arXiv subjects

M. Boettcher

Publications and source records attributed to M. Boettcher.

At least 109 records · Page 6Linked to original sources

arXiv2001

Monte-Carlo simulations of thermal/nonthermal radiation from a neutron-star magnetospheric accretion shell

We discuss the space-and-time-dependent Monte Carlo code we have developed to simulate the relativistic radiation output from compact astrophysical objects, coupled to a Fokker-Planck code to determine the self-consistent lepton populations. We have applied this code to model the emission from a magnetized neutron star accretion shell near the Alfven radius, reprocessing the radiation from the neutron sar surface. We explore the parameter space defined by the accretion rate, stellar surface field and the level of wave turbulence in the shell. Our results are relevant to the emission from atoll sources, soft-X-ray transient X-ray binaries containing weakly magnetized neutron stars, and to recently suggested models of accretion-powered emission from anomalous X-ray pulsars.

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arXiv2000

X-ray spectral features from GRBs: Predictions of progenitor models

We investigate the potentially observable prompt or delayed X-ray spectral features from the currently popular gamma-ray burst (GRB) models. During the evolution of many GRB progenitors, a disk around the central GRB source is produced. Shock heating as the GRB ejecta collide with the disk may produce observable X-ray features. We first summarize predictions deduced from previous calculations which invoke photoionization and relativistic blast waves. We then calculate the quasi-thermal X-ray line features produced assuming the ejecta are nonrelativistic (which is more likely for the disk interactions of many GRB models). In the framework of the Hypernova/Collapsar model, delayed (a few days - several months after the GRB) bursts of line-dominated, thermal X-ray emission may be expected. The He-merger scenario predicts similar X-ray emission line bursts <~ a few days after the GRB. These X-ray signatures should be observable with Chandra and XMM-Newton out to at least z ~ 1. Weak emission line features <~ a few days after the GRB may also result from the supranova GRB scenario. In all three cases, significant X-ray absorption features, in particular during the prompt GRB phase, are expected. No significant X-ray spectral features might result from compact-object binary mergers.

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arXiv2000

Line emission from gamma-ray burst environments

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.

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arXiv2000

A model for the alternating phase lags associated with QPOs in X-ray binaries

We present a theoretical model for the alternating phase lags associated with QPO fundamental and harmonic frequencies observed in some Galactic black-hole candidates. We assume that the accretion flow exhibits a transition from an outer cool, optically thick accretion disk to a hot, inner advection-dominated accretion flow (ADAF), and that the QPOs are related to small-scale oscillations of the accretion rate and the transition radius. We present an analytical estimate of the expected phase lags at the fundamental and first harmonic frequencies of the QPOs and perform detailed time-dependent Monte-Carlo simualtions of the radiation transport in the oscillating ADAF / cool disk system. We find that this model is well suited to reproduce alternating phase lags between the fundamental and the first harmonic. It also naturally explains the trend observed in GRS 1915+105 that, as the soft X-ray luminosity increases, the QPO frequency increases and the phase lag associated with the QPO fundamental frequency changes sign from positive to negative. The relation between the disk temperature and the QPO frequency observed in GRS 1915+105 is consistent with a secular instability modulating the disk evaporation at the transition radius.

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arXiv2000

Spectral Energy Distributions of Gamma Ray Bursts Energized by External Shocks

Sari, Piran, and Narayan have derived analytic formulas to model the spectra from gamma-ray burst blast waves that are energized by sweeping up material from the surrounding medium. We extend these expressions to apply to general radiative regimes and to include the effects of synchrotron self-absorption. Electron energy losses due to the synchrotron self-Compton process are also treated in a very approximate way. The calculated spectra are compared with detailed numerical simulation results. We find that the spectral and temporal breaks from the detailed numerical simulation are much smoother than the analytic formulas imply, and that the discrepancies between the analytic and numerical results are greatest near the breaks and endpoints of the synchrotron spectra. The expressions are most accurate (within a factor of ~ 3) in the optical/X-ray regime during the afterglow phase, and are more accurate when epsilon_e, the fraction of swept-up particle energy that is transferred to the electrons, is <~ 0.1. The analytic results provide at best order-of-magnitude accuracy in the self-absorbed radio/infrared regime, and give poor fits to the self-Compton spectra due to complications from Klein-Nishina effects and photon-photon opacity.

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arXiv2000

Multiwavelength observations of Mkn 501 during the 1997 high state

During the observation period 1997, the nearby Blazar Mkn 501 showed extremely strong emission and high variability. We examine multiwavelength aspects of this event using radio, optical, soft and hard X-ray and TeV data. We concentrate on the medium-timescale variability of the broadband spectra, averaged over weekly intervals. We confirm the previously found correlation between soft and hard X-ray emission and the emission at TeV energies, while the source shows only minor variability at radio and optical wavelengths. The non-linear correlation between hard X-ray and TeV fluxes is consistent with a simple analytic estimate based on an SSC model in which Klein-Nishina effects are important for the highest-energy electrons in the jet, and flux variations are caused by variations of the electron density and/or the spectral index of the electron injection spectrum. The time-averaged spectra are fitted with a Synchrotron Self-Compton (SSC) dominated leptonic jet model, using the full Klein-Nishina cross section and following the self-consistent evolution of relativistic particles along the jet, accounting for gamma-gamma absorption and pair production within the source as well as due to the intergalactic infrared background radiation. The contribution from external inverse-Compton scattering is tightly constrained by the low maximum EGRET flux and found to be negligible at TeV energies. We find that high levels of the X-ray and TeV fluxes can be explained by a hardening of the energy spectra of electrons injected at the base of the jet, in remarkable contrast to the trend found for gamma-ray flares of the flat-spectrum radio quasar PKS 0528+134.

