Swift/XRT observation of IGR J08408-4503
ATel #818; J. A. Kennea (PSU) and S. Campana (OAB)
on 22 May 2006; 20:07 UT
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Subjects: X-ray, Gamma Ray, Transient
Referred to by ATel #: 819
At 10:46UT on May 22nd, 2006, Swift/XRT began a Target of Opportunity observation of the newly discovered super-fast X-ray Transient (SF-XRT) IGR J08408-4503 (e.g. ATEL #814). In 2ks of XRT photon counting mode data we find a faint point source inside the INTEGRAL JEM-X error circle reported by Brandt et al. (ATEL #817) at the following coordinates: RA(J2000) = 08:40:47.97, Dec(J2000) = -45:03:29.8, with an estimated uncertainty of 5.4 arcseconds radius (90% containment).
This position lies less than 2 arcseconds from a bright (V=7.5) supergiant star HD 74194 (aka LM Vel) and we tentatively confirm that IGR J08408-4503 is associated with this star, as suggested by Masetti et al. (ATEL #815).
The X-ray source seen by XRT is faint, with only 10 background-subtracted counts, so no meaningful results can be obtained from spectral fitting. The estimated flux from this source is 1.9 x 10^-13 erg/s/cm^2 (0.5 â 10 keV), assuming a Crab-like spectrum. Assuming a distance of 3 kpc to the source, as estimated from the parallax of HD 74194 from the Hipparcos catalogue (e.g. Schroeder et al., 2004), gives an X-ray luminosity of ~2 x 10^32 erg/s (0.5-10 keV).
If this source is indeed IGR J08408-4503, it has faded significantly since its initial detection on May 15th, which would be expected if the source is a SF-XRT. Therefore we suggest that the source seen by Swift/XRT is quiescent X-ray emission from IGR J08408-4503. As the source is very faint, it is possible that Swift/XRT is detecting X-ray emission from HD 74194, and that IGR J08408-4503 may have faded below the level of detection. However, we consider this unlikely given the strong argument for HD 74194 being the counterpart to IGR J08408-4503 presented by Masetti et al. (ATEL #815). Deeper X-ray observations of this source to obtain a good X-ray spectrum and possibly future outbursts will help to confirm that HD 74194 is indeed the optical counterpart to IGR J08408-4503.