Toward a handle on the low-energy end of the electron distribution in kpc-scale jets: The case of PKS 0637-752
Martin Mueller (KIPAC/SLAC) , Dan Schwartz (Harvard-CfA)
We re-analyze the Chandra X-ray spectrum of the kpc-scale jet in PKS 0637-752 to investigate the possible low energy cut-off in the relativistic electron spectrum producing the non-thermal radiation in the scenario of inverse Compton emission off the cosmic microwave background. The soft X-ray band is the most promising to detect the effects of this cut-off, as the corresponding radio band at which the synchrotron spectrum starts to show deviations from power law shape is usually too low in frequency to permit observations. PKS 0637-752 was among the first objects targeted by the Chandra Observatory and gives a unique opportunity to study the low energy X-ray emission free of contamination. The power law index of the X-ray spectrum is consistent with that of the synchrotron component, but there is evidence for a broad excess of emission below 1 keV. Under the assumption that this soft excess is unrelated to the electron population responsible for the power law emission, the absence of any low energy turn-over in the X-ray spectrum implies a minimum Lorentz factor of the electron distribution of no higher than 75. In addition, the observed optical flux can be used to place a lower limit on ; the constraint is not very strong, but does suggest that must be higher than 1 to avoid overproducing the optical emission. An alternative phenomenological description of the soft excess is offered, where the low-energy end of the electron distribution is modified to account for the excess; a possible interpretation is that we are seeing the high-energy tail of the IC/CMB spectrum produced by the mildly relativistic bulk shocked electrons in the jet.