Glenn E. Allen

Cosmic-ray diffusion near the Bohm limit in the Cassiopeia A supernova remnant

The acceleration of cosmic rays in our Galaxy by means of diffusive shock (Fermi) acceleration is believed to occur primarily in supernova remnants (SNRs). Despite considerable theoretical work, the precise details are still unknown, in part because of the difficulty in directly observing nucleons that are accelerated to TeV energies in—and affect the structure of—SNR shocks. However, for the past ten years, X-ray observatories such as ASCA (Advanced Satellite for Cosmology and Astrophysics) and, more recently, Chandra, XMM-Newton and Suzaku, have made it possible to image the keV-scale synchrotron emission produced by cosmic-ray electrons accelerated in SNR shocks. Here, we describe a spatially resolved spectroscopic analysis of Chandra observations of the Galactic SNR Cassiopeia A to map the cutoff frequencies of electrons accelerated in the forward shock. We set upper limits on the diffusion coefficient and find locations where particles seem to be accelerated nearly as fast as theoretically possible (the Bohm limit).

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.

NON-THERMAL X-RAY EMISSION FROM THE NORTHWESTERN RIM OF THE GALACTIC SUPERNOVA REMNANT G266.2–1.2 (RX J0852.0-4622)

We present a detailed spatially resolved spectroscopic analysis of two observations (with a total integration time of 73280 s) made of the X-ray-luminous northwestern rim complex of the Galactic supernova remnant (SNR) G266.2–1.2 (RX J0852.0-4622) with the Chandra X-ray Observatory. G266.2–1.2 is a member of a class of Galactic SNRs which feature X-ray spectra dominated by non-thermal emission: in the cases of these SNRs, the emission is believed to have a synchrotron origin and studies of the X-ray spectra of these SNRs can lend insight into how SNRs accelerate cosmic-ray particles. The Chandra observations have clearly revealed fine structure in this rim complex (including a remarkably well-defined leading shock) and the spectra of these features are dominated by non-thermal emission. We have measured the length scales of the upstream structures at eight positions along the rim and derive lengths of 0.02-0.08 pc (assuming a distance of 750 pc to G266.2–1.2). We have also extracted spectra from seven regions in the rim complex (as sampled by the ACIS-S2, -S3, and -S4 chips) and fit these spectra with such models as a simple power law as well as the synchrotron models SRCUT and SRESC. We have constrained our fits to the latter two models using estimates for the flux densities of these filaments at 1 GHz as determined from radio observations of this rim complex made with the Australia Telescope Compact Array. Statistically acceptable fits to all seven regions are derived using each model: differences in the fit parameters (such as photon index and cutoff frequency) are seen in different regions, which may indicate variations in shock conditions and the maximum energies of the cosmic-ray electrons accelerated at each region. Finally, we estimate the maximum energy of cosmic-ray electrons accelerated along this rim complex to be approximately 40 TeV (corresponding to one of the regions of the leading shock structure assuming a magnetic field strength of 10 μG). We include a summary of estimated maximum energies for both Galactic SNRs as well as SNRs located in the Large Magellanic Cloud. Like these other SNRs, it does not appear that G266.2–1.2 is currently accelerating electrons to the knee energy (~3000 TeV) of the cosmic-ray spectrum. This result is not surprising, as there is some evidence that loss mechanisms which are not important for the accelerated cosmic-ray nucleons at energies just below the knee might cut off electron acceleration.

Flight spectral response of the ACIS instrument

We discuss the flight calibration of the spectral response of the Advanced CCD Imaging Spectrometer (ACIS) on-board the Chandra X-ray Observatory (CXO). The spectral resolution and sensitivity of the ACIS instrument have both been evolving over the course of the mission. The spectral resolution of the frontside-illuminated (FI) CCDs changed dramatically in the first month of the mission due to radiation damage. Since that time, the spectral resolution of the FI CCDs and the Backside-illuminated (BI) CCDs have evolved gradually with time. We demonstrate the efficacy of charge-transfer inefficiency (CTI) correction algorithms which recover some of the lost performance. The detection efficiency of the ACIS instrument has been declining throughout the mission, presumably due to a layer of contamination building up on the filter and/or CCDs. We present a characterization of the energy dependence of the excess absorption and demonstrate software which models the time dependence of the absorption from energies of 0.4 keV and up. The spectral redistribution function and the detection efficiency are well-characterized at energies from 1.5 to 8.0~keV primarily due to the existence of strong lines in the ACIS calibration source in that energy range. The calibration at energies below 1.5 keV is challenging because of the lack of strong lines in the calibration source and also because of the inherent non-linear dependence with energy of the CTI and the absorption by the contamination layer. We have been using data from celestial sources with relatively simple spectra to determine the quality of the calibration below 1.5 keV. We have used observations of 1E0102.2-7219 (the brightest supernova remnant in the SMC), PKS2155-304 (a bright blazar), and the pulsar PSR~0656+14 (nearby pulsar with a soft spectrum), since the spectra of these objects have been well-characterized by the gratings on the CXO. The analysis of these observations demonstrate that the CTI correction recovers a significant fraction of the spectral resolution of the FI CCDs and the models of the time-dependent absorption result in consistent measurements of the flux at low energies for data from a BI (S3) CCD.

