[Paper Review] The X-ray and Radio Loud Fast Blue Optical Transient AT2020mrf: Implications for an Emerging Class of Engine-Driven Massive Star Explosions

Yuhan Yao, Anna Y. Q. Ho+10 more|arXiv (Cornell University)|Dec 1, 2021

Gamma-ray bursts and supernovaePhysics and Astronomy153 references57 citations

TL;DR

This paper presents AT2020mrf, an extremely luminous X-ray and radio-bright fast blue optical transient (FBOT) at z = 0.1353, exhibiting a 3.7-day rise time and peak absolute magnitude Mg,peak = −20.0. It identifies the event as a member of a rare class of engine-driven massive star explosions, powered by a central engine—likely a millisecond magnetar or accreting black hole—based on its extreme X-ray luminosity (up to ∼2×10⁴³ erg s⁻¹) and persistent variability, which exceeds that of AT2018cow by a factor of ∼20.

ABSTRACT

We present AT2020mrf (SRGe J154754.2$+$443907), an extra-galactic ($z=0.1353$) fast blue optical transient (FBOT) with a rise time of $t_{g, m rise}=3.7$ days and a peak luminosity of $M_{g, m peak}=-20.0$. Its optical spectrum around peak shows a broad ($v\sim0.1c$) emission feature on a blue continuum ($T\sim2 imes10^4$ K), which bears a striking resemblance to AT2018cow. Its bright radio emission ($ u L_ u = 1.2 imes 10^{39}\,{ m erg\,s^{-1}}$; $ u_{ m 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 ($\dot M \lesssim 10^{-3}\,M_\odot \,{ m yr^{-1}}$ for $v_{ m w}=10^3\,{ m km\,s^{-1}}$). AT2020mrf occurs in a galaxy with $M_\ast \sim 10^8\,M_\odot$ and specific star formation rate $\sim 10^{-10}\, { m yr^{-1}}$, 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 ($L_{ m X}\sim 2 imes 10^{43}\,{ m erg\,s^{-1}}$; 0.3-10 keV) X-ray emission. The X-ray spectral shape ($f_ u \propto u^{-0.8}$) 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 $L_{ m X}\sim10^{42}\,{ m erg\,s^{-1}}$, 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.

Motivation & Objective

• To characterize the multiwavelength properties of AT2020mrf, a newly discovered fast blue optical transient (FBOT), across X-ray, radio, and optical bands.
• To investigate the origin of its extreme X-ray luminosity and variability, which exceed those of known AT2018cow-like events by a factor of ∼20.
• To determine whether AT2020mrf is powered by a central engine—such as a millisecond magnetar or accreting black hole—based on its energetic and variable emission.
• To assess the host galaxy environment and its implications for the progenitor system, particularly its preference for low-mass, star-forming dwarf galaxies.
• To predict detection rates of similar events in upcoming all-sky X-ray surveys such as SRG and Einstein Probe, based on observed luminosity and rate constraints.

Proposed method

• Conducted multi-epoch optical photometry using ZTF and ATLAS forced-photometry services to construct the rest-frame light curve of AT2020mrf.
• Analyzed X-ray data from SRG/eROSITA and Chandra, measuring luminosities and spectral shapes (fν ∝ν⁻⁰.⁸) to assess variability and spectral evolution.
• Used radio light curves at 7.4 GHz (νrest) to model synchrotron emission from a forward shock interacting with dense circumstellar material (CSM), inferring ˙M ≲10⁻³ M⊙ yr⁻¹.
• Fitted the optical light curve using a CSM shock breakout and cooling model (CSM SBO+SCE), incorporating systematic uncertainties via a constant variance term σ₀².
• Compared X-ray light curves of AT2020mrf with those of long-duration gamma-ray bursts (GRBs) and AT2018cow to assess luminosity and decay trends.
• Estimated detection rates for AT2018cow-like and AT2020mrf-like events in future all-sky surveys using luminosity functions and survey sensitivity curves.

Experimental results

Research questions

• RQ1 What is the origin of the extreme X-ray luminosity (LX ∼2×10⁴³ erg s⁻¹) observed in AT2020mrf at 35–37 days post-explosion?
• RQ2 Does the persistent X-ray variability on day-timescales and the soft X-ray spectrum (fν ∝ν⁻⁰.⁸) indicate the presence of a central engine, such as a millisecond magnetar or accreting black hole?
• RQ3 How does the radio luminosity (νLν = 1.2×10³⁹ erg s⁻¹ at 7.4 GHz) constrain the properties of the circumstellar medium (CSM), such as mass-loss rate and density?
• RQ4 What are the host galaxy properties of AT2020mrf, and how do they support the hypothesis that AT2018cow-like events preferentially occur in low-mass, star-forming dwarf galaxies?
• RQ5 What are the expected detection rates of AT2018cow-like and AT2020mrf-like events in upcoming X-ray all-sky surveys such as SRG and Einstein Probe?

Key findings

• AT2020mrf exhibits the highest X-ray luminosity among known FBOTs, reaching LX ∼2×10⁴³ erg s⁻¹ at 35–37 days, a factor of ∼20 greater than AT2018cow.
• The X-ray spectrum (fν ∝ν⁻⁰.⁸) and strong day-timescale variability are consistent with a central engine, such as a millisecond magnetar or accreting black hole, powering the emission.
• Radio emission at 7.4 GHz (νrest) with νLν = 1.2×10³⁹ erg s⁻¹ is best explained by synchrotron radiation from a forward shock interacting with a dense CSM, implying ˙M ≲10⁻³ M⊙ yr⁻¹ for vw = 10³ km s⁻¹.
• The host galaxy has a stellar mass M∗ ∼10⁸ M⊙ and a specific star formation rate ∼10⁻¹⁰ yr⁻¹, consistent with the preference of AT2018cow-like events for low-mass, star-forming dwarfs.
• At 328 days, Chandra detected AT2020mrf at LX ∼10⁴² erg s⁻¹, which is >200 times more luminous than AT2018cow and CSS161010, with ongoing day-timescale variability.
• The study predicts that SRG and Einstein Probe will detect AT2018cow-like events at a rate of ∼1–10 per year and AT2020mrf-like events at a rate of ∼0.1–1 per year, based on luminosity and survey sensitivity.