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Swift and GROND observation of OGLE16aaa

ATel #8579; J. Greiner, C. Delvaux (both MPE Garching), L. Wyrzykowski, Z. Kostrzewa-Rutkowska (both Warsaw Univ. Astronom. Observatory, Poland), T.-W. Chen, P. Schady (both MPE Garching)
on 22 Jan 2016; 16:59 UT
Distributed as an Instant Email Notice Transients
Credential Certification: J. Greiner (jcg@mpe.mpg.de)

Subjects: Optical, Ultra-Violet, X-ray, Transient

Referred to by ATel #: 8644

The transient source OGLE16aaa was discovered January 2, 2016 (Wyrzykowski et al. 2016, ATEL #8577) and since has been rising from below I~22 to presently I~19.5 mag (http://ogle.astrouw.edu.pl/ogle4/transients). The object is situated at the center of an unknown, non-active galaxy. PESSTO spectroscopy has revealed a blue continuum with no features, suggesting a tidal disruption event (Firth et al. 2016, ATEL #8559).

We performed a 1.96 ks Swift observation on January 19, 2016. No X-ray source is detected, implying an upper limit of 0.001 counts/s. Assuming a blackbody spectrum with a temperature of 70 eV, this translates into a limit of the unabsorbed flux of 2.6E-14 erg/cm^2/s (0.5-10 keV). However, a point source is clearly detected in the UVM2 filter, at 17.06+-0.04 (uvm2(AB)=18.74+-0.04) mag, consistent with the blue PESSTO spectrum.

We also observed the object with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La Silla Observatory (Chile). Observations were done on two epochs, with mid-times of January 18, 03:20 UT and January 21, 01:44 UT, confirming the blue continuum and the rising brightness.

Excluding the central 4 arcsec of the galaxy with the transient, we measure the following AB magnitudes for the host galaxy:

 g' = 18.0 +- 0.1 mag 
 r' = 17.7 +- 0.1 mag 
 i' = 17.5 +- 0.1 mag 
 z' = 17.4 +- 0.1 mag 
 J  = 17.1 +- 0.1 mag 
 H  = 17.2 +- 0.1 mag 
 K  > 17.0 mag 
The given magnitudes and limits are derived based on calibrating the images against GROND zeropoints and 2MASS field stars and are not corrected for the Galactic foreground extinction corresponding to a reddening of E(B-V)= 0.02 mag (Schlegel et al. 1998). This implies that there is likely only little contamination of the above uvm2 flux by the host galaxy.

At a redshift of 0.167 (Wyrzykowski et al. 2016, ATEL #8577), our Swift/XRT flux limit corresponds to a luminosity limit of 2E+42 erg/s (0.5-10 keV). This implies an upper limit of 1E+04 solar masses for the accreting black hole (if accreting at the Eddington limit), making an interpretation as a tidal disruption event unlikely.

We thank the Swift team, in particular Neil Gehrels, for the fast response time in excecuting this target-of-opportunity observation.

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