MOA 2012 BLG-320: Discovery and Observations of a Nova Candidate Towards the Galactic Bulge

ATel #4157; R. M. Wagner (OSU and LBTO), S. Dong (IAS), T. Bensby (Lund Obs.), J. Prieto (Princeton), I. Bond (Massey Univ.), P. Tristram (Mt. John University Observatory), D. Bennett (Notre Dame), K. Wada, T. Sumi (Osaka Univ.) and the MOA collaboration, A. Gould (OSU) and the microFUN collaboration, S. Starrfield (ASU)
on 7 Jun 2012; 20:05 UT
Distributed as an Instant Email Notice Novae
Credential Certification: Jose L. Prieto (jose@obs.carnegiescience.edu)

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We report the discovery and initial photometry and spectroscopy of a bright transient in the direction of the Galactic bulge. The Microlensing Observations in Astrophysics (MOA) collaboration detected a candidate microlensing event designated MOA 2012 BLG-320 on 2012 May 22.80 UT at I = 18.5 mag. The position of the transient is RA = 17:50:53.90 and DEC = -32:37:20.46 (J2000). No significant source appears at this position on either the DSS2 R or N survey plates.

MOA photometry of the source before the outburst shows that the progenitor was about I ~ 19.0-19.5 mag but with substantial variability on nightly timescales. Between 2012 May 14-16 UT the source began a slow monotonic increase in brightness and after 2012 May 24 UT the rate increased significantly. The light curve obtained between 2012 May 28-31 UT exhibits periodic modulations with an amplitude of ~0.1 mag and a period of ~1.6 hr superposed on the brightening source. Between 2012 June 1.77 and 2.55 UT, the transient brightened dramatically from I ~ 17 and reaching I ~ 11 on 2012 June 3.33 UT prompting more detailed follow-up by both MOA and the microFun (Microlensing Follow-Up Network) collaborations.

On 2012 June 4.08 UT, a high resolution optical spectrum of the transient (range: 375-950 nm; resolution R=42000) was obtained with the VLT and UVES. The spectrum exhibits strong emission lines with striking P Cygni profiles of the Balmer series of hydrogen, many multiplets of Fe II, as well as O I 777.4 and 844.6 nm. The equivalent widths of both H-beta emission and absorption are 1.7 and 0.3 nm respectively. The interstellar Na D lines are resolved and exhibit considerable structure at this spectral resolution. The total equivalent widths of the D1 and D2 lines are 0.16 and 0.19 nm respectively. The spectrum is reminiscent of either a slow "Fe II-type" classical nova or an intermediate luminosity red transient like V838 Mon. However, the measured H-beta and H-alpha FWHM of ~800 km/s (average expansion velocity of ~400 km/s) are more consistent with slow classical novae which have t_2 ~ 50-200 days and are thermonuclear events on accreting white dwarfs. Other historical slow novae include V450 Cyg and HR Del which were observed to have comparable expansion velocities and t_2 of 90 and 150 days respectively.

The most recent photometry of the nova obtained on 2012 June 7.29 from images obtained with the CTIO 1.3 m telescope and ANDICAM are V = 11.6 and I = 9.4 mag. The transient continues to brighten at a rate of ~0.4 mag/day (I-band) over the past 3 nights.

Additional observations at other wavelengths are encouraged.