VLA detection of radio emission from the new outburst of XTE J1908+094

ATel #5530; James C. A. Miller-Jones (ICRAR - Curtin), Gregory R. Sivakoff (U. Alberta), and Hans A. Krimm (CRESST/GSFC/USRA), on behalf of the JACPOT-XRB collaboration
on 30 Oct 2013; 20:25 UT
Credential Certification: Gregory R Sivakoff (sivakoff@ualberta.ca)

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Following the recent Swift/BAT detection of increasing hard X-ray flux from the known transient X-ray binary XTE J1908+094 (ATel #5523), we triggered radio observations with the Very Large Array. We observed from 22:51-23:21 UT on 2013 October 29 (MJD 56594.96) with the array in its relatively-extended B-configuration using two 1024-MHz basebands centred at 5.25 and 7.45 GHz. The source was significantly detected at both frequencies, and was fully consistent with being unresolved down to the beam size of 0.93×0.83 arcseconds at 7.45 GHz.

Fitting a point source model in the image plane gave flux densities of 868 ± 21 and 807 ± 29 microJy beam^-1 at 5.25 and 7.45 GHz, respectively. The fitted source position in the higher-frequency baseband (with the highest resolution) was
RA (J2000) = 19 08 53.076 ± 0.001
Dec (J2000) = 09 23 04.84 ± 0.01
where the quoted uncertainties are the statistical errors from the source fitting procedure. Nominal systematic uncertainties are estimated to be of order 10% of the beam size, i.e., 90×80 milliarcseconds. The source position is fully consistent with that determined by Rupen et al. (2002, IAUC 7874 ).

Our measured spectral index (defined by S_ν∝ν^α) is -0.21±0.14, approximately consistent with the flat spectrum expected from the compact jets known to exist in the hard spectral state of a black hole X-ray binary. Although we cannot rule out a steep declining spectrum at the 3σ level, the increasing Swift/BAT flux suggests that the source remains in the hard spectral state, and that the radio emission is therefore likely to arise from compact jets.

Although the distance is not well-determined, it is believed to lie in the range 3-10 kpc (Chaty et al. 2006, MNRAS, 365, 1387). Krimm et al. (ATel #5529) report an X-ray flux of 1.5±0.1×10^-9 erg s ^-1 cm^-2 (0.3-10 keV). Given the reported photon spectral index of 1.63±0.07 in the X-ray band, this corresponds to a flux of 1.2±0.1×10^-9 erg s ^-1 cm^-2 (1-10 keV). If XTE J1908+094 follows the standard (upper/radio-loud) track in the radio X-ray correlation (Corbel et al. 2013, MNRAS, 428, 2500), it would need to be beyond about 80 kpc. At this distance, the source would be too luminous to be a stellar mass black hole in the hard state. Our measured radio flux together with the X-ray flux suggest that, instead, XTE J1908+094 lies on the lower (radio quiet or X-ray bright) track in the radio/X-ray correlation with a distance of about 2-10 kpc.

We strongly encourage simultaneous multi-wavelength observations at all frequencies.

We thank the NRAO staff for rapid scheduling of these VLA observations.

JACPOT XRB