Search for gamma rays from VVV-WIT-01 or other possible Galactic Type Ia Supernovae using Fermi-GBM

ATel #4473; K. C. Y. Ng (Ohio State), S. Horiuchi (Ohio State, UC Irvine), J. F. Beacom (Ohio State), J. Siegal-Gaskins (Caltech), R. Preece (UAH), M. Smith (Penn State), J. Gelbord (Penn State)
on 10 Oct 2012; 02:05 UT
Credential Certification: Shunsaku Horiuchi (shunsaku.horiuchi@gmail.com)

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VVV-WIT-01 (RA = 16:10:53.45, Dec = -51:55:32.4; J2000) is an extreme transient discovered in March 2010 by the VVV survey (ATEL #4041). The object is heavily dust-attenuated and has shown a sharp decline in IR magnitude since discovery. The identity of this object is unknown, but it could be a supernova, nova, or variable star.

Motivated by this transient, we use the wide-FOV instrument GBM (energy range 10 keV - 30 MeV) on board the Fermi satellite to search for gamma-ray lines. If the object is a Galactic Type Ia Supernova (SNIa), approximately 0.7 Msun of radioactive 56Ni will be produced. The nuclear decays of 56Ni into 56Co and then 56Fe emit gamma-ray lines that are important observational signatures due to their high transmissivity through the dust (Guver and Ozel 2009). Assuming a distance of 20 kpc to the object, the 847 keV line flux (from 56Co decay) reaches a peak value of 10 ph/cm^2/s approximately two months after explosion and will last for about two months (Horiuchi and Beacom 2010). The large flux and long-lasting power makes 847 keV an ideal channel for GBM observations.

We use observations by one of GBM's NaI detectors from Aug 2008 to Nov 2011, as the time of the possible explosion is unknown. Even though GBM has no photon tracking capability, we obtain directional information through the angular dependence of the effective area. We therefore only use data obtained when the point of interest is within 30 degrees of the detector's pointing direction. We search at the location of VVV-WIT-01 and we find no excess of events that is consistent with a SNIa. In addition, we systematically search all sky directions and find no candidate signals. In the 847 keV channel, we observe a photon rate (10 +/- 2) ph/s in all directions, where the uncertainty is systematic only (the statistical uncertainty is negligible) and is chosen to contain nearly all (~90%) of the observed fluctuations. Using a conservative effective area, and requiring that the contribution from a point source does not contribute more than the quoted uncertainty, we obtain a point-source flux limit of 0.2 ph/cm^2/s for the 847 keV channel in any direction.

The interpretation of VVV-WIT-01 as a SNIa is strongly excluded, as it exceeds the flux limit by a factor of ~40. Interpreting the object as Type .Ia Supernova (Bildsten et al. 2007), which produces between 3-14% of the 56Ni yield of a normal SNIa, is also slightly disfavored. More generally, using the all-sky search, we conclude that there were no Galactic SNIa from Aug 2008 to Nov 2011.

For the case of a Core-Collapse Supernova (CCSN), the expected flux is reduced by a factor of ~1000 compared to a SNIa because of reduced production of 56Ni (~10% of SNIa) and heavy attenuation by the stellar envelope (factor of ~100). More sensitive gamma-ray satellites, e.g., INTEGRAL or Swift, could potentially detect gamma rays from VVV-WIT-01 in the CCSN case if they have data at the target location. Neutrino detectors can probe Galactic CCSN robustly, with a caveat that the signal only has an observing window of ~10 s (e.g., Ikeda et al. 2007, Aguilar-Arevalo et al. 2010).

Aguilar-Arevalo et al. ApJ669, 519 (2010)
Bildsten et al., ApJ 662, L95 (2007)
Guver and Ozel, MNRAS 400, 2050 (2009)
Horiuchi and Beacom, ApJ 723, 329 (2010)
Ikeda et al., PRD81, 032001 (2007)
ng.199@osu.edu, shoriuch@uci.edu, beacom.7@osu.edu, jsg@tapir.caltech.edu, rob.preece@nasa.gov, msmith@gravity.psu.edu, jgelbord@astro.psu.edu