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 (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 $<0.4\%$ of the local core-collapse supernova rate) H-rich transients that can launch mildly relativistic outflows.

An Analysis of the Mildly Relativistic Outflow from CSS161010

This paper presents a detailed investigation of the mildly relativistic outflow associated with the Fast Blue Optical Transient (FBOT) CSS161010, observed within a dwarf galaxy located at a distance of approximately 150 Mpc. The transient's luminous X-ray and rapidly decaying radio emissions, identified during observations spanning 69-531 days post-explosion, suggest significant divergence from standard supernova models. The authors attribute these emissions to synchrotron radiation from an expanding blastwave, pointing to the presence of a mildly relativistic outflow with a velocity of (\Gamma \beta c \geq 0.55c) at roughly 100 days post-explosion.

In examining CSS161010, the researchers observe that its inferred initial kinetic energy ((E_k \gtrsim 10^{51}) erg) parallels that typically associated with long Gamma-Ray Bursts (GRBs), distinguishing it with a considerably larger ejecta mass ((\sim0.01-0.1 M_{\odot})). Despite this, no gamma-rays were detected, indicating an engine-driven aspherical explosion potentially originating from a rare hydrogen-rich stellar progenitor's evolutionary path. The significant density of the surrounding medium ((\dot{M} \approx 2 \times 10^{-4} M_{\odot}) yr(^{-1})) further underscores this rare event nature.

The accompanying dwarf galaxy, with stellar mass (M_{*} \sim 10^{7} M_{\odot}), is among the least massive host-galaxies of such energetic transients. Its specific star formation rate ((\sim 0.3 \, \mathrm{Gyr}^{-1})), however, aligns with other transient hosts typically associated with massive stars' explosive deaths.

The conclusions drawn indicate that CSS161010 represents a novel category of rare H-rich transients capable of producing mildly relativistic outflows. Its characteristics challenge traditional perceptions of stellar explosions, demanding consideration of alternative scenarios such as a potential stellar tidal disruption event by a centrally located intermediate mass black hole. However, this hypothesis remains speculative owing to the absence of definitive evidence. Crucially, the research demonstrates that CSS161010 fulfills less than (0.4\%) of the local core-collapse supernova rate, underscoring its rarity.

This paper’s implications reverberate both practically and theoretically, suggesting the necessity to revise models encompassing transients like CSS161010. Further exploration into the dynamical interactions and progenitor environments leading to such phenomena can enrich our comprehension of cosmic transient mechanisms and their potential utility in identifying intermediate mass black holes within dwarf galaxies. Additionally, this study highlights the importance of multi-wavelength observations in discerning the multi-faceted nature of these explosive cosmic events.