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International audience; Saturn is permanently surrounded by 6 discrete proton radiation belts that are rigidly separated by the orbits of its inner moons and dense rings. These radiation belts are ideal environments to study the details of radial diffusion and the CRAND source process, yet progress has been hindered by the fact that the energy spectra are not known with certainty: Reanalysis of the response functions of the LEMMS instrument on-board the Cassini orbiter has shown that measurements of ≲ 10 MeV protons may be easily contaminated by ≳ 10 MeV protons and that many available measurements characterize a very broad energy range, so that the calculation of an energy-resolved spectrum is not as straightforward as previously assumed. Here we use forward modeling of the measurements based on the instrument response and combine this technique where useful with numerical modeling of the proton belt physics in order to determine Saturn's spectra with higher certainty. We find significant proton intensities up to ≈ 1 GeV. While earlier studies reported on proton spectra roughly following a power law with exponent ≈ - 2 , our more advanced analysis shows harder spectra with exponent ≈ - 1 . The observed spectra provide independent confirmation that Saturn's proton belts are sourced by CRAND and are consistent with the provided protons being subsequently cooled in the tenuous gas originating from Saturn or Enceladus. The intensities at Saturn are found to be lower than at Jupiter and Earth, which is also consistent with the source of Saturn being exclusively CRAND, while the other planets can draw from additional processes. Our new spectra can be used in the future to further our understanding of Saturn's proton belts and the respective physical processes that occur at other magnetized planets in general. Also, the spectra have applications for several topics of planetary science, such as space weathering of Saturn's moons and rings, and can be useful to constrain properties of the main rings through their production of secondary particles.

The detection of crystalline H2O-ice on multiple surfaces of Kuiper Belt Objects (KBOs) seems to contrast with what scientists understand about the surface environment of these objects, as previous estimates suggest that radiolysis should have easily amorphized these objects' surface over their lifetimes. Here, we use a detailed laboratory approach to show that crystalline H2O-ice can be amorphized by energetic electrons at temperatures as high as 70 K. However, the estimated time needed to completely amorphize the H2O-ice present on the surface of a KBO to the depth probed by near-infrared spectroscopy is only slightly less than the age of the solar system. Given the uncertainties involved in these types of extrapolations and the possibility of a resurfacing event occurring in these objects lifetime, the detection of crystalline or at least partially crystalline H2O-ice on KBOs should be expected.

We discuss the formation of aerosols within Titan's thermosphere-ionosphere and the different chemical pathways. Negative ion measurements by the Cassini Plasma Spectrometer (CAPS) Electron Spectrometer (ELS) give evidence for formation of unsaturated anion carbon chains, while positive ion measurements of the Cassini Ion Neutral Mass Spectrometer (INMS) indicate formation of more aromatic cation hydrocarbons. There is presently no direct observational evidence for large neutral molecule growth in Titan's thermosphere-ionosphere. The hydrocarbon cations are expected to form Polycyclic Aromatic Hydrocarbons (PAH), those with the addition of nitrogen being called PAHNs. We theorize anion carbon chains can eventually become long enough to fold into fullerene C60,70 carbon shells, of various charge states. Based on laboratory data the fullerenes can trap incoming O+ magnetospheric ions that have relatively high energy collisions with the fullerenes and, once trapped, protect the oxygen atom from Titan's reducing thermosphere-ionosphere. The fullerenes can form into larger onion fullerenes and condense into larger embryo aerosols (i.e., m/q > 10,000 amu/q anions as observed by CAPS/ELS) eventually falling onto Titan's surface and precipitating to the bottom of its hydrocarbon lakes. Molecule production composed of H, C, N is known to occur in Titan's atmosphere with energy input from the magnetosphere, solar UV, and deep-penetrating irradiation from galactic cosmic rays (GCR). Space radiation effects by GCR irradiation of Titan's surface and lakes can lead to the manufacture of exobiological molecules with oxygen as the new ingredient. We have developed a model of galactic cosmic ray irradiation of Titan's atmosphere, surface, subsurface and bottoms of Titan lakes. GCR would provide further energy for processing of the aerosols into more complex organic forms such as tholins and precursor molecules for amino acids. A second process called hydrolysis then converts the precursor molecules into amino acids. Hydrolysis is provided via meteor impacts with size >10 km and cryovolcanism both which can produce liquid water on Titan's surface for episodic periods > several 100 to 1000 years. Our model shows that GCR secondary particles can penetrate ~ 100 m below the ice surface (including the bottom of Titan's less dense hydrocarbon lakes ~ 150 m depths) and produce chemically significant dosages over very long timescales ~ 450 Myrs. The GCR model is combined with laboratory data from experiments in which dry methyl ices were irradiated to doses producing prebiotic amino acids such as glycine. The model calculations show glycine can form to ~ 2.5 ppb levels near the surface after ~ 450 Myrs of GCR proton irradiation and potentially to 5 ppb if heavy-ion GCRs up through Fe are included. If such molecules were detected, this would not only confirm this model but indicate that life forms different from ours may not be required.

