A Jet Model for the Broadband Spectrum of Xte J1118+480: Optically Thin Synchrotron X-rays in the Low/hard Spectral State

Observations have revealed strong evidence for powerful jets in the Low/Hard states of black hole candidate X-ray binaries. Correlations between the radio and IR, and radio and X-ray bands suggest that significant contributions to the observed broadband spectrum originate via synchrotron, and also possibly inverse Compton, emission from the jet. We show here that, for reasonable assumptions about the jet physical parameters, it is possible to have dominant synchrotron emission from radio through X-rays. We explore a relatively simple model for a relativistic, adiabatically expanding jet combined with a truncated thermal disk, in the context of the recently discovered black hole binary XTE J1118+480. The broadband spectrum can be almost entirely fit by synchrotron emission. In particular, the X-ray power-law emission can be explained as optically thin synchrotron emission from a shock acceleration region in the innermost part of the jet. The required jet power is consistent with a low accretion rate, and thus the acceleration and cooling time scales will allow high particle energies around γe & 10 . X-ray binaries in the Low/Hard-state could therefore be largely dominated by nonthermal emission from a jet, even without requiring strong relativistic boosting. Subject headings: binaries:close – ISM:jets and outflows – black hole physics – radiation mechanisms:nonthermal – accretion, accretion disks