Swift observations of TDE candidate OGLE17aaj

ATel #10006; K. Sokolovsky (IAASARS NOA/ASC Lebedev/SAI MSU), L. Wyrzykowski, A. Hamanowicz, M. Gromadzki (Warsaw Observatory)
on 25 Jan 2017; 21:03 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)

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The optical transient OGLE17aaj was discovered on 2017-01-01 by OGLE-IV. The transient coincides with the center of the galaxy GALEXASC J015624.70-710415.8. Six years of OGLE photometric monitoring show no previous sign of activity in this galaxy. Archival WISE colors do not indicate an AGN. Spectroscopic observations with VLT/FORS2 conducted on 2017-01-15 reveal a flat continuum with emission lines of the Balmer series, HeI, broad HeII, [OII], [OIII] and SII, at redshift z=0.116 (ATel #9977). The Hb/[OIII] and Ha/[NII] emission lines ratios are consistent with the absence of AGN activity and indicate a highly star-forming host if the emission lines belong to the host. This makes OGLE17aaj a promising tidal disruption event (TDE) candidate.

Swift observed OGLE17aaj for 1.7ks on 2017-01-24. No X-ray counterpart was detected down to the upper limit of 0.002+/-0.001 XRT counts/sec. Assuming power law emission with the photon index of 2 and HI column density of 1.56x10^20 cm^-2 (Kalberla & Haud 2015, A&A, 578, A78) this translates to the unabsorbed 0.3-10 keV flux limit of 7.3x10^-14 ergs/cm^2/s.

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

 
  JD(TT)      Filter   mag 
2457777.68372    V   >18.80 
2457777.67568    B   >19.53 
2457777.67404    U    18.74 +/-0.29 
2457777.67161   W1    18.55 +/-0.19 
2457777.68700   M2    18.31 +/-0.13 
2457777.67966   W2    18.14 +/-0.09 

Accounting for the Galactic reddening of E(B-V)=0.025 (Schlafly et al. 2011, ApJ, 737, 103) the measured magnitudes are consistent with a T=20000K black-body radiation, as expected for a TDE after the maximum light. Such high temperature rules out a supernova origin of this transient.

Further observations are required to distinguish between a TDE and a sudden onset of AGN activity in this otherwise quiet galaxy.

We thank the Swift team for rapid execution of this observation. We acknowledge the OGLE-IV Transient Detection System presented in ATel #4495, Kozlowski et al. 2013, AcA, 63, 1, 1; Wyrzykowski et al. 2014, AcA, 64, 197

OGLE-IV Transients