arXiv subjects

Matthew G. Baring

Publications and source records attributed to Matthew G. Baring.

At least 127 records · Page 7Linked to original sources

arXiv1999

Acceleration at Relativistic Shocks in Gamma-Ray Bursts

Most recent extragalactic models of gamma-ray bursts consider the expansion of a relativistic blast wave, emanating from a solar-mass type progenitor, into the surrounding interstellar medium as the site for their activity. The popular perception is that the optical afterglows result from the external shock interface, while the prompt transient gamma-ray signal arises from multiple shocks internal to the expansion. This paper illustrates a number of acceleration properties of relativistic and ultrarelativistic shocks that pertain to GRB models, by way of a standard Monte Carlo simulation. Computations of the spectral shape, the range of spectral indices, and the energy gain per shock crossing are presented, as functions of the shock speed and the type of particle scattering.

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arXiv1999

Inverse Bremsstrahlung in Shocked Astrophysical Plasmas

There has recently been interest in the role of inverse bremsstrahlung, the emission of photons by fast suprathermal ions in collisions with ambient electrons possessing relatively low velocities, in tenuous plasmas in various astrophysical contexts. This follows a long hiatus in the application of suprathermal ion bremsstrahlung to astrophysical models since the early 1970s. The potential importance of inverse bremsstrahlung relative to normal bremsstrahlung, i.e. where ions are at rest, hinges upon the underlying velocity distributions of the interacting species. In this paper, we identify the conditions under which the inverse bremsstrahlung emissivity is significant relative to that for normal bremsstrahlung in shocked astrophysical plasmas. We determine that, since both observational and theoretical evidence favors electron temperatures almost comparable to, and certainly not very deficient relative to proton temperatures in shocked plasmas, these environments generally render inverse bremsstrahlung at best a minor contributor to the overall emission. Hence inverse bremsstrahlung can be safely neglected in most models invoking shock acceleration in discrete sources such as supernova remnants. However, on scales > 100pc distant from these sources, Coulomb collisional losses can deplete the cosmic ray electrons, rendering inverse bremsstrahlung, and perhaps bremsstrahlung from knock-on electrons, possibly detectable.

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arXiv1998

Highly-Magnetized Pulsars and Integral

The complete absence of radio pulsars with periods exceeding a few seconds has lead to the popular notion of the existence of a high period death line. We have recently postulated the existence of another radio quiescence boundary at high magnetic fields ($B\gtrsim 4\times 10^{13}$G) in the upper portion of the period-period derivative diagram, a domain where no radio pulsars are observed. The origin of this high B boundary is also due to the suppression of magnetic pair creation, $γ\to e^{\pm}$, but mainly because of competition with the exotic QED process of magnetic photon splitting, $γ\toγγ$, coupled with ground state pair creation. This mechanism could also explain the low spectral cutoff energy of the gamma-ray pulsar PSR1509-58, which lies near the high B death-line. In this paper, we summarize the hypothesis of this new ``death line,'' and discuss some subtleties of pair suppression that relate to photon polarization and positronium formation. We identify several ways in which Integral will serve as a useful tool in probing high field pulsars, given that the radio quiescence boundary is transparent to the high energy bands.

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arXiv1998

Radio to Gamma-Ray Emission from Shell-type Supernova Remnants: Predictions from Non-linear Shock Acceleration Models

Supernova remnants (SNRs) are widely believed to be the principal source of galactic cosmic rays. Such energetic particles can produce gamma-rays and lower energy photons via interactions with the ambient plasma. In this paper, we present results from a Monte Carlo simulation of non-linear shock structure and acceleration coupled with photon emission in shell-like SNRs. These non-linearities are a by-product of the dynamical influence of the accelerated cosmic rays on the shocked plasma and result in distributions of cosmic rays which deviate from pure power-laws. Such deviations are crucial to acceleration efficiency and spectral considerations, producing GeV/TeV intensity ratios that are quite different from test particle predictions. The Sedov scaling solution for SNR expansions is used to estimate important shock parameters for input into the Monte Carlo simulation. We calculate ion and electron distributions that spawn neutral pion decay, bremsstrahlung, inverse Compton, and synchrotron emission, yielding complete photon spectra from radio frequencies to gamma-ray energies. The cessation of acceleration caused by the spatial and temporal limitations of the expanding SNR shell in moderately dense interstellar regions can yield spectral cutoffs in the TeV energy range; these are consistent with Whipple's TeV upper limits on unidentified EGRET sources. Supernova remnants in lower density environments generate higher energy cosmic rays that produce predominantly inverse Compton emission at super-TeV energies; such sources will generally be gamma-ray dim at GeV energies.

