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

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)=$(8.22\pm 2.04)\times 10^{-9}$ ph cm${}^{-2}$s${}^{-1}$ and a hard power law spectrum with spectral index $\Gamma =1.76\pm 0.09$. 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.

arXiv comment5 pages, 4 figures. Accepted for publication in Astronomy & Astrophysics

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