Gamma-ray Novae: Rare or Nearby?

Abstract

Classical novae were revealed as a surprise source of γ-rays in Fermi Large Area Telescope (LAT) observations. During the first 8 yr since the LAT was launched, 6 novae in total have been detected to >5σ in γ-rays, in contrast to the 69 discovered optically in the same period. We attempt to resolve this discrepancy by assuming all novae are γ-ray emitters, and assigning peak 1 d fluxes based on a flat distribution of the known emitters to a simulated population. To determine optical parameters, the spatial distribution and magnitudes of bulge and disc novae in M31 are scaled to the Milky Way, which we approximate as a disc with a 20 kpc radius and elliptical bulge with semimajor axis 3 kpc and axis ratios 2:1 in the xy plane. We approximate Galactic reddening using a double exponential disc with vertical and radial scaleheights of rd = 5 kpc and zd = 0.2 kpc, and demonstrate that even such a rudimentary model can easily reproduce the observed fraction of γ-ray novae, implying that these apparently rare sources are in fact nearby and not intrinsically rare. We conclude that classical novae with mR ≤ 12 and within ≈ 8 kpc are likely to be discovered in γ-rays using the Fermi LAT.