Supernova Remnant Shock Waves Research Articles

Electron–Ion Temperature Ratio in Astrophysical Shocks

Collisionless shock waves in supernova remnants and the solar wind heat electrons less effectively than they heat ions, as is predicted by kinetic simulations. However, the values of T e /T p inferred from the Hα profiles of supernova remnant shocks behave differently as a function of Mach number or Alfvén Mach number than what is measured in the solar wind or predicted by simulations. Here we determine T e /T p for supernova remnant shocks using Hα profiles, shock speeds from proper motions, and electron temperatures from X-ray spectra. We also improve the estimates of sound speed and Alfvén speed used to determine Mach numbers. We find that the Hα determinations are robust and that the discrepancies among supernova remnant shocks, solar wind shocks, and computer-simulated shocks remain. We discuss some possible contributing factors, including shock precursors, turbulence, and varying preshock conditions.

Modeling CO Line Profiles in Shocks of W28 and IC 443

Molecular emission arising from the interactions of supernova remnant (SNR) shock waves and molecular clouds provide a tool for studying the dispersion and compression that might kick-start star formation as well as understanding cosmic-ray production. Purely rotational CO emission created by magnetohydrodynamic shock in the SNR–molecular cloud interaction is an effective shock tracer, particularly for slow-moving, continuous shocks into cold inner clumps of the molecular cloud. In this work, we present a new theoretical radiative transfer framework for predicting the line profile of CO with the Paris–Durham 1D shock model. We generated line profile predictions for CO emission produced by slow, magnetized C shocks into gas of density ∼104 cm−3 with shock speeds of 35 and 50 km s−1. The numerical framework to reproduce the CO line profile utilizes the large velocity gradient (LVG) approximation and the omission of optically thick plane-parallel slabs. With this framework, we generated predictions for various CO spectroscopic observations up to J = 16 in SNRs W28 and IC 443, obtained with SOFIA, IRAM-30 m, APEX, and KPNO. We found that CO line profile prediction offers constraints on the shock velocity and pre-shock density independent of the absolute line brightness and requires fewer CO lines than diagnostics using a rotational excitation diagram.

Interaction of a supernova remnant with a wind of young massive star: MHD simulations

Fast stellar winds of young massive stars and supernovae play an essential role in the evolution of the interstellar medium in the Milky Way. They dominate the dynamics of the active starforming regions in starburst galaxies and in particularly their non-thermal radiation. The non-thermal emission from starburst galaxies NGC 253, M82, NGC 1068 was detected from radio to gamma-rays. To model the observed radiation, one needs to know the complex structure of the flows and magnetic fields in the starforming regions. In this work a 2.5D magnetohydrodynamic (MHD) simulation (i.e., we consider 3D vector fields, but assume a 2D axisymmetric geometry) of the interaction between a stellar wind and a supernova remnant shock wave is carried out using the MHD module of the code PLUTO. The structure of the flows in the collision region is obtained taking into account the magnetic fields of the rotating source stars. We present the profiles of the bulk plasma velocity, density and magnetic field in the collision region. The amplification of regular magnetic fields in such systems is studied.

Particle acceleration in interstellar shocks

This review presents the fundamentals of the particle acceleration processes active in interstellar medium (ISM), which are essentially based on the so-called Fermi mechanism theory. More specifically, the review presents here in more details the first order Fermi acceleration process -- also known as diffusive shock acceleration (DSA) mechanism. In this case, acceleration is induced by the interstellar (IS) shock waves. These IS shocks are mainly associated with emission nebulae (HII regions, planetary nebulae and supernova remnants). Among all types of emission nebulae, the strongest shocks are associated with supernova remnants (SNRs). Due to this fact they also provide the most efficient manner to accelerate ISM particles to become {high energy particles}, i.e.~cosmic-rays (CRs). The review therefore focuses on the particle acceleration at the strong shock waves of supernova remnants.

