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

We report the serendipitous discovery of transient, point-like high energy γ-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 ~8σ during a 200 day period which began on November 17th 2011 (MJD 55 882). The observed γ-ray emission is characterised by a moderate 0.2-300 GeV flux F_{0.2-300 GeV} = (8.22 ± 2.04) × 10^-9 ph cm^-2 s^-1 and a hard power law spectrum with spectral index Γ = 1.76 ± 0.09. These properties appear consistent with the known sub-population of TeV γ-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.