arXiv subjects

M. Boettcher

Publications and source records attributed to M. Boettcher.

119 records · Page 7Linked to original sources

arXiv1998

Radiation Feedback in Hot Accretion-Disk Corona Models

We present a detailed study of the observable effects of photoionization and Comptonization of line and continuum radiation from a cold accretion disk with a thin, warm, photoionized transition layer in the framework of self-consistent accretion-disk corona models for Galactic black-hole candidates. We use an iterative method to treat the non-linear radiation feedback between the transition layer and the hot corona numerically using a Monte-Carlo Comptonization code in combination with the photoionization and line transfer code XSTAR. The subset of the parameter space allowed in self-consistent accretion-disk corona systems on energetic grounds is explored, checking for the emergence of emission lines and/or absorption edges, with special emphasis on the spectral range $E \lesssim 1$ keV where such features might become observable with the advent of the AXAF satellite. Comparing our model calculations to the broadband spectrum of GX~339-4, we find good agreement with the observed spectral features and discuss how the future detection of the predicted features at lower energies can be used to constrain parameters.

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arXiv1998

Transrelativistic pair plasmas in AGN jets

Models of relativistic jets filled with ultrarelativistic pair plasma are very successful in explaining the broadband radiation of $γ$-ray blazars. Assuming that the initial injection and cooling of ultrarelativistic pair plasma in an AGN jet has occurred, producing the observed high-energy $γ$-ray radiation, we investigate the further evolution of the pair plasma as it continues to move out from the central engine. The effects of thermalization and reacceleration, the emission of pair bremsstrahlung and annihilation radiation and the bulk Compton process, and the possible application to MeV blazars are discussed. A model calculation to the special case of PKS~0208-512 is presented.

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arXiv1998

High Energy Gamma Rays from Ultrahigh Energy Cosmic Rays in Gamma Ray Bursts

It has recently been proposed that ultrahigh energy ($\gtrsim 10^{19}$ eV) cosmic rays (UHECRs) are accelerated by the blast waves associated with GRBs. We calculate the observed synchrotron radiation spectrum from protons and energetic leptons formed in the cascades initiated by photopion production, taking into account $γγ$ attenuation at the source. Normalizing to the emission characteristics of GRB~970508, we predict $\sim 10$ MeV - 100 GeV fluxes at a level which may have been observed with EGRET from bright GRBs, and could be detected with the proposed GLAST experiment or with ground-based air \v Cerenkov telescopes having thresholds $\lesssim $ several hundred GeV. The temporal decay of the UHECR-induced high-energy $γ$-ray afterglows is significantly slower than that of the lower-energy burst and associated synchrotron self-Compton (SSC) radiation, which provides a direct way to test the hadronic origin of a high-energy GRB afterglow. Besides testing the UHECR origin hypothesis, the short wavelength emission and afterglows can be used to probe the level of the diffuse intergalactic infrared radiation field or constrain redshifts of GRB sources.

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arXiv1998

On Compton Scattering Scenarios for Blazar Flares

The synchrotron reflection scenario recently proposed to explain $γ$-ray flares observed from blazar jets is studied. Our analysis takes into account the angular distribution of the beamed radiation, the finite extent of the scattering region, and light travel-time effects. We compare energy densities and powers for synchrotron, SSC, reflected synchrotron (RSy), and external Compton (EC) scattering processes. If the width of the scattering layer is much larger than $ΓR^\prime_B$, where $Γ$ and $ R^\prime_B$ denote the bulk Lorentz factor and comoving-frame radius of the plasma blob, respectively, then the ratio of the RSy and synchrotron energy densities $\sim 4 Γ^3 n_{BLR} σ_{T} R^\prime_B$, where $n_{BLR}$ is the mean particle density in the broad line region (BLR). Our results imply that Thomson-thick scattering regions of narrow extent must be present for the synchrotron reflection mechanism to operate effectively. This process seems unlikely to cause flares in lineless BL Lac sources, where X-ray and TeV flares are common and the BLR is thought to be weak or absent. We sketch time profiles of flares for various scenarios, including a model where the blob is energized by sweeping up surrounding material.

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arXiv1998

Comptonization signatures in the rapid aperiodic variability of Galactic black-hole candidates

We investigate the effect of inverse-Compton scattering of flares of soft radiation in different geometries of a hot, Comptonizing region and a colder accretion disk around a solar-mass black hole. The photon-energy dependent light curves, their Fourier transforms, power spectra and Fourier-period dependent time lags of hard photons with respect to softer photons are discussed. On the basis of a comparison with existing data we find arguments against Comptonization of external soft radiation as well as Comptonization in a homogeneous medium as dominant mechanisms for the rapid aperiodic variability in Galactic black-hole candidates. Possible further observational tests for the influence of Comptonization on the rapid aperiodic variability of Galactic black-hole candidates are suggested.

