Abstract:
We present AT2020mrf (SRGe J154754.2+443907), an extra-galactic (z = 0.1353) fast blue optical transient
(FBOT) with a rise time of tg,rise = 3.7 days and a peak luminosity of Mg,peak = −20.0. Its optical spectrum around
peak shows a broad (v ∼ 0.1c) emission feature on a blue continuum (T ∼ 2 × 104 K), which bears a striking
resemblance to AT2018cow. Its bright radio emission (νLν = 1.2 × 1039 erg s−1
; νrest = 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 (0.07–0.08c) forward shock and a dense environment ( M M - - 10 yr 3 1 for vw = 103 km s−1
).
AT2020mrf occurs in a galaxy with M* ∼ 108 Me and specific star formation rate ∼10−10 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 (LX ∼ 2 × 1043 erg s−1
; 0.3–10 keV)
X-ray emission. The X-ray spectral shape (fν ∝ ν−0.8) and erratic intraday variability are reminiscent of AT2018cow,
but the luminosity is a factor of ∼20 greater than AT2018cow. At 328 days, Chandra detected it at LX ∼ 1042 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 ∼1 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.
