arXiv subjects

Charles D. Dermer

Publications and source records attributed to Charles D. Dermer.

87 records · Page 5Linked to original sources

arXiv2000

External Shock Model for Gamma-Ray Bursts during the Prompt Phase

The hard X-ray and gamma-ray phenomenology of gamma-ray bursts (GRBs) can be explained by an external shock model where a single relativistic blast wave interacts with the surrounding medium. Besides reproducing the generic spectral behavior of GRB profiles, the external shock model provides quantitative fits to the peak flux distribution, the > 1 s t_{50} duration distribution, and the distribution of the peaks E_pk of the vF_v spectra of GRBs measured with BATSE. The apparent paradox between a relativistic beaming scenario and the empirical finding that E_pk values are preferentially measured within the triggering range of a GRB detector is resolved by this model when blast wave physics and detector triggering criteria are taken into account. Some surprising implications follow, namely that the fireball event rate is ~1 per 10^4 years per Milky Way galaxy for unbeamed sources, and proportionally more if fireball outflows are collimated. This is ~ 3 orders of magnitude larger than normally quoted. Most of the clean and dirty fireball transients are undetected due to telescope sensitivity and design limitations. Strongly variable GRB time histories with good radiative efficiencies are possible because of the strongly enhanced emissions when a blast wave interacts with density inhomogeneities located nearly along the line-of-sight to the observer. Arguments against short timescale variability in an external shock model are answered, and difficulties in an internal shock/colliding shell model are mentioned.

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arXiv1999

High Energy Radiation from Gamma Ray Bursts

Gamma-ray burst (GRB) engines are probed most intimately during the prompt gamma-ray luminous phase when the expanding blast wave is closest to the explosion center. Using GRBs 990123 and 940217 as guides, we briefly review observations of high-energy emission from GRBs and summarize some problems in GRB physics. γγtransparency arguments imply relativistic beaming. The parameters that go into the external shock model are stated, and we show numerical simulation results of gamma-ray light curves from relativistic blast waves with different amounts of baryon loading. A distinct component due to the synchrotron self-Compton process produces significant emission at GeV and TeV energies. Predictions for spectral and temporal evolution at these energies are presented for a blast wave expanding into uniform surroundings. Observations of the slow decay of GeV-TeV radiation provide evidence for ultra-high energy cosmic ray acceleration in GRBs.

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arXiv1999

Beaming, Baryon-Loading, and the Synchrotron Self-Compton Component in Gamma-Ray Burst Blast Waves Energized by External Shocks

We present detailed calculations of nonthermal synchrotron and synchrotron self-Compton (SSC) spectra radiated by blast waves that are energized by interactions with a uniform surrounding medium. Radio, optical, X-ray and gamma-ray light curves and spectral indices are calculated for a standard parameter set that yields hard GRB spectra during the prompt emission phase. Because no lateral spreading of the blast-wave is assumed, the calculated temporal breaks represent the sharpest breaks possible from collimated outflows in a uniform surrounding medium. Absence of SSC hardenings in observed GRB X-ray afterglows indicates magnetic field generation toward equipartition as the blast wave evolves. EGRET detections of 100 MeV-GeV photons observed promptly and 90 minutes after GRB 940217 are attributed to nonthermal synchrotron radiation and SSC emission from a decelerating blast wave, respectively. The SSC process will produce prompt TeV emission that could be observed from GRBs with redshifts $z \lesssim 0.1$, provided $γ$-$γ$ opacity in the source is small. Measurements of the time dependence of the 100 MeV-GeV spectral indices with the planned {\it GLAST} mission will chart the evolution of the SSC component and test the external shock scenario. Transient optical and X-ray emissions from misaligned GRBs are generally much weaker than on-axis emissions produced by dirty and clean fireballs that would themselves not trigger a GRB detector; thus detection of long wavelength transients not associated with GRBs will not unambiguously demonstrate GRB beaming.

