Swift X-ray detection of the nuclear transient Gaia17dbg
ATel #11709; K. Sokolovsky (ASC Lebedev/SAI MSU), M. Gromadzki, L. Wyrzykowski (Warsaw Observatory), N. Ihanec (University of Nova Gorica)
on 13 Jun 2018; 01:15 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)
Subjects: Ultra-Violet, X-ray, AGN, Tidal Disruption Event
The transient source ATLAS17lbg = Gaia17dbg = AT2017gul coinciding
with the nucleus of the galaxy GALEXASC J225413.32-214100.1 was
identified by ATLAS on 2017-08-01 and later by Gaia on 2017-11-25.
Spectroscopic observations with SALT revealed a blue
continuum with emission lines at z=0.192 (ATel #11049),
H-alpha, H-beta and HeI emission lines were composed of
narrow (<350 km/s) and broad (FWHM ~= 2500 km/s) components.
No X-ray counterpart of the optical transient was found during
the two Swift observations on 2017-12-05 and 2017-12-29.
A new Swift/XRT observation of Gaia17dbg was obtained on 2018-06-08
(after the solar conjunction gap) resulted in a non-detection.
However, when the three Swift/XRT data sets are combined together,
the resulting 4.8ks exposure image reveals a four-sigma (0.004
+/-0.001 cts/s) X-ray source at the position of the transient.
The source is too faint to model its spectrum. Assuming
a power law model with the photon index of 2 and the Galactic
n_H1 = 1.81x10^20 cm^-2 (Kalberla et al. 2005 A&A, 440, 775),
the observed Swift/XRT count rate corresponds to the unabsorbed
0.3-10 keV flux of 1.3x10^-13 ergs/cm^2/s.
Swift/UVOT photometry (AB system)
JD Band MagAB Err.
2458093.07 UVW1 19.12 0.07
2458117.30 UVW1 18.91 0.07
2458278.14 UVW1 19.33 0.08
2458093.07 UVM2 19.23 0.08
2458117.30 UVM2 19.31 0.08
2458278.13 UVM2 19.42 0.09
2458093.07 UVW2 19.13 0.06
2458117.29 UVW2 19.04 0.06
2458278.13 UVW2 19.03 0.06
shows the source remains bright in UV compared to the archival
GALEX detection at NUV=21.1 +/-0.2 (Bianchi et al. 2011 Ap&SS,
335, 161). The X-ray detection and blue UV colors are consistent
with both AGN and TDE explanations of the Gaia17dbg event.
We thank the Neil Gehrels Swift Observatory team and PI,
Brad Cenko, for performing these ToO observations.
Gaia17dbg = ATLAS17lbg = AT2017gul at the Transient Name Server
