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

TL;DR

The paper reclassifies four INTEGRAL hard X-ray transients previously labeled as AGN by integrating long-term hard X-ray monitoring with targeted soft X-ray follow-up. Through precise X-ray localization and multiwavelength characterization, the authors identify Galactic counterparts: IGR J16426+6536 as a very faint X-ray transient or low-mass X-ray binary, IGR J09446−2636 and IGR J21268+6203 as nearby flaring stars, and IGR J02447+7046 as a gamma-ray binary with a Be-star companion. The study highlights the importance of soft X-ray localization to distinguish Galactic from extragalactic origins and underscores the diverse Galactic population contributing to hard X-ray transients detected by INTEGRAL. The findings have significant implications for population studies and for understanding the true nature of hard X-ray transients in the Galaxy.

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 In addition, we performed a systematic analysis of unpublished archival soft X-ray observations below 10 keV, enabling a full exploration of their large INTEGRAL error circles in search of the most likely soft X-ray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, finally a gamma-ray binary nature for IGR J02447+7046.

Figures (11)

• Figure 1: XMM--Newton EPIC pn (right) and Swift/XRT (left) image of IGR J16426+6536 sky region with superimposed the ISGRI error circle (90% confidence). Two X-ray sources (n.1 and n.2) are detected inside of it in the XMM--Newton image. Conversely, only X-ray source n.1 is detected in the Swift/XRT image, where the small dashed circle marks the position of the undetected X-ray source n.2.
• Figure 2: XMM--Newton EPIC pn image (0.2-10 keV) of IGR J09446$-$2636 sky region with superimposed the ISGRI error circle at 90% confidence (dashed) and at 95% confidence. Two X-ray sources (n.1 and n.2) are detected inside of it.
• Figure 3: Swift/XRT image (0.2-10 keV) of IGR J21268+6203 sky region with superimposed the ISGRI error circle at 90% (dashed) and 95% confidence. Only one X-ray counterpart (n.1) is detected inside of it. The smallest dashed circle marks the position of the radio/infrared counterpart previously proposed in the literature, which is undetected by Swift/XRT.
• Figure 4: Swift/XRT image (0.2-10 keV, obsid 00033613001 in Table 3) of IGR J02447+7046 sky region with superimposed its IBIS/IGRI error circle at 90% confidence (4$\hbox{$^\prime$}$.78 radius, dashed) and at 95% confidence (5$\hbox{$^\prime$}$.45 radius). Only one X-ray counterpart (n.2) is detected inside the 95% confidence error circle. The detected X-ray source n.1, outside the 95% confidence error circle, is the AGN previously suggested in the literature as counterpart of IGR J02447+7046
• Figure 5: NVSS radio map at 20 cm (as taken from the NVSS team web page http://www.cv.nrao.edu/nvss/) of IGR J02447+7046 sky region. The small black circle marks the position of the Swift/XRT error circle of the best candidate X-ray counterpart of IGR J02447+7046 (source n.2 in Fig. 4)