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ATEL # 1472; M.A.P. Torres (CfA), D. Steeghs (Warick/CfA), G. Roelofs (CfA), P. G. Jonker (SRON/CfA), I. Thompson (Carnegie)
on 12 Apr 2008; 2:34 UT
Password Certification: Manuel Torres (email@example.com)
Subjects: Infra-Red, Binaries, Neutron Stars, Transients
Referred to by ATEL #: 1490
We report on follow-up observations of the field containing the currently active neutron star X-ray transient SAX J1750-2900 (Markwardt & Swank 2008, ATel #1425; see also ATels #1426, #1428). The observations consist of near-infrared imaging with the PANIC camera mounted on the 6.5m Baade telescope at Las Campanas Observatory.
In order to identify variable counterpart candidates, we obtained a series of 15s Ks-band images on 2008 Mar 30 09:52-09:59 UT to complement the Ks-band observations of the field acquired on 2008 Mar 18 09:06 UT (Torres et al. 2008, ATel #1446). Sky conditions were good during both nights with a similar seeing of ~0.4 arcsec. Additional 3s exposures of the field were obtained on Mar 18 to derive an astrometric and absolute flux calibration of the frames relative to 2MASS. The astrometric and photometric scales were defined using isolated 2MASS objects in the field of view achieving a positional RMS < 0.15 arcsec and a zero-point error < 0.1 mag.
Relative (PSF-fitting) photometry on the two data sets resulted in the identification of a variable source within the error circle for the improved X-ray location of SAX J1750.8-2900 (Steeghs et al. 2008, ATel #1431; Torres et al. 2008, ATel #1446). The measured magnitudes are Ks = 17.1 +/- 0.1 on Mar 18 and Ks = 17.8 +/- 0.2 on Mar 30. We determined a position of R.A(J2000)=17:50:24.43 and Dec(J2000)=-29:02:15.0 (+/- 0.1" uncertainty) for this source (see also finding chart). Note that the Mar 30 data set was acquired when the X-ray flux had declined by a factor of ~10 compared to the X-ray flux measured at the peak of the outburst on Mar 18 (Markwardt & Swank 2008, ATel #1425; Torres et al. 2008, ATel #1446).
We consider this variable source a likely infrared counterpart to SAX J1750.8-2900.Finding chart for J1750.8: http://hea-www.harvard.edu/~mtorres/SAXJ1750.8m2900.html