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arXiv1997

Spectral Variability in PKS 0528+134 at Gamma-Ray Energies

We present model calculations to the observed spectral variability in the gamma-ray spectrum of the ultraluminous blazar-type quasar PKS 0528+134. We argue that the observed appearance of a spectral break between the COMPTEL and the EGRET energy range can plausibly be explained by a variation of the Doppler beaming factor in the framework of a relativistic jet model for AGNs, combining the ERC and the SSC model. This may be an instructive example for the different dependence of these two spectral components on the Doppler factor, as predicted in earlier work.

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arXiv1997

The radio spectra of galactic nuclei

We present a model for the inverted radio spectra of active active galactic nuclei as well as the central regions of normal galaxies. The model is based on the unified scenario for active galaxies, stating that the central engines of active galaxies consists of a supermassive black hole surrounded by an accretion disk and a radio jet. The nuclei of normal (i. e. less active) galaxies are supposed to be scaled-down versions of the same phenomenon. We show that the radio emission of a jet component, becoming optically thin to the radio emission of a monoenergetic pair plasma at decreasing frequencies as it moves outward and expands, is well suited to explain the observational results. We present a model calculation for the special case of the nucleus of M~81.

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arXiv1997

Spectral modeling of gamma-ray blazars

We present model calculations reproducing broadband spectra of $γ$-ray blazars by a relativistic leptonic jet, combining the EIC and the SSC model. To this end, the evolution of the particle distribution functions inside a relativistic pair jet and of the resulting photon spectra is investigated. Inverse-Compton scattering of both external (EIC) as well as synchrotron photons (SSC) is treated using the full Klein-Nishina cross-section and the full angle-dependence of the external photon source. We present model fits to the broadband spectra of Mrk~421 and 3C279 and the X-ray and $γ$-ray spectrum of PKS~1622-297. We find that the most plausible way to explain both the quiescent and the flaring states of these objects consists of a model where EIC and SSC dominate the observed spectrum in different frequency bands. For both Mrk~421 and 3C279 the flaring states can be reproduced by a harder spectrum of the injected pairs.

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arXiv1997

The Pair Production Spectrum from Photon-Photon Annihilation

We present the first completely analytical computation of the full differential γ-γpair production rate from compact radiation fields, exact to 2nd order QED, and use this result to investigate the validity of previously known approximations.

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arXiv1997

Gamma-ray emission and spectral evolution of pair plasmas in AGN jets

The evolution of the particle distribution functions inside a relativistic jet containing an e^-e^+ pair plasma and of the resulting $γ$-ray and X-ray spectra is investigated. The first phase of this evolution is governed by heavy radiative energy losses. For this phase, approximative expressions for the energy-loss rates due to inverse-Compton scattering, using the full Klein-Nishina cross section, are given as one-dimensional integrals for both cooling by inverse-Compton scattering of synchrotron photons (SSC) and of accretion disk photons (EIC). We calculate instantaneous and time-integrated $γ$-ray spectra emitted by such a jet for various sets of parameters, deducing some general implications on the observable broadband radiation. Finally, we present model fits to the broadband spectrum of Mrk 421. We find that the most plausible way to explain both the quiescent and the flaring state of Mrk 421 consists of a model where EIC and SSC dominate the observed spectrum in different frequency bands. In our model the flaring state is primarily related to an increase of the maximum Lorentz factor of the injected pairs.

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arXiv1995

Pair annihilation radiation from relativistic jets in gamma-ray blazars

The contribution of the pair annihilation process in relativistic electron-positron jets to the gamma-ray emission of blazars is calculated. Under the same assumptions as for the calculation of the yield of inverse Compton scattered accretion disk radiation (Dermer and Schlickeiser 1993) we calculate the emerging pair annihilation radiation taking into account all spectral broadening effects due to the energy spectra of the annihilating particles and the bulk motion of the jet. It is shown that the time-integrated pair annihilation spectrum appears almost like the well-known gamma-ray spectrum from decaying $π^o$-mesons at rest, yielding a broad bumpy feature located between 50 and 100 MeV. We also demonstrate that for pair densities $> 10^9$ cm$^{-3}$ in the jet the annihilation radiation will dominate the inverse Compton radiation, and indeed may explain reported spectral bumps at MeV energies. The refined treatment of the inverse Compton radiation leads to spectral breaks of the inverse Compton emission in the MeV energy range with a change in spectral index $Δα$ larger than 0.5 as detected in PKS 0528+134 and 3C273.

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