We present quasi-simultaneous radio and X-ray observations of the black hole X-ray binary GRS 1739–278 of its 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 champaign was run by Swift in the X-rays and by JVLA in the radio ( at both 5 GHz and 8 GHz ) .
We find the brightest radio emission is actually achieved during the soft sate , where radio spectrum is unexpectedly flat with a spectral index \alpha \approx - 0.2 ( flux F _ { \nu } \propto \nu ^ { \alpha } ) .
For the radio emission in hard state , we also find a large diversity in the spectral index , i.e .
a majority of radio spectra are optically thick with -0.5 < \alpha < 0.5 , while a few are optically thin with \alpha < -1 .
We also 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 this source does not follow the standard correlation whose p \approx 0.6 ( in the form of L _ { R } \propto L _ { X } ^ { p } ) , but instead belongs to the “ outlier ” category that may follow a hybrid correlation .
For more than orders of magnitude variation in the X-ray luminosity this source exhibits a flat correlation with p \approx 0.16 .
Both the slope and the corresponding luminosity range agree well with those in H1743–322 , the prototype of the hybrid correlation .