CRTS discovery of an outburst from a short period binary system

ATel #9112; A. J. Drake, S. G. Djorgovski, A. A. Mahabal, M. J. Graham, C. Donalek, R. Williams (Caltech); M. Catelan (PUC Chile); E. Christensen, S. M. Larson (LPL/UA)
on 4 Jun 2016; 03:25 UT
Credential Certification: Andrew J. Drake (ajd@cacr.caltech.edu)

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Here we report the discovery of an outburst from known short period binary system 2MASS_J16211735+4412541. On 2016-06-03.45 UT, 2MASS_J162117 was detected by CRTS as a transient event ( CSS160603:162117+441254) with V_CSS=13.3. This system has an average magnitude of V_CSS=15.0 and an amplitude of 0.5 mags based on 359 prior observations taken between 2005-05-17 to 2016-05-14. No prior outbursts have been observed.

This system was previously identified as a contact eclipsing binary (WUMa) by Palaversa et al. (2013, AJ, 146, 101), Lohr et al. (2013, A&A 549, 86) and Drake et al. (2014a, ApJS, 213, 9) with a period of P=0.207852(1) days. This period places the system is among the ~0.1% of ultra-short period binary systems known to have periods shorter than the 0.22 day contact binary minimum (Rucinski 2007, MNRAS, 382, 393). Due to its unusual period the source was studied by Lohr et al. (2013) and Drake et al. (2014b, AJ, 790, 157).

2MASS_J162117 has SDSS magnitudes u =17.54, g=15.76, r=14.92, i=14.56 and z=14.37, and GALEX mags FUV=20.5, NUV=20.2. The extinction corrected SDSS colors are consistent with a MS+MS (WUMa) binary. However, the object is near the SDSS saturation limit. The GALEX magnitudes are most consistent with a WD+MS binary system. Based on Drake et al. (2014b) we find that the shape of phase-folded lightcurve is more consistent with the ellipsoidal (WD+MS) variables than WUMa.

If this system is a MS+MS contact binary, the outburst could be the beginning of a very rare binary merger event similar to V1309 Scorpii (Tylenda et al. 2011, A&A, 528, 114). However, based on the GALEX data and lightcurve shape, we suspect that the event is more likely an outburst from an unusual cataclysmic variable system.

We request photometric and spectroscopic follow-up to determine the nature of the system and the outburst.

Archival photometry for this source is public available from the CSDR2 pages. All CRTS transients are discovered within minutes of observation and openly published. Links to all CRTS transients can be found at http://crts.caltech.edu/. We are grateful to the all the observers involved in follow-up observations.