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arXiv1998

Direct Acceleration of Pickup Ions at the Solar Wind Termination Shock: the Production of Anomalous Cosmic Rays

We have modeled the injection and acceleration of pickup ions at the solar wind termination shock and investigated the parameters needed to produce the observed Anomalous Cosmic Ray (ACR) fluxes. A non-linear Monte Carlo technique was employed, which in effect solves the Boltzmann equation and is not restricted to near-isotropic particle distribution functions. This technique models the injection of thermal and pickup ions, the acceleration of these ions, and the determination of the shock structure under the influence of the accelerated ions. The essential effects of injection are treated in a mostly self-consistent manner, including effects from shock obliquity, cross-field diffusion, and pitch-angle scattering. Using recent determinations of pickup ion densities, we are able to match the absolute flux of hydrogen in the ACRs by assuming that pickup ion scattering mean free paths, at the termination shock, are much less than an AU and that modestly strong cross-field diffusion occurs. Simultaneously, we match the flux ratios He$^{+}$/H$^{+}$ or O$^{+}$/H$^{+}$ to within a factor $\sim 5$. If the conditions of strong scattering apply, no pre-termination-shock injection phase is required and the injection and acceleration of pickup ions at the termination shock is totally analogous to the injection and acceleration of ions at highly oblique interplanetary shocks recently observed by the Ulysses spacecraft. The fact that ACR fluxes can be modeled with standard shock assumptions suggests that the much-discussed "injection problem" for highly oblique shocks stems from incomplete (either mathematical or computer) modeling of these shocks rather than from any actual difficulty shocks may have in injecting and accelerating thermal or quasi-thermal particles.

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arXiv1998

Radio Quiet Pulsars with Ultra-Strong Magnetic Fields

The notable absence of radio pulsars having measured magnetic dipole surface field strengths above $B_0\sim 3\times 10^{13}$ Gauss naturally raises the question of whether this forms an upper limit to pulsar magnetization. Recently there has been increasing evidence that neutron stars possessing higher dipole spin-down fields do in fact exist, including a growing list of anomalous X-ray pulsars (AXPs) with long periods and spinning down with high period derivatives, implying surface fields of $10^{14}$--$10^{15}$ Gauss. Furthermore, the recently reported X-ray period and period derivative for the Soft Gamma-ray Repeater (SGR) source SGR1806-20 suggest a surface field around $10^{15}$ Gauss. None of these high-field pulsars have yet been detected as radio pulsars. We propose that high-field pulsars should be radio-quiet because electron-positron pair production in their magnetospheres, thought to be essential for radio emission, is efficiently suppressed in ultra-strong fields ($B_0\gtrsim 4\times 10^{13}$ Gauss) by the action of photon splitting, a quantum electrodynamical process in which a photon splits into two. Our computed radio quiescence boundary in the radio pulsar $P-\dot P$ diagram, where photon splitting overtakes pair creation, is located just above the boundary of the known radio pulsar population, neatly dividing them from the AXPs. We thus identify a physical mechanism that defines a new class of high-field radio-quiet neutron stars that should be detectable by their pulsed emission at X-ray and perhaps $γ$-ray energies.

