Tracing the jet contribution to the mid-IR over the 2005 outburst of GRO J1655-40 via broadband spectral modeling

Abstract

We present new results from a multi-wavelength (radio/infrared/optical/X-ray) study of the black hole X-ray binary GRO J1655-40 during its 2005 outburst. We detected, for the first time, mid-infrared emission at 24 um from the compact jet of a black hole X-ray binary during its hard state, when the source shows emission from a radio compact jet as well as a strong non-thermal hard X-ray component. These detections strongly constrain the optically thick part of the synchrotron spectrum of the compact jet, which is consistent with being flat over four orders of magnitude in frequency. Moreover, using this unprecedented coverage, and especially thanks to the new Spitzer observations, we can test broadband disk and jet models during the hard state. Two of the hard state broadband spectra are reasonably well fitted using a jet model with parameters overall similar to those previously found for Cyg X-1 and GX 339-4. Differences are also present; most notably, the jet power in GRO J1655-40 appears to be a factor of at least ~3-5 higher (depending on the distance) than that of Cyg X-1 and GX 339-4 at comparable disk luminosities. Furthermore, a few discrepancies between the model and the data, previously not found for the other two black hole systems for which there was no mid-IR/IR and optical coverage, are evident, and will help to constrain and refine theoretical models.