New Publication: “Electron heating in shocks: Statistics and comparison” by Michael Gedalin et al.

Supernova remnant (SNR) shocks are the highest Mach number non-relativistic shocks in electron-ion plasmas. These shocks are the most efficient particle accelerators in space. SNR shock parameters are inferred from measurements of electromagnetic radiation from heated and accelerated particles. Temperature of the shock heated electrons is one of the most important parameters in supernova remnant shocks. Knowledge of the downstream electron-to-ion temperature ratio or of the ratio of the downstream electron temperature to the incident ion energy is crucial for understanding physics of the very high-Mach number SNR shocks. Heliospheric shocks have substantially lower Mach numbers than SNR shocks but can be extensively studied in in situ observations with further extrapolation of the findings to higher Mach numbers. Magnetospheric Multiscale mission observations of the Earth bow shock are used to analyze dependence of the electron heating on the shock Mach number. It is found that the ratio of the downstream electron temperature to the incident ion energy decreases with the increase of the Mach number. At high Mach numbers this ratio and stabilizes at about 2.5%. The electron-to-ion temperature ratio stabilizes at about 10%. The peak electron temperature occurs at the overshoot maximum, further downstream electrons cool down. The mean ratio of the 4.5 s averages of the downstream and maximum electron temperatures is 0.85. Electron heating does not follow the thermodynamic adiabatic law. The heating and cooling behavior implies that the energy is provided by the overall cross-shock potential while small-scale electric fields rapidly isotropize the electron distribution.

Examples of two shock crossings included in the selection for the analysis. The shock crossing is zero time. The magnetic field magnitude (black line) is normalized on the maximum magnetic field inside the window of ±1,200 s around the crossing. The electron temperature is normalized on the maximum temperature in the same window

Full Article:
Gedalin, M. (SHARP), Golan, M., Vink, J. (SHARP), Ganushkina, N. (SHARP), & Balikhin, M. (2023). Electron heating in shocks: Statistics and comparison. Journal of Geophysical Research: Space Physics, 128, doi: 10.1029/2023JA0316

License: CC BY-NC-ND 4.0

SHARP Summer School on Collisionless Shocks in Space, August 21-25, 2023

In August, the SHARP consortium organised a summer school on collisionless shocks in space in Levi, Finland. In total 23 students from more than 10 different countries attended the school. The lectures covered all research areas of the SHARP project, including basic theory on collisionless shocks, heliospheric shocks and astrophysical shocks. The students also attended exercise sessions where they learned about data analysis of shocks and the computation of basic shock parameters.

The program of the summer school can be found here.

SHARP summer school participants

New Publication: “Non-locality of ion reflection at the shock front: Dependence on the shock angle” by Michael Gedalin

In typical heliospheric collisionless shocks most of the mass, momentum and energy are carried by ions. Therefore, the shock structure should be most affected by ions. With the increase of the Mach number, ion reflection becomes more and more important, and reflected ions participate in shaping the shock profile. Ion reflection at the collisionless shock is a non-local process: the reflected–transmitted ions re-enter the shock front far from the reflection point. The direction and the magnitude of this shift depend on the shock angle. The distance between the reflection point and the re-entry point is of the order of the upstream ion convective gyroradius and exceeds the shock width. The non-locality of ion reflection may have implications for shock rippling since reflected ions may carry perturbations along the shock front.

Two-dimensional cuts of the ion distribution at the red line position for θBn=75∘. (a) The reduced two-dimensional distribution function f(vx,vy,x=0). (b) The reduced two-dimensional distribution function f(vx,vz,x=0)

Full Article:
Gedalin, M. (SHARP) (2023). Non-locality of ion reflection at the shock front: Dependence on the shock angle. Journal of Plasma Physics, 89(4), doi: 10.1017/S0022377823000831

License: CC BY 4.0

New Publication: “Effect of the reflected ions on the magnetic overshoot of a collisionless shock” by Michael Gedalin and Prachi Sharma

A collisionless shock transfer of mass, momentum, and energy occurs from upstream to downstream. Most of the momentum and energy fluxes are carried by ions so the shock structure is affected mainly by ions. With the increase in the Mach number, the fraction of reflected ions increases and their influence on the shock structure becomes progressively more important. Here, we study the effect of the reflected ions on the overshoot strength. It is shown that directly transmitted ions are responsible for the overshoot formation and the interaction of the overshoot field with these ions alone might result in an unstable growth of the overshoot. On the contrary, reflected ions, at their second crossing of the shock, are accelerated along the shock normal and, thus, provide a stabilizing effect on the overshoot.

The reduced distribution function f(x,vx) (normalized on the maximum value, linear scale), for the shock crossing measured by MMS1.