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arXiv1998

Charged-Particle Motion in Electromagnetic Fields Having at Least One Ignorable Spatial Coordinate

We give a rigorous derivation of a theorem showing that charged particles in an arbitrary electromagnetic field with at least one ignorable spatial coordinate remain forever tied to a given magnetic-field line. Such a situation contrasts the significant motions normal to the magnetic field that are expected in most real three-dimensional systems. It is pointed out that, while the significance of the theorem has not been widely appreciated, it has important consequences for a number of problems and is of particular relevance for the acceleration of cosmic rays by shocks.

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arXiv1997

Gamma-Ray Bursts Above 1 GeV

One of the principal results obtained by the Compton Gamma Ray Observatory relating to the study of gamma-ray bursts was the detection by the EGRET instrument of energetic ($>$100 MeV) photons from a handful of bright bursts. The most extreme of these was the single 18 GeV photon from the GRB940217 source. Given EGRET's sensitivity and limited field of view, the detection rate implies that such high energy emission may be ubiquitous in bursts. Hence expectations that bursts emit out to at least TeV energies are quite realistic, and the associated target-of-opportunity activity of the TeV gamma-ray community is well-founded. This review summarizes the observations and a handful of theoretical models for generating GeV--TeV emission in bursts sources, outlining possible ways that future positive detections could discriminate between different scenarios. The power of observations in the GeV--TeV range to distinguish between spectral structure intrinsic to bursts and that due to the intervening medium between source and observer is also discussed.

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arXiv1997

Shock Acceleration and Gamma-Ray Emitting Supernova Remnants

Diffusive shock acceleration in the environs of a remnant's expanding shell is a popular candidate for the origin of SNR gamma-rays. In this paper, results from our study of non-linear effects in shock acceleration theory and their impact on the gamma-ray spectra of SNRs are presented. These effects describe the dynamical influence of the accelerated cosmic rays on the shocked plasma at the same time as addressing how the non-uniformities in the fluid flow force the distribution of the cosmic rays to deviate from pure power-laws. Such deviations are crucial to gamma-ray spectral determination. Our self-consistent Monte Carlo approach to shock acceleration is used to predict ion and electron distributions that spawn neutral pion decay, bremsstrahlung and inverse Compton emission components for SNRs. We demonstrate how the spatial and temporal limitations imposed by the expanding SNR shell quench acceleration above critical energies in the 500 GeV - 10 TeV range, thereby spawning gamma-ray spectral cutoffs that are quite consistent with Whipple's TeV upper limits to the EGRET unidentified sources that have SNR associations. We also discuss the role of electron injection in shocks and its impact on the significance of electromagnetic components to GeV--TeV spectral formation.

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arXiv1997

Supernova Remnants and Plerions in the Compton Gamma-Ray Observatory Era

Due to observations made by the Compton Gamma-Ray Observatory over the last six years, it appears that a number of galactic supernova remnants may be candidates for sources of cosmic gamma-rays. These include shell-type remnants such as IC443 and $γ$ Cygni, which have no known parent pulsars, but have significant associations with unidentified EGRET sources, and others that appear to be composite, where a pulsar is embedded in a shell (e.g. W44 and Vela), or are purely pulsar-driven, such as the Crab Nebula. This review discusses our present understanding of gamma-ray production in plerionic and non-plerionic supernova remnants, and explores the relationship between such emission and that in other wavebands. Focuses include models of the Crab and Vela nebulae, the composite nature of W44, the relationship of shell-type remnants to cosmic ray production, the relative importance of shock-accelerated protons and electrons, constraints on models placed by TeV, X-ray and radio observations, and the role of electrons injected directly into the remnants by parent pulsars.

