Increasing activity from optical transient MLS120331:131536-045337

ATel #5028; A. J. Drake, S. G. Djorgovski, A. A. Mahabal, M. J. Graham, R. Williams (Caltech); J. Prieto (Princeton); M. Catelan (PUC Chile); E. Christensen, S. M. Larson (LPL/UA)
on 29 Apr 2013; 21:32 UT
Credential Certification: Andrew J. Drake (ajd@cacr.caltech.edu)

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Optical transient source MLS120331:131536-045337 was discovered by CRTS in data from the Mount Lemmon Survey 1.5m telescope on 2012 Mar. 31.34UT. At discovery time the source was found to have varied between V=19.9 and 18.3 within four observations taken over a period of ~30 minutes. Archival MLS photometry shows that the object has a quiescent magnitude of V~21. Additional outbursts were detected in MLS data taken on 2013 Apr. 7 and Apr. 29UT, at which time it varied between V~19.5 and V~17.7 within ~28 mins and ~23 mins, respectively. The source was independently detected (as CSS130108:131536-045336) by CRTS in data from the Catalina Schmidt telescope on 2013 Jan. 8UT, and found to vary between V~19.5 and 18.5 in ~20 mins. Additional outbursts are seen in CSS data from 2012 Apr. 24 (reaching V=19.2) and 2012 Dec. 20 (reaching V=18.5). The nine years of archival CSS and MLS photometry suggest that the source was in a quiescent state prior to detection in MLS data.

The source is clearly seen in DSS2-B images, but not in DSS2-R and DSS2-I. No clear source is present in FIRST or NVSS radio data, or WISE and 2MASS near-IR data. However, a matching source is detected in GALEX AIS, with NUV=19.9, and in the Vista VHS survey with j=19.3 and y=20.0.

The significant level of rapid variation (>1.5 mags/30mins) of this object is similar to that of CSS080924:233423+391423 (Mahabal et al. 2008, ATel#1741; Quimby et al. 2008, ATel#1750) and CSS090826:223958+231837 (Djorgovski et al. 2010, ATel#2654). This suggests that the source is most likely cataclysmic variable rising into an active state. Nevertheless, since the parameter space of rapidly varying optical sources remains poorly constrained by past and current synoptic surveys, we request follow-up observations to determine the nature of this object.