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arXiv2000

A large high-energy gamma-ray flare from the blazar 3C 273

The Compton Gamma-Ray Observatory (CGRO) experiments EGRET and COMPTEL observed the Virgo sky region continuously for 7 weeks between December 10, 1996 and January 28, 1997. The prominent quasar 3C~273 was found to be the brightest source in gamma-rays and was significantly detected by EGRET and COMPTEL. The EGRET experiment observed a time-variable flux at energies above 100 MeV, which reached in a 2-week flaring period (December 30, 1996 to January 14, 1997) its highest flux level observed during the CGRO-era. COMPTEL, however, does not observe obvious time variability at energies below ~30 MeV contemporaneous to EGRET. In particular, no flare was observed, indicating that this outburst is solely a high-energy (>100 MeV) phenomenon. The energy spectrum between 3 MeV and 10 GeV is well represented by a simple power-law model. Below 3 MeV a spectral turnover is indicated. Performing spectral analysis for different time periods, we found evidence for a spectral hardening during the flaring period, which is consistent with the flare occurring mainly at the higher energies and with its absence at COMPTEL energies of a few MeV. This may be interpreted as an indication that the emission in the EGRET energy range is dominated by a different radiation mechanism than the MeV emission. We argue that the most likely mechanism for the high-energy flare is inverse-Compton scattering of reprocessed accretion-disk radiation.

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arXiv1999

Early GRB Afterglows from Relativistic Blast Waves in General Radiative Regimes

We present simple analytical expressions for the predicted spectral and temporal behavior of the early afterglow radiation from gamma-ray bursts in radiative regimes intermediate between the adiabatic and the fully radiative solutions of the blastwave hydrodynamic equations. Our expressions are valid as long as the relativistic electrons responsible for the observed synchrotron emission are in the fast cooling regime and the blast wave is relativistic. We show that even a slight deviation from a perfectly adiabatic evolution results in significant changes of the temporal characteristics of the afterglow emission.

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arXiv1999

Analyzing the Multiwavelength Spectrum and Variability of BL Lacertae During the July 1997 Outburst

The multiwavelength spectrum of BL Lacertae during its July 1997 outburst is analyzed in terms of different variations of the homogeneous leptonic jet model for the production of high-energy radiation from blazars. We find that a two-component gamma-ray spectrum, consisting of a synchrotron self-Compton and an external Compton component, is required in order to yield an acceptable fit to the broadband spectrum. Our analysis indicates that in BL Lac, unlike other BL Lac objects, the broad emission line region plays an important role for the high-energy emission. Several alternative blazar jet models are briefly discussed. In the appendix, we describe the formalism in which the process of Comptonization of reprocessed accretion disk photons is treated in the previously developed blazar jet simulation code which we use.

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arXiv1999

Leptonic Jet Models of Blazars: Broadband Spectra and Spectral Variability

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.

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arXiv1999

A New Approach to Statistics of Cosmological Gamma-Ray Bursts

We use a new method of analysis to determine parameters of cosmological gamma-ray bursts (GRBs), assuming that their distribution follows the star-formation history of the universe. Spectral evolution is calculated from an external shock model for fireball/blast wave evolution, and used to evaluate the measured peak flux, duration, and $νF_ν$ peak photon energy for a GRB source occuring at a given redshift and with given values of total energy, baryon-loading and environmental parameters. We then fit model distributions of GRB sources to the observed peak flux, duration and $νF_ν$ peak photon energy distributions. We find that the observed width of the $E_p$ and duration distributions can not be explained by cosmological redshift and time dilation effects. Rather, broad distributions of total blastwave energies and bulk Lorentz factors are necessary to explain the observed distributions simultaneously within the framework of our unifying GRB model. We discuss implications for source parameter distributions and determine a range of burst parameters consistent with the data.

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arXiv1999

Broadband Spectral Analysis of PKS 0528+134: A Report on Six Years of EGRET Observations

The multiwavelength spectra of PKS 0528+134 during six years of observations by EGRET have been analyzed using synchrotron self-Compton (SSC) and external radiation Compton (ERC) models. We find that a two-component model, in which the target photons are produced externally to the gamma-ray emitting region, but also including an SSC component, is required to suitably reproduce the spectral energy distributions of the source. Our analysis indicates that there is a trend in the observed properties of PKS 0528+134, as the source goes from a gamma-ray low state to a flaring state. We observe that during the higher gamma-ray states, the bulk Lorentz factor of the jet increases and the ERC component dominates the high-energy emission. Our model calculations indicate the trend that the energies of the electrons giving rise to the synchrotron peak decreases, and the power-ratio of the gamma-ray and low energy spectral components increases, as the source goes from a low to a high gamma-ray state.

