Recasting the nature of INTEGRAL hard X-ray transients previously classified as active galactic nuclei

¹ INAF–OAS, Osservatorio di Astrofisica e Scienza dello Spazio, Area della Ricerca del CNR, via Gobetti 101, I-1-40129 Bologna, Italy
² INAF–IASF, Istituto di Astrofisica Spaziale e Fisica Cosmica, via A. Corti 12, 20133 Milano, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 30 July 2025
Accepted: 3 October 2025

Abstract

We present new broad-band X-ray results aimed at the identification and characterization of four poorly studied hard X-ray transients discovered by INTEGRAL: IGR J16426+6536, IGR J09446−2636, IGR J21268+6203, and IGR J02447+7046. The key properties and X-ray behavior of these sources have remained largely unknown until now. We investigated the temporal, spectral, and energetic characteristics of their hard X-ray outbursts detected above 20 keV by INTEGRAL. In addition, we performed a systematic analysis of unpublished archival soft X-ray observations below 10 keV, enabling a full exploration of their large INTEGRAL error circles in order to search for the most likely soft X-ray counterparts. Within their arcsecond-sized X-ray error circles, we identified single optical/near-infrared counterparts for each source. We analyzed their photometric properties to constrain the nature of the systems. Our results show that the X-ray properties of these four transients are inconsistent with the previously proposed extragalactic AGN origin and instead support a Galactic nature for all of them. Specifically, we propose a very faint X-ray transient classification for IGR J16426+6536, a nearby flaring star for IGR J09446−2636 and IGR J21268+6203, and finally a γ-ray binary nature for IGR J02447+7046.