Models for Nonthermal Photon Spectra

We describe models of nonthermal photon emission from a homogeneous distribution of relativistic electrons and protons. Contributions from the synchrotron, inverse Compton, nonthermal bremsstrahlung, and neutral-pion decay processes are computed separately using a common parameterization of the underlying distribution of nonthermal particles. The models are intended for use in fitting spectra from multiwavelength observations and are designed to be accurate and efficient. Although our applications have focused on Galactic supernova remnants, the software is modular, making it straightforward to customize for different applications. In particular, the shapes of the particle distribution functions and the shape of the seed photon spectrum used by the inverse Compton model are defined in separate modules and may be customized for specific applications. We assess the accuracy of these models by using a recurrence relation and by comparing them with analytic results and with previous numerical work by other authors.

Evidence for cosmic-ray acceleration in supernova remnants from X-ray observations

Abstract Spatially-resolved X-ray spectroscopic observations over the past several years have led to the discovery of non-thermal X-ray emission arising in the shells of most young Galactic supernova remnants, most notably SN 1006 and Cas A. In addition, the X-ray emission from the shells of a few newly-discovered supernova remnants is dominated by a non-thermal component. This emission is thought to be synchrotron emission from electrons shock accelerated to hundreds of TeV, and thus represents strong evidence that cosmic rays are accelerated in SNR shocks. The inferences made using the X-ray observations are corroborated by the detection of TeV γ-rays from two of these remnants. We review the status of the X-ray observations and describe how they can be used to provide insight into the shock-acceleration process.

ASCA and RXTE observations of non-thermal X-ray emission from galactic supernova remnants: G156.2+5.7

We are conducting a survey of Galactic shell-type supernova remnants (SNRs) known or suspected to possess non-thermal components to their X-ray emission using new and archived observations made with such X-ray satellites as ROSAT, ASCA, RXTE, Chandra and XMM-Newton. This research is intended to probe the phenomenon of cosmic-ray acceleration by Galactic SNRs and estimate the maximum energy of cosmic-ray electrons accelerated by these sources. To illustrate this work, we examine the X-ray spectrum of the northwestern rim of an SNR suspected to have a non-thermal component to its X-ray emission, G156.2+5.7 (RX J04591+5147), over the energy range ≈0.7–12.0 keV using observations made by the ASCA GIS and the RXTE PCA. We compare fits made to the non-thermal component using two models, a simple power law and SRCUT. Both models give acceptable fits: the photon index derived from the fit made with the power law model (Γ=2.0+0.2−0.5) is comparable to values obtained for the bright rims of other SNRs with hard X-ray spectra. Using the SRCUT model, we derive a value of 2.42+0.24−0.23×1017 Hz for the cutoff frequency νcutoff: based on this value and assuming a mean magnetic field strength of 14 μG, we estimate the cutoff energy Ecutoff of cosmic-ray electrons accelerated by G156.2+5.7 to be ≈32 TeV. This energy value is well short of the “knee” feature of the cosmic-ray spectrum.

Spectral line imaging observations of 1E0102.2-7219

E0102-72 is the second brightest X-ray source in the Small Magellanic Cloud and the brightest supernova remnant in the SMC. We observed this SNR for ∼140 ksec with the High Energy Transmission Gratings (HETG) aboard the Chandra X-ray Observatory. The small angular size and high surface brightness make this an excellent target for HETG and we resolve the remnant into individual lines. We observe fluxes from several lines which include O VIII Lyα, Lyβ, and O VII along with several lines from Ne X, Ne IX and Mg XII. These line ratios provide powerful constraints on the electron temperature and the ionization age of the remnant.

The non-thermal X-ray emission of SN 1006 and the implications for cosmic rays

We present the results of a spectral analysis of RXTE, ASCA, and ROSAT data of SN 1006. These data were fit with several sets of thermal and non-thermal X-ray emission models to characterize the global spectral properties of the remnant. The present work represents the first attempt to model both the thermal and non-thermal X-ray emission over the entire X-ray energy band from 0.12–17 keV. The non-thermal X-ray spectrum is described by a broken power-law with low- and high-energy photon indices of 2.1 and 3.0, respectively. Since this spectrum steepens with increasing energy, our results support the claims that the emission is produced by synchrotron radiation from 100 TeV electrons. Using the radio and X-ray data, we estimate the parameters of the cosmic-ray electron, proton, and helium spectra. The results suggest that the ratio of the number densities of protons and electrons is 150 at 1 GeV and that the total energy in cosmic rays is 1050 erg. These results and the spectral index of the electrons at 1...