Possible Origin of a Newly Discovered GeV Gamma-ray Source Fermi J1242.5+3236

TL;DR

This study reports the discovery of a new steady GeV gamma-ray source, Fermi J1242.5+3236, with a hard spectrum, likely an unidentified background blazar, based on 13.4 years of Fermi data analysis.

Contribution

First detection and characterization of a new gamma-ray source with detailed spectral and positional analysis using extensive Fermi data.

Findings

Detected Fermi J1242.5+3236 with 5.64 sigma significance in 13.4 years.

The source has a hard photon index of about -1.60.

The source is likely a high-synchrotron-peaked blazar.

Abstract

Based on the first 13.4 yr of Fermi science data in the energy range from 300 MeV to 500 GeV, we discovered a bright GeV gamma-ray source with a $\sim$5.64 $\sigma$ detection, named Fermi J1242.5+3236, which has an offset of about 0fdg0996 from a nearby star-forming galaxy NGC 4631. When using the 12-yr data, the detection significance of Fermi J1242.5+3236 is about 4.72 $\sigma$. Fermi J1242.5+3236 is a steady point source without significant temporal variability and has a hard gamma-ray photon index of about -1.60 $\pm$0.24. The spatial offset and the hard gamma-ray spectrum disfavor this source as the diffuse gamma-ray emission from this galaxy. This new source might have a possible origin of an unidentified background blazar, which is more likely a high-synchrotron-peaked blazar for its hard gamma-ray photon index. A follow-up optical observation would help distinguish origin of Fermi J1242.5+3236.