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arXiv1997

The Escape of High-Energy Photons from Gamma-Ray Bursts

Eleven bright gamma-ray bursts (GRBs) detected by BATSE have also been seen at much higher energies by EGRET, six at energies above 10 MeV. Such observations imply that these bursts are optically thin to photon-photon pair production at all observed energies. For bursts more than about 30pc away, internal transparency can be achieved only if the source is moving with a relativistic bulk Lorentz factor $Γ\gg 1$, or if the radiation is highly beamed. Early calculations of $γγ\to e^+e^-$ considerations for GRBs were limited to cases of a beam with opening half-angle $\Thetab\sim 1/Γ$, or expansions of infinitely thin spherical shells. This paper presents our extension of pair production optical depth calculations in relativistically expanding sources to more general geometries, including shells of finite thickness and arbitrary opening angle. The problem is reduced analytically to a single integral in the broadly applicable case of observing photons along the axis of the expansion. We find that the minimum bulk Lorentz factor for the EGRET sources to be optically thin is only moderately dependent on the shell thickness and virtually independent of its opening solid angle if $\Thetab\gtrsim 1/Γ$. This insensitivity to $\Thetab$ relieves the commonly-perceived number problem for non-repeating sources at cosmological distances, i.e. it is not necessary to invoke small $\Thetab$ to effect photon escape.

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arXiv1997

A New Class of Radio Quiet Pulsars

The complete absence of radio pulsars with periods exceeding a few seconds has lead to the popular notion of the existence of a high $P$ death line. In the standard picture, beyond this boundary, pulsars with low spin rates cannot accelerate particles above the stellar surface to high enough energies to initiated pair cascades through curvature radiation, and the pair creation needed for radio emission is strongly suppressed. In this paper we postulate the existence of another pulsar ``death line,'' corresponding to high magnetic fields $B$ in the upper portion of the $\dot{P}$--$P$ diagram, a domain where few radio pulsars are observed. The origin of this high $B$ boundary, which occurs when $B$ becomes comparable to or exceeds $10^{13}$ Gauss, is again due to the suppression of magnetic pair creation $γ\to e^+e^-$, but in this instance, primarily because of ineffective competition with the exotic QED process of magnetic photon splitting. This paper describes the origin, shape and position of the new ``death line,'' above which pulsars are expected to be radio quiet, but perhaps still X-ray and $γ$-ray bright.

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arXiv1997

Gamma-ray Production in Supernova Remnants

Supernova remnants are widely believed to be a principal source of galactic cosmic rays, produced by diffusive shock acceleration in the environs of the remnant's expanding shock. This review discusses recent modelling of how such energetic particles can produce gamma-rays via interactions with the remnants' ambient interstellar medium, specifically via neutral pion decay, bremsstrahlung and inverse Compton emission. Predictions that relate to the handful of associations between EGRET unidentified sources and known radio/optical/X-ray emitting remnants are summarized. The cessation of acceleration above 1 TeV - 10 TeV energies in young shell-type remnants is critical to model consistency with Whipple's TeV upper limits; these observations provide important diagnostics for theoretical models.

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arXiv1997

Diffusive Shock Acceleration: the Fermi Mechanism

The mechanism of diffusive Fermi acceleration at collisionless plasma shock waves is widely invoked in astrophysics to explain the appearance of non-thermal particle populations in a variety of environments, including sites of cosmic ray production, and is observed to operate at several sites in the heliosphere. This review outlines the principal results from the theory of diffusive shock acceleration, focusing first on how it produces power-law distributions in test-particle regimes, where the shock dynamics are dominated by the thermal populations that provide the seed particles for the acceleration process. Then the importance of non-linear modifications to the shock hydrodynamics by the accelerated particles is addressed, emphasizing how these subsequently influence non-thermal spectral formation.

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arXiv1997

Magnetic Photon Splitting: Computations of Proper-time Rates and Spectra

The splitting of photons in the presence of an intense magnetic field has recently found astrophysical applications in polar cap models of gamma-ray pulsars and in magnetar scenarios for soft gamma repeaters. Numerical computation of the polarization-dependent rates of this third order QED process for arbitrary field strengths and energies below pair creation threshold is difficult: thus early analyses focused on analytic developments and simpler asymptotic forms. The recent astrophysical interest spurred the use of the S-matrix approach by Mentzel, Berg and Wunner to determine splitting rates. In this paper, we present numerical computations of a full proper-time expression for the rate of splitting that was obtained by Stoneham, and is exact up to the pair creation threshold. While the numerical results derived here are in accord with the earlier asymptotic forms due to Adler, our computed rates still differ by as much as factors of 3 from the S-matrix re-evaluation of Wilke and Wunner, reflecting the extreme difficulty of generating accurate S-matrix numerics for fields below about \teq{4.4\times 10^{13}}Gauss. We find that our proper-time rates appear very accurate, and exceed Adler's asymptotic specializations significantly only for photon energies just below pair threshold and for supercritical fields, but always by less than a factor of around 2.6. We also provide a useful analytic series expansion for the scattering amplitude valid at low energies.

