Classification of the optical transient ASASSN-19gt as a dwarf nova
ATel #12611; E. Aydi, K. V. Sokolovsky, J. Strader, L. Chomiuk, L. Shishkovsky, A. Kawash (MSU), K. Z. Stanek, C. S. Kochanek, J. Shields (OSU), B. J. Shappee (Univ. of Hawaii), and J. Prieto (Universidad Diego Portales)
on 26 Mar 2019; 02:44 UT
Credential Certification: Elias Aydi (eaydi@saao.ac.za)
Subjects: Optical, X-ray, Binary, Cataclysmic Variable, Star, Transient
We report optical, UV, and X-ray observations of the transient
ASASSN-19gt which was discovered by the All-Sky Automated Survey
for Supernovae (ASAS-SN) on 2019-03-18.2UT.
We obtained a 1200s low-resolution spectrum of ASASSN-19gt with
the Goodman spectrograph on the SOAR telescope on 2019-03-25.31UT.
The spectrum shows a blue continuum with Balmer and He I lines,
characterized by multiple components. Hbeta, Hgamma, and some of
the He I lines show an absorption component at a heliocentric
radial velocity of +100 +\- 50 km/s with a blueshifted emission
component at around -550 +\- 50 km/s. Halpha shows two emission
components: a blueshifted one at a heliocentric radial velocity
of -500 km/s +\- 50 km/s and a redshifted one at a heliocentric
radial velocity of +650 km/s +\- 50 km/s, both likely originating
from a high-inclination accretion disk. The FWHM of the blueshifted
and redshifted emission components of Halpha are 530 +\- 50 km/s
and 760 +\- 50 km/s, respectively (corrected for instrumental
width). The overall spectrum is characteristic of
a dwarf nova outburst.
We also observed ASASSN-19gt with the Neil Gehrels Swift
Observatory for 0.5ks split between 2019-03-20 and 2019-03-25.
Swift/XRT detected no X-ray source at the position of the transient
with an upper limit of 0.005 +/-0.003 cts/s. Assuming power law
emission with the photon index of 2 and HI column density of
1.43x10^22 cm^-2 this translates to the unabsorbed 0.3-10 keV flux
limit of 6.7x10^-13 ergs/cm^2/s. Swift/UVOT detected an ultraviolet
source at the position 11:40:33.14 -62:50:17.2 +/-0.2" J2000
(measured using SOAR and UVOT images relative to UCAC3 stars in
the field) with the following magnitudes (Vega system):
JD Band Mag. Err.
2458563.08 UVM2 15.59 0.09
2458567.85 UVW1 15.93 0.04
The ultraviolet source does not appear to be highly reddened from
comparing UVM2, UVW1 and ASAS-SN photometry and neglecting
variability between the non-simultaneous observations.
This suggests ASASSN-19gt is in front of most of the dust producing
E(B-V)=2.41 reddening in this direction (Schlafly & Finkbeiner 2011
ApJ, 737, 103).
Line identifications:
H I: 3889, 3970, 4102, 4340, 4861, and 6563 A.
He I: 5876 and 7178 A.
Unidentified line: 6281 A.
We thank the Neil Gehrels Swift Observatory team and PI, Brad Cenko, for
rapid scheduling of this ToO observation.
