arXiv subjects

Charles D. Dermer

Publications and source records attributed to Charles D. Dermer.

At least 73 records · Page 4Linked to original sources

arXiv2005

Cosmic Rays from Gamma Ray Bursts in the Galaxy

The rate of terrestrial irradiation events by galactic gamma-ray bursts (GRBs) is estimated using recent standard-energy results. We assume that GRBs accelerate high-energy cosmic rays, and present results of three-dimensional simulations of cosmic rays moving in the Galactic magnetic field and diffusing through pitch-angle scattering. An on-axis GRB extinction event begins with a powerful prompt gamma-ray and neutron pulse, followed by a longer-lived phase from cosmic-ray protons and neutron-decay protons that diffuse towards Earth. Our results force a reinterpretation of reported ~ 10^{18} eV cosmic-ray anisotropies and offer a rigorous test of the model where high-energy cosmic rays originate from GRBs, which will soon be tested with the Auger Observatory.

astro-ph↗

arXiv2004

Nonthermal Radiation Processes in X-ray Jets

Analytic approximations for synchrotron, synchrotron self-Compton (SSC), and external Compton (EC) processes are used to constrain model parameters for knot and hot-spot emission in extended jets of radio galaxies. Equipartition formulas are derived that relate the Doppler factor $δ$ and comoving magnetic field $B$ assuming a nonthermal synchrotron origin of the radio emission, and synchrotron, SSC and EC origins of the X-ray emission. Expressions are also derived for $δ$ and $B$ that minimize the total jet powers of the emitting region in synchrotron, SSC and EC models for the X-ray emission. The results are applied to knot WK7.8 of PKS 0637-752. Predictions to test two-component synchrotron and EC models are made for {\it Chandra} and {\it GLAST}.

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arXiv2004

Curvature Effects in Gamma Ray Burst Colliding Shells

An elementary kinematic model for emission produced by relativistic spherical colliding shells is studied. The case of a uniform blast-wave shell with jet opening angle $θ_j \gg 1/Γ$ is considered, where $Γ$ is the Lorentz factor of the emitting shell. The shell, with comoving width $Δr^\prime$, is assumed to be illuminated for a comoving time $Δt^\prime$ and to radiate a broken power-law $νL_ν$ spectrum peaking at comoving photon energy $\e_{pk,0}^{\prime}$. Synthetic GRB pulses are calculated, and the relation between energy flux and internal comoving energy density is quantified. Curvature effects dictate that the measured $νF_ν$ flux at the measured peak photon energy $\e_{pk}$ is proportional to $\e^3_{pk}$ in the declining phase of a GRB pulse. Possible reasons for discrepancy with observations are discussed, including adiabatic and radiative cooling processes that extend the decay timescale, a nonuniform jet, or the formation of pulses by external shock processes. A prediction of a correlation between prompt emission properties and times of the optical afterglow beaming breaks is made for a cooling model, which can be tested with Swift.

astro-ph↗

arXiv2004

The Collapsar and Supranova Models

This rapporteur review summarizes results presented in Parallel Session GBT2 (Gamma Ray Burst Theory 2) on the Collapsar and Supranova Models held 25 July 2003 at the 10th Marcel Grossmann Meeting on General Relativity, Rio de Janeiro, Brazil. A central issue in GRB studies is the process whereby energy is released from the GRB engine. One scenario is the collapsar model, where the evolved stellar core promptly collapses to a black hole surrounded by a massive, intermittently-accreting torus of nuclear density material. A second scenario is the supranova model, where the first step of a two-step collapse process leaves behind a rapidly rotating neutron star stabilized by rotation, which later collapses to a black hole while making the GRB. In the supranova model, a powerful pulsar wind lasting days to weeks after the supernova makes distinctive signatures from the heating of the supernova remnant (SNR) shell, which should be discovered with Swift. This model also predicts nonstandard reddened excesses from the cooling SNR due to the range of delays between the two collapse events, contrary to observations of SN emissions from low redshift GRBs which favor the collapsar model. The observational basis for both models is critically reviewed. An outline of a GRB model is proposed where the second collapse takes place within minutes to hours after the primary SN event, during which time loss of centrifugal support along the rotation axis of the neutron star provides a relatively baryon-clean polar environment along which a newly formed, rapidly spinning black hole drives collimated baryon-dilute outflows.

