Radio observations of XTE J1748-361 (=A1744-36?)

ATel #210; M. P. Rupen, V. Dhawan, A. J. Mioduszewski (NRAO)
on 26 Nov 2003; 00:48 UT
Credential Certification: Michael P. Rupen (mrupen@nrao.edu)

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Very Large Array (VLA) observations of the transient X-ray source XTE J1748-361 (ATEL #204) taken on November 10 UT at 4.86 GHz show:

• a marginal (3sigma) detection of the 1976 transient A1744-36 (ATEL #204, ATEL #205), at a nominal level of 196+/-67 microJy/beam. This is plausibly a detection because of the accuracy of the Chandra position (ATEL #205).
• one other source within the ~9 arcminute field-of-view of these observations, a 570+/-72 microJy source 84.4 arcseconds from A1744-36, and 4.1 arcminutes from the RXTE ASM position reported in ATEL #204, and well within its 12 arcminute error circle. The coordinates of this source, measured with respect to the VLA calibrator J1744-3116 (4.9 degrees away), are:

  17 48 13.148 +/- 0.014    -36 07 57.02 +/- 0.30 (J2000)

  where the error bars are statistical, based on fitting a point source plus planar background to the image. For such a faint source close to the phase center these statistical errors are expected to dominate the total error budget.

  no other sources within the field-of-view, roughly +/-4.4 arcminutes at the half-power point (i.e., the noise level 4.4 arcminutes from A1744-36 is twice that quoted above).

  a confusing source at the position of the possible counterpart detected at the ATCA (ATEL #208). The apparent total flux density of this source was ~2.6 mJy, before correcting for the sensitivity of the individual antenna elements; it is impossible to do this correction accurately for a source so distant (~9 arcminutes) from the pointing center. (See below for further details on this source.)

Note that there is as yet no evidence for variability of any of these sources. Further observations are planned.

Following the report of a possible radio counterpart (ATEL #208) at the position of the confusing source mentioned above, we observed that position with the VLA on November 21 UT and detected a complex, quite extended (~10 arcseconds) source, with total flux densities of 73 mJy at 4.86 and 44 mJy at 8.46 GHz respectively. There is also a strong source apparent in the NRAO VLA Sky Survey (NVSS; http://www.cv.nrao.edu/nvss/ ), with a total flux density of about 200 mJy at 1.4 GHz. These detections are consistent with a constant source with spectral index -0.9 between 1.4 and 8.5 GHz, and this together with its appearance (see http://www.aoc.nrao.edu/~mrupen/XRT/X1748-361/x1748-361.shtml for contour plots) suggest that this source is associated with a background active galactic nucleus (AGN) rather than the X-ray transient. There were no other sources within the field-of-view of the interferometer, to limiting rms noise levels at the pointing center of 0.17 mJy/beam at 4.86 GHz (half-power field-of-view: +/-4.4 arcmin) and 0.15 mJy/beam at 8.46 GHz (half-power field-of-view: +/-2.5 arcmin).

The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

VLA Observations of XTE J1748-361