Discovery of transient high-energy gamma-ray emission from the BL Lacertae object 5C 3.178

TL;DR

This paper reports the serendipitous detection of transient high-energy gamma-ray emission from the BL Lac object 5C 3.178 using Fermi LAT, revealing variability and spectral properties consistent with TeV-emitting BL Lacs, and suggesting new search strategies.

Contribution

The discovery of transient gamma-ray emission from 5C 3.178 provides new insights into BL Lac objects and demonstrates the potential for real-time searches to detect such transient events.

Findings

Detected gamma-ray emission at ~8 sigma significance over 200 days

Source exhibits a hard spectrum with spectral index 1.76

Temporal variability indicates potential for real-time detection strategies

Abstract

We report the serendipitous discovery of transient, point-like high energy gamma-ray emission coincident with the position of the suspected BL Lac object 5C 3.178. The source was detected using the Fermi Large Area Telescope (LAT) at a significance level of ~8 sigma during a 200 day period which began on MJD 55882. The observed gamma-ray emission is characterised by a moderate 0.2-300 GeV flux F(0.2-300 GeV)=$(8.22\pm2.04)\times10^{-9}$ ph cm$^{-2}$s$^{-1}$ and a hard power law spectrum with spectral index $\Gamma=1.76\pm0.09$. These properties appear consistent with the known sub-population of TeV gamma-ray-emitting BL Lac objects, implying that the source may be detectable using atmospheric Cherenkov telescope arrays. Moreover, the temporal variability of the source suggests that real-time searches of the Fermi-LAT all-sky dataset for weak emission on ~200 day timescales may represent a rewarding strategy.