Dust Destruction in Nonradiative Shocks

Supernova remnant (SNR) shock waves are the main place where interstellar dust grains are destroyed. However, the dust destruction efficiency in non-radiative shocks is still not well known. One way to estimate the fraction of dust destroyed is to compare the difference between postshock gas abundances and preshock medium total abundances when the preshock elemental depletion factors are known. We compare the postshock gas abundances of 16 SNRs in Large Magellanic Cloud (LMC) with the LMC interstellar medium abundances that we derived based on 69 slow-rotating early B-type stars. We find that, on average, $\sim$61\% of Si rich dust grains are destroyed in the shock while the fraction of dust destroyed is only $\sim$40\% for Fe rich dust grains. This result supports the idea that the high depletion of Fe in the diffuse neutral medium is not caused by the resilience of Fe rich grains but because of faster growth rate. This work also presents a potential way to constrain the chemical composition of interstellar dust.

NuSTAR Observations of the Supernova Remnant RX J1713.7–3946

Abstract The shock waves of supernova remnants (SNRs) are prominent candidates for the acceleration of the Galactic cosmic rays. SNR RX J1713.7−3946 is one well-studied particle accelerator in our Galaxy because of its strong nonthermal X-ray and gamma-ray radiation. We have performed Nuclear Spectroscopic Telescope Array (3–79 keV) observations of the northwest rim of RX J1713.7−3946, which is the brightest part in X-ray and has a shock speed of about 4000 km s−1. The spatially resolved X-ray emission from RX J1713.7−3946 is detected up to 20 keV for the first time. The hard X-ray image in 10–20 keV is broadly similar to the soft-band image in 3–10 keV. The typical spectrum is described by power-law model with exponential cutoff with the photon index Γ = 2.15 and the cutoff energy ε c = 18.8 keV. Using a synchrotron radiation model from accelerated electrons in the loss-limited case, the cutoff energy parameter ranges from 0.6–1.9 keV, varying from region to region. Combined with the previous measurement of the shock speed, the acceleration of electrons is close to the Bohm-limit regime in the outer edge, while the standard picture of accelerated particles limited by synchrotron radiation in SNR shock is not applicable in the inner edge and the filamentary structure.

Radiative transfer of hydrogen lines from supernova remnant shock waves: contributions of 2s-state hydrogen atoms

Abstract Radiative transfer in hydrogen lines in supernova remnant (SNR) shock waves is studied taking into account the population of the hydrogen atom 2s-state. Measurements of Balmer line emission, especially of H α, are often relied on to derive physical conditions in the SNR shock. On the other hand, Lyman series photons, especially Ly β, are mostly absorbed by upstream hydrogen atoms. As a result, atoms are excited to the 3p state, and then emit H α by the spontaneous transition from 3p to 2s. Thus, the nature of H α depends on how many Ly β photons are converted to H α photons. Moreover, the Balmer lines can be scattered by the 2s-state hydrogen atoms, which are excited not only by collisional excitation but also by the Lyman–Balmer conversion. It is shown for example that the H α photons are scattered if the shock propagates into an H i cloud with a density of ∼30 cm−3 and a size of ∼1 pc. We find that the line profile of H α becomes asymmetric resulting from the difference between line centre frequencies among the transitions from 3s to 2p, from 3p to 2s, and from 3d to 2p. We also find that the broad-to-narrow ratio of H α, which is often used to estimate the ion-electron temperature equilibrium, varies at most ≃ 10 per cent depending on the ionization degree of the upstream medium because of incomplete conversion of Lyman lines to Balmer lines.

ALMA Observations of Supernova Remnant N49 in the LMC. I. Discovery of CO Clumps Associated with X-Ray and Radio Continuum Shells

Abstract N49 (LHA 120-N49) is a bright X-ray supernova remnant (SNR) in the Large Magellanic Cloud. We present new 12CO (J = 1–0, 3–2), H i, and 1.4 GHz radio continuum observations of the SNR N49 using Mopra, ASTE, ALMA, and ATCA. We have newly identified three H i clouds using ATCA with an angular resolution of ∼20″: one associated with the SNR and the others located in front of the SNR. Both the CO and H i clouds in the velocity range from 281 to 291 km s−1 are spatially correlated with both the soft X-rays (0.2–1.2 keV) and the hard X-rays (2.0–7.0 keV) of N49 on a ∼10 pc scale. CO 3–2/1–0 intensity ratios indicate higher values of the CO cloud toward the SNR shell with an angular resolution of ∼45″, and thus a strong interaction was suggested. Using the ALMA, we have spatially resolved CO clumps embedded within or along the southeastern rim of N49 with an angular resolution of ∼3″. Three of the CO clumps are rim brightened on a 0.7–2 pc scale in both hard X-rays and the radio continuum: this provides further evidence for dynamical interactions between the CO clumps and the SNR shock wave. The enhancement of the radio synchrotron radiation can be understood in terms of magnetic field amplification around the CO clumps via a shock–cloud interaction. We also present a possible scenario in which the recombining plasma that dominates the hard X-rays from N49 was formed via thermal conduction between the SNR shock waves and the cold/dense molecular clumps.

