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Swift XRT and UVOT observations of AX J1749.1-2733

ATEL # 1040; P. Romano (INAF-OABrera), V. Mangano (INAF-IASF Palermo), A. Cucchiara (PSU), L. Sidoli (INAF-IASF Milano)
on 1 Apr 2007; 13:34 UT
Password Certification: Pat Romano (patrizia.romano@brera.inaf.it)

Subjects: Optical, X-ray, Request for Observations, Binaries, Transients

Following the discovery of a likely-periodicity of 185 days in the transient AX J1749.1-2733 by Zurita Heras (ATEL #1035) we obtained a 5ks ToO observation with Swift on March 30 16:05:23 to 19:13:50 UT.

The Swift/XRT data show strong contamination from single-reflection from stray light from outside the XRT field of view, with a reflection ring going through the only source within the putative <1 arcmin ASCA error circle (Sakano et al. 2002). Detailed spectroscopy of the source is impaired by this contamination. Fitting the source spectrum with an absorbed power-law, we obtain NH=(23-10+14)E+22 cm-2 and photon index 2.5-1.7+2.0 (chi2_red=1.53/9dof) in the 0.3-10 keV energy range, and a 2-10 keV observed flux of about 1E-11 ergs/cm2/s. The spectral parameters are consistent with the ones reported by Sakano et al. 2002 (NH=25E+22 cm-2 and photon index 2.1).

Using only the 3rd orbit of the XRT observations (~300s), which is less affected by the presence of the rings, we obtain a position at RA,Dec(J2000)=267.2784, -27.5418,
RA (J2000) = 17h 49m 06.8s
Dec(J2000) = -27d 32m 30.6s
with an estimated error radius of 6.3 arcseconds (90% confidence). This position is 31 arcseconds from the ASCA position (Sakano et al. 2002), RA,Dec(J2000)=17h 49m 06.00s,-27d 33m 00.0s.

Swift/UVOT also observed the field with the V filter. At the source location we can provide a 3-sigma upper limit of V=20.67 mag.

Within the XRT error circle we find no USNO-B1 counterparts. We do find 3 2MASS counterparts: 1) 17 49 06.99 -27 32 26.8 (J=12.572+/-0.021, H=10.270+/-0.022, K= 9.255+/-0.021) 2) 17 49 06.75 -27 32 36.9 (J=15.466+/-0.102, H=13.345+/-0.083, K=12.475+/-0.059) 3) 17 49 06.31 -27 32 28.6 (J=15.483+/-0.06, H=13.497+/-0.047, K=12.599+/-0.058). From the flux reported by Sguera et al (2006) during a brief outburst in 2003, and a typical luminosity for HMXRBs of 1E36 erg/s, we derive a likely source distance of 5-6 kpc. The high J-K colors of the three 2MASS proposed counterparts suggest an optical extinction around 20 mag (which is consistent with the UVOT upper limit). With these assumptions, among the three proposed 2MASS counterparts, star n.1 is more compatible with a supergiant star (Vela X-1 like counterpart), while stars n.2 and n.3 are more consistent with a main sequence B star, typical companions in Be-systems (e.g. B0V spectral type with Mv=-4; Johnson 1966). Thus, from the IR properties of the optical counterpart, nothing can be concluded about the nature of the X-ray source (Be vs. Supergiant system).

We would like to thank the Swift Team for making these observations possible, in particular N. Gehrels and D. Burrows, the duty scientists as well as the science planners.


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R. E. Rutledge , Editor
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