Optically thin synchrotron emission from XTE J1908+094 observed by the ATCA
ATel #5575; M. Coriat (Univ. Cape Town), Tasso Tzioumis (ATNF), S. Corbel (Univ. Paris Diderot & CEA Saclay), R. Fender (Oxford)
on 12 Nov 2013; 19:34 UT
Credential Certification: Mickael Coriat (mickael@coriat.eu)
Subjects: Radio, X-ray, Binary, Black Hole, Transient
Referred to by ATel #: 12628
The black hole candidate XTE J1908+094, currently in outburst (Krimm et al. ATel #5523, Atel #5529; Miller-Jones et al. Atel #5530; Rushton et al. Atel #5532), recently started a hard-to-soft state transition (Negoro et al. Atel #5549). Rushton et al. (ATel #5551) reported a significant increase of the radio flux density at 16.3 GHz from 0.93 mJy on 2013-11-04T14:22-18:19 to 7.3 mJy on 2013-11-05T14:27-20:24 UT suggesting the ejection of optically thin radio-emitting plasma as usually associated with a hard-to-soft state transition.
We report on ATCA observations of XTE J1908+094 conducted on 2013-11-07T04:57-08:29 at 5.5 GHz, 9 GHz, 17 GHz and 19 GHz. The source was detected at all frequency bands with the following flux densities:
5.5 GHz: 1.51 +/- 0.04 mJy
9 GHz: 1.05 +/- 0.04 mJy
17 GHz: 0.51 +/- 0.05 mJy
19 GHz: 0.27 +/- 0.06 mJy
which implies a spectral index alpha = -0.92 +/- 0.13 (S = k nu^+alpha). We therefore confirm the optically thin synchrotron nature of the radio flare reported by Rushton et al. (ATel #5551) which has significantly faded compare to the 7.3 mJy detected at 16.3 GHz two days earlier. However, multiple flares can be expected while the source is transiting toward the soft X-ray state. We therefore encourage a regular monitoring of the source at multiple wavelengths.
We did not resolve any radio emitting structure separated from the core down to a beam size of 0.3 by 12.5 arcsec (at 19 GHz) oriented 2.8 degrees west of north.
We thank Phil Edwards for the rapid scheduling of these observations
