ChandraX‐Ray Observations of Pictor A: High‐Energy Cosmic Rays in a Radio Galaxy

Abstract

We report X-ray observations of the nearby, powerful radio galaxy Pictor A with the Chandra Observatory and optical and near uv observations of its western radio hot spot with the Hubble Space Telescope. X-ray emission is detected from the nucleus, a 1. ′ 9 (110 kpc) long jet to the west of the nucleus, the western radio hot spot some 4. ′ 2 (240 kpc) from the nucleus, and the eastern radio lobe. The morphology of the western hot spot is remarkably similar to that seen at radio and optical wavelengths, where the emission is known to be synchrotron radiation. The X-ray spectrum of the hot spot is well described by an absorbed power law with photon index Γ = 2.07 ± 0.11. The X-ray jet coincides with a weak radio jet and is laterally extended by ≃ 2. ′′ 0 (1.9 kpc). The observed jet is up to ≃ 15 times brighter in X-rays than any counter jet, a difference ascribed to relativistic boosting as the western radio lobe is probably the closer. The jet's spectrum is well modelled by an absorbed power law with Γ = 1.94 +0.43 −0.49 and poorly fitted by a Raymond-Smith thermal plasma model.The emission processes responsible for the X-rays are discussed in detail. The radio-optical spectrum of the hot spot breaks or turns down at 10 13−14 Hz, and its X-ray spectrum is not a simple extension of the radio-optical spectrum to higher frequencies. Thermal models for the hot spot's X-ray emission are ruled out. Synchrotron self-Compton models involving scattering from the known population of electrons give the wrong spectral index for the hot spot's X-ray emission and are also excluded. A composite synchrotron plus synchrotron self-Compton model can match the X-ray observations but requires similar contributions from the two components in the Chandra band. We show that the hot spot's X-ray emission could be synchrotron self-Compton emission from a hitherto unobserved population of electrons emitting at low radio frequencies, but do not favor this model in view of the very weak magnetic field required.An inverse Compton model of the jet, in which it scatters microwave background photons but moves non-relativistically, requires a magnetic field a factor of ≃ 30 below equipartition, and ad hoc conditions to explain why the radio lobes are fainter than the jet in X-rays but brighter in the radio. These problems are alleviated if the jet moves relativistically, but models with an equipartition field require an implausibly small angle (θ) between the jet and the line of sight. A model with θ ≃ 23 • and a field a factor of 6 below equipartition seems viable.Synchrotron radiation is an alternative process for the X-ray emission. The expected synchrotron spectrum from relativistic electrons accelerated by strong shocks and subject to synchrotron radiation losses is in very good agreement with that observed for both the hot spot and jet. The possibility that the relativistic electrons result via photo-pion production by high energy protons accelerated in shocks (a 'proton induced cascade') is briefly discussed.

Integral field spectroscopy of the circum-nuclear region of the radio Galaxy Pictor A

“…This may indicate an interaction between the gas emitting these features and the radio jet. The redshifted region in PC4, however, is located to the west of the nucleus, and therefore corresponds to the near-side (approaching) jet, as indicated by the fact that both the X-ray jet (Wilson et al 2001) and the parsec-scale radio jet (Tingay et al 2000) are on the western side of the nucleus. One might naively expect a jet-gas interaction to generate blueshifts, rather than redshifts, on the approaching side and vice versa.…”

“…This result is consistent with the inclination estimated by Tingay et al (2000), who derived the jet inclination in a different manner, using the jet deflection model for a bent jet following Conway & Murphy (1993), finding an upper limit of θj < 51 • . Another estimate by Wilson et al (2001), incorporating projection relations from the radio maps of Perley et al (1997) …”

Observational evidence for gravitationally trapped massive axion(-like) particles

“…Their msec variability challenges the understanding of the coronal plasma [29], but also the down-flows along with the redshifted lines, which are seen in the transition region [200,201]. 9) Solar/celestial jets [202], including the puzzling X-ray tails behind Pulsars (or even Galaxies?) [203].…”

Recent Progress in Understanding the Large Scale Jets of Powerful Quasars

“…However, more powerful FR II (quasar) jets have very different spectral energy distributions (SEDs), with the observed X-rays being harder and at a higher flux than expected from extrapolating the radio-to-optical synchrotron spectrum, suggesting that they are part of a separate spectral component (Figure 1b). A synchrotron self-Compton (SSC) nature for the X-rays was disfavored [9,10], as the X-rays are at least a factor of 100 higher than the anticipated SSC flux (e.g., [9][10][11][12][13]). A controversy with serious implications for jet physics emerged over the nature of the X-rays.…”

“…It has been proposed [15,16] that the X-ray emission is due to the IC/CMB process, requiring relativistic jets (bulk Γ ∼ [10][11][12][13][14][15][16][17][18][19][20] oriented close to the line of sight and an electron energy distribution (EED) extending down to Lorentz factors γ ∼ 30-much lower than the γ ∼ 10 3 -10 4 traced by GHz emission-to explain the X-ray emission. In view of the low radiative efficiency of these electrons, reproducing the observed X-ray flux requires high power, sometimes in excess of the Eddington luminosity of the central black hole [17][18][19][20].…”