Heating and Nonequilibrium Distributions of Ions in a Reverse Shock Wave of the SN 1987A Remnant

A hydrodynamical description of supernova remnants is based on the approximation of locally equilibrium particle distributions. Shock waves in supernova remnants at various stages of ejecta propagation are collisionless and form nonequilibrium particle distributions that relax slowly to quasiequilibrium distributions within a time longer than the hydrodynamic time. A kinetic model of the heating of ions behind the front of a shock wave in the SN 1987A remnant is considered with allowance for a complex chemical composition of the ejecta, and nonequilibrium distributions of ions in the vicinity of this shock wave are calculated. In addition to the quasi-Maxwellian peak, which determines the effective temperature of a given charge state of an ion, nonequilibriumdistributions of ions contain, in some cases, a suprethermal component, which may describe to the injection of ions in the process of cosmic-ray acceleration.

Particle-in-cell simulation of shock waves with different kinds of ions.

The problem of Cosmic Ray (CR) origin and acceleration is a widely discussed problem of modern astrophysics. It is considered that most likely CR sources are supernova remnant shock waves. One of the methods to study particle injection and acceleration by shock waves is a particle-in-cell computer simulation. In this paper we present results of shock wave simulations based on our self-developed implicit particle-in-cell code. First, we test the code with the well-known two stream and Weibel instabilities and second, simulate shock waves in plasma with different particle composition. Using the obtained results we study the process of charged particle acceleration at the shock front.

Polarized Balmer line emission from supernova remnant shock waves efficiently accelerating cosmic rays

Linearly polarized Balmer line emissions from supernova remnant shocks are studied taking into account the energy loss of the shock owing to the production of nonthermal particles. The polarization degree depends on the downstream temperature and the velocity difference between upstream and downstream regions. The former is derived once the line width of the broad component of the H$\alpha$ emission is observed. Then, the observation of the polarization degree tells us the latter. At the same time, the estimated value of the velocity difference independently predicts adiabatic downstream temperature that is derived from Rankine-Hugoniot relations for adiabatic shocks. If the actually observed downstream temperature is lower than the adiabatic temperature, there is a missing thermal energy which is consumed for particle acceleration. It is shown that a larger energy loss rate leads to more highly polarized H$\alpha$ emission. Furthermore, we find that polarized intensity ratio of H$\beta$ to H$\alpha$ also depends on the energy loss rate and that it is independent of uncertain quantities such as electron temperature, the effect of Lyman line trapping and our line of sight.

Interstellar gas and X-rays toward the Young supernova remnant RCW 86; pursuit of the origin of the thermal and non-thermal X-ray

We have analyzed the atomic and molecular gas using the 21 cm Hi and 2.6/1.3 mm CO emissions toward the young supernova remnant (SNR) RCW 86 in order to identify the interstellar medium with which the shock waves of the SNR interact. We have found an Hi intensity depression in the velocity range between −46 and −28 kms−1 toward the SNR, suggesting a cavity in the interstellar medium. The Hi cavity coincides with the thermal and non-thermal emitting X-ray shell. The thermal X-rays are coincident with the edge of the Hi distribution, which indicates a strong density gradient, while the non-thermal X-rays are found toward the less dense, inner part of the Hi cavity. The most significant non-thermal X-rays are seen toward the southwestern part of the shell where the Hi gas traces the dense and cold component. We also identified CO clouds which are likely interacting with the SNR shock waves in the same velocity range as the Hi, although the CO clouds are distributed only in a limited part of the SNR shell. The most massive cloud is located in the southeastern part of the shell, showing detailed correspondence with the thermal X-rays. These CO clouds show an enhanced CO J=2–1/1–0 intensity ratio, suggesting heating/compression by the shock front. We interpret that the shock-cloud interaction enhances non-thermal X-rays in the southwest and the thermal X-rays are emitted by the shock-heated gas of density 10–100 cm−3. Moreover, we can clearly see an Hi envelope around the CO cloud, suggesting that the progenitor had a weaker wind than the massive progenitor of the core-collapse SNR RX J1713.7–3949. It seems likely that the progenitor of RCW 86 was a system consisting of a white dwarf and a low-mass star with low-velocity accretion winds.

