Cara E. Rakowski

A Physical Relationship Between Electron-Proton Temperature Equilibration and Mach Number in Fast Collisionless Shocks

The analysis of Balmer-dominated optical spectra from nonradiative (adiabatic) SNRs has shown that the ratio of the electron to proton temperature at the blast wave is close to unity at vS 400 km s-1 but declines sharply down to the minimum value of me/mp dictated by the jump conditions at shock speeds exceeding 2000 km s-1. We propose a physical model for the heating of electrons and ions in non-cosmic-ray-dominated, strong shocks (vS > 400 km s-1) wherein the electrons are heated by lower hybrid waves immediately ahead of the shock front. These waves arise naturally from the cosmic ray pressure gradient upstream from the shock. Our model predicts a nearly constant level of electron heating over a wide range of shock speeds, producing a relationship (Te/Tp)0 v (M-2) that is fully consistent with the observations.

Electron Heating and Cosmic Rays at a Supernova Shock from Chandra X-Ray Observations of 1E 0102.2–7219

In this Letter, we use the unprecedented spatial resolution of the Chandra X-Ray Observatory to carry out, for the first time, a measurement of the postshock electron temperature and proper motion of a young supernova remnant, specifically to address questions about the postshock partition of energy among electrons, ions, and cosmic rays. The expansion rate, 0.100% ± 0.025% yr-1, and inferred age, ~1000 yr, of 1E 0102.2-7219, from a comparison of X-ray observations spanning 20 yr, are fully consistent with previous estimates based on studies of high-velocity oxygen-rich optical filaments in the remnant. With a radius of 6.4 pc for the blast wave estimated from the Chandra image, our expansion rate implies a blast-wave velocity of ~6000 km s-1 and a range of electron temperatures 2.5-45 keV, dependent on the degree of collisionless electron heating. Analysis of the Chandra CCD spectrum of the immediate postshock region reveals a thermal plasma with abundances and column density typical of the Small Magellanic Cloud and an electron temperature of 0.4-1 keV. The measured electron temperature is significantly lower than the plausible range above, which can be reconciled only if we assume that a significant fraction of the shock energy, rather than contributing to the heating of the postshock electrons and ions, has gone into generating cosmic rays.

EVIDENCE FOR PARTICLE ACCELERATION TO THE KNEE OF THE COSMIC RAY SPECTRUM IN TYCHO'S SUPERNOVA REMNANT

Supernova remnants (SNRs) have long been assumed to be the source of cosmic rays (CRs) up to the knee of the CR spectrum at 1015 eV, accelerating particles to relativistic energies in their blast waves by the process of diffusive shock acceleration (DSA). Since CR nuclei do not radiate efficiently, their presence must be inferred indirectly. Previous theoretical calculations and X-ray observations show that CR acceleration significantly modifies the structure of the SNR and greatly amplifies the interstellar magnetic field. We present new, deep X-ray observations of the remnant of Tychos supernova (SN 1572, henceforth Tycho), which reveal a previously unknown, strikingly ordered pattern of non-thermal high-emissivity stripes in the projected interior of the remnant, with spacing that corresponds to the gyroradii of 1014-1015 eV protons. Spectroscopy of the stripes shows the plasma to be highly turbulent on the (smaller) scale of the Larmor radii of TeV energy electrons. Models of the shock amplification of magnetic fields produce structure on the scale of the gyroradius of the highest energy CRs present, but they do not predict the highly ordered pattern we observe. We interpret the stripes as evidence for acceleration of particles to near the knee of the CR spectrum in regions of enhanced magnetic turbulence, while the observed highly ordered pattern of these features provides a new challenge to models of DSA.

