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Optical counterpart to SAX J1818.6-1703

ATel #831; Ignacio Negueruela (U. Alicante), David M. Smith (U. C. Santa Cruz)
on 9 Jun 2006; 11:55 UT
Credential Certification: Ignacio Negueruela (ignacio@dfists.ua.es)

Subjects: Infra-Red, Optical, X-ray, Binary, Transient

Referred to by ATel #: 915, 1482

The recurrent hard X-ray transient SAX J1818.6-1703 has been observed by several satellites. It displays strong very short flares (Grebenev & Sunyaev 2005, Ast. Lett. 31, 672; Sguera et al. 2005, A&A 444, 221), similar to those from the well described Supergiant Fast X-ray Transients XTE J1739-302 (Smith et al. 2006, ApJ 638, 982) and IGR J17544-2619 (Pellizza et al. 2006, astro-ph/0605559).

Motivated by the similarity to known SFXTs, we searched archival (2MASS, DENIS, USNO) data of its error circle for objects that could be early type stars. The bright early-type star HD 168074 is within the error circle, but it is a B3 IV star without any peculiarity (Negueruela et al. 2006, ESA SP 604, 165). The only other object compatible with being a reddened early-type star is 2MASS J18183790-1702479 (K=7.85+-0.03), which is visible in the optical as USNO-B1.0 0729-0750578 (R2=17.4).

Low resolution spectroscopy of this object was obtained on the night of April 23rd 2006 with the VLT (UT1) and FORS2 over the 6000-10000A region (programme ESO 077.D-0055B). The I band spectrum is typical of an OB supergiant, with well resolved Paschen lines visible to Pa 20, strong HeI 8779 and prominent diffuse interstellar bands. In the red, H-alpha is in emission and HeI 6678, 7065 A are clearly visible in absorption. The lack of OI 7774 A at this resolution puts the spectral type at earlier than B3, but an accurate spectral type requires a better spectrum.

Pending a better localisation of SAX J1818.6-1703, this object is most likely the optical counterpart to this system, which is hence a SFXT. Further observations are in progress in order to achieve a better characterisation of this system.