The X-ray and Radio Loud Fast Blue Optical Transient AT2020mrf: Implications for an Emerging Class of Engine-Driven Massive Star Explosions (2112.00751v2)

Abstract: We present AT2020mrf (SRGe J154754.2$+$443907), an extra-galactic ($z=0.1353$) fast blue optical transient (FBOT) with a rise time of $t_{g,\rm rise}=3.7$ days and a peak luminosity of $M_{g,\rm peak}=-20.0$. Its optical spectrum around peak shows a broad ($v\sim0.1c$) emission feature on a blue continuum ($T\sim2\times10^4$ K), which bears a striking resemblance to AT2018cow. Its bright radio emission ($\nu L_\nu = 1.2\times 10^{39}\,{\rm erg\,s^{-1}}$; $\nu_{\rm rest}= 7.4$ GHz; 261 days) is similar to four other AT2018cow-like events, and can be explained by synchrotron radiation from the interaction between a sub-relativistic ($\gtrsim0.07$-$0.08c$) forward shock and a dense environment ($\dot M \lesssim 10^{-3}\,M_\odot \,{\rm yr^{-1}}$ for $v_{\rm w}=10^3\,{\rm km\,s^{-1}}$). AT2020mrf occurs in a galaxy with $M_\ast \sim 10^8\,M_\odot$ and specific star formation rate $\sim 10^{-10}\, {\rm yr^{-1}}$, supporting the idea that AT2018cow-like events are preferentially hosted by dwarf galaxies. The X-ray luminosity of AT2020mrf is the highest among FBOTs. At 35-37 days, SRG/eROSITA detected luminous ($L_{\rm X}\sim 2\times 10^{43}\,{\rm erg\,s^{-1}}$; 0.3-10 keV) X-ray emission. The X-ray spectral shape ($f_\nu \propto \nu^{-0.8}$) and erratic intraday variability are reminiscent of AT2018cow, but the luminosity is a factor of $\sim20$ greater than AT2018cow. At 328 days, Chandra detected it at $L_{\rm X}\sim10^{42}\,{\rm erg\,s^{-1}}$, which is $>200$ times more luminous than AT2018cow and CSS161010. At the same time, the X-ray emission remains variable on the timescale of $\sim1$ day. We show that a central engine, probably a millisecond magnetar or an accreting black hole, is required to power the explosion. We predict the rates at which events like AT2018cow and AT2020mrf will be detected by SRG and Einstein Probe.

• The paper identifies AT2020mrf as an engine-driven FBOT with a rapid 3.7-day rise and a peak luminosity of M_g,peak = -20.0.
• The paper reports exceptional radio (1.2×10^39 erg s^-1) and X-ray (2×10^43 erg s^-1) emissions indicating strong shock interactions with the circumstellar material.
• The paper posits that either a rapidly spinning magnetar or an accreting black hole powers the explosion, challenging traditional models of massive star deaths.

Insights into the Fast Blue Optical Transient AT2020mrf

The paper by Yao et al. presents an in-depth investigation of the Fast Blue Optical Transient (FBOT) AT2020mrf, highlighting its unique properties and postulating its inclusion in a new category of engine-driven supernovae. Located at a redshift of z = 0.1353, AT2020mrf is characterized by an exceptionally fast rise time and elevated peak luminosity in optical wavelengths, positioning it as a significant subject in the paper of transient astronomical phenomena.

AT2020mrf's observations exhibit several intriguing aspects. Notably, its optical light curve displayed a rapid rise time of 3.7 days and a peak luminosity marked by M_g,peak = -20.0. Its spectral analysis near peak reveals broad emission features on a blue continuum, reminiscent of the prototypical AT2018cow. Radio observations report impressive radiative output, with a radio luminosity of νL_ν = 1.2 × 10^39 erg s^-1 at 7.4 GHz, 261 days post-explosion, suggesting significant synchrotron emission from interactions between shocked material and dense circumstellar material (CSM).

The X-ray emission from AT2020mrf is particularly exceptional, with luminosities reported to reach 2 × 10^43 erg s^-1 in the 0.3–10 keV range at 35–37 days post-explosion. This figure represents the highest recorded among FBOTs to date, and the transient X-ray luminosity exceeds that of AT2018cow by a factor of 20.

The paper posits the necessity of a central engine to sustain such a powerful emission profile. Two candidates are proposed: a rapidly spinning magnetar or an accreting black hole, both of which could account for the remarkable energy output and rapid variability observed. There is also a strong association with a host galaxy marked by low metallicity and a star formation rate typical of dwarf galaxies, consistent with other events of this kind.

In terms of broader implications, AT2020mrf underscores the complexity of massive star deaths and challenges existing paradigms. It suggests the presence of diverse explosion mechanisms beyond traditional models, emphasizing the potential roles of compact remnants as persistent energy sources.

The paper raises significant speculative points about the development of transient survey technology and theoretical modeling. Continued observations with facilities capable of high-cadence multi-wavelength monitoring, like SRG/eROSITA and the upcoming Einstein Probe, offer promising prospects for cataloging and understanding these extraordinary events.

Considering current limitations in detection efficiency, the estimated detection rates of emissions similar to AT2020mrf in these surveys remain speculative but point toward a considerable population of such transients yet to be discovered. This broad potential for discovery challenges future models to incorporate complex physics of mass loss, interaction, and possible connections between transient types such as AT2018cow-like events and other well-known categories like superluminous supernovae (SLSNe) and gamma-ray bursts (GRBs).