Abstract
The process responsible for the Chandra-detected X-ray emission from the large-scale jets of pow- erful quasars is not clear yet. The two main models are inverse Compton scattering o the cosmic microwave background photons (IC/CMB) and synchrotron emission from a population of electrons separate from those producing the radio-IR emission. These two models imply radically di erent conditions in the large scale jet in terms of jet speed, kinetic power, and maximum energy of the particle acceleration mechanism, with important implications for the impact of the jet on the larger-scale environment. Georganopoulos et al. (2006) proposed a diagnostic based on a fundamental di erence between these two models: the production of synchrotron X-rays requires multi-TeV electrons, while the EC/CMB model requires a cuto in the electron energy distribution be- low TeV energies. This has significant implications for the -ray emission predicted by these two models. Here we present new Fermi observations that put an upper limit on the gamma-ray flux from the large-scale jet of 3C 273 that clearly violates the flux expected from the IC/CMB X-ray interpretation found by extrapolation of the UV to X-ray spectrum of knot A, thus ruling out the IC/CMB interpretation entirely for this source. Further, the upper limit from Fermi puts a limit on the Doppler beaming factor of at least < 9, assuming equipartition fields, and possibly as low as < 5 assuming no major deceleration of the jet from knots A through D1.
Highlights
Large-scale jets with sizes up to ∼ Mpc are routinely seen in radio observations of radio-loud AGN, but only more recently has high-resolution imaging with the Hubble Space Telescope (HST) and the Chandra X-ray observatory shown that the knots in many of these large-scale jets often produce significant high-energy radiation
The IC/CMB model has since been applied to other jets with X-rays inconsistent with their radio-optical synchrotron spectra, including the wellstudied source 3C 273 [20], and many more Fanaroff and Riley (FR) II X-ray jets subsequently discovered (e.g., [2, 17, 21])
The IC/CMB model requires that the jet remain highly relativistic out to the location of the X-ray knots, point close to our line of sight, and have an electron energy distribution (EED) extending down to energies ∼ 10 − 100 MeV, significantly lower than the ∼ 1 − 10 GeV electron energies traced by GHz synchrotron radio emission
Summary
Large-scale jets with sizes up to ∼ Mpc are routinely seen in radio observations of radio-loud AGN, but only more recently has high-resolution imaging with the Hubble Space Telescope (HST) and the Chandra X-ray observatory shown that the knots in many of these large-scale jets often produce significant high-energy radiation. Because the synchrotron emission mechanism is far more efficient than IC/CMB, it does not require the high Lorentz factors, extreme jet lengths or near-Eddington jet powers, as the IC/CMB model does in several cases [7, 15, 24]. It is not clear what physical mechanism might produce this second EED, and in many cases the observed SED [e.g. 17] requires the high-energy particle population to have a difficult-to-explain low-energy cutoff at ∼ TeV energies, where fast cooling is unavoidable.
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