[Paper Review] Unveiling the nature of INTEGRAL objects through optical spectroscopy. VI. A multi-observatory identification campaign

N. Masetti, E. Mason+19 more|ArXiv.org|Feb 7, 2008

Astrophysical Phenomena and ObservationsPhysics and Astronomy65 references89 citations

TL;DR

This study identifies 39 previously unknown or poorly characterized INTEGRAL hard X-ray sources using optical spectroscopy across eight telescopes in both hemispheres and archival sky surveys. The key finding is that 74% are active galactic nuclei (AGNs), with the remainder comprising X-ray binaries, cataclysmic variables, and other stellar objects, demonstrating the effectiveness of multi-wavelength cross-correlation and spectroscopy for identifying hard X-ray sources.

ABSTRACT

Using 8 telescopes in the northern and southern hemispheres, plus archival data from two on-line sky surveys, we performed a systematic optical spectroscopic study of 39 putative counterparts of unidentified or poorly studied INTEGRAL sources in order to determine or at least better assess their nature. This was implemented within the framework of our campaign to reveal the nature of newly-discovered and/or unidentified sources detected by INTEGRAL. Our results show that 29 of these objects are active galactic nuclei (13 of which are of Seyfert 1 type, 15 are Seyfert 2 galaxies and one is possibly a BL Lac object) with redshifts between 0.011 and 0.316, 7 are X-ray binaries (5 with high-mass companions and 2 with low-mass secondaries), one is a magnetic cataclysmic variable, one is a symbiotic star and one is possibly an active star. Thus, the large majority (74%) of the identifications in this sample belongs to the AGN class. When possible, the main physical parameters for these hard X-ray sources were also computed using the multiwavelength information available in the literature. These identifications further underscore the importance of INTEGRAL in studying the hard X-ray spectra of all classes of X-ray emitting objects, and the effectiveness of a strategy of multi-catalogue cross-correlation plus optical spectroscopy to securely pinpoint the actual nature of still unidentified hard X-ray sources.

Motivation & Objective

• To determine the nature of 39 unidentified or poorly studied INTEGRAL hard X-ray sources detected by the IBIS instrument.
• To reduce ambiguity in source identification by combining multi-wavelength cross-correlation with high-signal-to-noise optical spectroscopy.
• To assess the prevalence of different source classes—especially AGNs and X-ray binaries—among hard X-ray sources in the Galactic and extragalactic sky.
• To provide secure identifications for sources lacking precise soft X-ray positions, particularly in the Zone of Avoidance and Galactic Plane.
• To support future identification efforts by validating a strategy of catalogue cross-correlation followed by spectroscopic follow-up.

Proposed method

• Conducted optical spectroscopy using eight major observatories across the northern and southern hemispheres, including ESO, AAO, Apache Point, and others.
• Combined new spectroscopic data with archival data from the 2MASS and Sloan Digital Sky Survey to improve source localization and classification.
• Performed multiwavelength cross-correlation between INTEGRAL source positions and optical/near-infrared catalogs to identify potential counterparts.
• Used emission-line diagnostics (e.g., [O III] λ5007, Hα, Hβ) to classify AGNs, including Seyfert 1, Seyfert 2, and BL Lac candidates.
• Applied redshift measurements from emission lines to estimate distances and luminosities for AGN candidates.
• Evaluated X-ray binary candidates based on optical spectral features such as Balmer emission, He II λ4686, and evidence of mass transfer.

Experimental results

Research questions

• RQ1 What is the true nature of 39 hard X-ray sources from the INTEGRAL 3rd IBIS survey with no clear counterparts at other wavelengths?
• RQ2 How effective is a multi-observatory spectroscopic campaign in identifying the physical class of previously unclassified INTEGRAL sources?
• RQ3 What fraction of the unidentified hard X-ray sources are active galactic nuclei, and how do their redshifts and luminosities compare to known AGN populations?
• RQ4 Can optical spectroscopy resolve ambiguities in source association when soft X-ray positions are uncertain or unavailable?
• RQ5 What role do heavily obscured or dust-embedded sources play in the overall population of hard X-ray sources, particularly in the Galactic Plane?

Key findings

• Seventy-four percent (29 out of 39) of the identified sources are active galactic nuclei, including 13 Seyfert 1 galaxies, 15 Seyfert 2 galaxies, and one likely BL Lac object.
• Seven sources are X-ray binaries: five high-mass X-ray binaries (HMXBs) and two low-mass X-ray binaries (LMXBs).
• One source is a magnetic cataclysmic variable, one is a symbiotic star, and one is possibly an active star.
• Redshifts for the AGNs range from z = 0.011 to z = 0.316, indicating a mix of nearby and moderately distant objects.
• For four sources (IGR J06292+4858, IGR J07437−5137, IGR J08023−6954, IGR J14561−3738), the optical counterpart remains tentative due to poor soft X-ray localization.
• The study confirms that optical spectroscopy combined with multi-catalogue cross-correlation is a highly effective method for identifying hard X-ray sources, especially in regions of high confusion like the Galactic Plane.