Polarimetry and the High-Energy Emission Mechanisms in Quasar Jets

📝 Abstract

The emission mechanisms in extragalactic jets include synchrotron and various inverse-Compton processes. At low (radio through infrared) energies, it is widely agreed that synchrotron emission dominates in both low-power (FR I) and high-power (FR II and quasar) jets, because of the power-law nature of the spectra observed and high polarizations. However, at higher energies, the emission mechanism for high-power jets at kpc scales is hotly debated. Two mechanisms have been proposed: either inverse-Compton of cosmic microwave background photons or synchrotron emission from a second, high-energy population of electrons. Here we discuss optical polarimetry as a method for diagnosing the mechanism for the high-energy emission in quasar jets, as well as revealing the jet’s three-dimensional energetic and magnetic field structure. We then discuss high-energy emission mechanisms for powerful jets in the light of the HST polarimetry of PKS 1136-135.

💡 Analysis

The emission mechanisms in extragalactic jets include synchrotron and various inverse-Compton processes. At low (radio through infrared) energies, it is widely agreed that synchrotron emission dominates in both low-power (FR I) and high-power (FR II and quasar) jets, because of the power-law nature of the spectra observed and high polarizations. However, at higher energies, the emission mechanism for high-power jets at kpc scales is hotly debated. Two mechanisms have been proposed: either inverse-Compton of cosmic microwave background photons or synchrotron emission from a second, high-energy population of electrons. Here we discuss optical polarimetry as a method for diagnosing the mechanism for the high-energy emission in quasar jets, as well as revealing the jet’s three-dimensional energetic and magnetic field structure. We then discuss high-energy emission mechanisms for powerful jets in the light of the HST polarimetry of PKS 1136-135.

📄 Content

arXiv:0909.0297v1 [astro-ph.HE] 1 Sep 2009 Polarimetry and the High-Energy Emission Mechanisms in Quasar Jets M. Cara∗, E. S. Perlman∗, Y. Uchiyama†, S. Jester∗∗, M. Georganopoulos‡, C. C. Cheung§, R. M. Sambruna§, W. B. Sparks¶, A. Martel¶, C. P. O’Dea∥, S. A. Baum∥, D. Axon∥, M. Begelman††, D. M. Worrall‡‡, M. Birkinshaw‡‡, C. M. Urry§§, P. Coppi¶¶ and Ł. Stawarz∗∗∗ ∗Department of Physics & Space Sci., Florida Institute of Technology, Melbourne, FL 32901, USA †SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA ∗∗Max Planck Institute for Astronomy, 69117 Heidelberg, Germany ‡Joint Center for Astrophysics, University of Maryland, Baltimore, MD 21250, USA §NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA ¶Space Telescope Science Institute, Baltimore, MD 21218, USA ∥CIS and Department of Physics, Rochester Institute of Technology, Rochester, NY 14623, USA ††Joint Institute for Laboratory Astrophysics, University of Colorado, Boulder, CO 80309, USA ‡‡Department of Physics, University of Bristol, Bristol BS8 1TL, UK §§Department of Physics, Yale University, New Haven, CT 06520-8121, USA ¶¶Department of Astronomy, Yale University, New Haven, CT 06520-8101, USA ∗∗∗KIPAC, Stanford University, Stanford, CA 94305, USA Abstract. The emission mechanisms in extragalactic jets include synchrotron and various inverse- Compton processes. At low (radio through infrared) energies, it is widely agreed that synchrotron emission dominates in both low-power (FR I) and high-power (FR II and quasar) jets, because of the power-law nature of the spectra observed and high polarizations. However, at higher energies, the emission mechanism for high-power jets at kpc scales is hotly debated. Two mechanisms have been proposed: either inverse-Compton of cosmic microwave background photons or synchrotron emis- sion from a second, high-energy population of electrons. Here we discuss optical polarimetry as a method for diagnosing the mechanism for the high-energy emission in quasar jets, as well as reveal- ing the jet’s three-dimensional energetic and magnetic field structure. We then discuss high-energy emission mechanisms for powerful jets in the light of the HST polarimetry of PKS 1136−135. Keywords: Radiation and spectra, Galaxies, Other PACS: 98.54.-h,98.54.Aj,98.62.Nx,98.54.Cm INTRODUCTION The jets of radio-loud AGN carry energy and matter out from the nucleus to cluster- sized lobes, over distances of hundreds of kpc. While found in only ∼10% of AGN, jets can have a power output (including both luminosity and kinetic energy flux) comparable to that of the host galaxy and AGN [1], and can profoundly influence the evolution of their host galaxy and nearby neighbors. AGN jets are completely ionized flows, and the radiation we see from them is non-thermal in nature. That the radio emission arise from synchrotron radiation is supported by strong linear polarization and power-law spectra seen in both lower- and higher-power jets. However, at higher energies, the nature of the emission from higher-power large-scale jets is less clear. In low-power FR I radio galaxies, the optical and X-ray fluxes fit on extrapolations of the radio spectra (e.g., [2–4]), and high polarizations are seen in the optical (typically ∼20 −30%, [5, 6]) suggesting synchrotron emission. These jets exhibit a wide variety of polarization properties [6–8], often correlated with X-ray emission. For example, in the jet of M87 [4], a strong anti-correlation between X-ray emission and optical polarization was found in the knots, accompanied by changes in the magnetic field direction, suggesting a strong link between the jet’s dynamical structure and high-energy processes in the jet interior, where shocks compress the magnetic field and accelerate particles in situ. For the more powerful FR II and quasar jets, the nature of both the optical and X- ray emission is under active debate. In many FR II jets, the optical emission can lie well below an interpolation between the radio and X-rays (e.g., [9]), sometimes by decades (e.g., PKS 0637−752, [10, 11]), resulting in a characteristic double-humped shape of spectral energy distribution. In some jets the optical emission appears linked to the X-ray emission by a common component. This is the case in both 3C 273 and PKS 1136−135, where deep HST, Chandra and Spitzer imaging [12–16] has shown that a second component, distinct from the lower-energy synchrotron emission, arises in the near-IR/optical and dominates the jet emission at optical and higher energies, at least up to 10 keV. Competing mechanisms have been proposed: either synchrotron radiation from very high-energy particles or inverse-Comptonization (see [17, 18]), however, the nature of this component cannot be constrained by multi-waveband spectra alone [19]. POLARIMETRY AS A DIAGNOSTIC TOOL Polarimetry is a powerful diagnostic for jets because synchrotron emission is naturally polarized, with the inferred direction of the magnetic field vector indicating

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