2004CBET...59....1R

M. P. Rupen, A. J. Mioduszewski, and V. Dhawan, National Radio Astronomy Observatory, report strong, on-going radio flares from the microquasar candidate associated with GSC 7861.1088 = 1RXS J162848.1-415241 (cf. IAUC 7968). Observations with the Very Large Array (VLA) on Jan. 6, 11, and 22 UT gave flux densities at 8.46 GHz of 0.35 ± 0.08, 13.8 ± 0.8, and 6.6 ± 0.4 mJy, respectively. Quasi-simultaneous observations at 4.86 GHz on the last day gave a flux density of 4.9 ± 0.3 mJy, implying that the flux density goes as frequency to the power 0.54 ± 0.16. Such an inverted radio spectrum is almost invariably associated with the early, optically thick stage of a strong radio outburst, and this – together with the changing radio flux density – suggests that the source is currently undergoing a series of radio flares. The most reliable radio position, based on the weather and the inverted spectrum (which implies that the source is small, and presumably close to the binary-star system), is that from the Jan. 22 observations at 4.86 GHz: R.A. = 16h28m47s.283 ± 0s.003, Decl. = -41o52'39".05 ± 0".05 (equinox 2000.0), where the error bars are statistical, based on a fit using a point source plus a planar background. This is in excellent agreement with the UCAC2 optical position (Zacharias et al. 2003, from the Vizier On-line Data Catalog), which gives position end figures 47s.286, 39".01 (quoted accuracy 0".01). The VLA data show no obvious signs of extension, with the best limits coming from the Jan. 11 observations, which had a beam size of 3".5 x 0".6, with the long axis oriented toward p.a. 14 degrees. A Gaussian fit to those data gave a nominal upper limit to the extent of 0".3 (full-width at half-maximum). Further VLA observations are planned, and observations at other wavelengths (particularly radio, x-ray, and optical) are urgently requested.