Possible Association of the Gamma-ray Transient Fermi J0639+0548 with Nova Mon 2012

ATel #4310; C. C. Cheung (NRC/NRL), S. N. Shore (U. Pisa and INFN), I. De Gennaro Aquino (U. Pisa), S. Charbonnel (Durtal Observatory, France), J. Edlin (Idaho Falls, Idaho), E. Hays (NASA/GSFC), R. H.D. Corbet (UMBC, NASA/GSFC), D. L. Wood (NRL); on behalf of the Fermi Large Area Telescope Collaboration
on 17 Aug 2012; 18:30 UT
Credential Certification: Teddy Cheung (ccheung@milkyway.gsfc.nasa.gov)

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In ATEL #4224, we reported the detection by the Fermi Large Area Telescope of Fermi J0639+0548, a new gamma-ray transient in the Galactic plane, beginning on 2012 June 22. The transient was seen through the Monoceros region of our Galaxy with Galactic coordinates, (l, b) = (206.42 deg, 0.03 deg). We noted the gamma-ray source was close to the Sun at the time of detection, thus precluding prompt follow-up with Swift (and also ground-based optical observers).

The region is now observable by ground-based optical observers and on 2012 August 9, S. Fujikawa (Kagawa, Japan) reported the discovery of a possible nova in Monoceros at a magnitude of 9.4 at a position (J2000) of R.A. = 06 39 38.57, Decl. = +05 53 53.4 (+/- 1".5) measured by S. Nakano (http://www.cbat.eps.harvard.edu/unconf/followups/J06393874+0553520.html; CBET #3202). Its position is consistent with that of Fermi J0639+0548, being within the preliminary 68% confidence LAT error circle radius of 0.12 deg (statistical uncertainty only). Analysis by one of us (S.N.S.) of optical spectra obtained at the Observatoire de Haute Provence (France) on August 14 UT 05:00-06:30 with a LHIRES III spectrograph (R~18000) on a 0.3m telescope by S.C. and a LISA spectrograph (R=800) on a 0.28m reflector by J.E., and a FIES high resolution spectrum (R ~ 67000) obtained by S.N.S. at the 2.6m Nordic Optical Telescope on August 16 UT 05:50 confirm this to be a nova whose spectrum bears striking resemblance to that of the fast ONe nova V382 Vel 1999 well after peak brightness. If Nova Mon 2012 is an analog of V382 Vel (and V1974 Cyg 1992, both of which were bright Galactic ONe novae), the spectra observed after announcement are consistent with a late time, optically thin nebular spectrum taken > 50 days after optical discovery. Assuming similar light curves, the Fermi detection could have occurred at the time of the explosion. Scaling the light variations imply a maximum V ~ 4.5-5, about 1.5-2 mag fainter than V382 Vel but still naked-eye and comparable to V1974 Cyg, implying this nova would have been one of the visually brightest of the last 20 years but it was not possible to detect the optical peak from the ground due to its proximity to the Sun. Thus, the Fermi detection appears to be the first time a gamma-ray detection of a nova preceded the optical discovery. Scaling to V382 Vel, and assuming a possible E(B-V) ~ 0.2-0.5, suggests a distance of ~ 3 - 4 kpc.

After the discovery of gamma-ray emission during a nova outburst from the symbiotic binary V407 Cyg (CBET #2199, ATEL #2487; The Fermi LAT collaboration 2010, Science, 329, 817), and the recently reported possible association of the gamma-ray transient Fermi J1750-3243 with Nova Sco 2012 (CBET #3136, ATEL #4157, ATEL #4284), this would be the third case of a gamma-ray transient detection from a nova by the Fermi-LAT.

In consideration of the ongoing activity of this source we strongly encourage multiwavelength observations. For this source the Fermi LAT contact persons are C.C. Cheung (Teddy.Cheung.ctr@nrl.navy.mil) and E. Hays (elizabeth.a.hays@nasa.gov).

The Fermi LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.

The Nordic Optical Telescope on the island of La Palma is jointly operated by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. This observation was part of the OPTICON program.