The radio/X-ray domain of black hole X-ray binaries at the lowest radio luminosities

[Abridged] We report on deep, coordinated radio and X-ray observations of the black hole X-ray binary XTE J1118+480 in quiescence. The source was observed with the Karl G. Jansky Very Large Array for a total of 17.5 hrs at 5.3 GHz, yielding a 4.8 pm 1.4 microJy radio source at a position consistent with the binary system. At a distance of 1.7 kpc, this corresponds to an integrated radio luminosity between 4-8E+25 erg/s, depending on the spectral index. This is the lowest radio luminosity measured for any accreting black hole to date. Simultaneous observations with the Chandra X-ray Telescope detected XTE J1118+480 at 1.2E-14 erg/s/cm^2 (1-10 keV), corresponding to an Eddington ratio of ~4E-9 for a 7.5 solar mass black hole. Combining these new measurements with data from the 2005 and 2000 outbursts available in the literature, we find evidence for a relationship of the form ellr=alpha+beta*ellx (where ell denotes logarithmic luminosities), with beta=0.72pm0.09. XTE J1118+480 is thus the third system, together with GX339-4 and V404 Cyg, for which a tight, non-linear radio/X-ray correlation has been reported over more than 5 dex in ellx. We then perform a clustering and linear regression analysis on what is arguably the most up-to-date collection of coordinated radio and X-ray luminosity measurements from quiescent and hard state black hole X-ray binaries, including 24 systems. At variance with previous results, a two-cluster description is statistically preferred only for random errors <=0.3 dex in both ellr and ellx, a level which we argue can be easily reached when the known spectral shape/distance uncertainties and intrinsic variability are accounted for. A linear regression analysis performed on the whole data set returns a best-fitting slope beta=0.61pm0.03 and intrinsic scatter sigma_0=0.31pm 0.03 dex.

Observational constraints on the powering mechanism of transient relativistic jets

We revisit the paradigm of the dependency of jet power on black hole spin in accreting black hole systems. In a previous paper we showed that the luminosity of compact jets continuously launched due to accretion onto black holes in X-ray binaries (analogous to those that dominate the kinetic feedback from AGN) do not appear to correlate with reported black hole spin measurements. It is therefore unclear whether extraction of the black hole spin energy is the main driver powering compact jets from accreting black holes. Occasionally, black hole X-ray binaries produce discrete, transient (ballistic) jets for a brief time over accretion state changes. Here, we quantify the dependence of the power of these transient jets (adopting two methods to infer the jet power) on black hole spin, making use of all the available data in the current literature, which includes 12 BHs with both measured spin parameters and radio flares over the state transition. In several sources, regular, well-sampled radio monitoring has shown that the peak radio flux differs dramatically depending on the outburst (up to a factor of 1000) whereas the total power required to energise the flare may only differ by a factor ~< 4 between outbursts. The peak flux is determined by the total energy in the flare and the time over which it is radiated (which can vary considerably between outbursts). Using a Bayesian fitting routine we rule out a statistically significant positive correlation between transient jet power measured using these methods, and current estimates of black hole spin. Even when selecting subsamples of the data that disregard some methods of black hole spin measurement or jet power measurement, no correlation is found in all cases.

Zooming in on a sleeping giant: milliarcsecond HSA imaging of the black hole binary V404 Cyg in quiescence

Observations of the black hole X-ray binary V404 Cyg with the very long baseline interferometer HSA (the High Sensitivity Array) have detected the source at a frequency of 8.4 GHz, providing a source position accurate to 0.3 mas relative to the calibrator source. The observations put an upper limit of 1.3 mas on the source size (5.2 AU at 4 kpc) and a lower limit of 7 x 10^6 K on its brightness temperature during the normal quiescent state, implying that the radio emission must be non-thermal, most probably synchrotron radiation, possibly from a jet. The radio lightcurves show a short flare, with a rise time of about 30 min, confirming that the source remains active in the quiescent state.