Eveline A. Helder

Measuring the Cosmic-Ray Acceleration Efficiency of a Supernova Remnant

Cosmic Shock Waves Cosmic rays are high-energy charged particles that bombard Earth from all directions in the sky; those originating from within our Galaxy are thought to be accelerated in the shockwaves produced by supernova explosions. Helder et al. (p. 719, published online 25 June; see the Perspective by Raymond) measured the velocity of a section of the blast wave created by supernova RCW 86, an exploding star believed to have been witnessed by Chinese astronomers in 185 A.D., and the post-shock proton temperature. The post-shock proton temperature was much lower than would be expected without any cosmic ray acceleration, which implies that the pressure induced by cosmic ray exceeds the thermal pressure behind the shock. The pressure induced by cosmic rays produced by the explosion of a star exceeds the thermal pressure behind the shock wave. Cosmic rays are the most energetic particles arriving at Earth. Although most of them are thought to be accelerated by supernova remnants, the details of the acceleration process and its efficiency are not well determined. Here we show that the pressure induced by cosmic rays exceeds the thermal pressure behind the northeast shock of the supernova remnant RCW 86, where the x-ray emission is dominated by synchrotron radiation from ultrarelativistic electrons. We determined the cosmic-ray content from the thermal Doppler broadening measured with optical spectroscopy, combined with a proper-motion study in x-rays. The measured postshock proton temperature, in combination with the shock velocity, does not agree with standard shock heating, implying that >50% of the postshock pressure is produced by cosmic rays.

Characterizing the nonthermal emission of Cassiopeia A

We report on our analysis of the 1 Ms Chandra observation of the supernova remnant Cas A in order to localize, characterize, and quantify the nonthermal X-ray emission. More specifically, we investigated whether the X-ray synchrotron emission from the inside of the remnant is from the outward shock, but projected toward the inner ring, or from the inner shell. We tackled this problem by employing a Lucy-Richardson deconvolution technique and measuring spectral indices in the 4.2-6 keV band. We show that most of the continuum emission is coming from an inner ring that coincides with the previously reported location of the reverse shock. This inner ring also includes filaments whose X-ray emission has been found to be dominated by X-ray synchrotron emission. The X-ray emission from these filaments, both at the forward shock and from the inner ring, have relatively hard spectra with spectral index >–3.1. The regions emitting hard X-ray continuum contribute about 54% of the total X-ray emission in the 4.2-6 keV. This is lower than that suggested by extrapolating the hard X-ray spectrum as measured by BeppoSAX PDS and INTEGRAL. This can be reconciled by assuming a gradual steepening of the spectrum toward higher energies. We argue that the X-ray synchrotron emission is mainly coming from the western part of the reverse shock. The reverse shock in the west is almost at rest in our observation frame, corresponding to a relatively high reverse shock velocity of ~6000 km s−1 in the frame of the freely expanding ejecta.

THE RELATION BETWEEN POST-SHOCK TEMPERATURE, COSMIC-RAY PRESSURE, AND COSMIC-RAY ESCAPE FOR NON-RELATIVISTIC SHOCKS

Supernova remnants (SNRs) are thought to be the dominant source of Galactic cosmic rays. This requires that at least 5% of the available energy is transferred to cosmic rays, implying a high cosmic-ray pressure downstream of SNR shocks. Recently, it has been shown that the downstream temperature in some remnants is low compared to the measured shock velocities, implying that additional pressure supported by accelerated particles is present. Here we use a two-fluid thermodynamic approach to derive the relation between post-shock fractional cosmic-ray pressure and post-shock temperature, assuming no additional heating beyond adiabatic heating in the shock precursor and with all non-adiabatic heating occurring at the subshock. The derived relations show that a high fractional cosmic-ray pressure is only possible if a substantial fraction of the incoming energy flux escapes from the system. Recently, a shock velocity and a downstream proton temperature were measured for a shock in the SNR RCW 86. We apply the two-fluid solutions to these measurements and find that the downstream fractional cosmic-ray pressure is at least 50% with a cosmic-ray energy flux escape of at least 20%. In general, in order to have 5% of the supernova energy to go into accelerating cosmic rays, on average the post-shock cosmic-ray pressure needs to be 30% for an effective cosmic-ray adiabatic index of γcr = 4/3.

COSMIC-RAY ACCELERATION EFFICIENCY VERSUS TEMPERATURE EQUILIBRATION: THE CASE OF SNR 0509-67.5

We study the 0509-67.5 supernova remnant in the Large Magellanic Cloud with the VLT/FORS2 spectrograph. We detect a broad component in the Hα emission with an FWHM of 2680 ± 70 km s −1 and 3900 ± 800 km s −1 for the southwest (SW) and northeast (NE) shocks, respectively. For the SW, the proton temperature appears to be too low for the shock velocity, which we attribute to a cosmic-ray pressure behind the shock front of at least 20% of the total pressure. For the NE, the post-shock proton temperature and the shock velocity are compatible, only if the plasma behind the shock front has a degree of thermal equilibrium of over 20%, which is at odds with current models for temperature equilibration behind fast shocks, which do not accelerate cosmic rays. If we assume the electron temperature to be less than 10% of the proton temperature, we find a post-shock cosmic-ray pressure of at least 7%.

