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Search Results: 1 - 10 of 401524 matches for " M. Boettcher "
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The synchrotron peak shift during high-energy flares of blazars
M. Boettcher
Physics , 1999, DOI: 10.1086/311967
Abstract: A prediction for the energy shift of the synchrotron spectrum of flat-spectrum radio quasars (FSRQs) during high-energy flares is presented. If the $\gamma$-ray emission of FSRQs is produced by Comptonization of external radiation, then the peak of the synchrotron spectrum is predicted to move to lower energies in the flare state. This is opposite to the well-known broadband spectral behavior of high-frequency peaked BL-Lac objects where the external radiation field is believed to be weak and synchrotron-self Compton scattering might be the dominant $\gamma$-ray radiation mechanism. The synchrotron peak shift, if observed in FSRQs, can thus be used as a diagnostic to determine the dominant radiation mechanism in these objects. I suggest a few FSRQs as promising candidates to test the prediction of the external-Comptonization model.
X-Ray Lines and Absorption Edges in GRBs and Their Afterglows
M. Boettcher
Physics , 2002,
Abstract: Absorption and Reprocessing of Gamma-ray burst radiation in the environment of cosmological GRBs can be used as a powerful probe of the elusive nature of their progenitors. In particular, transient X-ray emission line and absorption features in the prompt and early afterglows of GRBs are sensitive to details of the location and density structure of the reprocessing and/or absorbing material. To date, there have been only rather few detections of such features, and the significance is marginal in most individual cases. However, transient X-ray emission lines in GRB afterglows have now been found by four different X-ray satellites, which may justify a more detailed theoretical investigation of their origin. In this paper, I will first present a brief review of the status of observations of transient X-ray emission line and absorption features. I will then discuss general physics constraints which those results impose on isotropy, homogeneity, and location of the reprocessing material with respect to the GRB source, and review the various currently discussed, specific models of GRBs and their environments in which the required conditions could arise.
Leptonic Jet Models of Blazars: Broadband Spectra and Spectral Variability
M. Boettcher
Physics , 1999, DOI: 10.1063/1.1291341
Abstract: The current status of leptonic jet models for gamma-ray blazars is reviewed. Differences between the quasar and BL-Lac subclasses of blazars may be understood in terms of the dominance of different radiation mechanisms in the gamma-ray regime. Spectral variability patterns of different blazar subclasses appear to be significantly different and require different intrinsic mechanisms causing gamma-ray flares. As examples, recent results of long-term multiwavelength monitoring of PKS 0528+134 and Mrk 501 are interpreted in the framework of leptonic jet models. A simple quasi-analytic toy model for broadband spectral variability of blazars is presented.
Physics Input from Multiwavelength Observations of AGN
M. Boettcher
Physics , 2001,
Abstract: The current status of leptonic jet models for blazars is reviewed. Differences between the quasar and BL-Lac subclasses of blazars may be understood in terms of the dominance of different radiation mechanisms in the gamma-ray regime. Spectral variability patterns of different blazar subclasses appear to be significantly different and require different intrinsic mechanisms causing gamma-ray flares. As examples, recent results of long-term multiwavelength monitoring of PKS 0528+134, 3C 279, and Mrk 501 are interpreted in the framework of leptonic jet models. Short-term variability patterns give important additional clues about the source geometry and the relevant radiation mechanisms in blazars. Challenges for future observational efforts are discussed.
Coordinated Multiwavelength Observations of BL Lacertae in 2000
M. Boettcher
Physics , 2003, DOI: 10.1086/378156
Abstract: BL Lacertae was the target of an extensive multiwavelength monitoring campaign in the second half of 2000. Simultaneous or quasi-simultaneous observations were taken at radio (UMRAO and Metsaehovi) and optical(WEBT collaboration) frequencies, in X-rays (BeppoSAX and RXTE), and at VHE gamma-rays (HEGRA). The WEBT optical campaign achieved an unprecedented time coverage, virtually continuous over several 10 - 20 hour segments. It revealed intraday variability on time scales of ~ 1.5 hours and evidence for spectral hardening associated with increasing optical flux. During the campaign, BL Lacertae underwent a major transition from a rather quiescent state prior to September 2000, to a flaring state for the rest of the year. This was also evident in the X-ray activity of the source. BeppoSAX observations on July 26/27 revealed a rather low X-ray flux and a hard spectrum, while a BeppoSAX pointing on Oct. 31 - Nov. 2, 2000, indicated significant variability on time scales of < a few hours, and provided evidence for the synchrotron spectrum extending out to ~ 10 keV during that time. During the July 26/27 observation, there is a tantalizing, though not statistically significant, indication of a time delay of ~ 4 - 5 hr between the BeppoSAX and the R-band light curve. Also, a low-significance detection of a time delay of 15 d between the 14.5 GHz and the 22 GHz radio light curves is reported. Several independent methods to estimate the co-moving magnetic field in the source are presented, suggesting a value of ~ 2 e_B^{2/7} G, where e_B is the magnetic-field equipartition factor w.r.t. the electron energy density in the jet.
