Swift detection of the new WZ Sge-type dwarf nova ASASSN-19hl
ATel #12629; K. Sokolovsky, E. Aydi, L. Chomiuk, A. Kawash, J. Strader (MSU), K. Mukai (NASA/GSFC), I. Mereminskiy (IKI), D. Denisenko (SAI), K. Z. Stanek, C. S. Kochanek, J. Shields (OSU), B. J. Shappee (Univ. of Hawaii)
on 4 Apr 2019; 15:50 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)
Subjects: Ultra-Violet, X-ray, Cataclysmic Variable, Transient
Referred to by ATel #: 12947
The optical transient
ASASSN-19hl
discovered on 2019-03-27.03 UT was identified as a WZ Sge-type
dwarf nova based on its spectrum (ATel #12613), white color
(vsnet-alert 23090) and early superhumps detected in its
lightcurve (vsnet-alert 23103).
Swift/XRT observed ASASSN-19hl for 0.7ks on 2019-03-28.639 and
detected a hard X-ray source with the net count rate of
0.21 +/-0.02 cts/s at the position of the transient. The X-ray
spectrum can be fit with a combination of two thermal components
(apec) having kT=0.14 +/-0.03 keV and kT>5 keV with negligible
foreground absorption. The 0.3-10 keV flux of that model
is 8x10^-12 ergs/cm^2/s. The corresponding ultraviolet source is
saturated in the Swift/UVOT images preventing accurate photometry
and astrometry (needed to confirm the Gaia source association).
At the distance of 320pc to the possible Gaia counterpart,
the observed 0.3-10 keV flux translates to the luminosity of almost
10^32 ergs/s. This is high compared with those of dwarf novae
in quiescence in general (Byckling et al. 2010 MNRAS, 408, 2298),
let alone those of optically faint cataclysmic variable similar to
quiescent WZ Sge type dwarf novae (Reis et al. 2013, MNRAS, 430,
1994). This high luminosity is particularly remarkable, in
the context of the paradigm in which dwarf novae in outburst become
less X-ray luminous than in quiescence (Wheatley et al. 2003, MNRAS,
345, 49). It may be that the X-ray luminosity of the WZ Sge type
dwarf novae increases during outburst - cf. GW Lib (Byckling et al.
2009, MNRAS, 399, 1576), SSS J122221.7-311525 (Neustroev et al. 2018,
A&A, 611, 13), V805 Aur (ATel #4954), ASASSN-18fs (Parikh &
Wijnands 2018, RNAAS, 2d, 201) and TCP J06373299-0935420
(ATel #12531).
We thank the Neil Gehrels Swift Observatory team and
PI, Brad Cenko, for rapid scheduling of this ToO observation.
ASAS-SN lightcurve of ASASSN-19hl
