Radio non-detection of V4641 Sgr implies jet quenching in a low-luminosity soft state

ATel #7908; J. Miller-Jones (ICRAR Curtin), A. Bahramian (Alberta), M. Middleton (Cambridge), G. Sivakoff (Alberta) on behalf of a larger XRB collaboration
on 14 Aug 2015; 08:03 UT
Credential Certification: James Miller-Jones (james.miller-jones@curtin.edu.au)

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Following the recent outburst of the black hole X-ray binary V4641 Sgr (ATel #7858, #7874, #7904), we took Director's Discretionary Time observations with the Karl G. Jansky Very Large Array. We observed from 01:49 - 02:09 UT on 2015 August 12 (MJD 57246.076 - 57246.090), in two 1024-MHz basebands centered at frequencies of 5.25 and 7.45 GHz, achieving 3.3 minutes of on-source time in each baseband. The array was in its most extended A-configuration. We used 3C286 to set the amplitude scale, and J1820-2528 to determine the complex gains of the instrument. V4641 Sgr was not detected in either baseband, to 3-sigma upper limits of 83 and 75 microJy/beam at 5.25 and 7.45 GHz, respectively. By stacking the two basebands, we reached a marginally more constraining 3-sigma upper limit of 58 microJy/beam.

Although not strictly simultaneous, recent Swift XRT Windowed Timing Mode observations imply that the source is in a soft thermal state (ATel #7904). We follow the same methods discussed in ATel #7904, concentrating only on the last four observations, which includes a new observation since ATel #7904. As these observations have start times of August 9, 08:42; August 10, 20:00; August 11, 00:34, and August 13, 10:04, the last two epochs bracket our VLA observation. We include the other two observations to provide an indication of potential variability. The disk blackbody fits imply 1 - 10 keV luminosities of 2.7e36, 4.7e35, 1.8e36, and 1.8e36 erg/s, respectively (for an assumed distance of 6.2 kpc; Macdonald et al. 2014, ApJ, 784, 2). Based on Figure 10 from Deller et al. 2015 (ApJ, 809, 13), these imply predicted 5.5-GHz radio flux densities of 3.2, 1.1, 2.5, and 2.5 mJy/beam if the source fell on the "standard", radio-loud branch of the radio/X-ray correlation for black holes. At an X-ray luminosity of 1.8e36 erg/s, we would expect a 5.5-GHz flux density of 0.36 mJy/beam if the source were to fall on the fainter radio luminosity end of the radio-quiet, "outlier" branch. Thus, the radio emission is a factor of at least 6 - 40 times fainter than expected in the radio band for an assumed X-ray luminosity of 1.8e36 erg/s. Given the soft, disc-dominated X-ray spectra reported in ATel #7904, we infer that the radio jet is quenched in this faint, soft state, with an Eddington fraction of <1%. This confirms that the presence of radio jets appears to depend strongly on the existence of a hard power-law X-ray spectrum, from a vertically-extended corona or jet base, regardless of the source luminosity.

We thank NRAO staff for the rapid assessment and scheduling of these DDT observations.