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Emission-line early-type stars as possible optical counterparts of INTEGRAL hard X-ray sources IGR J08408-4503 and IGR J15539-6142

ATel #815; N. Masetti, L. Bassani (INAF/IASF, Bologna), A. Bazzano (INAF/IASF, Rome), A. J. Dean (Univ. Southampton), J. B. Stephen (INAF/IASF, Bologna), R. Walter (ISDC, Versoix)
on 19 May 2006; 15:23 UT
Credential Certification: Nicola Masetti (masetti@iasfbo.inaf.it)

Subjects: Optical, X-ray, A Comment, Binary, Star, Transient

Referred to by ATel #: 817, 818, 819, 1034, 3586

We retrieved and analyzed ESO archival spectra of the blue supergiant star HD 74194, putatively associated with the hard X-ray source IGR J08408-4503 (Gotz et al., ATel #813; Mereghetti et al., ATel #814).
The spectra were acquired on 2003 Jan 9, for a total exposure time of 10 s, under programme 70.C-0396(A) at the ESO 3.6m equipped with EFOSC, and cover the 6000-10000 Angstrom range.

The analysis show the Halpha line in emission (with EW ~ 1 Angstrom) and HeI absorption lines at 6675 and 7065 Angstrom. This presence of Halpha emission strongly indicates, by analogy with other systems (see e.g. Masetti et al., 2006, A&A, 449, 1139 and references therein), that HD 74194 is indeed the optical counterpart of IGR J08408-4503 and that this source belongs to the growing class of Supergiant Fast X-ray Transients (see e.g. Sguera at al., astro-ph/0603756) as suggested in ATels #813 and #814.

Likewise, we note that an emission-line early-type star, HD 141689 (classified as B2/B3ne by Henize 1976, ApJS, 30, 491) is present in the published error box of the newly-discovered INTEGRAL source IGR J15593-6142 (Keek et al., ATel #810).
Waiting for a refinement of the ISGRI error box, we propose this star (besides the galaxy ESO 136-6) as a further possible optical counterpart of the hard X-ray emission detected with INTEGRAL. Followup optical spectroscopy is therefore encouraged for these two objects.

For both INTEGRAL sources, we remark that pointed soft X-ray observations with satellites affording arcsecond localizations (such as Chandra, XMM-Newton or Swift) are conclusive to confirm the above associations.

This research has made use of data retrieved from the ESO/ST-ECF Science Archive and of the SIMBAD database, operated at CDS, Strasbourg, France.