A new model is presented on how chemically driven cryovolcanism might contribute to episodic outgassing at the icy moon Enceladus and potentially elsewhere including Europa and Kuiper Belt Objects. Exposed water ices can become oxidized from radiolytic chemical alteration of near-surface water ice by space environment irradiation. In contact with primordially abundant reductants such as NH3, CH4, and other hydrocarbons, the product oxidants can react exothermically to produce volatile gases driving cryovolcanism via gas-piston forces on any subsurface liquid reservoirs. Radiolytic oxidants such as H2O2 and O2 can continuously accumulate deep in icy regoliths and be conveyed by rheological flows to subsurface chemical reaction zones over million-year time scales indicated by cratering ages for active regions of Enceladus and Europa. Surface blanketing with cryovolcanic plume ejecta would further accelerate regolith burial of radiolytic oxidants. Episodic heating from transient gravitational tides, radioisotope decay, impacts, or other geologic events might occasionally accelerate chemical reaction rates and ignite the exothermic release of cumulative radiolytic oxidant energy. The time history for the suggested "Old Faithful" model of radiolytic gas-driven cryovolcanism at Enceladus and elsewhere therefore consists of long periods of chemical energy accumulation punctuated by much briefer episodes of cryovolcanic activity. The most probable sequence for detection of activity in the current epoch is a long evolutionary phase of slow but continuous oxidant accumulation over billions of years followed by continuous but variable high activity over the past 10(exp 7)-10(exp 8) years. Detectable cryovolcanic activity could then later decline due to near-total oxidation of the rheologically accessible ice crust and depletion the accessible reductant abundances, as may have already occurred for Europa in the more intense radiation environment of Jupiter's magnetosphere. Astrobiological potential of Enceladus could correspondingly be higher than at Europa due to a less extreme state of oxidation and greater residual abundance of organics.

The Advanced Compton Telescope (ACT) has been selected by NASA for a one-year “vision mission” study. The study’s main goal is to determine feasible instrument configurations to achieve ACT’s sensitivity requirements, and to give recommendations for technology development. Space-based instruments operating in the energy range of nuclear lines are subject to complex backgrounds generated by cosmic rays, earth albedo radiations, trapped particles, and diffuse gamma rays; typically measurements are significantly background-dominated. Therefore accurate, detailed simulations of the background induced in different ACT configurations, and exploration of event selection and reconstruction techniques for reducing these backgrounds, are crucial to determining the capabilities of a given instrument configuration. The ACT simulation team has assembled a complete suite of tools that allows the generation of particle backgrounds for a given orbit, their propagation through any instrument and spacecraft geometry – including delayed photon emission from instrument activation – as well as the selection and reconstruction of Compton events in the given detectors. We describe here the scope of the ACT simulation effort and the suite of tools used.

We have developed MGGPOD, a suite of Monte Carlo codes built around the widely used GEANT (Version 3.21) package, to simulate ab initio the physical processes relevant for the production of instrumental backgrounds. These include the build-up and delayed decay of radioactive isotopes as well as the prompt de-excitation of excited nuclei, both of which give rise to a plethora of instrumental γ-ray background lines in addition to continuum background. We demonstrate the capabilities of the MGGPOD suite by modelling high resolution γ-ray spectra recorded by the Transient Gamma-Ray Spectrometer (TGRS) on board Wind and the SPI spectrometer on board the recently launched INTEGRAL observatory.

In this paper we present analysis of X-ray spectra from the supernova remnant IC 443 obtained using the PCA on RXTE. The spectra in the 3–20 keV band are well fit by a two-component model consisting of thermal and non-thermal components. We compare these results with recent results of other X-ray missions and discuss the need for a cut-off in the non-thermal spectrum. Recent Chandra and XMM-Newton observations suggest that much of the non-thermal emission from IC 443 can be attributed to a pulsar wind nebula. We present the results of our search for periodic emission in the RXTE PCA data. We then discuss the origin of the non-thermal component and its possible association with the unidentified EGRET source.

