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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