AGILE detection of transient γ-ray emission from the region of the supergiant fast X-ray transient source IGR J17354-3255
Context. On April 14, 2009, the AGILE satellite detected a γ-ray flare from an unknown transient source. Subsequent X-ray follow-up observations with Swift and INTEGRAL identified the supergiant fast X-ray transient (SFXT) IGR J17354-3255 as the best candidate counterpart, based on positional coincidence and a similar temporal behaviour. Aside from this hint, no SFXT has been firmly detected at high energies to date. Overall, SFXTs comprise a subclass of high-mass X-ray binaries (HMXBs) that host a massive OB supergiant star as a companion donor. They tend to display the most extreme X-ray variability among HMXBs. These systems might be able to emit photons at MeV-TeV energies in the form of fast flares lasting from hours to a few days, with a low-duty cycle. Aims. In this work, we analyse archival AGILE data to search for γ-ray flares consistent with IGR J17354-3255 and evaluate their possible physical origin. Methods. We identified a transient source, AGL J1736-3250, which emitted 19 γ-ray flares and was seen to be positionally consistent with IGR J17354-3255. Most flares, detected on a 1 d timescale, concentrate most of their emission on two, four, and six hour timescales, resembling those observed in the X-ray band from IGR J17354-3255. Results. An orbital phase analysis revealed that approximately half of the γ-ray activity occurs around the apastron passage of the compact object hosted in the binary system. We also incorporated archival Swift and INTEGRAL observations to provide phase-folded light curves at lower energies. Our collected results strongly support a physical association between IGR J17354-3255 and AGL J1736-3250, offering evidence that SFXTs could constitute a new class of Galactic high-energy transient emitters.