Gamma-ray Novae: Rare or Nearby?

TL;DR

This paper investigates whether gamma-ray novae are intrinsically rare or simply nearby, using population modeling and Galactic distribution assumptions to explain the observed gamma-ray detection rate.

Contribution

The study introduces a population simulation approach that accounts for Galactic distribution and reddening, suggesting gamma-ray novae are more common and nearby than previously thought.

Findings

Gamma-ray novae are likely nearby and not intrinsically rare.

A rudimentary Galactic model reproduces the observed gamma-ray nova fraction.

Classical novae within 8 kpc and brighter than magnitude 12 are detectable in gamma-rays.

Abstract

Classical Novae were revealed as a surprise source of gamma-rays in Fermi LAT observations. During the first 8 years since the LAT was launched, 6 novae in total have been detected to > 5 sigma in gamma-rays, in contrast to the 69 discovered optically in the same period. We attempt to resolve this discrepancy by assuming all novae are gamma-ray emitters, and assigning peak one-day 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 semi major 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 scale heights of r_d = 5 kpc and z_d = 0.2 kpc, and demonstrate that even such a rudimentary model can easily reproduce the observed fraction of gamma-ray novae, implying that these apparently rare sources are in fact nearby and not intrinsically rare. We conclude that classical novae with m_R < 12 and within ~8 kpc are likely to be discovered in gamma-rays using the Fermi LAT.