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Near-infrared counterpart candidate to SLX 1746-331

ATEL # 1252; M.A.P. Torres (CfA), D. Steeghs (Warick/CfA), P. G. Jonker (SRON/CfA), J. Kennea (PSU), D. K. Erb (CfA), S. Gonzalez, C. R. Burns, W. L. Freedman (Carnegie)
on 30 Oct 2007; 4:26 UT
Password Certification: Manuel Torres (

Subjects: Infra-Red, Binaries, Black Holes, Transients

We report near-infrared observations with the 6.5m Magellan Baade telescope at Las Campanas observatory of the field containing the currently active black hole X-ray transient SLX 1746-331 (Markwardt & Swank 2007, ATel #1235).

We have acquired a series of 15s Ks-band images on 2007 Oct 17 23:40 - 23:52 UT and again on 2007 Oct 22 23:45-23:57 using the PANIC camera and totaling 450s on source for both nights. The frames were acquired under good sky conditions with seeing of 0.5 arcsec. Additional 3s exposures of the field were obtained for an absolute flux calibration of the images. The astrometric and photometric scale were defined using 2MASS sources in the field of view achieving a positional RMS of 0.1 arcsec and a zero-point error of 0.1 mag.

Our imaging (see finding chart) revealed a large number of point sources within the 4 arcsec error circle for the improved X-ray location of SLX 1746-331 (Kennea et al. 2007; ATel #1237), Our relative (PSF-fitting) photometry on the two data sets resulted in the identification of only one variable object with Ks = 16.93 +/- 0.06 on Oct 17 and Ks = 17.29 +/- 0.08 on Oct 22. We determined a position of R.A(J2000)=17:49:48.94 and Dec(J2000)=-33:12:11.6 (+/- 0.1" uncertainty) for this source. Note that the Oct 22 (MJD 54395.99) data set was acquired when the X-ray flux had declined by a factor of ~2 compared to the X-ray flux measured on Oct 17 ((MJD 54390.99; see public RXTE ASM light curve).

This variable source matches the Swift and ROSAT X-ray positions and thereby we consider it a likely infrared counterpart to SLX 1746-331. We plan to acquire additional imaging when the source reaches quiescence to firmly establish the counterpart (if visible at infrared wavelengths).

Finding Chart

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R. E. Rutledge , Editor
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