An Unusual Fast Transient Detected by CRTS

ATel #4586; A Mahabal, A Drake, S G Djorgovski, G Hallinan, M Vallisneri, V Rana, M Graham, R Williams, C Donalek (Caltech), T Nagayama (Nagoya Univ.), S Barway (SAAO), S Hodgkin (IoA, Cambridge), G Vianello (Stanford), D Thompson (NASA), D Kocevski (KIPAC), J Chiang (Stanford), L Petrov (Astrogeo Center), V Mohan (IUCAA), S Larson, E Christensen (LPL)
on 19 Nov 2012; 06:43 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Ashish Mahabal (aam@astro.caltech.edu)

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We report on a fast optical transient discovered by CRTS on 2012-11-06 UT, at the J2000 coordinates 01 44 19.91 +08 23 11.2 (MLS121106:014420+082311). The 1.5m Mt. Lemmon Survey (MLS) telescope of the CRTS takes 4 images separated by ~10 minutes. The transient was discovered in the second image of the set. The epochs and magnitudes for the four images are given below. As always, the transient was reported on the CRTS webpages at http://crts.caltech.edu.

UT	V Mag
05 38 55	>21.5
05 47 36	17.87
05 56 18	19.47
05 05 05	19.95

The obvious possibility is that the progenitor is a low-mass star. However, the master comparison MLS image from several co-added images from before does not reveal the source, nor do SDSS, FIRST, WISE, 2MASS, UKIDSS etc. In particular, the non-detection in the UKIDSS image suggests the K magnitude to be fainter than ~20 (epoch: Nov 2010). Considering the magnitudes of the prototypical flare star UV Ceti, one would expect a V-K of ~6.5 thus leading to a quiescent V magnitude of ~25.5 for the transient even by considering a 3-sigma preliminary K detection limit of 19 for the UKIDSS image. Given the flare magnitude of ~18 (correcting for the bandpass and T0), we have to ascribe the flare a brightening of well above 7.5 mags though we have used very conservative calculations. During the several years of operation of CRTS we have not seen anything of this scale before. The typical extreme whitelight flares on dMs would be 5-6 magnitudes in U (e.g. Kowalski et al. 2010 ApJL, 714, 98). Besides being a very extreme event, given the faint magnitude the source would be at a distance of several hundred pc. Given the high volume-density of dMs, if this were a dM, we would have seen many such sources nearer us. As a result it is either not a flaring star, or a really extreme one. (We have accounted for the possibility of proper motion while looking at 2MASS, UKIDSS and fresh NIR images described in the next paragraph).

On 2012-11-07 UT a 15-minute spectrum at the Palomar 5m telescope with the Double Beam Spectrograph (DBSP) shows no trace of the source (down to ~21st mag). Subsequent images in g/r/i/z from Palomar 1.5m down to r~23 do not show the transient either. Simultaneous J/H/Ks images were obtained with the SAAO IRSF 1.4m telescope on 2012-11-14 UT. The conditions were not ideal and the 24-minute exposures go to ~17 in Ks and do not reveal the source either.

One alternative to the source being an extreme flaring star is that it is an orphan GRB afterglow. Assuming a powerlaw (it is not exponentially decaying) with alpha=-1 the burst occurred ~3 minutes after the first image was taken. The decay would be consistent with the source falling below detection limits of the Palomar spectrum and images as well as the NIR images.

Swift XRT observations were carried out on 2012-11-17 UT to look for possible afterglow signature (we had to wait to eliminate the possibility of it being a flare star). No source was detected with XRT in ~3.8ks at the position of the transient. Given that the source is 10 days old an afterglow would likely have been close to the detection limits so this does not rule out an orphan afterglow, but it does rule out a much brighter exotic X-ray source. Unfortunately the source was covered by the Earth for Fermi-LAT around T0 and analysis at the source position at the nearest time intervals does not reveal a detection. We are also scheduling radio observations, and contacting GW experiments for possible simultaneous data. Further observations are requested.

All CRTS discoveries are available from http://crts.caltech.edu/.

We acknowledge the use of the Swift data archive.

Additional details about MLS121106:014420+082311