The Physics of Supernova Remnant Blast Waves. II. Electron-Ion Equilibration in DEM L71 in the Large Magellanic Cloud

We present analysis and modeling of X-ray spectra from the blast wave shock of DEM L71 in the Large Magellanic Cloud. This remnant exhibits widespread Balmer-dominated emission characteristic of nonradiative shocks in partially neutral gas. We have used Chandra ACIS-S data and optical Fabry-Perot spectra of the blast wave to measure the electron and proton temperatures, respectively. In principle, when combined, these measurements can determine the degree of electron-ion temperature equilibration (g0 ≡ Te/Tp) immediately behind the shock front. In our X-ray analysis we fit Chandra spectra of three nested regions behind the blast wave under three different scenarios: (1) a planar, initially unequilibrated shock (g0 = me/mp), where the downstream electron and proton temperatures equilibrate through Coulomb collisions, (2) a planar, immediately equilibrated shock (g0 = 1), and (3) a spherical, equilibrated shock under Sedov evolution. Using independent measurements of Te and Tp, we find that the X-ray spectra from the fastest blast wave locations (Vs ~ 700-1000 km s-1) are consistent with little or no equilibration at the shock front and are inconsistent with full equilibration. In contrast, spectra from regions showing slower blast wave speeds (Vs ~ 400-600 km s-1) allow full equilibration but exclude zero equilibration. In order to directly constrain the initial equilibration, we incorporated our knowledge of the proton temperatures into our X-ray models to build planar shock models that allow for a variable g0. This model confirmed and strengthened the above results. Specifically, we found that X-ray spectra from an intermediate-velocity shock (Vs ~ 800 km s-1) were consistent with intermediate equilibration, excluding both g0 = me/mp and g0 = 1 at greater than 1 σ. Overall, our results support the picture of decreasing electron-ion equilibration with increasing shock speed found from previous studies of optical spectra in other Balmer-dominated supernova remnants.

Iron-rich Ejecta in the Supernova Remnant DEM L71

Chandra X-ray observations of DEM L71, a supernova remnant (SNR) in the Large Magellanic Cloud (LMC), reveal a clear double-shock morphology consisting of an outer blast-wave shock surrounding a central bright region of reverse-shock-heated ejecta. The abundances of the outer shock are consistent with LMC values, while the ejecta region shows enhanced abundances of Si, Fe, and other species. However, oxygen is not enhanced in the ejecta; the Fe/O abundance ratio there is 5 times the solar ratio. Based on the relative positions of the blast-wave shock and the contact discontinuity in the context of SNR evolutionary models, we determine a total ejecta mass of ~1.5 M☉. Ejecta mass estimates based on emission measures derived from spectral fits are subject to considerable uncertainty because of the lack of knowledge of the true contribution of hydrogen continuum emission. Maximal mass estimates, i.e., assuming no hydrogen, result in 1.5 M☉ of Fe and 0.24 M☉ of Si. Under the assumption that an equal quantity of hydrogen has been mixed into the ejecta, we estimate 0.8 M☉ of Fe and 0.12 M☉ of Si. These characteristics support the view that in DEM L71, we see Fe-rich ejecta from a Type Ia supernova several thousand years after explosion.

Ion Charge States in Halo Coronal Mass Ejections: What Can We Learn about the Explosion?

We describe a new modeling approach to develop a more quantitative understanding of the charge state distributions of the ions of various elements detected in situ during halo coronal mass ejection (CME) events by the Advanced Composition Explorer (ACE) satellite. Using a model CME hydrodynamic evolution based on observations of CMEs propagating in the plane of the sky and on theoretical models, we integrate time-dependent equations for the ionization balance of various elements to compare with ACE data. We find that plasma in the CME core typically requires further heating following filament eruption, with thermal energy input similar to the kinetic energy input. This extra heating is presumably the result of posteruptive reconnection. Plasma corresponding to the CME cavity is usually not further ionized, since whether heated or not, the low density gives freeze-in close the Sun. The current analysis is limited by ambiguities in the underlying model CME evolution. Such methods are likely to reach their full potential when applied to data to be acquired by STEREO when at optimum separation. CME evolution observed with one spacecraft may be used to interpret CME charge states detected by the other.

Electron–ion temperature equilibration at collisionless shocks in supernova remnants

Abstract The topic of this review is the current state of our knowledge about the degree of initial equilibration between electrons, protons and ions at supernova remnant (SNR) shocks. Specifically, the question has been raised as to whether there is an inverse relationship between the shock velocity and the equilibration. This review aims to compile every method that has been used to measure the equilibration and every SNR on which they have been tested. I review each method, its problems and uncertainties and how those would effect the degree of equilibration (or velocity) inferred. The final compilation of observed electron to proton temperature ratios as a function of shock velocity gives an accurate, conservative picture of the state of our knowledge and the avenues we need to pursue to make progress in our understanding of the relation between the velocity of a shock and the degree of equilibration.

