Constraints on the Low-Energy Cutoff in the Electron Distribution of the PKS 0637-752 Jet

Abstract

We re-analyze the Chandra ACIS spectrum of the kpc-scale jet in PKS 0637-752 to investigate the possible low energy cutoff in the relativistic electron spectrum producing the non-thermal radiation in the scenario of inverse Compton emission off the cosmic microwave background. This 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 detector contamination. As previously reported in the literature, the spectrum can be fit by a power law, with the slope predicted by the radio spectrum, modified by low energy absorption through the Galaxy as determined from the spectrum of the quasar core and by HI 21 cm observations. We obtain a marginally better fit with an model of inverse Compton emission produced by an electron population that exhibits a cutoff at (gamma_min delta_10) between about 50 and 80 (assuming Gamma = delta). This range for gamma_min is higher than has previously been assumed in broad-band spectral fits to the jet emission. The observed optical flux can be used to place a lower limit on gamma_min; the constraint is not very strong, but does suggest that gamma_min must be higher than 1 to avoid overproducing the optical emission. We investigate the effect of uncertainties in the column density for galactic absorption as well as the calibration of Chandra for these early observations. Finally, we discuss the implication of these limits on the jet luminosity in this source.