Full Article:
Gedalin, M. (SHARP), Sharma, P. (SHARP) (2023). Effect of the reflected ions on the magnetic overshoot of a collisionless shock. Physics of Plasmas, 30 (7), doi: 10.1063/5.0154840

License: CC BY 4.0

New Publication: “Evidence for Thermal X-Ray Emission from the Synchrotron-dominated Shocks in Tycho’s Supernova Remnant” by Amaël Ellien et al.

Young supernova remnant (SNR) shocks are believed to be the main sites of galactic cosmic-ray production, showing X-ray synchrotron-dominated spectra in the vicinity of their shock. While a faint thermal signature left by the shocked interstellar medium (ISM) should also be found in the spectra, proofs for such an emission in Tycho’s SNR have been lacking. We perform an extended statistical analysis of the X-ray spectra of five regions behind the blast wave of Tycho’s SNR using Chandra archival data. We use Bayesian inference to perform extended parameter space exploration and sample the posterior distributions of a variety of models of interest. According to Bayes factors, spectra of all five regions of analysis are best described by composite three-component models taking nonthermal emission, ejecta emission, and shocked ISM emission into account. The shocked ISM stands out the most in the northern limb of the SNR. We performed an extended analysis of the northern limb and show that the measured synchrotron cutoff energy is not well constrained in the presence of a shocked ISM component. Such results cannot currently be further investigated by analyzing emission lines in the 0.5–1 keV range, because of the low Chandra spectral resolution in this band. We show with simulated spectra that Athena X-ray Integral Field Unit future performances will be crucial to address this point.

Left: broadband Chandra image of Tycho’s SNR. Right: schematic view of Tycho’s SNR. The five black boxes are the regions over which our shock spectra were extracted and analyzed. The contours are drawn from the contrast image computed from the normalized 1.7–1.95 keV and 4.0–6.0 keV images. Note that the contrast image has been smoothed with a 1σ Gaussian kernel before drawing the contours.

Full Article:
Ellien, A. (SHARP), Greco, E. (SHARP) and Vink, J. (SHARP) (2023). Evidence for Thermal X-Ray Emission from the Synchrotron-dominated Shocks in Tycho’s Supernova Remnant. The Astrophysical Journal, 951, doi: 10.3847/1538-4357/accc85

License: CC BY 4.0

New Publication: “Scattering of Ions at a Rippled Shock” by Michael Gedalin et al.

In a collisionless shock the energy of the directed flow is converted to heating and acceleration of charged particles, and to magnetic compression. In low-Mach number shocks the downstream ion distribution is made of directly transmitted ions. In higher-Mach number shocks ion reflection is important. With the increase of the Mach number, rippling develops, which is expected to affect ion dynamics. Using ion tracing in a model shock front, downstream distributions of ions are analyzed and compared for a planar stationary shock with an overshoot and a similar shock with ripples propagating along the shock front. It is shown that rippling results in the distributions, which are substantially broader and more diffuse in the phase space. Gyrotropization is sped up. Rippling is able to generate backstreaming ions, which are absent in the planar stationary case.

The two-dimensional surface of the magnetic field magnitude for the rippled shock. Y is in the direction or rippling propagation. The global shock normal is along x. The local shock normal is determined by the steepest gradient of the magnetic field magnitude, depends on Y, and differs from the global normal. The maximum overshoot magnetic field also depends on Y.

Full Article:
Gedalin, M. (SHARP), Pogorelov, N. V. and Roytershteyn, V. (2023). Scattering of Ions at a Rippled Shock. The Astrophysical Journal, 951, doi: 10.3847/1538-4357/acd63c

License: CC BY 4.0

New Publication: “Investigating the Time Evolution of the Thermal Emission from the Putative Neutron Star in SN 1987A for 50+ Years” by Akira Dohi, Emanuele Greco et al.

Observations collected with the Atacama Large Millimeter/submillimeter Array (ALMA) and analysis of broadband X-ray spectra have recently suggested the presence of a central compact object (CCO) in SN 1987A. However, no direct evidence of the CCO has been found yet. Here we analyze Chandra X-ray observations of SN 1987A collected in 2007 and 2018, and synthesize 2027 Chandra and 2037 Lynx spectra of the faint inner region of SN 1987A. We estimate the temporal evolution of the upper limits of the intrinsic luminosity of the putative CCO in three epochs (2018, 2027, and 2037). We find that these upper limits are higher for higher neutron star (NS) kick velocities due to increased absorption from the surrounding cold ejecta. We compare NS cooling models with both the intrinsic luminosity limits obtained from the X-ray spectra and the ALMA constraints with the assumption that the observed blob of SN 1987A is primarily heated by thermal emission. We find that the synthetic Lynx spectra are crucial to constrain the physical properties of the CCO, which will be confirmed by future observations in the 2040s. We draw our conclusions based on two scenarios, namely the nondetection and detection of the NS by Lynx. If the NS is not detected, its kick velocity should be ≃700 km s−1. Furthermore, nondetection of the NS would suggest rapid cooling processes at the age of 40 yr, implying strong crust superfluidity. Conversely, in the case of NS detection, the mass of the NS envelope must be high.

