ROSAT HRI observations confirm the X-ray emission from the jet of 3C 273 as suspected on the basis of the EINSTEIN HRI observations. They furthermore show that the X-rays originate from all along the jet, probably including the hot spot. While the jet's radio emission is highly peaked towards the outer end, the optical emission is more or less constant along the jet. This trend with wavelength continues at X-rays in that these are strongly peaked now at the inner end (knots A/B).
Despite the considerably improved data base the problems with the interpretation of the jet's X-ray emission (see Harris & Stern 1987) still remain. Whereas the X-ray emission from the jet of M 87 might well be due to the synchrotron emission process (Neumann et al. 1997a, Röser & Meisenheimer 1999), the situation is less clear for the jet of 3C 273. An extrapolation of the radio-to-optical synchrotron continuum could only explain the X-ray emission from the innermost knot A, it fails for the rest of the jet. Any X-ray emission from the hot spot up to the level we found can be accounted for by synchrotron-self-Compton emission. The emission mechanism for knots B to D remains a mystery, as - except for an hypothetical high-energy synchrotron component - all three mechanisms discussed above cannot account for the X-ray emission.
The forthcoming observations of 3C 273 by CHANDRA will provide X-ray data at higher resolution and with spectral information. High spatial resolution will test if the X-ray emission does indeed coincide with the radio-optical synchrotron continuum emission, not necessarily the case for thermal bremsstrahlung emission. X-ray spectra will set important constraints on the emission mechanism mainly via the spectral slope in the X-ray band, which could directly reveal a second population of relativistic particles emitting synchrotron X-rays. Thus we can expect further insight into the mystery of X-ray emission from the jet of 3C 273 within the immediate future.
© European Southern Observatory (ESO) 2000
Online publication: July 27, 2000