astro-ph↗

arXiv2004

Frontiers in Cosmic Rays

This rapporteur review covers selected results presented in the Parallel Session HEA2 (High Energy Astrophysics 2) of the 10th Marcel Grossmann Meeting on General Relativity, held in Rio de Janeiro, Brazil, July 2003. The subtopics are: ultra high energy cosmic ray anisotropies, the possible connection of these energetic particles with powerful gamma ray bursts, and new exciting scenarios with a strong neutrino-nucleon interaction in the atmosphere.

hep-ph↗

arXiv2004

High-Energy Cosmic Rays from Gamma-Ray Bursts

A model is proposed for the origin of cosmic rays (CRs) from ~10^14 eV to the highest energies, >10^20 eV. Gamma-Ray Bursts (GRBs) are assumed to inject CR protons and ions into the interstellar medium of star-forming galaxies--including the Milky Way--with a power law spectrum extending to a maximum energy ~10^20 eV. The CR spectrum near the knee is fit with CRs trapped in the Galactic halo that were accelerated and injected by an earlier Galactic GRB. These CRs diffuse in the disk and halo of the Galaxy due to gyroresonant pitch-angle scattering with MHD turbulence in the Galaxy's magnetic field. The preliminary (2001) KASCADE data through the knee of the CR spectrum are fit by a model with energy-dependent propagation of CR ions from a single Galactic GRB. Ultra-high energy CRs (UHECRs), with energies above the ankle are assumed to propagate rectilinearly with their spectrum modified by photo-pion, photo-pair, and expansion losses. We fit the measured UHECR spectrum assuming comoving luminosity densities of GRB sources consitent with possible star formation rate histories of the universe. For power-law CR proton injection p>2 this model implies that the nonthermal content in the GRB blast waves is hadronically dominated by a factor ~60-200. Calculations show that 100 TeV-100 PeV neutrinos could be detected several times per year from all GRBs in kilometer-scale neutrino detectors such as IceCube, for GRB blast-wave Doppler factors <~200. GLAST measurements of gamma-ray components and cutoffs will constrain the product of nonthermal baryon loading and radiative efficiency, limit the Doppler factor, and test this senario.

astro-ph↗

arXiv2003

High Energy Neutrinos from Gamma Ray Bursts

We treat high-energy neutrino production in GRBs. Detailed calculations of photomeson neutrino production are presented for the collapsar model, where internal nonthermal synchrotron radiation is the primary target photon field, and the supranova model, where external pulsar-wind synchrotron radiation provides important additional target photons. Detection of > 10 TeV neutrinos from GRBs with Doppler factors > 200, inferred from gamma-ray observations, would support the supranova model. Detection of < 10 TeV neutrinos is possible for neutrinos formed from nuclear production. Only the most powerful bursts at fluence levels > 3x10^{-4} erg/cm^2 offer a realistic prospect for detection of muon neutrinos.

astro-ph↗

arXiv2003

External Shock Model for the Prompt Phase of Gamma Ray Bursts: Implications for GRB Source Models

An external shock model for the prompt gamma-ray luminous phase of gamma-ray bursts (GRBs) is treated both analytically and numerically. A widely cited derivation claiming that an external shock model for rapidly variable GRBs must be very inefficient employs an incorrect expression for the angular timescale. Numerical results show that variable GRBs can be formed with >10% efficiency to transform the directed kinetic energy of the relativistic fireball to gamma-rays. Successes of the external shock model and difficulties with an internal wind/colliding shell model are summarized. An impulsive external shock model is consistent with the supranova model.

