Identification of high-energy sources

A new class of X-ray binaries, recently discovered by the Integral high-energy observatory, is composed of intrinsically obscured supergiant high mass X-ray binaries. This discovery of a dust component confined in a disk-like structure around the supergiant binary system enables us to address fundamental but still unsolved questions about these intriguing sources, namely their formation, evolution, and also the nature of their environment.

IGR J16318-4848 pair formed by a supergiant star
emitting an intense wind as a heated dust disk,
inside which is orbiting a compact object, neutron star or black hole
(credit ESA / S. Chaty)

In a paper, we present new ESO VLT VISIR mid-infrared (MIR) spectroscopic observations, which unveiled the supergiant X-ray binary IGR J16318-4848, composed of a compact object orbiting a supergiant B[e] star.
First, emission line diagnostics allowed us to characterize the unusually intense absorption of this source, which is due to the presence of absorbing material (dust and cold gas) shrouding the whole binary system.
Second, by fitting broadband infrared SED (ESO NTT/SofI, VLT/VISIR and Spitzer data) - with a phenomenological model for sgB[e] stars, we show that the supergiant star is surrounded by an irradiated rim heated to a temperature of 3,800-5,500 K, and then by a viscous disk of dust component at an inner temperature of 750 K, with the compact object orbiting inside this disk.

Both new data and a novel method led to this discovery:

1. New ESO/VISIR spectra played an essential role by excluding a spherical geometry for the dust component.
2. We adapted a model dedicated to Herbig Ae/Be stars, allowing us for the first time to accurately model the complex environment of this peculiar high-mass X-ray binary system.

This discovery is important because:

1. We accurately modelled for the first time the complex environment of a supergiant sgB[e] star.
2. It brings constraints on the formation and evolution of such rare and short-lived high-mass X-ray binary systems in our Galaxy.

Read also ESA's paper: "Integral reveals exotic and dusty binary systems"

References:

• S. Chaty and F. Rahoui, "Broadband ESO/VISIR-Spitzer infrared spectroscopy of the obscured supergiant X-ray Binary IGR J16318?4848", Astrophysical Journal accepted, June 2012
• Sylvain Chaty, Farid Rahoui, Cédric Foellmi, John A. Tomsick, Jérôme Rodriguez, Roland Walter, "Multi-wavelength observations of Galactic hard X-ray sources discovered by INTEGRAL. I. The nature of the companion star", arXiv:0802.1774v2
• Farid Rahoui, Sylvain Chaty, Pierre-Olivier Lagage, Eric Pantin, "Multi-wavelength observations of Galactic hard X-ray sources discovered by Integral. II. The environment of the companion star", arXiv:0802.1770v2