arXiv subjects

C. D. Dermer

Publications and source records attributed to C. D. Dermer.

87 records · Page 5Linked to original sources

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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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

Synchrotron and SSC Emission and the Blast-Wave Model of Gamma-Ray Bursts

We investigate the dynamics and radiation from a relativistic blast-wave which decelerates as it sweeps up ambient matter. The bulk kinetic energy of the blast-wave shell is converted into internal energy by the process of accreting external matter. If it takes the form of non-thermal electrons and magnetic fields, then this internal energy will be emitted as synchrotron and synchrotron self-Compton radiation. We perform analytic and numerical calculations for the deceleration and radiative processes and present time-resolved spectra throughout the evolution of the blast-wave. We also examine the dependence of the burst spectra and light curves on various parameters describing the magnetic field and non-thermal electron distributions. We find that for bursts such as GRB~910503, GRB~910601 and GRB~910814, the spectral shapes of the prompt gamma-ray emission at the peaks in $νF_ν$ strongly constrain the magnetic fields in these bursts to be well below ($\la 10^{-2}$) the equipartition values. These calculations are also considered in the context of the afterglow emission from the recently detected gamma-ray burst counterparts.

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arXiv1997

Multiwavelength Observations of a Flare from Markarian 501

We present multiwavelength observations of the BL Lacertae object Markarian 501 (Mrk 501) in 1997 between April 8 and April 19. Evidence of correlated variability is seen in very high energy (VHE, E > 350 GeV) gamma-ray observations taken with the Whipple Observatory gamma-ray telescope, data from the Oriented Scintillation Spectrometer Experiment of the Compton Gamma-Ray Observatory, and quicklook results from the All-Sky Monitor of the Rossi X-ray Timing Explorer while the Energetic Gamma-Ray Experiment Telescope did not detect Mrk 501. Short term optical correlations are not conclusive but the U-band flux observed with the 1.2m telescope of the Whipple Observatory was 10% higher than in March. The average energy output of Mrk 501 appears to peak in the 2 keV to 100 keV range suggesting an extension of the synchrotron emission to at least 100 keV, the highest observed in a blazar and ~100 times higher than that seen in the other TeV-emitting BL Lac object, Mrk 421. The VHE gamma-ray flux observed during this period is the highest ever detected from this object. The VHE gamma-ray energy output is somewhat lower than the 2-100 keV range but the variability amplitude is larger. The correlations seen here do not require relativistic beaming of the emission unless the VHE spectrum extends to >5 TeV.

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arXiv1997

OSSE Observations of the Ultraluminous Infrared Galaxies Arp 220, Mrk 273 and Mrk 231

We report results of soft gamma-ray observations of the ultraluminous infrared galaxies Arp 220, Mrk 273, and Mrk 231 in order to test whether the infrared radiation from these sources originates from buried active galactic nuclei (AGNs). Only upper limits are measured, implying that the emergent soft gamma-ray luminosities are 1-2 orders of magnitude smaller than the infrared luminosities. Monte Carlo simulations of radiation transport through tori are used to infer the minimum column densities $N_H$ required to block transmission of soft gamma-rays from a buried AGN, assuming that spectra of AGNs in such sources are similar to those of radio-quiet quasars. Lack of measured gamma-ray emission provides no supporting evidence for the existence of buried AGNs in these galaxies, but is consistent with an origin of the infrared luminosity from starburst activity.

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arXiv1997

Annihilation Fountain in the Galactic Center Region

Two different model-independent mapping techniques have been applied to OSSE, SMM, TGRS and balloon data and reveal a feature in the 0.511 MeV electron-positron annihilation radiation pattern of our galaxy centered roughly at l=-2 deg. and b=10 deg. with a flux of 5x10^(-4) 0.511 MeV ph/cm^2/s. If near the galactic center, then positron sources are producing approximately 10^42 positrons/s which annihilate 1-2 kpc above the galactic plane. A starburst episode within the inner few hundred pc of our galaxy would drive hot pair-laden gas into the halo, with the one-sidedness pointing to the site of initial pressure release at the onset of the starburst activity. Positrons lose energy and annihilate as they are convected upward with the gas flow, and we calculate high-latitude annihilation patterns and fluxes in accord with the observations. Changes in the ionization state when the escaping gas cools could give annihilation radiation substructure. The fountain of hot (10^6-10^7 K) gas rising into the galactic halo would be seen through its enhanced dispersion measure, thermal emission, and recombination radiation.

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arXiv1997

The Beaming Pattern of Doppler Boosted Thermal Annihilation Radiation: Application to MeV Blazars

The beaming pattern of thermal annihilation radiation is broader than the beaming pattern produced by isotropic nonthermal electrons and positrons in the jets of radio-emitting active galactic nuclei which Compton scatter photons from an external isotropic radiation field. Thus blueshifted thermal annihilation radiation can provide the dominant contribution to the high-energy radiation spectrum at observing angles theta > 1/Gamma, where Gamma is the bulk Lorentz factor of the outflowing plasma. This effect may account for the spectral features of MeV blazars discovered with the Compton Telescope on the Compton Gamma Ray Observatory. Coordinated gamma-ray observations of annihilation line radiation to infer Doppler factors and VLBI radio observations to measure transverse angular speeds of outflowing plasma blobs can be used to determine the Hubble constant.