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arXiv1999

The External Shock Model of Gamma-Ray Bursts: Three Predictions and a Paradox Resolved

In the external shock model, gamma-ray burst (GRB) emissions are produced by the energization and deceleration of a thin relativistic blast wave due to its interactions with the circumburst medium (CBM). We study the physical properties of an analytic function which describes temporally-evolving GRB spectra in the limit of a smooth CBM with density n(x)\propto x^(-η), where x is the radial coordinate. The hard-to-soft spectral evolution and the intensity-hardness correlation of GRB peaks are reproduced. We predict that (1) GRB peaks are aligned at high photon energies and lag at low energies according to a simple rule; that (2) temporal indices at the leading edge of a GRB peak display a well-defined shift with photon energy; and that (3) the change in the spectral index values between the leading and trailing edges of a GRB peak decreases at higher photon energies. The reason that GRBs are usually detected with vF_v peaks in the 50 keV - several MeV range for detectors which trigger on peak flux over a fixed time interval is shown to be a consequence of the inverse correlation of peak flux and duration of the radiation emitted by decelerating blast waves.

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arXiv1999

Variability in Blazars

The kinetic energy of bulk relativistic plasma ejected from the central engine of blazars is converted into nonthermal particle energy in the comoving frame through a process of sweeping up material from the surrounding medium. The resulting deceleration of the bulk plasma introduces a number of effects which must be included in blazar modeling. For example, the varying Doppler factor means that model fits must employ time integrations appropriate to the observing times of the detectors. We find that the ratio of the peak synchrotron fluxes reached at two different photon energies is largest when viewing along the jet axis, and becomes smaller at large angles to the jet axis. This effect is important in studies of the statistics of jet sources. Variability due either to bulk plasma deceleration or radiative cooling must be distinguished in order to apply recently proposed tests for beaming from correlated X-ray and TeV observations. The blast-wave physics developed to analyze these problems implies that most of the energy injected in the comoving frame is originally in the form of nonthermal hadrons. Because plasmoid deceleration can produce rapid variability due to a changing Doppler factor, arguments against hadronic blazar models related to the long radiative cooling time scale of hadrons are not compelling.

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arXiv1999

Short Timescale Variability in the External Shock Model of Gamma Ray Bursts

We have developed a computer model to calculate gamma ray burst (GRB) light curves and efficiencies from the interaction of a single, thin blast wave with clouds in the external medium. Large amplitude, spiky variability occurs when the clouds have radii r << R/Gamma, where R is the mean distance of a cloud from the GRB source and Gamma is the blast-wave Lorentz factor. Efficiencies >~ 10% require a large number of small clouds, each with sufficiently large column densities to extract most of the available blast-wave energy in the region of interaction. For such efficiencies, we find that erratic variability persists in the simulated GRB light curves. If GRB sources are surrounded by clouds with such properties, then short timescale variability of GRBs is possible in the external shock model.

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arXiv1998

Variability and Unification of Blazars and Gamma Ray Bursts

Most models for blazars and gamma-ray bursts involve relativistic plasma outflows powered by accretion processes onto black holes. The blast wave physics developed for cosmological models of GRBs is reviewed. Two points relevant for blazar modeling are made: (1) The injection of nonthermal relativistic particles in the comoving frame is simply treated though a process of energizing the plasma as it sweeps up material from the surrounding medium. (2) The primary energy source of blazar radiation derives from the bulk kinetic energy of the outflowing plasma. Thus deceleration of the plasma blast wave must be included in blazar flaring calculations, and this process will introduce temporal and spectral effects in addition to those produced by acceleration and radiative cooling.