Acceleration of particles and generation of nonthermal emission in old supernova remnants

Particle acceleration by shock waves in old supernova remnants is studied. The radiative loss of gas behind the front of a shock wave, the streaming instability of magnetohydrodynamic waves, and the damping of waves on neutral atoms are considered. The calculated spectra of electromagnetic emissions are compared to observations of supernova remnant IC 443.

A SYSTEMATIC STUDY OF THE THERMAL AND NONTHERMAL EMISSION IN THE SUPERNOVA REMNANT RCW 86 WITH SUZAKU

ABSTRACT Diffusive shock acceleration by the shockwaves in supernova remnants (SNRs) is widely accepted as the dominant source for Galactic cosmic rays. However, it is unknown what determines the maximum energy of accelerated particles. The surrounding environment could be one of the key parameters. The SNR RCW 86 shows both thermal and nonthermal X-ray emission with different spatial morphologies. These emission originate from the shock-heated plasma and accelerated electrons respectively, and their intensities reflect their density distributions. Thus, the remnant provides a suitable laboratory to test possible association between the acceleration efficiency and the environment. In this paper, we present results of spatially resolved spectroscopy of the entire remnant with Suzaku. The spacially resolved spectra are well reproduced with a combination of a power-law for synchrotron emission and a two-component optically thin thermal plasma, corresponding to the shocked interstellar medium (ISM) with kT of 0.3–0.6 keV and Fe-dominated ejecta. It is discovered that the photon index of the nonthermal component becomes smaller when decreasing the emission measure of the shocked ISM, where the shock speed has remained high. This result implies that the maximum energy of accelerated electrons in RCW 86 is higher in the low-density and higher shock speed regions.

The plasma physics of cosmic rays in star-forming regions

It is largely accepted that Galactic cosmic rays, which pervade the interstellar medium, originate by means of shock waves in supernova remnants. Cosmic rays activate the rich chemistry that is observed in a molecular cloud and they also regulate its collapse timescale, determining the efficiency of star and planet formation, but they cannot penetrate up to the densest part of a molecular cloud, where the formation of stars is expected, because of energy loss processes and magnetic field deflections. Recently, observations towards young protostellar systems showed a surprisingly high value of the ionisation rate, the main indicator of the presence of cosmic rays in molecular cloud. Synchrotron emission, the typical feature of relativistic electrons, has also been detected towards the bow shock of a T Tauri star. Nevertheless, the origin of these signatures peculiar to accelerated particles is still puzzling. Here we show that particle acceleration can be driven by shock waves occurring in protostars through the first-order Fermi acceleration mechanism. We expect that shocks in protostellar jets can be efficient accelerators of protons, which can be boosted up to mildly relativistic energies. A strong acceleration can also take place at the protostellar surface, where shocks produced by infalling material during the phase of collapse are powerful enough to accelerate protons. Our model shows that thermal particles can experience an acceleration during the first phases of a system similar to the proto-Sun, and can also be used to explain recent observations. The presence of a local source of cosmic rays may have an unexpected impact over the process of the formation of stars and planets, as well as on the pre-biotic molecule formation.

Revealing Collisionless Heating of Electrons at the Supernova Remnant Shock Wave with X-Ray Spectroscopy of Non-Equilibrium Plasmas(Research)

Revealing Collisionless Heating of Electrons at the Supernova Remnant Shock Wave with X-Ray Spectroscopy of Non-Equilibrium Plasmas(Research)

CARBON, HELIUM, AND PROTON KINETIC TEMPERATURES IN A CYGNUS LOOP SHOCK WAVE

ABSTRACT Observations of SN 1006 have shown that ions and electrons in the plasma behind fast supernova remnant shock waves are far from equilibrium, with the electron temperature much lower than the proton temperature and ion temperatures approximately proportional to ion mass. In the ∼360 km s−1shock waves of the Cygnus Loop, on the other hand, electron and ion temperatures are roughly equal, and there is evidence that the oxygen kinetic temperature is not far from the proton temperature. In this paper, we report observations of the He ii λ1640 line and the C iv λ1550 doublet in a 360 km s−1shock in the Cygnus Loop. While the best-fit kinetic temperatures are somewhat higher than the proton temperature, the temperatures of He and C are consistent with the proton temperature and the upper limits are 0.5 and 0.3 times the mass-proportional temperatures, implying efficient thermal equilibration in this collisionless shock. The equilibration of helium and hydrogen affects the conversion between proton temperatures determined from Hα line profiles and shock speeds, and the efficient equilibration found here reduces the shock speed estimates and the distance estimate to the Cygnus Loop of Medina et al. to about 800 pc.

