A Mildly Relativistic Outflow from the Energetic, Fast-rising Blue Optical Transient CSS161010 in a Dwarf Galaxy
Lundqvist, P.; Reichart, D. E.; Kochanek, C. S.; Nugent, P. E.; Ridnaia, A.; Dong, Y.; Laskar, T.; Zhang, B.; Fong, W.; Sand, D. J.; Valenti, S.; Meisner, A.; Guidorzi, C.; Terreran, G.; Yang, S.; Milisavljevic, D.; Margutti, R.; Drout, M. R.; Chilingarian, I. V.; Alexander, K. D.; Bietenholz, M.; Coppejans, D. L.; Coughlin, E. R.; Paterson, K.; Zauderer, B. A.; Stroh, M. C.; Baldeschi, A.; Svinkin, D.; Nayana, A. J.; Frohmaier, C.; Chandra, P.; Frederiks, D.; Ward, C.; Shappee, B.; Caprioli, D.; Hurley, K. H.; MacLeod, M.; Esquivia, C.
United States, Canada, United Arab Emirates, India, United Kingdom, South Africa, Russia, Italy, Sweden, Denmark
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 (LX ∼ 5 × 1039 erg s-1) and radio (Lν ∼ 1029 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 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 (Ek ≳ 1051 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 ( $\sim 0.01\mbox{--}0.1\,{M}_{\odot }$ ). 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* ∼ 107 M⊙ 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 (rate < 0.4% of the local core-collapse supernova rate) H-rich transients that can launch mildly relativistic outflows.