We performed a joint fit to radio, recent Chandra X-ray, and γ-ray data for the supernova remnant SN 1006 using models of synchrotron and inverse-Compton emission. The results are used to determine the magnetic field strength and the properties of the electron spectrum for the eastern rim of the remnant.

In this paper we discuss the methods developed for the production of the INTEGRAL/SPI instrument response. The response files were produced using a suite of Monte Carlo simulation software developed at NASA/GSFC based on the GEANT-3 package available from CERN. The production of the INTEGRAL/SPI instrument response also required the development of a detailed computer mass model for SPI. We discuss our extensive investigations into methods to reduce both the computation time and storage requirements for the SPI response. We also discuss corrections to the simulated response based on our comparison of ground and inflight calibration data with MGEANT simulations.

The SPI spectrometer on INTEGRAL features a camera system with 19 Ge detector modules, imaging photons through a tungsten coded mask. Background is reduced by an anticoincidence detector system surrounding these. The specifics of this instrument lead to data correction and analysis methods which are described here. Raw data for science analysis are detector event messages and spectra for different categories of detector hits and pulse shapes. Preprocessing combines calibrated spectra from these, which are then interpreted using the imaging and spectral response function for measured spectra where parts of the detector plane are occulted by the mask. Background dominates the overall signal, tailored background estimates and models are based on instrument-specific signatures, their correlations, and trends.

Recent observations of the Galactic center region by the GRIS balloon-borne germanium spectrometer have determined that the diffuse 1809 keV emission resulting from the decay of 26Al has an intrinsic width of 5.4 keV FWHM. This line width indicates that the 26Al either is at a temperature of ~4.5 × 108 K or has a nonthermal velocity of ~500 km s-1. Previous authors have suggested that the 26Al must be trapped within dust grains in the interstellar medium (ISM) in order for these conditions to persist over the 106 yr lifetime of the aluminum. We discuss the results of our model, in which 26Al dust grains are produced in Type II supernovae and are subsequently reaccelerated in the ISM by ambient supernova remnant (SNR) shocks. Our results show that dust grains can be maintained at a velocity sufficient to explain the GRIS observation for ISM densities of ~0.2 cm-3, dust-grain sizes near 10-5 cm, and distances between SNR shocks in the ISM of 100-200 pc.

Assuming that supernova shocks accelerate nonthermal particles, we model the temporally evolving nonthermal particle and photon spectra at different stages in the lifetime of a standard shell-type supernova remnant (SNR). A characteristic νFν spectrum of an SNR consists of a peak at radio through optical energies from nonthermal electron synchrotron emission and another high-energy gamma-ray peak due primarily to secondary pion production, nonthermal electron bremsstrahlung, and Compton scattering. We find that supernova remnants are capable of producing maximum gamma-ray luminosities 1035 ergs s-1 if the density of the local interstellar medium is 10 cm-3. This emission will persist for 105 yr after the supernova explosion because of the long energy loss timescales for electrons with kinetic energy ~1 GeV. This long gamma-ray lifetime implies that SNRs with a wide range of ages could be gamma-ray sources and could constitute some of the unidentified EGRET sources.

We report OSSE observations of the bursting X-ray pulsar GRO J1744-28 made in 1995 December and 1996 January at hard X-ray energies >35 keV. The pulse profile of the persistent (i.e., nonbursting) pulsar emission is fitted with a sinusoid in the energy range 35-90 keV. Residuals reveal a second harmonic amplitude of 3.0% ± 0.5% of the fundamental. The distribution of time intervals between bursts measured in January is characterized by a broad flat-topped function with width 35 minutes and mean 33 minutes. The burst profile averaged over an ensemble of 104 bursts in the 35-60 keV energy range has FWHM width of 3.6 ± 0.3 s and displays a factor of 2 faster rise time than decay and a pronounced dip in persistent emission after the burst. The phase of the sinusoidal pulse profile during bursts lags the phase prior to bursts by 90 ms (1.2 radians), and a 29 ± 6 ms (0.39 ± 0.08 radians) lag persists following the burst. There are no statistically significant spectral differences between the hard X-ray spectra of the bursting and persistent emission in the OSSE energy range, nor is there any evidence of annihilation or neutron capture line emission or cyclotron absorption.

