Constraints on the Low-Energy Cutoff in the Electron Distributionof the PKS 0637--752 Jet
We re-analyze the Chandra X-ray 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 contamination. As previously noted, 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 report evidence for a broad excess of emission below 1 keV, but are unable to constrain the shape very well. If we assume that this soft excess is unrelated to the electron population responsible for the power law emission, and that the electron spectrum cuts off at an energy of {gamma}{sub min}m{sub e}c{sup 2}, then we must have {gamma}{sub min} ({Lambda}/10) {approx}< 75 due to the absence of any low energy turn-over in the X-ray spectrum. This predicts that the observed radio spectrum should extend unbroken down below 2 MHz for the estimated jet restframe magnetic field B = 10 {micro} G. In addition, the observed optical flux can be used to place a lower limit on {gamma}{sub min}; the constraint is not very strong, but does suggest that {gamma}{sub min} must be higher than 1 to avoid overproducing the optical emission. The implication of these limits on the jet luminosity is discussed. An alternative phenomenological description of the soft excess is offered where the low-energy end of the electron spectrum is modified to account for the excess.
- Research Organization:
- SLAC National Accelerator Lab., Menlo Park, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC02-76SF00515
- OSTI ID:
- 909310
- Report Number(s):
- SLAC-PUB-12620; ASJOAB; TRN: US200722%%1213
- Journal Information:
- Astrophysical Journal, Journal Name: Astrophysical Journal; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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