GeV-TeV cosmic-ray spectral anomaly as due to reacceleration by weak shocks in the Galaxy

Recent cosmic-ray measurements have found an anomaly in the cosmic-ray energy spectrum at GeV-TeV energies. Although the origin of the anomaly is not clearly understood, suggested explanations include effect of cosmic-ray source spectrum, propagation effects, and the effect of nearby sources. In this paper, we propose that the spectral anomaly might be an effect of re-acceleration of cosmic rays by weak shocks in the Galaxy. After acceleration by strong supernova remnant shock waves, cosmic rays undergo diffusive propagation through the Galaxy. During the propagation, cosmic rays may again encounter expanding supernova remnant shock waves, and get re-accelerated. As the probability of encountering old supernova remnants is expected to be larger than the younger ones due to their bigger sizes, re-acceleration is expected to be mainly due to weaker shocks. Since weaker shocks generate a softer particle spectrum, the resulting re-accelerated component will have a spectrum steeper than the initial cosmic-ray source spectrum produced by strong shocks. For a reasonable set of model parameters, it is shown that such re-accelerated component can dominate the GeV energy region while the non-reaccelerated component dominates at higher energies, explaining the observed GeV-TeV spectral anomaly.

TRIGGERING COLLAPSE OF THE PRESOLAR DENSE CLOUD CORE AND INJECTING SHORT-LIVED RADIOISOTOPES WITH A SHOCK WAVE. III. ROTATING THREE-DIMENSIONAL CLOUD CORES

A key test of the supernova triggering and injection hypothesis for the origin of the solar system's short-lived radioisotopes is to reproduce the inferred initial abundances of these isotopes. We present here the most detailed models to date of the shock wave triggering and injection process, where shock waves with varied properties strike fully three dimensional, rotating, dense cloud cores. The models are calculated with the FLASH adaptive mesh hydrodynamics code. Three different outcomes can result: triggered collapse leading to fragmentation into a multiple protostar system; triggered collapse leading to a single protostar embedded in a protostellar disk; or failure to undergo dynamic collapse. Shock wave material is injected into the collapsing clouds through Rayleigh-Taylor fingers, resulting in initially inhomogeneous distributions in the protostars and protostellar disks. Cloud rotation about an axis aligned with the shock propagation direction does not increase the injection efficiency appreciably, as the shock parameters were chosen to be optimal for injection even in the absence of rotation. For a shock wave from a core-collapse supernova, the dilution factors for supernova material are in the range of $\sim 10^{-4}$ to $\sim 3 \times 10^{-4}$, in agreement with recent laboratory estimates of the required amount of dilution for $^{60}$Fe and $^{26}$Al. We conclude that a type II supernova remains as a promising candidate for synthesizing the solar system's short-lived radioisotopes shortly before their injection into the presolar cloud core by the supernova's remnant shock wave.

Cosmic ray acceleration in young supernova remnants

We investigate the appearance of magnetic field amplification resulting from a cosmic ray escape current in the context of supernova remnant shock waves. The current is inversely proportional to the maximum energy of cosmic rays, and is a strong function of the shock velocity. Depending on the evolution of the shock wave, which is drastically different for different circumstellar environments, the maximum energy of cosmic rays as required to generate enough current to trigger the non-resonant hybrid instability that confines the cosmic rays follows a different evolution and reaches different values. We find that the best candidates to accelerate cosmic rays to ∼ few PeV energies are young remnants in a dense environment, such as a red supergiant wind, as may be applicable to Cassiopeia A. We also find that for a typical background magnetic field strength of 5 μG the instability is quenched in about 1000 years, making SN1006 just at the border of candidates for cosmic ray acceleration to high energies.