Can Ejecta-dominated Supernova Remnants be Typed from Their X-Ray Spectra? The Case of G337.2–0.7

In this paper we use recent X-ray and radio observations of the ejecta-rich Galactic SNR G337.2-0.7 to determine properties of the SN explosion that formed this source. H I absorption measurements from ATCA constrain the distance to G337.2-0.7 to lie between 2.0 ? 0.5 and 9.3 ? 0.3 kpc. Combined with a clear radio image of the outer blast wave, this distance allows us to estimate the dynamical age (between 750 and 3500 yr) from the global X-ray spectrum obtained with the XMM-Newton and Chandra observatories. The presence of ejecta is confirmed by the pattern of fitted relative abundances, which show Mg, Ar, and Fe to be less enriched (compared to solar) than Si, S, or Ca, and the ratio of Ca to Si to be 3.4 ? 0.8 times the solar value (under the assumption of a single electron temperature and single ionization timescale). With the addition of a solar abundance component for emission from the blast wave, these abundances (with the exception of Fe) resemble the ejecta of a Type Ia, rather than core-collapse, SN. Comparing directly to models of the ejecta and blast wave X-ray emission calculated by evolving realistic SN Ia explosions to the remnant stage allows us to deduce that one-dimensional delayed-detonation and pulsed delayed-detonation models can indeed reproduce the major features of the global spectrum. In particular, stratification of the ejecta, with the Fe shocked most recently, is required to explain the lack of prominent Fe K emission.

The Physics of Supernova Blast Waves. I. Kinematics of DEM L71 in the Large Magellanic Cloud

We present the results from Fabry-Perot imaging spectroscopy of the Balmer-dominated supernova remnant DEM L71 (0505-67.9) in the LMC. Spectra extracted from the entire circumference of the blast wave reveal the broad- and narrow-component Hα line emission characteristic of nonradiative shocks in partially neutral gas. The new spectra of DEM L71 include portions of the rim that have not been previously observed. We find that the broad-component width varies azimuthally along the edge of DEM L71, ranging from 450 ± 60 km s-1 along the eastern edge to values as high as 985 km s-1 along the faint western edge. The latter width is nearly 60% larger than the value determined by earlier spectroscopy of the brightest Balmer-dominated filaments. In part of the faint northern rim the broad component is not detected, possibly indicating a lower density in these regions and/or a broad-component width in excess of 1000 km s-1. Between the limits of zero and full electron-ion temperature equilibration at the shock front, the allowed range of shock velocities is 430-560 km s-1 along the east rim and 700-1250 km s-1 along other parts of the blast wave. The Hα broad-to-narrow flux ratios vary considerably around the remnant, ranging from 0.4 to 0.8. These ratios lie below the values predicted by our shock models. We find that narrow-component Hα emission from a cosmic-ray precursor may be the cause of the discrepancy. The least decelerated portions of the blast wave (i.e., regions excluding the brightest filaments) are well characterized by Sedov models with a kinetic energy E51 = (0.37 ± 0.06)D, where D50 is the LMC distance in units of 50 kpc. The corresponding age for DEM L71 is (4360 ± 290)D50 yr. This is the first time that velocity information from the entire blast wave has been utilized to study the global kinematics of a nonradiative supernova remnants at a known distance.

On the Remote Detection of Suprathermal Ions in the Solar Corona and their Role as Seeds for Solar Energetic Particle Production

Forecasting large solar energetic particle (SEP) events associated with shocks driven by fast coronal mass ejections (CMEs) poses a major difficulty in the field of space weather. Besides issues associated with CME initiation, the SEP intensities are difficult to predict, spanning three orders of magnitude at any given CME speed. Many lines of indirect evidence point to the pre-existence of suprathermal seed particles for injection into the acceleration process as a key ingredient limiting the SEP intensity of a given event. This paper outlines the observational and theoretical basis for the inference that a suprathermal particle population is present prior to large SEP events, explores various scenarios for generating seed particles and their observational signatures, and explains how such suprathermals could be detected through measuring the wings of the H I Ly{alpha} line.