The optical/near-infrared counterpart of the integral obscured source IGR J16318-4848: An sgB[e] in a high-mass X-ray binary?

The X-ray source IGR J16318-4848 was the first source discovered by INTEGRAL on 2003 January 29. The high-energy spectrum exhibits such a high column density that the source is undetectable in X-rays below 2 keV. On 2003 February 23-25, we triggered our Target of Opportunity (ToO) program using the EMMI and SOFI instruments on the New Technology Telescope of the European Southern Observatory (La Silla) to obtain optical and near-infrared (NIR) observations. We confirm the already proposed NIR counterpart and for the first time extended detection into optical. We report here photometric measurements in the R, I, and J bands, upper flux limits in the Bb and V bands, and lower flux limits in the H and K_s bands. We also obtain NIR spectroscopy between 0.95 and 2.52 μm, revealing a large number of emission lines, including forbidden iron lines and P Cygni profiles, and showing a strong similarity with CI Cam, another strongly absorbed source. Together with the spectral energy distribution (SED), these data point to a high-luminosity, high-temperature source, with an intrinsic optical/NIR absorption greater than the interstellar absorption but 2 orders of magnitude below the X-ray absorption. We propose the following picture to match the data: the source is a high-mass X-ray binary (HMXB) at a distance between 0.9 and 6.2 kpc, and the optical/NIR counterpart corresponds to the mass donor, which is an early-type star, maybe an sgB[e] star, surrounded by a dense and absorbing circumstellar material. This would make the second HMXB with an sgB[e] star as the mass donor after CI Cam. Such sources may represent a different evolutionary state of X-ray binaries previously undetected with the lower energy space telescopes; if so, a new class of strongly absorbed X-ray binaries is being unveiled by INTEGRAL.