The possible cause of the initial gamma ray emission from Nova Mon 2012; Continued Infrared observations.

ATel #4764; Dipankar P. K. Banerjee, Vishal Joshi and N. M. Ashok (Astronomy and Astrophysics Division, Physical Research Laboratory, Ahmedabad, India)
on 26 Jan 2013; 09:48 UT
Credential Certification: Dipankar P.K. Banerjee (orion@prl.res.in)

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The cause of the initial gamma ray burst from Nova Mon 2012 (Atel # 4224) is a mystery. Till date, few gamma ray detections accompanying novae outbursts have been made with the best documented case being that of the symbiotic nova V407 Cyg comprising of a white dwarf and a dusty Mira variable companion. The gamma ray burst from nova V407 Cyg, is understood to arise from the diffusive acceleration of particles (Fermi acceleration) across the shock front formed as a consequence of the high velocity ejecta from the nova colliding with the extended envelope of the mass lost in the Mira wind. The accelerated particles can then either inverse-Compton scatter ambient optical and IR photons to higher energies or alternatively proton-proton inter-collisions can form unstable neutral pions that decay through emission of gamma rays. A similar diffusive-acceleration mechanism was invoked for the generation of high-energy TeV particles from the shock-front of the recurrent nova RS Oph (Tatischeff and Hernanz, 2007, ApJ, 663, L101).

We have looked at archival data on Nova Mon 2012 and note that the source was detected by the Wide-field Infrared Survey Explorer (WISE) prior to outburst in all four bands W1, W2, W3 and W4 (at 3.4, 4.6, 12 and 22 microns respectively) in March 2010 with magnitudes of 14.732, 14.740, 10.831 and 8.720 respectively. The very large [W4-W1] and [W3 - W1] colors are clearly indicative of a large mid-IR excess that is likely to be associated with a dusty extended envelope of a companion star. The SED plotted using the pre-outburst 2MASS J,H,K fluxes and the WISE data clearly establish this large mid-IR excess. Source association appears to be correct. WISE reports only one source in a 10" diameter circle around the position of the nova whose coordinates match the 2MASS counterpart coordinates closely and also closely match the coordinates of the nova reported independently by several other observers (3 reports in CBET # 3202, ATel # 4572) and archival optical coordinates (USNO-B1 etc). We thus propose that the WISE/2MASS source being seen at the nova's position is the secondary companion of the WD in the nova system. The gamma ray emission could then have arisen, in a similar manner as in V407 Cyg, by the interaction of the ejecta and the extended cool envelope that the companion is implied to harbor from its mid-IR colors. The companion is also variable in the near-IR and could be a semi-regular variable. Its JHK magnitudes as reported by 2MASS are 15.759, 15.231 and 15.036 (epoch 29 Feb 2000) whereas measurements from UKIDSS in October 2008 give corresponding values of 16.263, 15.709 and 15.420 respectively (Lucas et al., 2008, MNRAS 391, 136). H-alpha and short term variability is also reported in ATel # 4365.

Near infrared spectra in the 1.08 to 2.25 micron region have continued to be taken at regular intervals since out first report in early November 2012 (ATel # 4542); the latest data have been obtained on 23 Jan 2013. The spectra have been obtained at R ~ 1000 using the NICMOS-3 Imager/Spectrograph and the 1.2m Mount Abu Observatory telescope of the Physical Research Laboratory, India. Strong coronal lines continue to be seen among which we detect [Si VI] 1.9641, [S IX] 1.2520 combined with He I 1.2534, a prominent line at 1.55 microns ( [Cr XI] 1.5503 ? ), [Ti VI] 1.7155, [P VIII] 1.7362 blended with Br 10, [Al IX] 2.040, [Ca VIII] 2.323 micron and unidentified coronal lines at 1.1101, 1.1908 and 2.09 micron. The [Al IX] line has strengthened considerably since reported in early November 2012 (ATel # 4542). The He I lines detected are those at 1.0831, 1.7002, 2.1120 + 2.1132 and 2.0581 micron. The H lines seen are Pa alpha, Pa beta; Br gamma, Br 8 & Br 10 to 17 in the H band. The 1.1287 and 1.3164 micron lines of O I have weakened but persist even in the latest spectra.