Search for gamma-ray emission from Galactic novae with the Fermi -LAT

¹ Deutsches Elektronen Synchrotron DESY, 15738 Zeuthen, Germany
e-mail: anna.franckowiak@desy.de
² CNRS, IRAP, 31028 Toulouse Cedex 4, France
³ GAHEC, Université de Toulouse, UPS-OMP, IRAP, 31400 Toulouse, France
⁴ W. W. Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics and SLAC National Accelerator Laboratory, Stanford University, Stanford, CA 94305, USA
⁵ Space Science Division, Naval Research Laboratory, Washington, DC 20375-5352, USA
⁶ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁷ NASA Postdoctoral Program Fellow, USA

Received: 5 July 2017
Accepted: 10 October 2017

Abstract

Context. A number of novae have been found to emit high-energy gamma rays (>100 MeV). However, the origin of this emission is not yet understood. We report on the search for gamma-ray emission from 75 optically detected Galactic novae in the first 7.4 years of operation of the Fermi  Large Area Telescope using the Pass 8 data set.

Aims. We compile an optical nova catalog including light curves from various resources and estimate the optical peak time and optical peak magnitude in order to search for gamma-ray emission to determine whether all novae are gamma-ray emitters.

Methods. We repeated the analysis of the six novae previously identified as gamma-ray sources and developed a unified analysis strategy that we then applied to all novae in our catalog. We searched for emission in a 15 day time window in two-day steps ranging from 20 days before to 20 days after the optical peak time. We performed a population study with Monte Carlo simulations to set constraints on the properties of the gamma-ray emission of novae.

Results. Two new novae candidates have been found at ~ 2σ global significance. Although these two novae candidates were not detected at a significant level individually, taking them together with the other non-detected novae, we found a sub-threshold nova population with a cumulative 3σ significance. We report the measured gamma-ray flux for detected sources and flux upper limits for novae without significant detection. Our results can be reproduced by several gamma-ray emissivity models (e.g., a power-law distribution with a slope of 2), while a constant emissivity model (i.e., assuming novae are standard candles) can be rejected.