A joint spectral analysis of some Chandra ACIS X-ray data and Molonglo Observatory Synthesis Telescope radio data was performed for 13 small regions along the bright northeastern rim of the supernova remnant SN 1006. These data were fitted with a synchrotron radiation model. The nonthermal electron spectrum used to compute the photon emission spectra is the traditional exponentially cut off power law, with one notable difference: The power-law index is not a constant. It is a linear function of the logarithm of the momentum. This functional form enables us to show, for the first time, that the synchrotron spectrum of SN 1006 seems to flatten with increasing energy. The effective power-law index of the electron spectrum is 2.2 at 1 GeV (i.e., radio synchrotron-emitting momenta) and 2.0 at about 10 TeV (i.e., X-ray synchrotron-emitting momenta). This amount of change in the index is qualitatively consistent with theoretical models of the amount of curvature in the proton spectrum of the remnant. The evidence of spectral curvature implies that cosmic rays are dynamically important instead of being "test" particles. The spectral analysis also provides a means of determining the critical frequency of the synchrotron spectrum associated with the highest-energy electrons. The critical frequency seems to vary along the northeastern rim, with a maximum value of 1.1e17 (0.6e17 - 2.1e17) Hz. This value implies that the electron diffusion coefficient can be no larger than a factor of ~4.5-21 times the Bohm diffusion coefficient if the velocity of the forward shock is in the range 2300-5000 km/s. Since the coefficient is close to the Bohm limit, electrons are accelerated nearly as fast as possible in the regions where the critical frequency is about 1.0e17 Hz., 41 pages, 8 figures, accepted by ApJ

We present the results of a broad-band X-ray study of the enigmatic Be star Gamma Cassiopeiae (herein gamma Cas) based on observations made with both the Suzaku and INTEGRAL observatories. gamma Cas has long been recognized as the prototypical example of a small subclass of Be stars with moderately strong X-ray emission dominated by a hot thermal component in the 0.5-12 keV energy range Lx ~ 10^32 - 10^33 erg s^-1. This places them at the high end of the known luminosity distribution for stellar emission, but several orders of magnitude below typical accretion powered Be X-ray binaries. The INTEGRAL observations spanned an 8 year baseline and represent the deepest measurement to date at energies above ~50 keV. We find that the INTEGRAL data are consistent within statistics to a constant intensity source above 20 keV, with emission extending up to ~100 keV and that searches for all of the previously reported periodicities of the system at lower energies led to null results. We further find that our combined Suzaku and INTEGRAL spectrum, which we suggest is the most accurate broad-band X-ray measurement of gamma Cas to date, is fitted extremely well with a thermal plasma emission model with a single absorption component. We found no compelling need for an additional non-thermal high-energy component. We discuss these results in the context of a currently favored models for gamma Cas and its analogs., 28 pages, 5 figures, accepted for publication in ApJ

Intense and complex instrumental backgrounds, against which the much smaller signals from celestial sources have to be discerned, are a notorious problem for low and intermediate energy gamma-ray astronomy (~50 keV - 10 MeV). Therefore a detailed qualitative and quantitative understanding of instrumental line and continuum backgrounds is crucial for most stages of gamma-ray astronomy missions, ranging from the design and development of new instrumentation through performance prediction to data reduction. We have developed MGGPOD, a user-friendly suite of Monte Carlo codes built around the widely used GEANT (Version 3.21) package, to simulate ab initio the physical processes relevant for the production of instrumental backgrounds. These include the build-up and delayed decay of radioactive isotopes as well as the prompt de-excitation of excited nuclei, both of which give rise to a plethora of instrumental gamma-ray background lines in addition to continuum backgrounds. The MGGPOD package and documentation are publicly available for download from http://sigma-2.cesr.fr/spi/MGGPOD/. We demonstrate the capabilities of the MGGPOD suite by modeling high resolution gamma-ray spectra recorded by the Transient Gamma-Ray Spectrometer (TGRS) on board Wind during 1995. The TGRS is a Ge spectrometer operating in the 40 keV to 8 MeV range. Due to its fine energy resolution, these spectra reveal the complex instrumental background in formidable detail, particularly the many prompt and delayed gamma-ray lines. We evaluate the successes and failures of the MGGPOD package in reproducing TGRS data, and provide identifications for the numerous instrumental lines., Comment: 60 pages, 13 figures, 7 tables, accepted for publication in ApJS