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arXiv1997

Gamma-Rays from Supernova Remnants and the Signatures of Diffusive Shock Acceleration

While the definitive detection of gamma-rays from known supernova remnants (SNRs) remains elusive, the collection of unidentified EGRET sources that may be associated with SNRs has motivated recent modelling of TeV emission from these sources. Current theoretical models use power-law shock-accelerated protons and electrons in their predictions of expected gamma-ray TeV fluxes from those unidentified EGRET sources with remnant associations. In this paper, we explore a more detailed non-linear shock acceleration model, which generates non-thermal proton distributions and includes a self-consistent determination of shock hydrodynamics. We obtain gamma-ray spectra for SNRs allowing for the cessation of acceleration to high energies that is due to the finite ages and sizes of remnants. Gamma-ray spectral cutoffs can be observed in the TeV range for reasonable remnant parameters, and deviations from power-law behaviour are found at all energies ranging from 1 MeV up to the cutoff. Correlated observations by INTEGRAL, Whipple and other instruments may provide stringent constraints to our understanding of supernova remnants.

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arXiv1997

The Attenuation of Gamma-Ray Emission in Strongly-Magnetized Pulsars

Gamma-rays from pulsars can be efficiently attenuated in their magnetospheres via the mechanism of single-photon pair production and also the exotic QED process of photon splitting, which become prolific in fields approaching the quantum critical value of $B_{cr}=4.41\times 10^{13}$ Gauss. Recently we have published results of our modelling of strongly-magnetized $γ$-ray pulsars, which focused on the escape or attenuation of photons emitted near the pole at the neutron star surface in dipole fields, in a Schwarzschild metric. We found that pair production and splitting totally inhibit emission above around 10--30 MeV in PSR1509-58, whose surface field is inferred to be as high as $0.7B_{cr}$. Our model pulsar spectra are consistent with the EGRET upper limits for PSR1509-58 for a wide range of polar cap sizes. Here we review the principal predictions of our attenuation analysis, and identify how its powerful observational diagnostic capabilities relate to current and future gamma-ray experiments. Diagnostics include the energy of the gamma-ray turnover and the spectral polarization, which constrain the estimated polar cap size and field strength, and can determine the relative strength of splitting and pair creation.

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arXiv1997

Pair Production Absorption Troughs in Gamma-Ray Burst Spectra: A Potential Distance Discriminator

Relativistic bulk motion with large Lorentz factors has recently been inferred for gamma-ray bursts regardless of whether they are of galactic or cosmological origin. This conclusion results from calculations of internal pair production transparency in bursts that usually assume an infinite power-law source spectrum for simplicity, an approximation that is quite adequate for some bursts detected by EGRET. However, for a given bulk Lorentz factor \teqΓ, photons above the EGRET range can potentially interact with sub-MeV photons in such calculations. Hence it is essential to accurately address the spectral curvature in bursts seen by BATSE. In this paper we present the major properties induced in photon-photon opacity considerations by such spectral curvature. The observed spectral breaks around 1 MeV turn out to be irrelevant to opacity in cosmological bursts, but are crucial to estimates of source transparency in the 1 GeV -- 1 TeV range for sources located in the galactic halo. We find that broad absorption troughs can arise at these energies for suitable bulk motion parameters \teqΓ. Such troughs are probably an unambiguous signature of a galactic halo population, and if observed by experiments such as Whipple, MILAGRO and GLAST, would provide powerful evidence that such bursts are not at cosmological distances.

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