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 (firstname.lastname@example.org)
Subjects: Optical, Ultra-Violet, X-ray, Tidal Disruption Event
Referred to by ATel #: 10776
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)
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