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arXiv1995

High-Energy Spectral Complexity from Thermal Gradients in Black Hole Atmospheres

We show that Compton scattering of soft photons with energies near 100 eV in thermally stratified black-hole accretion plasmas with temperatures in the range 100 keV - 1 MeV can give rise to an X-ray spectral hardening near 10 keV. This could produce the hardening observed in the X-ray spectra of black holes, which is generally attributed to reflection or partial covering of the incident continuum source by cold optically thick matter. In addition, we show that the presence of very hot (kT=1 MeV) cores in plasmas leads to spectra exibiting high energy tails similar to those observed from Galactic black-hole candidates.

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arXiv1995

Reverberation mapping of the central regions of active galactic nuclei using high-energy gamma-ray observations

We calculate the time- and energy-dependent opacity of high-energy gamma-rays attenuated by pair production interaction with accretion-flare photons that are scattered by gas and dust surrounding thenuclei of active galaxies. We show that the temporal behavior of the high-energy opacity cutoff can be used in conjunction with the time history of the accretion flare to determine the location of the gamma-ray emission site, as well as the column density and scale height of the material surrounding the central engine. Reverberation mapping using this technique is possible for nearby BL Lac objects and will be particularly valuable when the 10 -- 100 GeV gamma-ray window is opened, because there the attenuation of distant blazar radiation by the IIBR is negligible.

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arXiv1994

Thermal Comptonization in Mildly Relativistic Pair Plasmas

We use a Monte Carlo simulation to calculate the spectra of mildly relativistic thermal plasmas in pair balance. We use the exact integral expression for the electron-positron thermal annihilation spectrum, and provide accurate expressions for the Gaunt factors of electron-ion, electron-electron, and electron-positron thermal bremsstrahlung in the transrelativistic temperature regime. The particles are assumed to be uniformly distributed throughout a sphere, and the pair opacity is self-consistently calculated from the energy and angular distribution of scattered photons. The resultant photon spectra are compared with the nonrelativistic diffusion treatment of Sunyaev and Titarchuk, the bridging formulas of Zdziarski, and the relativistic corrections proposed by Titarchuk. We calculate allowed pair-balanced states of thermal plasmas with no pair escape which include bremsstrahlung and internal soft photons. The results are presented in the spectral index/compactness plane, and can be directly compared with observations of spectra from AGNs and Galactic black hole candidates. By comparing with X-ray spectral indices of Seyfert AGNs and compactnesses inferred from X-ray variability data, we find that the allowed solutions for pair equilibrium plasma imply that the temperatures of Seyfert galaxies are $\lesssim 300$ keV. This prediction can be tested with more sensitive gamma-ray observations of Seyfert galaxies. We find that if the X-ray variability time scale gives an accurate measure of the compactness, pair-dominated solutions are inconsistent with the data.

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arXiv1994

Association of Unidentified, Low Latitude EGRET Sources with Supernova Remnants

We propose that some of the unidentified Galactic plane (-10 degrees < b < 10 degrees) sources listed in the first EGRET catalog (Fichtel et al. 1994) are associated with nearby supernova remnants. The probability that the association of strong EGRET sources with supernova remnants is due to chance alignment is found to be < 1/2200. Three of the most convincing associations are radio-bright, nearby shell remnants and a fourth is associated with a plerion. We examine possible gamma-ray production mechanisms and conclude that pion production and decay from high-energy protons interacting with the remnant gas could produce the gamma rays. The energetics of this process are in accord with theories of cosmic ray production through supernova shock acceleration. The identification of pion-decay features in the gamma-ray spectra of supernova remnants would support this interpretation.

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arXiv1994

Compton Scattering in Jets: A Mechanism for $\sim$ 0.4 and $\lsim$ 0.2 Mev Line Production

We show that gamma ray line emission at $\sim$0.4 MeV and $\lsim 0.2$ MeV can be produced by Compton scattering of beamed radiation in the jets of Galactic black hole candidates. This mechanism has the novel feature of not invoking the presence of $e^+$--$e^-$ pairs. To produce the two lines we employ a symmetric double sided jet with bulk flow velocity of about 0.5 c and incident beam radiation with a hard energy spectrum. We show that the two lines can be seen at viewing angle cosines relative to the jet ranging from 0.2 to 0.6. This comprises 40\% of the total solid angle. In addition, the line radiation is approximately 10\% polarized. Depending on the bulk flow and viewing angle the model can produce lines at other energies as well. In particular a broad feature near 1 MeV can be seen by viewing the jet close to its axis. Our model can also accommodate single line spectra if the beamed gamma ray emission or the jets themselves are asymmetric.

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arXiv1994

Is the High-Energy Emission from Centaurus A Compton-Scattered Jet Radiation?

We consider whether the hard X-ray and soft gamma-ray emission from Centaurus A is beamed radiation from the active nucleus which is Compton-scattered into our line-of-sight. We derive the spectrum and degree of polarization of scattered radiation when incident beamed radiation is scattered from a cold ($kT<<m_ec^2$) electron cloud moving with bulk relativistic motion along the jet axis, and calculate results for an unpolarized, highly-beamed incident power-law photon source. We fit the OSSE data from Centaurus A with this model and find that if the scatterers are not moving relativistically, then the angle the jet makes with respect to our line-of-sight is $61^\circ\pm 5^\circ$. We predict a high degree of polarization of the scattered radiation below $\sim300$ keV. Future measurements with X-ray and gamma-ray polarimeters could be used to constrain or rule out such a scenario.

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