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arXiv1999

GRB990123: The Case for Saturated Comptonization

The recent simultaneous detection of optical, X-ray and gamma-ray photons from GRB990123 during the burst provides the first broadband multi-wavelength characterization of the burst spectrum and evolution. Here we show that a direct correlation exists between the time-varying gamma-ray spectral shape and the prompt optical emission. This combined with the unique signatures of the time-resolved spectra of GRB990123 convincingly supports earlier predictions of the saturated Comptonization model. Contrary to other suggestions, we find that the entire continuum from optical to gamma-rays can be generated from a single source of leptons (electrons and pairs). The optical flux only appears to lag the gamma-ray flux due to the high initial Thomson depth of the plasma. Once the plasma has completely thinned out, the late time afterglow behavior of our model is the same as in standard models based on the Blandford-McKee (1976) solution.

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arXiv1999

The synchrotron peak shift during high-energy flares of blazars

A prediction for the energy shift of the synchrotron spectrum of flat-spectrum radio quasars (FSRQs) during high-energy flares is presented. If the $γ$-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 $γ$-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.

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arXiv1998

Prompt and Delayed High-Energy Emission from Cosmological Gamma-Ray Bursts

In the cosmological blast-wave model for gamma ray bursts (GRBs), high energy (> 10 GeV) gamma-rays are produced either through Compton scattering of soft photons by ultrarelativistic electrons, or as a consequence of the acceleration of protons to ultrahigh energies. We describe the spectral and temporal characteristics of high energy gamma-rays produced by both mechanisms, and discuss how these processes can be distinguished through observations with low-threshold Cherenkov telescopes or GLAST. We propose that Compton scattering of starlight photons by blast wave electrons can produce delayed flares of GeV -- TeV radiation.

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arXiv1998

A New Model for the Hard Time Lags in Black Hole X-Ray Binaries

The time-dependent Comptonized output of a cool soft X-ray source drifting inward through an inhomogeneous hot inner disk or corona is numerically simulated. We propose that this scenario can explain from first principles the observed trends in the hard time lags and power spectra of the rapid aperiodic variability of the X-ray emission of Galactic black-hole candidates.

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arXiv1998

Time-dependent photoelectric absorption, photoionization and fluorescence line emission in gamma-ray burst environments

If gamma-ray bursts are associated with dense star-forming regions in young galaxies, photoelectric absorption by the dense circumburster material (CBM) will occur. As the burst evolves, the surrounding material is photoionized, leading to fluorescence line emission and reduced photoelectric absorption opacity. We have analyzed this process in detail, accounting for the time-dependent photoelectric absorption, photoionization and fluorescence line emission from the CBM. We find that even if GRBs are hosted in dense star-forming regions, photoionization of the GRB environment leads to a constant, but very weak level of delayed fluorescence line emission on timescales of weeks to years after the burst. A temporally evolving iron~K edge absorption feature can serve as diagnostic tool to reveal the density structure of the CBM and may provide an opportunity for redshift measurements. We also investigated whether photoelectric absorption could be responsible for the spectral evolution of the low-energy slopes of some bright BATSE $γ$-ray bursts displaying extremely hard spectra below the peak energy, inconsistent with the optically-thin synchrotron shock model. We find that a very strong metal enrichment (~ 100 times solar-system abundances) in the gamma-ray burst environment and a rather peculiar spatial distribution of the CBM would be necessary in order to account for the observed hard spectra below a few 100 keV and their temporal evolution.

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arXiv1998

Fireballs Loading and the Blast Wave Model of Gamma Ray Bursts

A simple function for the spectral power $P(ε,t) \equiv νL(ν) $ is proposed to model, with 9 parameters, the spectral and temporal evolution of the observed nonthermal synchrotron power flux from GRBs in the blast wave model. Here $ε= hν/$m$_e$c$^2$ is the observed dimensionless photon energy and $t$ is the observing time. Assumptions and an issue of lack of self-consistency are spelled out. The spectra are found to be most sensitive to the baryon loading, expressed in terms of the initial bulk Lorentz factor $Γ_0$, and an equipartition term $q$ which is assumed to be constant in time and independent of $Γ_0$. Expressions are given for the peak spectral power $P_p(t) = P(ε_p,t)$ at the photon energy $ε= ε_p(t)$ of the spectral power peak. A general rule is that the total fireball particle kinetic energy $E_0 \sim Π_0 t_d$, where $t_d \propto Γ_0^{-8/3}$ is the deceleration time scale and $Π_0 \equiv P(ε_p,t_d) \propto Γ_0^{8/3}$ is the maximum measured bolometric power output in radiation, during which it is carried primarily by photons with energy ${\cal E}_0 = ε_p(t_d) \propto qΓ_0^4$.

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