The kinematics and chemical stratification of the type Ia supernova remnant 0519-69.0 - An XMM-Newton and Chandra study

We present a detailed analysis of the XMM-Newton and Chandra X-ray data of the young type Ia supernova remnant SNR 0519-69.0, which is situated in the Large Magellanic Cloud. We used data from both the Chandra ACIS and XMM-Newton EPIC MOS instruments, and high resolution X-ray spectra obtained with the XMM-Newton reflection grating spectrometer (RGS).
Our analysis of the spatial distribution of X-ray line emission using the Chandra data shows that there is a radial stratification of oxygen, intermediate mass elements (IME) and iron, with the emission from more massive elements peaking more toward the center. Using a deprojection technique we measure a forward shock radius of 4.0±0.3 pc and a reverse shock radius of 2.7±0.4 pc.
We took the observed stratification of the shocked ejecta into account in the modeling of the X-ray spectra, for which we used multi-component non-equilibrium ionization models, with the components corresponding to layers dominated by one or two elements. An additional component was added in order to represent the shocked interstellar medium, which mostly contributed to the continuum emission. This multicomponent model fits the data adequately, and was also employed to characterize the spectra of distinct regions extracted from the Chandra data. From our spectral analysis we find that the approximate fractional masses of shocked ejecta for the most abundant elements are: ≈ 32%, ≈ 7%/5%, ≈ 1% and ≈ 55%. From the continuum component we derive a circumstellar density of nH = 2.4 ± 0.2 cm-3. This density, together with the measurements of the forward and reverse shock radii suggest an age of 0519-69.0 of 450±200 yr, somewhat lower than, but consistent with the age estimate based on the extent of the light echo (600±200 yr).
Finally, from the high resolution RGS spectra we measured a Doppler broadening of σ = 1873 ± 50 km s-1, from which we derive a forward shock velocity of vFS = 2770 ± 500 km s-1. We discuss our results in the context of single degenerate explosion models, using semi-analytical and numerical modeling, and compare the characteristics of 0519-69.0 with those of other type Ia supernova remnants.

EFFECTS OF NEUTRAL HYDROGEN ON COSMIC-RAY PRECURSORS IN SUPERNOVA REMNANT SHOCK WAVES

Many fast supernova remnant shocks show spectra dominated by Balmer lines. The Hα profiles have a narrow component explained by direct excitations and a thermally Doppler broadened component due to atoms that undergo charge exchange in the post-shock region. However, the standard model does not take into account the cosmic-ray shock precursor, which compresses and accelerates plasma ahead of the shock. In strong precursors with sufficiently high densities, the processes of charge exchange, excitation, and ionization will affect the widths of both narrow and broad line components. Moreover, the difference in velocity between the neutrals and the precursor plasma gives rise to frictional heating due to charge exchange and ionization in the precursor. In extreme cases, all neutrals can be ionized by the precursor. In this Letter we compute the ion and electron heating for a wide range of shock parameters, along with the velocity distribution of the neutrals that reach the shock. Our calculations predict very large narrow component widths for some shocks with efficient acceleration, along with changes in the broad-to-narrow intensity ratio used as a diagnostic for the electron-ion temperature ratio. Balmer lines may therefore provide a unique diagnostic of precursor properties. We show that heating by neutrals in the precursor can account for the observed Hα narrow component widths and that the acceleration efficiency is modest in most Balmer line shocks observed thus far.

Suzaku X-Ray Imaging and Spectroscopy of Cassiopeia A

Suzaku X-ray observations of a young supernova remnant, Cassiopeia A, were carried out. K-shell transition lines from highly ionized ions of various elements were detected, including Chromium (Cr-K˛ at 5.61 keV). The X-ray continuum spectra were modeled in the 3.4–40 keV band, summed over the entire remnant, and were fitted with a simplest combination of the thermal bremsstrahlung and the non-thermal cut-off power-law models. The spectral fits with this assumption indicate that the continuum emission is likely to be dominated by non-thermal emission with a cut-off energy at >1 keV. The thermal-to-nonthermal fraction of the continuum flux in the 4–10 keV band is best estimated as 0.1. Non-thermal-dominated continuum images in the 4–14 keV band were made. The peak of the non-thermal X-rays appears at the western part. The peak position of the TeV -rays measured with HEGRA and MAGIC is also shifted at the western part with the 1-sigma confidence. Since the location of the X-ray continuum emission was known to be presumably identified with the reverse shock region, the possible keV–TeV correlations give a hint that the accelerated multi-TeV hadrons in Cassiopeia A are dominated by heavy elements in the reverse shock region.

TEMPERATURE EQUILIBRATION BEHIND THE SHOCK FRONT: AN OPTICAL AND X-RAY STUDY OF RCW 86

We study the electron-proton temperature equilibration behind several shocks of the RCW 86 supernova remnant. To measure the proton temperature, we use published and new optical spectra, all from different locations on the remnant. For each location, we determine the electron temperature from X-ray spectra, and correct for temperature equilibration between the shock front and the location of the X-ray spectrum. We confirm the result of previous studies that the electron and proton temperatures behind shock fronts are consistent with equilibration for slow shocks and deviate for faster shocks. However, we can not confirm the previously reported trend of the electron temperature to proton temperature ratio of 1/v^2.

Cosmic-ray acceleration efficiency of the supernova remnants RCW 86 and SNR 0509-67.5

We study the effect of cosmic-ray acceleration on the plasma temperature behind shock fronts, in order to characterize the cosmic-ray acceleration efficiencies. We combine post-shock plasma temperatures with shock velocities and show that this leads to a direct measurement of the cosmicray pressure behind the shock front. Additionally, we tie this pressure directly to the fraction of energy taken away from the shock by escaping particles. We show that at least some young supernova remnants transfer a significant amount of their energy into cosmic rays and these cosmic rays take away a substantial amount of the supernova energy by escaping the remnant.