Swift/XRT detection of a bright X-ray transient associated with ASASSN-18abj = AT 2018jro
ATel #12294; Kirill Sokolovsky, Elias Aydi, Laura Chomiuk (Michigan State University), Koji Mukai (GSFC)
on 14 Dec 2018; 01:32 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)
Subjects: Optical, Ultra-Violet, X-ray, Cataclysmic Variable, Nova, Transient, Variables
The g=11.2 optical transient ASASSN-18abj (AT 2018jro) was
discovered on 2018-12-12.109 UT by the ASAS-SN survey and reported
to the Transient Name Server. R. Fidrich confirmed the object on
2018-12-12.292 UT at V=10.8 (vsnet-alert 22827). T. Vanmunster
found fast irregular variability with ~0.05mag amplitude based
on a 3.6hr-long time series (vsnet-alert 22828).
We observed ASASSN-18abj with Swift for 2 ks on 2018-12-13.363 UT.
Swift/XRT detected a bright (0.101 +/-0.007 cts/s) X-ray source at
the position of the transient. The source spectrum is soft implying
negligible Galactic absorption and can be fit by the thermal
plasma model (APEC) with kT=10 +/-5 keV. The power-law model with
the photon index of 1.5 +/-0.1 also provides an acceptable fit.
The integrated 0.3-10 keV fluxes are (4.0 +/-0.4)x10^-12 and
(4.2 +/-0.4)x10^-12 ergs/cm^2/s for the APEC and power-law models,
respectively. Introducing an absorption component with free n_H1
into the model results in n_H1<10^21 cm^-2, consistent with zero
within the uncertainties. The source is saturated in the UVOT
ultraviolet image suggesting its magnitude UVW2<11 (Vega system).
The total Galactic absorbing column in the direction of this source
is n_H1 = 5.33x10^21 cm^-2 (Kalberla et al. 2005 A&A, 440, 775)
and the optical extinction is E(B-V)=0.824 (Schlafly & Finkbeiner
2011, ApJ, 737, 103), implying a UVW2 band extinction of 6.8mag
(Roming et al. 2009, ApJ, 690, 163). Both the absence of detectable
X-ray absorption and UV brightness of the source suggest that
ASASSN-18abj is nearby, foreground to much of the Galactic column.
Therefore, this object is unlikely to be a classical nova but may
be a high-amplitude, WZ Sge-type, dwarf nova, perhaps similar to,
but more distant than, GW Lib (Hiroi et al. 2009 PASJ, 61, 697 and
Byckling et al. 2009, MNRAS, 399, 1567). Spectroscopic observations
are needed to test this hypothesis.
We thank the Neil Gehrels Swift Observatory team and PI, Brad Cenko,
for rapid scheduling of this ToO observation.
ASASSN-18abj = AT 2018jro at the Transient Name Server
