[Paper Review] Unveiling the nature of INTEGRAL objects through optical spectroscopy. V. Identification and properties of 21 southern hard X-ray sources

N. Masetti, L. Morelli+14 more|ArXiv.org|Aug 18, 2006

Astrophysical Phenomena and ObservationsPhysics and Astronomy39 references93 citations

TL;DR

This study identifies 21 southern hard X-ray sources detected by INTEGRAL using optical spectroscopy at the CTIO 1.5-meter telescope, revealing 12 Active Galactic Nuclei (AGNs), 5 magnetic Cataclysmic Variables (CVs), and 4 High-Mass X-ray Binaries (HMXBs), including one in the Large Magellanic Cloud. The results confirm INTEGRAL's power in probing nearby extragalactic sources and highlight the effectiveness of cross-correlation with soft X-ray catalogs followed by spectroscopy for identifying unknown hard X-ray objects.

ABSTRACT

Optical spectroscopic identification of the nature of 21 unidentified southern hard X-ray objects is reported here in the framework of our campaign aimed at determining the nature of newly-discovered and/or unidentified sources detected by INTEGRAL. Our results show that 5 of these objects are magnetic Cataclysmic Variables (CVs), 4 are High-Mass X-ray Binaries (HMXBs; one of which is in the Large Magellanic Cloud) and 12 are Active Galactic Nuclei (AGNs). When feasible, the main physical parameters for these hard X-ray sources are also computed using the multiwavelength information available in the literature. These identifications further underscore the importance of INTEGRAL in the study of the hard X-ray spectrum of AGNs, HMXBs and CVs, and the usefulness of a strategy of catalogues cross-correlation plus optical spectroscopy to securely pinpoint the actual nature of the X-ray sources detected with INTEGRAL.

Motivation & Objective

• To identify the nature of 21 previously unknown southern hard X-ray sources detected by the INTEGRAL satellite.
• To determine whether these sources are AGNs, HMXBs, CVs, or other high-energy emitting objects through optical spectroscopy.
• To assess the effectiveness of combining multiwavelength cross-correlation with optical spectroscopy for identifying INTEGRAL sources.
• To measure key physical parameters such as black hole masses and broad-line region velocities for identified AGNs.
• To improve the understanding of the source population in the hard X-ray sky, particularly in the Galactic plane and near the Galactic plane.

Proposed method

• Optical spectroscopy was performed on 21 southern INTEGRAL sources using the 1.5-meter telescope at Cerro Tololo Interamerican Observatory (CTIO).
• Positional cross-correlation between INTEGRAL IBIS error boxes and soft X-ray catalogs (ROSAT and Swift/XRT) was used to reduce error box size to <10 arcsec.
• Spectra were analyzed to identify emission and absorption lines, enabling classification of the optical counterparts as AGNs, CVs, or HMXBs.
• For AGNs, broad-line region (BLR) velocities and black hole masses were estimated using the virial method, assuming $ M_{\text{BH}} \propto v_{\text{BLR}}^2 R_{\text{BLR}} $.
• Sources with ambiguous soft X-ray positions were flagged as tentative identifications, requiring confirmation with high-resolution X-ray telescopes like Chandra or XMM-Newton.
• The identification strategy was validated by comparing results with previous INTEGRAL source identifications from Papers I–IV.

Experimental results

Research questions

• RQ1 What is the true nature of 21 southern hard X-ray sources previously unidentified by INTEGRAL?
• RQ2 How effective is the combination of soft X-ray cross-correlation and optical spectroscopy in identifying INTEGRAL sources?
• RQ3 What fraction of unidentified INTEGRAL sources are AGNs, HMXBs, or CVs in the southern sky?
• RQ4 Can physical parameters such as black hole mass and BLR velocity be reliably estimated for INTEGRAL-identified AGNs?
• RQ5 Why do some sources remain unidentified despite multiwavelength cross-correlation?

Key findings

• Of the 21 identified sources, 12 are AGNs (6 Seyfert 1 and 6 Seyfert 2), 5 are magnetic Cataclysmic Variables (CVs), and 4 are High-Mass X-ray Binaries (HMXBs), one of which is located in the Large Magellanic Cloud.
• The study confirms that a significant fraction (~57%) of the sample consists of AGNs, indicating that INTEGRAL is highly effective in detecting nearby extragalactic sources.
• Five AGNs in the sample have measured broad-line region (BLR) velocities between 1100 km s⁻¹ and 5500 km s⁻¹, with black hole masses ranging from 0.1 × 10⁷ M☉ to 20 × 10⁷ M☉.
• For three sources (IGR J14175−4641, IGR J14452−5133, IGR J18244−5622), the optical counterpart remains tentative due to lack of arcsecond-accurate soft X-ray positions, requiring follow-up with Chandra or XMM-Newton.
• The identification success rate of this method—cross-correlation with soft X-ray catalogs followed by optical spectroscopy—has led to the identification of 18 out of 56 (32%) previously unknown sources from the 2nd IBIS Galactic Plane Survey.
• The results demonstrate that INTEGRAL is not only detecting Galactic X-ray binaries but also probing the nearby extragalactic sky, especially through AGNs, and is effective in penetrating the Galactic plane's Zone of Avoidance.