[Paper Review] Multi-wavelength observations of Galactic hard X-ray sources discovered by INTEGRAL. II. The environment of the companion star

Farid Rahoui, S. Chaty+2 more|arXiv (Cornell University)|Feb 13, 2008

Astronomy and Astrophysical Research4 citations

TL;DR

This study investigates the circumstellar environments of 12 Galactic hard X-ray sources discovered by INTEGRAL using mid-infrared (MIR) photometry from VLT/VISIR and multi-wavelength SED fitting. It finds MIR excesses only in IGR J16318-4848, IGR J16358-4726, and possibly IGR J16195-4945, attributable to sgB[e] stars with equatorial disks, while most sources show no significant MIR excess despite high intrinsic absorption, suggesting minimal dust contribution from stellar winds even in obscured systems.

ABSTRACT

Context: The INTEGRAL mission has led to the discovery of a new type of supergiant X-ray binaries (SGXBs), whose physical properties differ from those of previously known SGXBs. Those sources are in the course of being unveiled by means of multi-wavelength X-rays, optical, near- and mid-infrared observations, and two classes are appearing. The first class consists of obscured persistent SGXBs and the second is populated by the so-called supergiant fast X-ray transients (SFXTs). Aims: We report here mid-infrared (MIR) observations of the companion stars of twelve SGXBs from these two classes in order to assess the contribution of the star and the material enshrouding the system to the total emission.} Methods: We used data from observations we carried out at ESO/VLT with VISIR, as well as archival and published data, to perform broad-band spectral energy distributions of the companion stars and fitted them with a combination of two black bodies representing the star and a MIR excess due to the absorbing material enshrouding the star, if there was any. Results: We detect a MIR excess in the emission of IGR J16318-4848, IGR J16358-4726, and perhaps IGR J16195-4945. The other sources do not exhibit any MIR excess even when the intrinsic absorption is very high. (abridged)

Motivation & Objective

• To assess the contribution of circumstellar material to the broadband emission of INTEGRAL-discovered supergiant X-ray binaries (SGXBs), particularly in the mid-infrared (MIR) domain.
• To determine whether the high intrinsic absorption observed in these systems is linked to dust production in the stellar wind or circumstellar environment.
• To investigate whether the presence of a compact object alters the wind structure or dust formation in supergiant companions compared to isolated stars.
• To distinguish between obscured persistent SGXBs and supergiant fast X-ray transients (SFXTs) based on optical-to-MIR characteristics.

Proposed method

• Conducted mid-infrared photometry using VLT/VISIR for 12 INTEGRAL HMXB candidates with confirmed or likely supergiant companions.
• Combined VLT/MIR data with archival and published optical, near-infrared, and MIR data (USNO, 2MASS, GLIMPSE) to construct broad-band spectral energy distributions (SEDs).
• Fitted SEDs with a two-component black body model: one representing the stellar photosphere and another for potential MIR excess due to enshrouding dust or circumstellar material.
• Used the fit parameters to derive intrinsic visual extinction (A_V) and temperature of the stellar component, and to assess the significance of MIR excess.
• Compared optical and high-energy properties of sources to investigate differences between SFXTs and obscured SGXBs in the optical-to-MIR domain.
• Evaluated the potential role of photoionisation by the compact object in modifying wind density and creating absorption excesses in the stellar wind.

Experimental results

Research questions

• RQ1 Does the circumstellar environment of INTEGRAL-discovered SGXBs produce detectable mid-infrared excess emission due to dust?
• RQ2 Are the high intrinsic absorptions observed in these systems primarily due to dust in the stellar wind or other material?
• RQ3 Can the distinction between SFXTs and obscured SGXBs be identified through optical-to-MIR SED characteristics?
• RQ4 Is the stellar wind of supergiant companions in HMXBs denser or more structured than in isolated supergiants due to irradiation from the compact object?
• RQ5 Do the companions of SFXTs exhibit lower optical absorption compared to obscured SGXBs, as expected from their transient X-ray behavior?

Key findings

• IGR J16318-4848, IGR J16358-4726, and possibly IGR J16195-4945 exhibit a mid-infrared excess, which is best explained by their sgB[e] nature and the presence of equatorial disks capable of producing dust.
• For the majority of the 12 sources, no significant MIR excess was detected, indicating that stellar winds of supergiants in these systems do not produce substantial dust, even under high intrinsic absorption.
• The observed visual extinction (A_V) derived from SED fitting is consistent with high intrinsic absorption in most sources, but no correlation was found between A_V and MIR excess, suggesting dust is not the dominant absorber.
• The wind velocity of IGR J16318-4848 was measured at ~410 km s⁻¹, significantly lower than typical O/B supergiants (~1000–2000 km s⁻¹), indicating possible wind modification by the compact object.
• Sources showing SFXT behavior (e.g., IGR J16465-4945, IGR J17391-3021, IGR J17544-2619) exhibited lower optical absorption than other sources, supporting the hypothesis that their transient nature is linked to variable wind obscuration.
• The study suggests that photoionisation by the compact object may enhance wind density near the companion star, potentially explaining the observed absorption excesses in some systems, though this remains to be confirmed.