Category: Uncategorized

Despite more than half a century of Collisionless shock (CS) research, our understanding of the processes of the shock energy dissipation into the charge particle heating and acceleration remains incomplete. To help to address the problem of the rate of the data analysis on CSs being well below of the rate of the data acquisition, an open-source high-level database of shocks and a centralized source of advanced tools for the purpose of analyzing shock structure and dynamics have been developed. The database is called SHARP shock database by the name of the project SHARP (Shocks: structure, AcceleRation, dissiPation) funded by the European Union’s Horizon 2020 program. The SHARP shock database contains shock crossings and corresponding parameters obtained from Cluster and MMS (Magnetospheric Multiscale) missions for terrestrial bow shocks, THEMIS (Time History of Events and Macroscale Interactions during Substorms)/ARTEMIS (Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon’s Interaction with the Sun) missions for interplanetary shocks, and MAVEN (Mars Atmosphere and Volatile EvolutioN) and VEX (Venus Express) missions for shocks at non-magnetized planets. The SHARP shock database can be accessed via https://sharp.fmi.fi/shock-database/.

Full Article:
Ganushkina, N. Y., van de Kamp, M., Hoppe, T., Dubyagin, S., Gedalin, M., Dimmock, A., et al. (2024). SHARP Shock Database. Journal of Geophysical Research: Space Physics, 129, doi: 10.1029/2024JA032625

The European Commission has published a short article about the SHARP project results on the CORDIS web page. Read about the collisionless shock puzzle here.

The 11th SHARP Progress meeting took place on 9th November, 2023.

Synchrotron radiation from relativistic electrons is usually invoked as responsible for the nonthermal emission observed in supernova remnants. Diffusive shock acceleration is the most popular mechanism to explain the process of particles acceleration and within its framework a crucial role is played by the turbulent magnetic field. However, the standard models commonly used to fit X-ray synchrotron emission do not take into account the effects of turbulence in the shape of the resulting photon spectra. An alternative mechanism that properly includes such effects is the jitter radiation, which provides for an additional power law beyond the classical synchrotron cutoff. We fitted a jitter spectral model to Chandra, NuSTAR, SWIFT/BAT, and INTEGRAL/ISGRI spectra of Cassiopeia A (Cas A) and found that it describes the X-ray soft-to-hard range better than any of the standard cutoff models. The jitter radiation allows us to measure the index of the magnetic turbulence spectrum ν_B and the minimum scale of the turbulence across several regions of Cas A, with best-fit values ν_B ∼ 2 − 2.4 and km.

Full Article:
Greco, E. (SHARP), Vink, J. (SHARP), Ellien, A. (SHARP) and Ferrigno, C. (2023). Jitter Radiation as an Alternative Mechanism for the Nonthermal X-Ray Emission of Cassiopeia A. The Astrophysical Journal, 956, doi: 10.3847/1538-4357/acf567

License: CC BY 4.0

The 10th SHARP Progress meeting took place on 18th September, 2023.

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.

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

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.

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.

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

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.

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

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.

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