Spatially resolved images of the galactic supernova remnant G78.2+2.1 (gamma-Cygni) in hard X-ray energy bands from 25 keV to 120 keV are obtained with the IBIS-ISGRI imager aboard the International Gamma-Ray Astrophysics Laboratory INTEGRAL. The images are dominated by localized clumps of about ten arcmin in size. The flux of the most prominent North-Western (NW) clump is (1.7 +/- 0.4) 10^{-11} erg/cm^2/s in the 25-40 keV band. The observed X-ray fluxes are in agreement with extrapolations of soft X-ray imaging observations of gamma-Cygni by ASCA GIS and spatially unresolved RXTE PCA data. The positions of the hard X-ray clumps correlate with bright patches of optical line emission, possibly indicating the presence of radiative shock waves in a shocked cloud. The observed spatial structure and spectra are consistent with model predictions of hard X-ray emission from nonthermal electrons accelerated by a radiative shock in a supernova interacting with an interstellar cloud, but the powerful stellar wind of the O9V star HD 193322 is a plausible candidate for the NW source as well., Comment: 5 pages, 5 figures, Astronomy and Astrophysics Letters

GRB030320 is the 5th Gamma-ray burst (GRB) detected by INTEGRAL in the field of view (FoV). It is so far the GRB with the largest off-axis angle with respect to the INTEGRAL pointing direction, near to the edge of the FoV of both main instruments, IBIS and SPI. Nevertheless, it was possible to determine its position and to extract spectra and fluxes. The GRB nature of the event was confirmed by an IPN triangulation. It is a ~ 60 s long GRB with two prominent peaks separated by ~ 35 s. The spectral shape of the GRB is best represented by a single power law with a photon index Gamma ~ 1.7. The peak flux in the 20 - 200 keV band is determined to ~ 5.7 photons cm-2 s-1 and the GRB fluence to 1.1 x 10-5 erg cm-2. Analysing the spectral evolution of the GRB, a ``hard-to-soft'' behaviour emerges. A search for an optical counterpart has been carried out, but none was found., 5 pages, 3 figures. Accepted for publication in A&AL (INTEGRAL issue)

(Abridged) We present an analysis of the X-ray spectrum of the Galactic shell-type supernova remnant (SNR) G347.3-0.5 (RX J1713.7-3946). By performing a joint spectral analysis of data from observations made of G347.3-0.5 using the ROSAT PSPC, the ASCA GIS and the RXTE PCA, we have fit the spectra of particular regions of this SNR (including the bright northwestern and southwestern rims, the northeastern rim and the interior diffuse emission) over the approximate energy range of 0.5 through 30 keV. Based on the parameters of the best fit to the spectra using the SRCUT model, we estimate the maximum energy of cosmic-ray electrons accelerated by the rims of G347.3-0.5 to be 19-25 TeV, assuming a magnetic field strength of 10 microGauss. We present a broadband (radio to gamma-ray) photon energy flux-spectrum for the northwestern rim of the SNR, using a synchrotron-inverse Compton model with a variable magnetic field strength to fit the spectrum. Our fit derived from this model yields a maximum energy of only 8.8 TeV for the accelerated cosmic-ray electrons and a magnetic field strength of 150 microGauss. However, our derived ratio of volumes for TeV emission and X-ray emission (approximately 1000) is too large to be physically acceptable. We argue that neither non-thermal bremsstrahlung nor neutral pion decay can adequately describe the TeV emission from this rim, and therefore the physical process responsible for this emission is currently uncertain. Finally, we compare the gross properties of G347.3-0.5 with other SNRs known to possess X-ray spectra dominated by non-thermal emission., 46 pages, 16 figures, accepted for publication in the Astrophysical Journal (Volume 593, 10 August 2003 Issue)

Three calibration campaigns of the spectrometer SPI have been performed before launch in order to determine the instrument characteristics, such as the effective detection area, the spectral resolution and the angular resolution. Absolute determination of the effective area has been obtained from simulations and measurements. At 1 MeV, the effective area is 65 cm^2 for a point source on the optical axis, the spectral resolution ~2.3 keV. The angular resolution is better than 2.5 deg and the source separation capability about 1 deg. Some temperature dependant parameters will require permanent in-flight calibration., Comment: 9 pages, 12 figures, 2 tables. Accepted for publication in A&AL (INTEGRAL Special issue)

On Oct. 17, 2002, the ESA INTEGRAL observatory was launched into a highly elliptical orbit. SPI, a high resolution Ge spectrometer covering an energy range of 20-8000 keV, is one of its two main instruments. We use data recorded early in the mission (i.e. in March 2003) to characterize the instrumental background, in particular the many gamma-ray lines produced by cosmic-ray interactions in the instrument and spacecraft materials. More than 300 lines and spectral features are observed, for about 220 of which we provide identifications. An electronic version of this list, which will be updated continuously, is available for download at CESR. We also report first results from our efforts to model these lines by ab initio Monte Carlo simulation.

