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Swift observations of Gaia17acy/2017it

ATel #10029; K. Sokolovsky (IAASARS NOA/ASC Lebedev/SAI MSU), L. Wyrzykowski, A. Hamanowicz, M. Gromadzki (Warsaw Observatory)
on 31 Jan 2017; 10:54 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)

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

The optical transient Gaia17acy was discovered on 2017-01-14 coinciding with an uncataloged galaxy. Follow-up spectroscopic observations by NOT (ATel #9990) and PESSTO (ATel #10012) show blue continuum and emission lines at z = 0.175. There is an indication of a broad H-alpha emission component hinting this might be a luminous Type IIn supernova (ATel #10012).

Swift observed Gaia17acy for 2.5ks spread across two visits between 2017-01-27 13:40 and 2017-01-28 09:12 UT. No X-ray source is detected down to the upper limit of about 0.0004+/-0.0004 XRT counts/sec (a single photon counted within the source region). Assuming power law emission with the photon index of 2 and HI column density of 8.78x10^20 cm^-2 (Kalberla & Haud 2015, A&A, 578, A78) this translates to the unabsorbed 0.3-10 keV flux limit of 1.5x10^-14 ergs/cm^2/s.

An optical-UV source is detected at the position of the transient with the following UVOT magnitudes (Vega system):

Filter  mag   err 
  V    18.55 0.21 
  B    19.15 0.15 
  U    18.61 0.14 
 W1    18.75 0.17 
 M2    18.78 0.15 
 W2    19.32 0.16 

Accounting for the Galactic reddening of E(B-V)=0.089 (Schlafly et al. 2011, ApJ, 737, 103) and assuming no additional extinction from the host galaxy, the measured magnitudes are consistent with a T=11000K black-body radiation expected for a luminous supernova. Further optical (photometry and spectroscopy) and UV observations are necessary to observe the temperature evolution. In the supernova scenario the temperature should eventually drop below 10000K, while if this is a tidal disruption event or an AGN flare, but diminished by internal extinction, the temperature would remain high for weeks or months.

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).

Gaia Photometric Science Alerts