astro-ph↗

arXiv2003

Neutral Beams from Blazar Jets

We treat the production of neutrons, photons, and neutrinos through photomeson interactions of relativistic protons with ambient photons in the compact inner jets of blazars. We show that the presence of the external UV radiation field makes possible strong energy losses already for protons with energies > 1 PeV, while without this component effective energy losses of protons begin only at E > 10^{18} eV. We develop a model describing the production and escape of neutrons from a comoving spherical blob, which continue to interact with the ambient external radiation field on the parsec-scale broad line region (BLR). Neutrons may carry ~10% of the overall energy of the protons accelerated beyond E ~ 1 PeV outside the BLR. Ultra-high energy (UHE) gamma rays produced in photomeson interactions of neutrons outside the blob can also escape the BLR. The escaping neutrons, gamma rays and neutrinos form a collimated neutral beam with a characteristic opening angle ~ 1/Gamma, where Gamma is the bulk Lorentz factor of the inner jet. The energy and the momentum of such beam is then mostly deposited in the extended jet due to neutron decay at distances ~ (E_n/10^{17} eV}) kpc, and through pair-production attenuation of gamma rays with energies E_g ~ 10^{15}-10^{18} eV which can propagate to distances beyond (10-100) kpc. In this scenario, neutral beams of UHE gamma rays and neutrons can be the reason for straight extended jets such as in Pictor A. Fluxes of neutrinos detectable with km-scale neutrino telescopes are predicted from flat spectrum radio quasars such as 3C 279.

astro-ph↗

arXiv2002

Nonthermal Particles and Radiation Produced by Cluster Merger Shocks

We have developed a numerical model for the temporal evolution of particle and photon spectra resulting from nonthermal processes at the shock fronts formed in merging clusters of galaxies. Fermi acceleration is approximated by injecting power-law distributions of particles during a merger event, subject to constraints on maximum particle energies. We consider synchrotron, bremsstrahlung, Compton, and Coulomb processes for the electrons, nuclear, photomeson, and Coulomb processes for the protons, and knock-on electron production. Broadband radio through gamma-ray light curves radiated by nonthermal protons and primary and secondary electrons are calculated both during and after the merger event. Using ROSAT observations to establish typical parameters for the matter density profile of clusters of galaxies, we find that merger shocks are weak and accelerate particles with relatively soft spectra. Our results suggest that only a minor contribution to the diffuse extragalactic gamma-ray background can originate from cluster merger shocks.

astro-ph↗

arXiv2002

Supranova Model for the Delayed Reddened Optical Excesses Detected in Several GRBs

Excess reddened optical emission has been measured in the power-law decay afterglow light curves of several GRBs some tens of days after the GRB events. This emission is claimed to be a signature of a supernova (SN) taking place at the same time as the GRB event, in support of the collapsar scenario. Observations of features in prompt and afterglow X-ray spectra of GRBs require high density, high metallicity material near GRB sources, supporting the supranova (SA) scenario. These conflicting lines of evidence are resolved in likely regimes of parameter space if the supernova remnant (SNR) shell is illuminated and heated by a pulsar wind (PW), and cools following the GRB event. The heating of the SNR shell by the PW is parameterized, and simplified expressions for the synchrotron, synchrotron self-Compton, and thermal radiation fields within the SNR shell and pulsar wind bubble are derived for monoenergetic injection of wind electrons. A cooling SNR shell could produce the excess emission detected from GRB 970228, GRB 980326, and GRB 011121. X-ray synchrotron radiation from the PW electrons provides an important source of ionizing radiation.

astro-ph↗

arXiv2002

Neutrino, Neutron, and Cosmic Ray Production in the External Shock Model of Gamma Ray Bursts

