A jet model for the broadband spectrum of XTE J1118+480: Synchrotron emission from radio to X-rays in the Low/Hard spectral state

摘要

Observations have revealed strong evidence for powerful jets in the Low/Hardstates of black hole candidate X-ray binaries. Correlations, both temporal andspectral, between the radio -- infrared and X-ray bands suggest that jetsynchrotron as well as inverse Compton emission could also be significantlycontributing at higher frequencies. We show here that, for reasonableassumptions about the jet physical parameters, the broadband spectrum fromradio through X-rays can be almost entirely fit by synchrotron emission. Weexplore a relatively simple model for a relativistic, adiabatically expandingjet combined with a truncated thermal disk conjoined by an ADAF, in the contextof the recently discovered black hole binary XTE J1118+480. In particular, theX-ray power-law emission can be explained as optically thin synchrotronemission from a shock acceleration region in the innermost part of the jet,with a cutoff determined by cooling losses. For synchrotron cooling-limitedparticle acceleration, the spectral cutoff is a function only of dimensionlessplasma parameters and thus should be around a ``canonical'' value for sourceswith similar plasma properties. It is therefore possible that non-thermal jetemission is important for XTE J1118+480 and possibly other X-ray binaries inthe Low/Hard state.