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Radio detection of Sw J1745-26 with the VLA

ATel #4394; J. C. A. Miller-Jones (ICRAR - Curtin), G. R. Sivakoff (U. Alberta), on behalf of the larger JACPOT XRB collaboration
on 19 Sep 2012; 09:14 UT
Credential Certification: James Miller-Jones (james.miller-jones@curtin.edu.au)

Subjects: Radio, Binary, Black Hole, Transient

Referred to by ATel #: 4410, 4450, 4456, 4760

Following the detection of bright and increasing hard X-ray emission from the new transient source Sw J1745-26 (ATel #4381, #4383), we triggered radio observations with the Karl G. Jansky Very Large Array (VLA) on 2012 September 17-18 (MJD 56187.99), with the array in the extended hybrid BnA configuration. We made simultaneous observations in two 1024-MHz frequency bands centered at 5.0 and 7.45 GHz.

A highly-significant radio source was detected at a position that was consistent with the variable source identified by GROND (ATel #4380). The radio co-ordinates, which should now supersede previous X-ray and optical positions, are

RA (J2000) = 17:45:10.849 ± 0.001
Dec (J2000) = -26:24:12.60 ± 0.01

where the quoted uncertainties are from the statistical errors to Gaussian fits of the visibility data. Under typical conditions, the nominal astrometric accuracy of the array is thought to be of order 10% of the beam size, which in this case equates to about 81x46 milliarcseconds.

Our measured flux densities of 6.8 ± 0.1 and 6.2 ± 0.1 mJy at 5.0 and 7.45 GHz, respectively, imply a spectral index of -0.22 ± 0.09 (defining spectral index α via Sν ∝ να). Again, the quoted uncertainties are statistical only, and do not account for systematic errors on the flux density scale, which is believed to be good to 5-10%.

Combining our measured radio flux with the quasi-simultaneous X-ray flux reported by Swift (ATel #4383) would imply that at the distance of the Galactic Center, the source would lie on the "standard" radio/X-ray correlation for black hole X-ray binaries, well above the radio-quiet "outlier" branch. As a neutron star system would be expected to be much fainter in the radio band, this strongly supports the identification of the source as a black hole candidate (ATel #4381, #4393).

Since the measured spectral index is neither flat nor steep, we cannot conclusively determine whether the radio emission arises from a compact jet. However, since the source lies on the "standard" radio/X-ray correlation (for a distance of 8 kpc), it would suggest that we are detecting hard state emission from a compact jet, consistent with the X-ray properties determined a few hours later (ATel #4393).

Multi-wavelength follow-up observations are encouraged.

JACPOT XRB collaboration