The hypothesis that ultra-high energy (>~ 10^19 eV) cosmic rays (UHECRs) are accelerated by gamma-ray burst (GRB) blast waves is assumed to be correct. Implications of this assumption are then derived for the external shock model of gamma-ray bursts. The evolving synchrotron radiation spectrum in GRB blast waves provides target photons for the photomeson production of neutrinos and neutrons. Decay characteristics and radiative efficiencies of the neutral particles that escape from the blast wave are calculated. The diffuse high-energy GRB neutrino background and the distribution of high-energy GRB neutrino events are calculated for specific parameter sets, and a scaling relation for the photomeson production efficiency in surroundings with different densities is derived. GRBs provide an intense flux of high-energy neutrons, with neutron-production efficiencies exceeding ~ 1% of the total energy release. The radiative characteristics of the neutron beta-decay electrons from the GRB "neutron bomb" are solved in a special case. Galaxies with GRB activity should be surrounded by radiation halos of ~ 100 kpc extent from the outflowing neutrons, consisting of a nonthermal optical/X-ray synchrotron component and a high-energy gamma-ray component from Compton-scattered microwave background radiation. The luminosity of sources of GRBs and relativistic outflows in L* galaxies such as the Milky Way is at the level of ~10^40+-1 ergs/s. This is sufficient to account for UHECR generation by GRBs. We briefly speculate on the possibility that hadronic cosmic rays originate from the subset of supernovae that collapse to form relativistic outflows and GRBs. (abridged)

astro-ph↗

arXiv2002

Gamma Ray Bursts and the Collapsar Model: a comment on astro-ph/0203391 and astro-ph/0203467

In an essay published in 1997, M. J. Harris pointed out that the development of a theoretical explanation, or "synthesis," of a group of astronomical results can lead to a situation where subsequent observations are forced to fit within that framework. Moreover, new results may be reported that fit within the framework but which can no longer be reproduced following the advent of more sensitive observations that overturn the synthesis. Although Harris considered gamma-ray line transients, a similar situation regarding the collapsar model for gamma-ray bursts now holds. In two recent papers posted on astro-ph by J. S. Bloom et al., and by P. A. Price et al., the synthesis effect is illustrated with respect to the collapsar model. These authors dismiss observations that contradict the collapsar model, appeal to "simplicity" to justify their preferred model, and dismiss (or better yet, ignore) competing models with unscientific reasoning. Whether or not the collapsar model is correct, these papers serve as textbook examples of the synthesis effect in science.

astro-ph↗

arXiv2002

X-ray Synchrotron Spectral Hardenings from Compton and Synchrotron Losses in Extended Chandra Jets

Chandra observations of knots and hot spots in spatially resolved X-ray jets of radio galaxies show that the X-ray fluxes often lie above an extrapolation from the radio-to-optical continuum fluxes. We show that combined synchrotron and Compton losses on a single power-law electron injection function can produce a hardening in the electron spectrum at electron Lorentz factors γ\~ 2e8/[Γ(1+z)] due to KN energy losses on the cosmic microwave background radiation. Here Γis the bulk Lorentz factor of the outflow, and z is the source redshift. This produces a flattening in the spectrum at frequencies >~ 8e16 δB_{μG}/[Γ^2(1+z)^3] Hz, where B_{μG} is the magnetic field in the comoving plasma frame in units of micro-Gauss and δis the Doppler factor. A single population of synchrotron-emitting electrons may therefore produce the radio-to-X-ray continuum in some radio galaxy knots, such as those in 3C 273.

astro-ph↗

arXiv2001

Maximum Particle Energies by Fermi Acceleration and the Origin of Cosmic Rays above the Knee

We derive the maximum accelerated particle energy from first-order and second-order Fermi acceleration at nonrelativistic and relativistic shocks for explosions taking place in a uniform surrounding medium. Second-order stochastic processes in relativistic flows are shown to be capable of accelerating cosmic rays to ultra-high energies. Cosmic rays above the knee of the cosmic ray spectrum can be accelerated by the second-order Fermi mechanism in relativistic flows, such as those occuring in gamma-ray bursts and unusual supernovae like SN 1998bw.

