SNhunt133, an AGN outburst?

ATel #4279; A. J. Drake (Caltech)
on 26 Jul 2012; 20:57 UT
Credential Certification: Andrew J. Drake (ajd@cacr.caltech.edu)

Tweet

Supernova candidate SNhunt133 (PSN J17124620+2313265) was recently discovered within ~1" of the nucleus of NGC 6315 using image subtraction of CSS data. McCrum et al. (2012; ATel#4274) obtained spectroscopic follow-up of SNhunt133 and noted that "The spectrum is very similar to the archival spectrum of the nucleus of NGC 6315 from the Sloan Digital Sky Survey.". We searched SDSS DR8 archive for archival for matching spectra and found the closest match to be RA=17:12:45.04 Dec=+23:13:27.8, corresponding to ~15" East and ~3" South of the nucleus of NGC 6315. Consequently the closest SDSS spectrum is not that of an AGN in NGC 6315.

As we noted in our CBAT tocp posting the outburst of SNhunt133 occurred between 2012 May 13.41 and May 27.37. Averaging the 16 unfiltered ISSP photometry data points given by McCrum et al. (2012) over the two years prior to 2012 May 15 we find that NGC 6315 had an average mag of 15.168 with std deviation 0.024. The source clearly exhibits little variability prior to detection in ISSP photometry. However, on 2012 Jun 26.936, McCrum et al. (2012) find that this source to have brightened to mag 14.91. This corresponding 10 sigma increase in brightness is equivalent to the addition of a source with mag=16.6. Assuming a the distance modulus of NGC 6315 to be 34.87 (from NED), the additional flux is consistent with a source having absolute magnitude Mu~-18.3.

To further constrain the photometric variability of NGC 6315 we downloaded the publicly available CSS data for this galaxy from the Catalina Surveys Data Release 1 (CSDR1; www.catalinadata.org). This data consists of 293 observations spanning the ~2500 days between 2005 April 10 and 2012 April 25th. The photometry gives an average CSS V magnitude of 14.245 with a standard deviation 0.029 (in good agreement with the low level of variability observed by McCrum et al.). On Jun 8.4 the source was observed in CSS data at V~14.15 and Jun 15.3 at V~14.12. These corresponding 5 sigma brightness increases correspond to additional flux equivalent to a source with V=16.8 and V=16.5, respectively. In excellent agreement McCrum et al. (2012).

McCrum et al. (2012) note that the follow-up photometry and spectrum "suggest the transient is not a supernova, but it is an AGN (sic)". However, our analysis of the narrow features in this low S/N follow-up spectrum suggests that the host is a normal starforming galaxy, rather than an AGN. Inspecting FIRST and NVSS survey radio images we find no significant nuclear radio source. However, the central nucleus is clearly seen GALEX observations with magnitudes NUV=18.2 and FUV=17.1.

Given the low level of prior photometric variability observed in NGC 6315 by both CSS and ISSP, NGC 6315 appears to be undergoing a significant ongoing outburst located in, or near, its nucleus. The increase in flux and lack of strong features appears consistent with expectations for tidal disruption events. However, we cannot not currently rule out other possible sources of variability.

We encourage further follow-up to determine the nature of this outburst and wish to thank A. Pastorello and M. Fraser for providing their follow-up spectrum and photometry.