Broad (0.5–7 keV) Chandra/ACIS-S exposure-corrected count-rate maps of SN 1987A in 2007 and 2018, using subpixel sampling. Upper panels: source and background regions are shown in white and dashed red, respectively. Lower panels: same as the upper panels but the count-rate maps are deconvolved by the Chandra PSF through the Lucy algorithm.

Full Article:
Dohi, A., Greco, E. (SHARP), Nagataki, S., Ono, M., Miceli, M., Orlando,S. and Olmi, B. (2023). Investigating the Time Evolution of the Thermal Emission from the Putative Neutron Star in SN 1987A for 50+ Years. The Astrophysical Journal, 949, doi: 10.3847/1538-4357/acce3

License: CC BY 4.0

New Publication: “Role of the overshoot in the shock self-organization” by Michael Gedalin et al.

A collisionless shock is a self-organized structure where fields and particle distributions are mutually adjusted to ensure a stable mass, momentum and energy transfer from the upstream to the downstream region. This adjustment may involve rippling, reformation or whatever else is needed to maintain the shock. The fields inside the shock front are produced due to the motion of charged particles, which is in turn governed by the fields. The overshoot arises due to the deceleration of the ion flow by the increasing magnetic field, so that the drop of the dynamic pressure should be compensated by the increase of the magnetic pressure. The role of the overshoot is to regulate ion reflection, thus properly adjusting the downstream ion temperature and kinetic pressure and also speeding up the collisionless relaxation and reducing the anisotropy of the eventually gyrotropized distributions.

The magnetic field magnitude, normalized to the upstream magnetic field magnitude (black curve) and the reduced ion distribution function.

Full Article:
Gedalin, M. (SHARP), Dimmock, A. (SHARP), Russell, C. (SHARP), Pogorelov, N., & Roytershteyn, V. (2023). Role of the overshoot in the shock self-organization. Journal of Plasma Physics, 89(2), doi: 10.1017/S0022377823000090

License: CC BY 4.0

New Publication: “X-Ray Polarimetry Reveals the Magnetic-field Topology on Sub-parsec Scales in Tycho’s Supernova Remnant” by Riccardo Ferrazzoli et al.

Supernova remnants are commonly considered to produce most of the Galactic cosmic rays via diffusive shock acceleration. However, many questions regarding the physical conditions at shock fronts, such as the magnetic-field morphology close to the particle acceleration sites, remain open. Here we report the detection of a localized polarization signal from some synchrotron X-ray emitting regions of Tycho’s supernova remnant made by the Imaging X-ray Polarimetry Explorer. The derived degree of polarization of the X-ray synchrotron emission is 9% ± 2% averaged over the whole remnant, and 12% ± 2% at the rim, higher than the value of polarization of 7%–8% observed in the radio band. In the west region, the degree of polarization is 23% ± 4%. The degree of X-ray polarization in Tycho is higher than for Cassiopeia A, suggesting a more ordered magnetic field or a larger maximum turbulence scale. The measured tangential direction of polarization corresponds to the radial magnetic field, and is consistent with that observed in the radio band. These results are compatible with the expectation of turbulence produced by an anisotropic cascade of a radial magnetic field near the shock, where we derive a magnetic-field amplification factor of 3.4 ± 0.3. The fact that this value is significantly smaller than those expected from acceleration models is indicative of highly anisotropic magnetic-field turbulence, or that the emitting electrons either favor regions of lower turbulence, or accumulate close to where the orientation of the magnetic field is preferentially radially oriented due to hydrodynamical instabilities.

Polarization map in the 3–6 keV energy band with a 60” pixel size. Only the pixels with significance higher than 1σ are shown. The blue bars represent the direction of the polarization (that is, the direction of the electric vector polarization angle) and their length is proportional to the degree of polarization. The thicker cyan bars mark the pixels with significance higher than 2σ. The orientation of the magnetic field is perpendicular to the direction of the polarization. Superimposed in green are the 4–6 keV Chandra contours.

Full Article:
Ferrazzoli, R., Slane, P., Prokhorov, D. (SHARP), Zhou, P., Vink, J. (SHARP), et al. (2023). X-Ray Polarimetry Reveals the Magnetic-field Topology on Sub-parsec Scales in Tycho’s Supernova Remnant. The Astrophysical Journal, 945, doi: 10.3847/1538-4357/acb496

License: CC BY 4.0