astro-ph↗

arXiv2001

Adiabatic Losses and Stochastic Particle Acceleration in Gamma-Ray Burst Blast Waves

We treat the problem of adiabatic losses and stochastic particle acceleration in gamma-ray burst (GRB) blast waves that decelerate by sweeping up matter from an external medium. The shocked fluid is assumed to be represented by a homogeneous expanding shell. The energy lost by nonthermal particles through adiabatic expansion is converted to the bulk kinetic energy of the outflow, permitting the evolution of the bulk Lorentz factor Gamma of the blast wave to be self-consistently calculated. The behavior of the system is shown to reproduce the hydrodynamic self-similar solutions in the relativistic and nonrelativistic limits, and the formalism is applicable to scenarios that are intermediate between the adiabatic and fully radiative regimes. Nonthermal particle energization through stochastic gyroresonant acceleration with magnetic turbulence in the blast wave is treated by employing energy-gain rates and diffusive escape timescales based upon expressions derived in the quasilinear regime. If the magnetic field in the shocked fluid approaches its equipartition value, this process can accelerate escaping particles to >~10^{20} eV energies, consistent with the hypothesis that ultra-high energy cosmic rays (UHECRs) are accelerated by GRB blast waves. Due to particle trapping by the magnetic turbulence, only the highest energy particles can escape during the prompt and afterglow phases of a GRB for acceleration by a Kolmogorov spectrum of MHD turbulence. Lower energy particles begin to escape as the blast wave becomes nonrelativistic and shock Fermi acceleration becomes more important.

astro-ph↗

arXiv2000

Gamma Ray Bursts, Cosmic Ray Origin, and the Unidentified EGRET Sources

Statistical arguments show that the volume- and time-averaged kinetic power of GRBs and fireball transients (FTs) into an L* galaxy like the Milky Way is at the level of 10^40 ergs/s. This number, though with wide uncertainties related to the internal or external shock efficiency, is sufficient to power hadronic cosmic rays observed locally. The release of energy by the high-mass progenitor stars of GRBs and FTs is sufficient to power the ultrahigh energy cosmic rays, as already shown by Waxman and Vietri in 1995. It is sufficient to power the cosmic rays above the knee of the cosmic-ray spectrum. Indeed, all hadronic cosmic rays could originate from the high-mass (>~ 100 M_o) stars that collapse to black holes, in the process forming GRBs and FTs. This source class represents a new solution to the problem of cosmic-ray origin. The ~10^4 - 10^7 black holes made by these stars could make their presence known by radiating as they accrete from the ISM, by microlensing background radiations, and by forming luminous binary systems. Some unidentified EGRET sources could be isolated black holes that accrete from the ISM. Better imaging and sensitivity with GLAST and TeV observatories will test this model for the unidentified gamma-ray sources, and this theory for cosmic-ray origin.

astro-ph↗

arXiv2000

Flash-Heating of Circumstellar Clouds by Gamma Ray Bursts

The blast-wave model for gamma-ray bursts (GRBs) has been called into question by observations of spectra from GRBs that are harder than can be produced through optically thin synchrotron emission. If GRBs originate from the collapse of massive stars, then circumstellar clouds near burst sources will be illuminated by intense gamma radiation, and the electrons in these clouds will be rapidly scattered to energies as large as several hundred keV. Low-energy photons that subsequently pass through the hot plasma will be scattered to higher energies, hardening the intrisic spectrum. This effect resolves the "line-of-death" objection to the synchrotron shock model. Illuminated clouds near GRBs will form relativistic plasmas containing large numbers of electron-positron pairs that can be detected within ~ 1-2 days of the explosion before expanding and dissipating. Localized regions of pair annihilation radiation in the Galaxy would reveal past GRB explosions.

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