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., 10 pages, 2 figures, uuencoded gzipped tarred file. Uses AAS macros v. 4. Revised shortened paper, accepted for publication in Astrophysical Journal Letters

Electrons accelerated to relativistic energies in pulsar magnetospheres will Compton scatter surface thermal emission and nonthermal optical, UV, and soft X-ray emission to gamma-ray energies, thereby initiating a pair cascade through synchrotron radiation and magnetic pair production. This process is proposed as the origin of the high-energy radiation that has been detected from six isolated pulsars. We construct an analytic model of magnetic Compton scattering near the polar cap of isolated pulsar magnetospheres and present approximate analytic derivations for scattered spectra, electron energy-loss rates, and photon luminosities. A Monte Carlo simulation is used to model the pair cascade induced by relativistic electrons scattering photons through the cyclotron resonance. For simplicity, the primary electrons are assumed to be monoenergetic and the nonresonant emission is omitted. Assuming that the angle phi(sub B) between the magnetic and spin axes is approximately equal to the polar-cap angle theta(sub pc), this model can produce both double-peaked and broad single-peaked pulse profiles and account for the trend of harder gamma-ray spectra observed from older pulsars.

We present results of a model for the energetics of electrons accelerated by the large electric fields generated by a rotating highly magnetized neutron star. The energy loss mechanisms we consider in our calculations include magnetic Compton scattering of thermal x-ray photons, triplet pair production, and curvature radiation emission. The electron acceleration mechanism is assumed to operate only to a height above the polar cap approximately equal to the polar cap radius. We find several interesting results. First, magnetic Compton scattering is the dominant energy loss process when the electron Lorentz factors are < few x10^6 for typical gamma-ray pulsar magnetic fields and surface temperatures measured by ROSAT. The amount of energy converted to photons by accelerated electrons ranges from ~10% to ~100% of gamma_0 mc^2 where gamma_0 is the maximum Lorentz factor an electron can attain with no radiative losses. We also find that if B > 10^13 G and T > 3x10^6 K, the Lorentz factors of the electrons can be limited to values < 10^3 assuming values for the size of the neutron star thermal polar cap comparable to the polar cap size determined by the open field lines. Such small Lorentz factors may be capable of explaining the gamma-ray emission from PSR 1509-58 which is observed only at energies, Postscript, 17 pages, 9 figures accepted for publication in The Astrophysical journal

We model spectra and pulse profiles of gamma-ray pulsars assuming that the pulsar's magnetic axis is nearly aligned with its rotation axis. In this model, the nonthermal energy of electrons flowing outward along field lines connected to the light cylinder is efficiently converted to gamma rays via magnetic Compton scattering of optical and soft X-ray photons. The hard photons initiate a pair cascade in the pulsar magnetosphere through magnetic pair production followed by synchrotron emission. The calculated spectra are used to fit gamma-ray pulsar observations. This model produces a hollow cone of emission which can reproduce both the broad single-peaked and narrow double-peaked pulse profiles observed from gamma-ray pulsars.

We examine a nearly aligned pulsar model with polar cap acceleration in order to explain the energetics and number of the known gamma-ray pulsars. In this model, the efficiency of converting spin-down luminosity to gamma-ray luminosity increases with decreasing spin-down luminosity, a trend recently emphasized by Ulmer. The predicted gamma-ray flux is proportional to dot P(exp 3/4)/P(exp 5/4) d(exp 2), where P is the period, dot P is the period derivative, and d is the distance to the pulsar. For initial spin periods between approximately equals 10 and 30 ms and neutron star polar magnetic fields between approximately equals 1 and 4 TG, this model accounts for the number and age distribution of the five pulsars which have been observed to emit gamma rays at energies greater than 100 MeV. Implications for pulsar studies are considered.

We show that scattering atmospheres exist in the magnetospheres of luminous compact objects from considerations of resonance effects in magnetic Compton scattering. These regions of stable equilibria are formed by a balance between the forces of gravity and resonant Compton scattering, and could be important in spectral models of white dwarfs and neutron stars