Evolution of the X-ray jets from 4U 1755-33

We report on new X-ray observations of the large-scale jets recently discovered in X-rays from the black hole candidate 4U 1755-33. Our observations in 2004 show that the jets found in 2001 are still present in X-rays. However, sensitive radio observations in 2004 failed to detect the jets. We suggest that synchrotron radiation is a viable emission mechanism for the jets and that thermal bremsstrahlung and inverse Compton emission are unlikely on energetic grounds. In the synchrotron interpretation, the production of X-rays requires acceleration of electrons up to ∼60 TeV, the jet power is ∼4 × 10³⁵ ergs s^-1, and the radio nondetection requires a spectral index α > -0.65 (S_v α V^α), which is similar to the indexes found in lobes surrounding some other compact objects. We find an upper limit on the flux of 4U 1755-33 in quiescence of 5 × 10^-16 ergs cm^-2 s^-1 in the 0.3-8 keV band.