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On-going radio flare in Aql X-1

ATel #286; M. P. Rupen, A. J. Mioduszewski, V. Dhawan (NRAO)
on 26 May 2004; 20:19 UT
Credential Certification: Michael P. Rupen (mrupen@nrao.edu)

Subjects: Radio, Infra-Red, Optical, X-ray, Binary, Neutron Star, Transient, Variables

Referred to by ATel #: 2000

Very Large Array (VLA) observations of the transient neutron star X-ray binary system Aql X-1, currently active at X-ray, optical, and infrared wavelengths (ATEL #259, #260, #279, #280), show that a faint radio outburst is also underway. Our 8.46 GHz observations gave the following nominal flux densities at the location of the source:

Non-detections:
  •  9 March 2004:   70 +/-  60 microJy/beam
  • 13 March 2004:   19 +/-  47 microJy/beam
  • 21 March 2004:   53 +/-  61 microJy/beam
  • 27 March 2004:   11 +/-  54 microJy/beam
  •  2 April 2004: -17 +/- 120 microJy/beam
Detections:
  • 19 May   2004: 210 +/-  50 microJy/beam
  • 26 May   2004: 214 +/-  35 microJy/beam
Combining all the March data gave a nominal value of 26 +/- 28 microJy/beam at this frequency. The 26 May 2004 data in particular show a very clear detection, and quasi-simultaneous 4.86 GHz observations give a flux density of 175 +/- 75 microJy/beam. The corresponding spectral index (flux density going as nu^alpha) on 26 May 2004 is thus alpha=+0.4 +/- 0.8. Such a spectral index is consistent with the optically-thick emission often seen from X-ray binaries in the low/hard X-ray state.

These radio data are consistent with a flare corresponding to the optical/IR rise seen by the SMARTS consortium (cf. ATEL #279). Interestingly, no similar radio emission was seen during the last optical/IR flare in March/April 2003, which was also a strong soft X-ray event as seen with RXTE's All-Sky Monitor; the radio rms noise levels were 50-70 microJy/beam at 8.46 GHz. The radio emission thus seems to track the hard rather than the soft X-rays. Further radio observations are planned.

Observational details:

  • The March and April 2004 data were taken in the VLA's C configuration, giving a typical beam size (FWHM) at 8.46 GHz of 4 x 3 arcseconds. The 19 May 2004 data were taken during the move from C to DnC configuration, and should be treated with some caution; the beam size here was 6.0 x 3.4 arcseconds. The 26 May 2004 data were taken in DnC configuration, with a beam size of 8.4 x 4.4 arcseconds (oriented east/west) at 8.46 GHz, and 15 x 5 arcseconds (also east/west) at 4.86 GHz.
  • The (1sigma) error bars reflect a combination of thermal noise measured near the source, and differences between flux densities measured using various methods (different uv-weighting schemes, total vs. peak flux densities, etc.).
  • The source position (J2000), measured by fitting a point source plus planar background to the 26 May 2004 8.46 GHz image, is
    19 11 16.01 +/- 0.03    00 35 06.7 +/- 0.3
    where the error bars are 1sigma, and reflect only the statistical uncertainties due to noise. This position is referenced to that of the VLA calibrator 1950+081, 12.8 degrees away.

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