X-ray emission from transient jet model in black hole binaries

While the non-thermal radio through at least near-infrared emission in the hard state in X-ray binaries (XRBs) is known to originate in jets, the source of the non-thermal X-ray component is still uncertain. We introduce a new model for this emission, which takes into account the transient nature of outflows, and show that it can explain the observed properties of the X-ray spectrum. Rapid radiative cooling of the electrons naturally accounts for the break often seen below around 10keV, and for the canonical spectral slope F _νν^-1/2 observed below the break. We derive the constraints set by the data for both synchrotron- and Compton-dominated models. We show that for the synchrotron-dominated case, the jet should be launched at radii comparable to the inner radius of the disk (few 100 r_s for the 2000 outburst of XTE J1118+480), with typical magnetic field B ≳ 10 ⁶ G. We discuss the consequences of our results for the possible connection between the inflow and outflow in the hard state of XRBs.