Swift, Loiano and LCOGT observations of ASASSN-17gs/AT2017egv (Fermi J1544-0649) - awakening of a dormant high-energy peaked blazar?
ATel #10642; K. Sokolovsky (IAASARS NOA/ASC Lebedev/SAI MSU), F. Cusano (INAF-OABo), M. Dominik (University of St Andrews), S. Hodgkin (IoA/Cambridge), K. A. Rybicki, L. Wyrzykowski, M. Gromadzki, A. Hamanowicz (Warsaw Observatory), T. W.-S. Holoien (Ohio State) and S. Ciprini (SSDC-ASI Rome and INFN), C. C. Cheung (Naval Research Laboratory), on behalf of the Fermi LAT Collaboration
on 15 Aug 2017; 14:11 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)
Subjects: Optical, Ultra-Violet, X-ray, Request for Observations, AGN, Black Hole, Blazar, Quasar, Supernovae, Transient, Tidal Disruption Event
Referred to by ATel #: 11434
ASASSN-17gs is an extragalactic transient of uncertain nature
detected in GeV gamma rays (Fermi LAT), X-rays (Swift, MAXI) and
optical bands (ATel #10482, #10491, #10495) near the center of
the galaxy 2MASX J15441967-0649156. Swift observed ASASSN-17gs for
21.7ks split in 15 epochs between 2017-05-26 and 2017-08-09
following up the discovery of the Fermi LAT transient. Swift/XRT
detected a bright X-ray source with a net count rate of
0.856 +/-0.006 cts/s at the position of the transient. Fixing
the absorbing column density to the Galactic value
n_H = 8.7x10^20 cm^-2 (Kalberla et al. 2005 A&A, 440, 775),
the average spectrum can be fit by an absorbed power law with
the photon index of 1.86 +/-0.01 and the unabsorbed 0.3-10 keV flux
of 4.7x10^-11 ergs/cm^2/s. The XRT count rate varies irregularly by
up to a factor of four between the epochs and shows
no obvious decline.
Swift/UVOT and ground-based Loiano 1.5m and LCOGT 1m photometry
reveal irregular variations with the amplitude of up to 1 mag in UV
bands (0.6 mag in i-band) that do not seem to be correlated with
the XRT count rate. The mean observed magnitudes are W2=16.87,
M2=16.99, W1=16.89, U=16.95, B=17.56, V=17.00. Taking into account
the source redshift and the Galactic reddening of E(B-V)=0.139
(Schlafly et al. 2011 ApJ, 737, 103) the UV magnitudes (expected to
be less affected by the host light) correspond to the color
temperature ~20000 K.
We use Swift/UVOT images to measure the transient's position with
respect to the UCAC3 (Zacharias et al. 2010 AJ, 139, 2184) stars in
the field of view, RA = 15:44:19.671, Decl. = -06:49:15.35 (J2000)
which is 0.4" from the 2MASX galaxy position and 1.7" away from
the position originally reported by ASASSN.
The host galaxy of ASASSN-17gs is not listed in X-ray catalogs, but
is a radio source NVSS J154419-064913 (46.6 +/-1.5 mJy at 1.4 GHz;
Condon et al. 1998 AJ, 115, 1693), GLEAM J154419-064930
(77 +/-18 mJy in the 170-231 MHz band; Hurley-Walker et al. 2017
MNRAS, 464, 1146) suggesting previous AGN activity. One of the
likely explanations for the transient event is a bright flare of
a previously unknown GeV blazar with its relativistic jet producing
most of the observed GeV-to-radio emission. We encourage further
multiwavelength observations to confirm this hypothesis and follow
the development of the flare.
We thank the Swift team for performing ToO monitoring observations
of this unusual transient and Dr. D. J. Thompson, Dr. E. Cavazzuti,
Dr. S. Digel, Dr. S. Buson and Dr. P. T. Tam for the suggestions
that helped to improve this telegram. The photometric calibrations
of Loiano and LCOGT data were performed using the
Cambridge Photometric Calibration Server (CPCS),
designed and maintained by Sergey Koposov and Lukasz Wyrzykowski.
X-ray/UV/optical lightcurve of ASASSN-17gs/AT2017egv
