Steven John Sturner

Temporal Evolution of Nonthermal Spectra from Supernova Remnants

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.

Magnetic Compton-induced pair cascade model for gamma-ray pulsars

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.

MGGPOD: a Monte Carlo Suite for Modeling Instrumental Line and Continuum Backgrounds in Gamma-Ray Astronomy

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 γ-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 γ-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 (ver. 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 backgrounds. The MGGPOD package and documentation are publicly available online. We demonstrate the capabilities of the MGGPOD suite by modeling high-resolution γ-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-8 MeV range. Because of its fine energy resolution, these spectra reveal the complex instrumental background in formidable detail, particularly the many prompt and delayed γ-ray lines. We evaluate the successes and failures of the MGGPOD package in reproducing TGRS data and provide identifications for the numerous instrumental lines.

INTEGRAL/SPI ground calibration

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.

Monte Carlo simulations and generation of the SPI response

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.

First identification and modelling of SPI background lines

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.

ON THE ENERGETICS AND NUMBER OF 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.

On the spectra and pulse profiles of gamma-ray pulsars

We model spectra and pulse profiles of gamma-ray pulsars assuming that the pulsars 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.

RXTE, ROSAT, and ASCA Observations of G347.3–0.5 (RX J1713.7–3946): Probing Cosmic-Ray Acceleration by a Galactic Shell-Type Supernova Remnant

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). This SNR is a member of a growing class of dynamically young, shell-type SNRs that emit nonthermal X-rays from specific regions on their outer shells. 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 fitted the spectra of particular regions of this SNR (including the bright northwestern and southwestern rims, the northeast rim, and the interior diffuse emission) over the approximate energy range of 0.5-30 keV. We find that fits to the spectra of this SNR over this energy range using the SRCUT model were superior to a simple power-law model or the SRESC model. We also find that the inclusion of a thermal model with the SRCUT model helps to improve the fit to the observed X-ray spectrum: this represents the first detection of thermal X-ray emission from G347.3-0.5. Thermal emission appears to be more clearly associated with the diffuse emission in the interior of the SNR than with the bright X-ray-emitting rims. A weak emission feature seen near 6.4 keV in the RXTE PCA spectrum most likely originates from diffuse X-ray emission from the surrounding Galactic ridge rather than from G347.3-0.5 itself. We have analyzed our RXTE PCA data to search for pulsations from a recently discovered radio pulsar (PSR J1713-3949) which may be associated with G347.3-0.5, and we do not detect any X-ray pulsations at the measured radio period of 392 ms. Using the best-fit parameters obtained from the SRCUT model, we estimate the maximum energy of cosmic-ray electrons accelerated by the rims of G347.3-0.5 to be 36-48 TeV (assuming a magnetic field strength of B = 10 μG). We present a broadband (radio to γ-ray) photon energy-flux spectrum for the northwestern rim of G347.3-0.5, where we have fitted the spectrum using a more sophisticated synchrotron-inverse Compton model with a variable magnetic field strength. Our fit derived from this model yields a maximum energy of only 8.8 TeV for the accelerated cosmic-ray electrons and a much greater magnetic field strength of 150 μG; however, our derived ratio of volumes for TeV emission and X-ray emission based on this fit, VTeV/VX-ray ≈ 1000, is too large to be physically acceptable. We argue that neither nonthermal bremsstrahlung nor neutral pion particle decay can adequately explain the TeV emission from this rim, and therefore the physical process responsible for this emission at this site 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 nonthermal emission.

SPI-specific analysis method and software overview

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.