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arXiv1998

On Spectral and Temporal Variability in Blazars and Gamma Ray Bursts

A simple model for variability in relativistic plasma outflows is studied, in which nonthermal electrons are continuously and uniformly injected in the comoving frame over a time interval dt. The evolution of the electron distribution is assumed to be dominated by synchrotron losses, and the energy- and time-dependence of the synchrotron and synchrotron self-Compton (SSC) fluxes are calculated for a power-law electron injection function with index s = 2. The mean time of a flare or pulse measured at photon energy E with respect to the onset of the injection event varies as E^{-1/2} and E^{-1/4} for synchrotron and SSC processes, respectively, until the time approaches the limiting intrinsic mean time (1+z)dt/(2 D), where z is the redshift and D is the Doppler factor. This dependence is in accord with recent analyses of blazar and GRB emissions, and suggests a method to discriminate between external Compton and SSC models of high-energy gamma radiation from blazars and GRBs. The qualititative behavior of the X-ray spectral index/flux relation observed from BL Lac objects can be explained with this model. This demonstrates that synchrotron losses are primarily responsible for the X-ray variability behavior and strengthens a new test for beaming from correlated hard X-ray/TeV observations.

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arXiv1997

Electron Acceleration and Synchrotron Radiation in Decelerating Plasmoids

An equation is derived to calculate the dynamics of relativistic magnetized plasma which decelerates by sweeping up matter from the ISM. Reduction to the non-radiative and radiative regimes is demonstrated. The evolving electron momentum distribution function in the comoving fluid frame is used to calculate the observed synchrotron radiation spectrum, assuming that a fixed fraction of the comoving proton power is instantaneously transformed into a power-law, "shock"-like electron distribution function. This permits an analytic solution for the case of a relativistic plasmoid with a constant internal magnetic field. Breaks in the temporal behavior of the primary burst emission and long wavelength afterglows occur on two time scales for external ISM density distributions. The first represents the time when the outflow sweeps up approximately M/G_0 of material from the ISM, where M is the mass in the outflow ejecta and G_0 is the initial Lorentz factor of the plasmoid. The second represents the time when synchrotron cooling begins strongly to regulate the number of nonthemal electrons that are producing radiation which is observed at a given energy. Results are applied to GRB and blazar variability. The slopes of the time profiles of the X-ray and optical light cuves of the afterglows of GRB 970228 and GRB 970508 are explained by injection of hard electron spectra with indices $s < 2$ in a deceleration regime near the radiative limit. We also propose that blazar flares are due to the transformation of the directed kinetic energy of the plasmoid when interacting with the external medium, as would occur if relativistic outflows pass through clouds orbiting the central supermassive black hole.

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arXiv1997

Decelerating Plasmoid Model for Gamma-Ray Burst Afterglows

The flaring and fading radio, optical, and X-ray afterglows from GRB 970508 are modeled by a highly relativistic plasma sphere which decelerates by sweeping up ambient gas. The afterglow emission is assumed to be synchrotron radiation emitted by nonthermal electrons in the magnetized plasmoid. The temporal behavior of the delayed emission is controlled by the evolution of the Doppler factor and by adiabatic expansion losses of the nonthermal electrons in the plasmoid. Model fits to the optical data of GRB 970508 are provided, and the relative delay of the radio peak to the optical peak is found to result from a decrease in the observed self-absorption frequency as the plasmoid expands and decelerates. A variety of afterglow behaviors occurs for different observing angles and plasmoid parameters. The degree of collimation inferred from our fit to GRB 970508 implies a space density of GRB sources which exceeds the estimates from scenarios involving coalescing compact objects. This model can be verified through observations of superluminal motion in the delayed radio emission.

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arXiv1996

On the Nature of the Bursting X-Ray Pulsar GRO J1744-28

The unusual properties of the bursting X-ray pulsar GRO J1744-28 are explained in terms of a low-mass X-ray binary system consisting of an evolved stellar companion transferring mass through Roche-lobe overflow onto a neutron star, implying that the inclination of the system is < 18 degrees. Interpretation of the QPO at frequency nu(QPO) = 40 Hz using the beat-frequency model of Alpar \& Shaham and the measured period derivative with the Ghosh \& Lamb accretion-torque model implies that the persistent X-ray luminosity of the source is approximately equal to the Eddington luminosity and that the neutron star has a surface equatorial magnetic field = 2e10 [40 Hz/nu(QPO)] G for standard neutron star parameters. This implies a distance to GRO J1744-28 of ~ 5 [nu(QPO)/40 Hz]^{1/6} b^{1/2} kpc, where b < 1 is a correction factor that depends on the orientation of the neutron star.