Line emission from gamma-ray burst environments
M. Boettcher
Physics , 1999, DOI: 10.1086/309209
Abstract: The time and angle dependent line and continuum emission from a dense torus around a cosmological gamma-ray burst source is simulated, taking into account photoionization, collisional ionization, recombination, and electron heating and cooling due to various processes. The importance of the hydrodynamical interaction between the torus and the expanding blast wave is stressed. Due to the rapid deceleration of the blast wave as it interacts with the dense torus, the material in the torus will be illuminated by a drastically different photon spectrum than observable through a low-column-density line of sight, and will be heated by the hydrodynamical interaction between the blast wave and the torus. A model calculation to reproduce the Fe K-alpha line emission observed in the X-ray afterglow of GRB 970508 is presented. The results indicate that ~ 10^{-4} solar masses of iron must be concentrated in a region of less than 10^{-3} pc. The illumination of the torus material due to the hydrodynamic interaction of the blast wave with the torus is the dominant heating and ionization mechanism leading to the formation of the iron line. These results suggest that misaligned GRBs may be detectable as X-ray flashes with pronounced iron emission line features.
Coronal cooling and its signatures in the rapid aperiodic variability of Galactic black-hole candidates
M. Boettcher
Physics , 2000, DOI: 10.1086/320982
Abstract: The most popular models for the complex phase and time lags in the rapid aperiodic variability of Galactic X-ray binaries are based Comptonization of soft seed photons in a hot corona, where small-scale flares are induced by flares of the soft seed photon input (presumably from a cold accretion disc). However, in their original version, these models have neglected the additional cooling of the coronal plasma due to the increased soft seed photon input, and assumed a static coronal temperature structure. In this paper, our Monte-Carlo/Fokker-Planck code for time-dependent radiation transfer and electron energetics is used to simulate the self-consistent coronal response to the various flaring scenarios that have been suggested to explain phase and time lags observed in some Galactic X-ray binaries. It is found that the predictions of models involving slab-coronal geometries are drastically different from those deduced under the assumption of a static corona. However, with the inclusion of coronal cooling they may even be more successful than in their original version in explaining some of the observed phase and time lag features. The predictions of the model of inward-drifting density perturbations in an ADAF-like, two-temperature flow also differ from the static-corona case previously investigated, but may be consistent with the alternating phase lags seen in GRS 1915+105 and XTE J1550-564. Models based on flares of a cool disc around a hot, inner two-temperature flow may be ruled out for most objects where significant Fourier-frequency-dependent phase and time lags have been observed.
Statistical Models on Spherical Geometries
S. Boettcher,M. Moshe
Physics , 1995, DOI: 10.1103/PhysRevLett.74.2410
Abstract: We use a one-dimensional random walk on $D$-dimensional hyper-spheres to determine the critical behavior of statistical systems in hyper-spherical geometries. First, we demonstrate the properties of such a walk by studying the phase diagram of a percolation problem. We find a line of second and first order phase transitions separated by a tricritical point. Then, we analyze the adsorption-desorption transition for a polymer growing near the attractive boundary of a cylindrical cell membrane. We find that the fraction of adsorbed monomers on the boundary vanishes exponentially when the adsorption energy decreases towards its critical value.
Pair annihilation radiation from relativistic jets in gamma-ray blazars
M. Boettcher,R. Schlickeiser
Physics , 1995,
Abstract: 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 $\pi^o$-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 $\Delta \alpha $ larger than 0.5 as detected in PKS 0528+134 and 3C273.
The Pair Production Spectrum from Photon-Photon Annihilation
M. Boettcher,R. Schlickeiser
Physics , 1997,
Abstract: We present the first completely analytical computation of the full differential \gamma-\gamma pair 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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