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Swift and SALT observations the nuclear transient Gaia17dbg = ATLAS17lbg = AT2017gul

ATel #11049; K. Sokolovsky (IAASARS NOA/ASC Lebedev/SAI MSU), M. Gromadzki, L. Wyrzykowski (Warsaw Observatory), N. Ihanec (University of Nova Gorica), B. Van Soelen (UFS) and D. Buckley (SAAO)
on 11 Dec 2017; 18:21 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)

Subjects: Optical, Ultra-Violet, X-ray, AGN, Supernovae, Transient, Tidal Disruption Event

The optical transient ATLAS17lbg = Gaia17dbg = AT2017gul was discovered by ATLAS on 2017-08-01.5 UT and independently by Gaia on 2017-11-25.0. The ATLAS flux reported at the TNS is below the reported detection limit and the previous ATLAS non-detection is listed on the same night. However, this report may be a hint of the transient being active since August 1st, consistent with the latest Gaia non-detection on 2017-07-13. The transient is coinciding with the center of the galaxy GALEXASC J225413.32-214100.1

We obtained the first SALT/RSS spectrum of Gaia17dbg on 2017-11-28.8 (spectral range 370.0-900.0 nm) with an (averaged) spectral resolution of 350 and exposure time of 1200s. The spectrum consists of a blue continuum with superposed Balmer, [OII], [OIII], [NIII], [SII], HeII 468.6 nm and HeI 578.6 nm emission lines at z=0.192. The second spectrum obtained on 2017-11-29.8 (spectral range 518.7-821.4 nm, R~1100, 1200s exposure) reveals H-alpha, H-beta and HeI emission lines composed of unresolved (<350 km/s) and resolved (FWHM ~2500 km/s) components.

Swift observed Gaia17dbg for 1.8ks on 2017-12-05.6. Swift/XRT detected no X-ray source at the position of the transient with an upper limit of 0.004 +/-0.002 cts/s. Assuming power law emission with the photon index of 2 and HI column density of 1.81x10^20 cm^-2 this translates to the unabsorbed 0.3-10 keV flux limit of 1.5x10^-13 ergs/cm^2/s.

An ultraviolet source is detected at the position of the transient with the following Swift/UVOT magnitudes (AB system):

 
 Band  MagAB  Err.  
 UVW2  19.13  0.06  
 UVM2  19.23  0.08  
 UVW1  19.12  0.07  

suggesting a brightening with respect to the archival GALEX detection at NUV=21.1 +/-0.2 (Bianchi et al. 2011 Ap&SS, 335, 161). Taking into account the Galactic reddening of E(B-V)=0.024 (Schlafly et al. 2011 ApJ, 737, 103), the Swift/UVOT magnitudes correspond to a black-body emission with the temperature ~27000 K (source frame). The high color temperature suggest Gaia17dbg is unlikely to be a supernova and is rather an AGN flare or a TDE.

We thank the Swift team for rapid execution of this observation. We acknowledge ESA Gaia, DPAC and the Photometric Science Alerts Team (Rixon et al, 2014, ATel #6593).

Gaia17dbg = ATLAS17lbg = AT2017gul at the Transient Name Server