2020ApJ...891...31X

We present quasi-simultaneous radio and X-ray observations of the black hole X-ray binary GRS 1739-278 during the 2015-2016 mini-outbursts, i.e., between 2015 June 10 and 2016 October 31, with the X-ray-to-radio time interval being less than one day. The monitor campaign was run by Swift in the X-rays and by VLA in the radio (at both 5 and 8 GHz). We find that the brightest radio emission is actually achieved during the soft sate, and the spectrum is marginally optically thick with the spectral index α ≃ -0.28 ± 0.17 (flux F_ν ∝ ν^α). For the radio emission in the hard state, we find a large diversity in the spectral index, i.e., a majority of radio spectra are optically thick with -0.5 <= α <= 0.5, while a few are optically thin, with α being lower than -1 in certain cases. We then investigate the correlation between the luminosities in radio (monochromatic at 5 GHz, L_R) and 1-10 keV X-rays (L_X) during the hard state. We find that for more than two orders of magnitude variation in the X-ray luminosity, this source exhibits a flat correlation with p ≃ 0.16 (in the form of L_R∝L_X^p), i.e., it belongs to the "outlier" (to the standard correlation with p ≃ 0.6) category that may follow a hybrid correlation. Both the slope and the corresponding luminosity range agree well with those in H1743-322, the prototype of the hybrid correlation. Theoretical implications of our results are discussed.