AGILE detection of transient γ-ray emission from the region of the supergiant fast X-ray transient source IGR J17354–3255

¹ INAF – Osservatorio di Astrofisica e Scienza dello spazio di Bologna (OAS), Via Piero Gobetti 93/3, 40129, Bologna, Italy
² INAF – Istituto di Astrofisica e Planetologia Spaziali (IAPS), Via del Fosso del Cavaliere 100, 00133, Roma, Italy
³ Dipartimento di Fisica, Università di Roma Tor Vergata, Via della Ricerca Scientifica 1, I-00133, Roma, Italy
⁴ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807, Merate, Italy
⁵ CISAS G. Colombo University of Padua, Padua, Italy
⁶ Dipartimento Fisica, Università di Trieste, Via A. Valerio 2, I-34127, Trieste, Italy
⁷ INFN Sezione di Trieste and Università degli Studi di Trieste, Via Valerio 2, I-34127, Trieste, Italy
⁸ Università del Salento, Dipartimento di Matematica e Fisica “E. De Giorgi”, Lecce, Italy
⁹ INFN, Sezione di Lecce, Lecce, Italy
¹⁰ Department of Mathematics, University of Pavia, Pavia, Italy
¹¹ ASI Space Science Data Center (SSDC), Via del Politecnico snc, I-00133, Roma, Italy
¹² INAF-Osservatorio Astronomico di Roma, Via di Frascati 33, I-00078, Monte Porzio Catone, Italy
¹³ School of Physics, Wits University, Johannesburg, South Africa
¹⁴ INAF – Istituto di Radioastronomia (IRA), I-40129, Bologna, Italy

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Received: 8 July 2025
Accepted: 13 April 2026

Abstract

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.