti, S, Zauderer, A, Zhang, B
• Format: Article
• Language: English
• Subjects: Black holes; Disruption; Dwarf galaxies; Ejecta; Gamma ray bursts; Gamma rays; Kinetic energy; Massive stars; Outflow; Radio astronomy; Radio emission; Radio observation; Relativistic effects; Relativistic velocity; Star & galaxy formation; Star formation rate; Stellar evolution; Stellar mass; Transients (astronomy); X-rays
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2003.10503

We present X-ray and radio observations of the Fast Blue Optical Transient (FBOT) CRTS-CSS161010 J045834-081803 (CSS161010 hereafter) at t=69-531 days. CSS161010 shows luminous X-ray (\(L_x\sim5\times 10^{39}\,\rm{erg\,s^{-1}}\)) and radio (\(L_{\nu}\sim10^{29}\,\rm{erg\,s^{-1}Hz^{-1}}\)) emission. The radio emission peaked at ~100 days post transient explosion and rapidly decayed. We interpret these observations in the context of synchrotron emission from an expanding blastwave. CSS161010 launched a mildly relativistic outflow with velocity \(\Gamma\beta c\ge0.55c\) at ~100 days. This is faster than the non-relativistic AT2018cow (\(\Gamma\beta c\sim0.1c\)) and closer to ZTF18abvkwla (\(\Gamma\beta c\ge0.3c\) at 63 days). The inferred initial kinetic energy of CSS161010 (\(E_k\gtrsim10^{51}\) erg) is comparable to that of long Gamma Ray Bursts (GRBs), but the ejecta mass that is coupled to the mildly relativistic outflow is significantly larger (\(\sim0.01-0.1\,\rm{M_{\odot}}\)). This is consistent with the lack of observed gamma-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 \(M_{*}\sim10^{7}\,\rm{M_{\odot}}\) and specific star formation rate \(sSFR\sim 0.3\,\rm{Gyr^{-1}}\). 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 (rate \(