- Coppejans, DL
- Margutti, R
- Terreran, G
- Nayana, AJ
- Coughlin, ER
- Laskar, T
- Alexander, KD
- Bietenholz, M
- Caprioli, D
- Chandra, P
- Drout, MR
- Frederiks, D
- Frohmaier, C
- Hurley, KH
- Kochanek, CS
- MacLeod, M
- Meisner, A
- Nugent, PE
- Ridnaia, A
- Sand, DJ
- Svinkin, D
- Ward, C
- Yang, S
- Baldeschi, A
- Chilingarian, IV
- Dong, Y
- Esquivia, C
- Fong, W
- Guidorzi, C
- Lundqvist, P
- Milisavljevic, D
- Paterson, K
- Reichart, DE
- Shappee, B
- Stroh, MC
- Valenti, S
- Zauderer, BA
- Zhang, B
- et al.
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 ∼ 5 × 10 erg s ) and radio (L ∼ 10 erg s Hz ) 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 (E ⪆ 10 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 (∼ 0.01-0.1 M⊙). 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 M and specific star formation rate sSFR ∼ 0.3 Gyr . 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. x ν k * o˙ 39 -1 29 -1 -1 51 7 -1