Coordinated RXTE and multiwavelength observations of blazars

Abstract

Results from recent multiwavelength observations of blazars are reviewed, with particular emphasis on those involving the Rossi X-ray Timing Explorer (RXTE). I discuss blazars’ spectral energy distributions, their correlated variability at various energies, and the insights they offer on the physical processes in the jet. New results on Mrk 501, PKS 2155–304, and PKS 2005–489 are highlighted. 1. Blazars and Their spectral energy distributions (SEDs) Blazars are radio-loud Active Galactic Nuclei characterized by polarized, highly luminous, and rapidly variable non-thermal continuum emission (Angel & Stockmann 1980) from a relativistic jet oriented close to the line of sight (Blandford & Rees 1978). As such, blazars provide fortuitous natural laboratories to study the jet processes and ultimately how energy is extracted from the central black hole. The radio through gamma-ray spectral energy distributions (SEDs) of blazars exhibit two broad humps (Figure 1). The first component peaks at IR/optical in “red” blazars and at UV/X-rays in their “blue” counterparts, and is most likely due to synchrotron emission from relativistic electrons in the jet (see Ulrich, Maraschi, & Urry 1997 and references therein). The second component extends from X-rays to gamma-rays (GeV and TeV energies), and its origin is less well understood. A popular scenario is inverse Compton (IC) scattering of ambient photons, either internal (synchrotron-self Compton, SSC; Tavecchio, Maraschi, & Ghisellini 1998) or external to the jet (external Compton, EC; see Böttcher 1999 and references therein). In the following discussion I will assume the synchrotron and IC scenarios, keeping in mind, however, that a possible alternative for the production of gamma-rays is provided by the hadronic models (proton-induced cascades; see Rachen 1999 and references therein). Red and blue blazars are just the extrema of a continuous distribution of SEDs. This is becoming increasingly apparent from recent multicolor surveys (Laurent-Muehleisen et al. 1998; Perlman et al. 1998), which find sources with intermediate spectral shapes, and trends with bolometric luminosity were discovered (Sambruna, Maraschi, & Urry 1996; Fossati et al. 1998). In the more luminous red blazars the synchrotron and IC peak frequencies are lower, the Compton dominance (ratio of the synchrotron to IC peak luminosities) is larger, and the luminosity of the optical emission lines/non-thermal blue bumps is larger than in their blue counterparts (Sambruna 1997). A possible interpretation is that the different types of blazars are due to the different predominant electrons’ cooling mechanisms (Ghisellini et al. 1998). In a simple homoFig. 1. Spectral energy distributions (SEDs) of 3C279 [(a), Left] and Mrk 501 [(b), Right]. Data are from Maraschi et al. (1994), Wehrle et al. (1998), and Pian et al. (1998). Blazars’ SEDs typically have two broad humps, the first peaking anywhere from IR/optical (in red blazars like 3C279) to hard X-rays (in blue blazars like Mrk 501) and due to synchrotron emission from a relativistic jet. The second component, extending to gamma-rays, is less well understood. A popular explanation is inverse Compton scattering of ambient seed photons off the jet’s

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@inproceedings{Sambruna1999CoordinatedRA, title={Coordinated RXTE and multiwavelength observations of blazars}, author={Rita M . Sambruna}, year={1999} }