SIMBAD references

R. M. Wagner, Ohio State University (OSU) and Large Binocular Telescope Observatory; S. Dong, Institute for Advanced Study, Princeton; T. Bensby, Lund Observatory; J. Prieto, Princeton University; I. Bond, Massey University; P. Tristram, Mt. John University Observatory; D. Bennett, University of Notre Dame; K. Wada and T. Sumi, Osaka University, and the Microlensing Observations in Astrophysics (MOA) collaboration; A. Gould, OSU, and the Microlensing Follow-Up Network (microFUN) collaboration; and S. Starrfield, Arizona State University, report the discovery and initial photometry and spectroscopy of a bright transient in the direction of the Galactic bulge. The MOA collaboration detected a candidate microlensing event, designated MOA 2012 BLG-320, on 2012 May 22.80 UT at magnitude I = 18.5, located at R.A. = 17:50:53.90, Decl. = -32:37:20.5 (equinox 2000.0). No significant source appears at this position on either the R or N survey plates of the Digitized Sky Survey.

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

On June 4.08 UT, a high-resolution optical spectrum (range 375-950 nm; resolution R = 42000) of the transient was obtained with the Very Large Telescope (+ UVES). The spectrum exhibits strong emission lines with striking P-Cyg profiles of the Balmer series of hydrogen and 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 about 800 km/s (average expansion velocity of about 400 km/s) are more consistent with slow classical novae that have t_2 around 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 this new nova obtained on June 7.29, from images obtained with the Cerro Tololo 1.3-m telescope (+ ANDICAM) are magnitudes V = 11.6 and I = 9.4. The transient continues to brighten at a rate of about 0.4 mag/day in the I band over the past three nights.