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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

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