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Register a new userModels of Comptonization
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Added by
Pierre-Olivier Petrucci
Publication date
2008
fields
Physics
and research's language is
English
Authors
Pierre-Olivier Petrucci
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After a rapid introduction about the models of comptonization, we present some simulations that underlines the expected capabilities of Simbol-X to constrain the presence of this process in objects like AGNs or XRB.
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We used high quality BeppoSAX data of 6 Seyfert galaxies to test realistic thermal Comptonization models. Our main effort was to adopt a Comptonization model taking into account the anisotropy of the soft photon field. The best fit parameter values of the temperature and optical depth of the corona and of the reflection normalization obtained fitting this class of models to the data are substantially different from those derived fitting the same data with the power law + cut--off model commonly used. The two models also provide different trends and correlation between the physical parameters, which has major consequences for the physical interpretation of the data
We present an empirical model of Comptonization for fitting the spectra of X-ray binaries. This model, simpl, has been developed as a package implemented in XSPEC. With only two free parameters, simpl is competitive as the simplest empirical model of Compton scattering. Unlike other empirical models, such as the standard power-law model, simpl incorporates the basic physics of Compton scattering of soft photons by energetic coronal electrons. Using a simulated spectrum, we demonstrate that simpl closely matches the behavior of physical Comptonization models which consider the effects of optical depth, coronal electron temperature, and geometry. We present fits to RXTE spectra of the black-hole transient H1743-322 and a BeppoSAX spectrum of LMC X-3 using both simpl and the standard power-law model. A comparison of the results shows that simpl gives equally good fits and a comparable spectral index, while eliminating the troublesome divergence of the standard power-law model at low energies. Importantly, simpl is completely flexible and can be used self-consistently with any seed spectrum of photons. We show that simpl - unlike the standard power law - teamed up with diskbb (the standard model of disk accretion) gives results for the inner-disk radius that are unaffected by strong Comptonization, a result of great importance for the determination of black hole spin via the continuum-fitting method.
The X-ray spectra of many active galactic nuclei (AGN) show a soft X-ray excess below 1-2 keV on top of the extrapolated high- energy power law. The origin of this component is uncertain. It could be a signature of relativistically blurred, ionized reflection, or the high-energy tail of thermal Comptonization in a warm (kT $sim$ 1 keV), optically thick ($tausimeq$ 10-20) corona producing the optical/UV to soft X-ray emission. The purpose of the present paper is to test the warm corona model on a statistically significant sample of unabsorbed, radio-quiet AGN with XMM-newton archival data, providing simultaneous optical/UV and X-ray coverage. The sample has 22 objects and 100 observations. We use two thermal comptonization components to fit the broad-band spectra, one for the warm corona emission and one for the high-energy continuum. In the optical-UV, we also include the reddening, the small blue bump and the Galactic extinction. In the X-rays, we include a WA and a neutral reflection. The model gives a good fit (reduced $chi^2 <1.5$) to more than 90% of the sample. We find the temperature of the warm corona to be uniformly distributed in the 0.1-1 keV range, while the optical depth is in the range $sim$10-40. These values are consistent with a warm corona covering a large fraction of a quasi-passive accretion disc, i.e. that mostly reprocesses the warm corona emission. The disk intrinsic emission represents no more than 20% of the disk total emission. According to this interpretation, most of the accretion power would be released in the upper layers of the accretion flow.
Quasi-thermal Comptonization is an attractive alternative to the synchrotron process to explain the spectra of GRBs, even if we maintain other important properties of the internal shock scenario, implying a compact emitting region and an equipartition magnetic field. Photon-photon absorption and electron-positron pairs can play a crucial role: this process may lock the effective temperature in a narrow range and may be the reason why burst spectra have high energy cut-offs close to the rest mass-energy of the electron. If the progenitors of GRB are hypernovae, the circum-burst matter is dominated by the wind of the pre-hypernova star. The presence of this dense material has strong effects on the generation of the radiation of the burst and its afterglow.
We study the time dependent spectra produced via the bulk Compton process by a cold, relativistic shell of plasma moving (and accelerating) along the jet of a blazar, scattering on external photons emitted by the accretion disc and reprocessed in the broad line region. Bulk Comptonization of disc photons is shown to yield a spectral component contributing in the far UV band, and would then be currently unobservable. On the contrary, the bulk Comptonization of broad line photons may yield a significant feature in the soft X-ray band. Such a feature is time-dependent and transient, and dominates over the non thermal continuum only when: a) the dissipation occurs close to, but within, the broad line region; b) other competing processes, like the synchrotron self-Compton emission, yield a negligible flux in the X-ray band. The presence of a bulk Compton component may account for the X-ray properties of high redshift blazars that show a flattening (and possibly a hump) in the soft X-rays, previously interpreted as due to intrinsic absorption. We discuss why the conditions leading to a detectable bulk Compton feature might be met only occasionally in high redshift blazars, concluding that the absence of such a feature in the spectra of most blazars should not be taken as evidence against matter--dominated relativistic jets. The detection of such a component carries key information on the bulk Lorentz factor and kinetic energy associated to (cold) leptons.
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