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arXiv1995

Method for Organizing the Multiwavelength Data of Radio-Loud Active Galactic Nuclei

The broadband emission observed from radio galaxies, BL Lac objects and quasars is thought to be produced by energetic electrons confined in plasma blobs which are ejected from supermassive black holes at relativistic speeds. The lower energy radio-through-optical component is almost certainly nonthermal electron synchrotron radiation, while the higher-energy $γ$-ray component is probably due to Compton scattering of target photons by these same electrons. If the high-energy component is formed by jet electrons Compton-scattering radiation from outside the jet, such as the direct or rescattered accretion-disk photons, then the ratio of the power in the high-energy Compton component to the power in the low-energy synchrotron component stands in a simple relation depending on the observing angle to the jet and the plasma outflow speed. When combined with contemporaneous VLBI measurements of apparent transverse speed, we find that a simple diagram relates different classes of radio-loud active galactic nuclei (AGNs) and makes definite predictions for multiwavelength observations of these sources. If an equipartition parameter remains constant between episodes of plasma ejection, then multiple observations of a single source can in principle determine the Hubble constant.

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arXiv1995

Stochastic Particle Acceleration near Accreting Black Holes

We consider the stochastic acceleration of particles which results from resonant interactions with plasma waves in black hole magnetospheres. We calculate acceleration rates and escape time scales for protons and electrons resonating with Alfvén waves, and for electrons resonating with whistlers. Assuming either a Kolmogorov or Kraichnan wave spectrum, accretion at the Eddington limit, magnetic field strengths near equipartition, and turbulence energy densities $\sim 10\%$ of the total magnetic field energy density, we find that Alfvén waves accelerate protons to Lorentz factors $\lte 10^4$--$10^6$ before they escape from the system. Acceleration of electrons by fast mode and whistler waves can produce a nonthermal population of relativistic electons whose maximum energy is determined by a competition with radiation losses. Particle energization and outflow is not possible at lower accretion rates, magnetic field strengths, or turbulence levels due to dominant Coulomb losses. Increases in the accretion luminosity relative to the Eddington luminosity can trigger particle acceleration out of the thermal background, and this mechanism could account for the differences between radio-quiet and radio-loud active galactic nuclei. Observations of outflowing radio-emitting components following transient X-ray events in galactic X-ray novae and gamma-ray flares in blazars are in accord with this scenario.

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arXiv1995

Cosmological Models of Gamma-Ray Bursts

We review models of cosmological gamma-ray bursts (GRBs). The statistical and $γ$-ray transparency issues are summarized. Neutron-star and black-hole merger scenarios are described and estimates of merger rates are summarized. We review the simple fireball models for GRBs and the recent work on non-simple fireballs. Alternative cosmological models, including models where GRBs are analogs of active galactic nuclei and where they are produced by high-field, short period pulsars, are also mentioned. The value of neutrino astronomy to solve the GRB puzzle is briefly reviewed.

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arXiv1993

On the Location of the Acceleration and Emission Sites in Gamma-Ray Blazars

Compton scattering of external radiation by nonthermal particles in outflowing blazar jets is dominated by accretion-disk photons rather than scattered radiation to distances $\sim 0.01-0.1$ pc from the central engine for standard parameters, thus clarifying the limits of validity of the model by the present authors and the model of Sikora, Begelman, \& Rees. On the basis of contemporaneous Ginga X-ray and EGRET gamma-ray observations, we estimate the radius of 3C 279's gamma-ray photosphere to be smaller than estimated by Blandford. There is thus no need to require that the acceleration and emission sites of gamma-ray blazars to be located farther than $\sim 10^{2-3}$ gravitational radii from the central engine. We argue that lineless BL Lac objects, rather than quasars, are more likely to be detected in the TeV energy range.

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