The accretion/ejection coupling in accreting black hole binaries has been described by empirical relations between the X-ray/radio and X-ray/optical-infrared luminosities. These correlations were initially supposed to be universal. However, recently many sources have been found to produce jets that, given certain accretion-powered luminosities, are fainter than expected from the correlations. This shows that black holes with similar accretion flows can produce a broad range of outflows in power. Here we discuss whether typical parameters of the binary system, as well as the properties of the outburst, produce any effect on the energy output in the jet. We also define a jet-toy model in which the bulk Lorentz factor becomes larger than ~1 above ~0.1% of the Eddington luminosity. We finally compare the "radio quiet" black holes with the neutron stars.
Relativistic ejections (jets) are a common consequence of accretion processes onto stellar-mass black holes. In the low/hard state (LHS) and in the quiescent state of black hole candidates (BHCs) a compact, steady jet is on. The jet is highly quenched in the high/soft state (HSS) of BHCs (see Fender 2010 for a review). Corbel et al. (2003) and Gallo et al. (2003) found that the radio luminosity of many BHCs in the LHS correlates over several orders of magnitude with the X-ray luminosity. They proposed that a correlation of the form L X ∝ L 0.58±0.16 R (where L X and L R are the X-ray and radio luminosities) could be universal and also valid for sources in quiescence (Gallo et al. 2006). This relation describes a coupling between the accretion processes and the ejection mechanisms. A similar correlation also hold between the the X-ray and the optical/infrared (IR) luminosities (Russell et al. 2006). However, in the past few years, several "radio quiet" outliers have been found (Xue & Cui 2007;Gallo 2007). These sources seem to feature similar X-ray luminosities to other BHCs but are characterized by a radio emission that, given a certain X-ray luminosity, is fainter than expected from the radio/X-ray correlation. It is possible that a correlation with similar slope but lower normalization than the other BHCs could describe this discrepancy, at least in a few sources (e.g. Soleri et al. 2010). If confirmed, this would suggest that some other parameters might be tuning the accretion-ejection coupling, allowing accretion flows with similar radiative efficiency to produce a broad range of outflows. Casella & Pe'er (2009) suggested that different values of the jet magnetic field can cause a quenching of the radio emission, without influencing the energy output in the X rays. Gallo et al. (2006) Figure 1. Left-hand panel: Radio and near-IR normalizations as a function of the orbital period and the size of the Roche lobe of the BHCs. Our BHC sample is listed in the inset. The inset also shows a key to the symbols. Right-hand panel: Values of the radio luminosity expected from our toy model for 10 viewing angles, in Eddington units. See the left-hand panel for a key to the symbols. Fender et al. (2010) showed that, if our measures of the spin and the estimates of the jet power are correct, the spin does not play any role in powering jets from BHCs.
In this conference Proceedings we investigate whether there is a connection between the values of some binary parameters and properties of the outburst of 17 BHCs (listed in Figure 1, left-hand panel) and the compact steady-jet power. We will follow the approach presented in Fender et al. (2010) to use the normalizations of the radio/X-ray and IR/Xray correlations as a proxy for the jet power. The data used to calculate the normalizations are from Gallo et al. (2003), Gallo et al. (2006), Gallo (2007), Russell et al. (2006), Russell et al. (2007) and Soleri et al. (2010). We develop a jet-toy model to study whether deboosting effects can explain the scatter around the radio/X-ray correlation. We also compare the “radio quiet” BHCs to the accreting neutron star (NS) X-ray binaries.
Since the accretion disc occupies ∼ 70% of the Roche lobe of the black hole, we calculated the size of the Roche lobe of the accretor as a measure of the disc size. Figure 1 (left-hand panel) shows the radio and near-IR normalizations as a function of the size of the Roche lobe of the black hole and the orbital period of the binary. To test whether there is any correlation between the jet power and these two orbital parameters, we calculated the Spearman rank correlation coefficients. The values of the correlation coefficients ρ, as well as the null hypothesis probabilities (the probability that the data are not correlated), are reported in Table 1. Clearly no correlation is present.
We also investigate the dependence of the radio and near-IR normalizations on the inclination between the jet axis and the line of sight. We will refer to this angle i as either inclination or viewing angle. Casella et al (2010) recently showed that compactsteady jets from BHCs can have rather high bulk Lorentz factor Γ > 2. This result suggests that de-boosting effects can become important, not only at high viewing angles. In our analysis we are assuming that the X-ray emission is un-beamed (but see Fender 2010 and references therein). To test if a correlation exists, we calculated the Spearman coefficients ρ. We show them in Table 1. In the case of the near-IR normalizations, we obtained ρ ∼ -0.9, with a probability for the null hypothesis of ∼ 2%. This suggests that there is an anticorrelation between the inclination angle and the near-IR normalization. However, the lack of data points (only 7) might have biased this result.
During an outburst, BHCs usually show a transition to the HSS. However, some sources spend the whole outburst in the LHS (or in the LHS and in the intermediate states), without transitin
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