Supernova Remnant Shock Front Research Articles

Supernova connection of unidentified ultra-high-energy gamma-ray source LHAASO J2108+5157

ABSTRACT We present a simple phenomenological model of hadronic interaction between protons accelerated in an old supernova remnant (SNR) and cold protons situated within the associated molecular clouds (MCs). The accelerated protons from the old SNR escaped the SNR shock front, and got injected into the MCs at an earlier time, producing ultra-high-energy gamma-rays and neutrinos through inelastic proton–proton interaction. We also take into account the acceleration and subsequent escape of electrons from the SNR shock front. The escaped electrons produce gamma-rays through various radiative cooling mechanisms, after getting injected into the MCs. We use the model discussed in this letter to explain the multiwavelength spectral energy distribution of unidentified Galactic ultra-high-energy gamma-ray source LHAASO J2108+5157. We also discuss the feasibility of applying this model in other cases as well. Future observations can test the viability of the model discussed in this letter, which will in turn confirm that the SNRs can, in fact, accelerate particles up to PeV energies.

Revealing a new region of gamma-ray emission in the vicinity of HESS J1825–137

HESS J1825-137 is a bright very-high-energy (VHE) gamma-ray source that has been firmly established as a pulsar wind nebula (PWN), and one of the most extended gamma-ray objects within this category. The progenitor supernova remnant (SNR) for this PWN has not been firmly established. We carried out an analysis of gamma-ray observations in the region of HESS J1825-137 with the Fermi-LAT which reveal emission in the direction away from the Galactic plane. The region lies beyond the PWN and reaches a distance from the pulsar compatible with the supposed location of the SNR shock front. The spectrum of the gamma-rays is hard with a photon index of $\sim 1.9$ in the 10-250 GeV range. Several scenarios for the origin of the emission are discussed, including the SNR as a source of high-energy particles and the "leakage"' of leptons from the PWN.

Anomalous Distributions of Primary Cosmic Rays as Evidence for Time-dependent Particle Acceleration in Supernova Remnants

Abstract Recent precise measurements of cosmic-ray (CR) spectra show that the energy distribution of protons is softer than those of heavier nuclei, and there are spectral hardenings for all nuclear compositions above ∼200 GV. Models proposed for these anomalies generally assume steady-state solutions of the particle acceleration process. We show that if the diffusion coefficient has a weak dependence on the particle rigidity near shock fronts of supernova remnants (SNRs), time-dependent solutions of the linear diffusive shock acceleration at two stages of SNR evolution can naturally account for these anomalies. The high-energy component of CRs is dominated by acceleration in the free expansion and adiabatic phases with enriched heavy elements and a high shock speed. The low-energy component may be attributed to acceleration by slow shocks propagating in dense molecular clouds with low metallicity in the radiative phase. Instead of a single power-law distribution, the spectra of time-dependent solutions soften gradually with the increase of energy, which may be responsible for the “knee” of CRs.

Radio spectral characteristics of the supernova remnant Puppis A and nearby sources

This paper presents a new study of the spectral index distribution of the supernova remnant (SNR) Puppis A. The nature of field compact sources is also investigated according to the measured spectral indices. This work is based on new observations of Puppis A and its surroundings performed with the Australia Telescope Compact Array in two configurations using the Compact Array Broad-band Backend centred at 1.75 GHz. We find that the global spectral index of Puppis A is α = −0.563 ± 0.013. Local variations have been detected, however this global index represents well the bulk of the SNR. At the SE, we found a pattern of parallel strips with a flat spectrum compatible with small-scale filaments, although not correlated in detail. The easternmost filament agrees with the idea that the SNR shock front is interacting with an external cloud. There is no evidence of the previously suggested correlation between emissivity and spectral index. A number of compact features are proposed to be evolved clumps of ejecta based on their spectral indices, although dynamic measurements are needed to confirm this hypothesis. We estimate precise spectral indices for the five previously known field sources, two of which are found to be double (one of them, probably triple), and catalogue 40 new sources. In the light of these new determinations, the extragalactic nature previously accepted for some compact sources is now in doubt.

INTERACTION BETWEEN CASSIOPEIA A AND NEARBY MOLECULAR CLOUDS

We present spectroscopy of the supernova remnant Cassiopeia A (Cas A) observed at infrared wavelengths from 10 - 40 micron with the Spitzer Space Telescope and at millimeter wavelengths in 12CO and 13CO J = 2 - 1 (230 and 220 GHz) with the Heinrich Hertz Submillimeter Telescope. The IR spectra demonstrate high-velocity features toward a molecular cloud coincident with a region of bright radio continuum emission along the northern shock front of Cas A. The millimeter observations indicate that CO emission is broadened by a factor of two in some clouds toward Cas A, particularly to the south and west. We believe that these features trace interactions between the Cas A shock front and nearby molecular clouds. In addition, some of the molecular clouds that exhibit broadening in CO lie 1 - 2' away from the furthest extent of the supernova remnant shock front. We propose that this material may be accelerated by ejecta with velocity significantly larger than the observed free-expansion velocity of the Cas A shock front. These observations may trace cloud interactions with fast-moving outflows such as the bipolar outflow along the southwest to northeast axis of the Cas A supernova remnant, as well as fast-moving knots seen emerging in other directions.

Stochastic electron motion driven by space plasma waves

Abstract. Stochastic motion of relativistic electrons under conditions of the nonlinear resonance interaction of particles with space plasma waves is studied. Particular attention is given to the problem of the stability and variability of the Earth's radiation belts. It is found that the interaction between whistler-mode waves and radiation-belt electrons is likely to involve the same mechanism that is responsible for the dynamical balance between the accelerating process and relativistic electron precipitation events. We have also considered the efficiency of the mechanism of stochastic surfing acceleration of cosmic electrons at the supernova remnant shock front, and the accelerating process driven by a Langmuir wave packet in producing cosmic ray electrons. The dynamics of cosmic electrons is formulated in terms of a dissipative map involving the effect of synchrotron emission. We present analytical and numerical methods for studying Hamiltonian chaos and dissipative strange attractors, and for determining the heating extent and energy spectra.

Supernova remnants and the origin of cosmic rays

AbstractSupernova remnants have long been considered to be the dominant sources of Galactic cosmic rays. For a long time the prime evidence consisted of radio synchrotron radiation from supernova remnants, indicating the presence of electrons with energies of several GeV. However, in order to explain the cosmic ray energy density and spectrum in the Galaxy supernova remnant should use 10% of the explosion energy to accelerate particles, and about 99% of the accelerated particles should be protons and other atomic nuclei.Over the last decade a lot of progress has been made in providing evidence that supernova remnant can accelerate protons to very high energies. The evidence consists of, among others, X-ray synchrotron radiation from narrow regions close to supernova remnant shock fronts, indicating the presence of 10-100 TeV electrons, and providing evidence for amplified magnetic fields, gamma-ray emission from both young and mature supernova remnants. The high magnetic fields indicate that the condition for accelerating protons to >1015 eV are there, whereas the gamma-ray emission from some mature remnants indicate that protons have been accelerated.

THE NONLINEAR SATURATION OF THE NON-RESONANT KINETICALLY DRIVEN STREAMING INSTABILITY

A non-resonant instability for the amplification of the interstellar magnetic field in young Supernova Remnant (SNR) shocks was predicted by Bell (2004), and is thought to be relevant for the acceleration of cosmic ray (CR) particles. For this instability, the CRs streaming ahead of SNR shock fronts drive electromagnetic waves with wavelengths much shorter than the typical CR Larmor radius, by inducing a current parallel to the background magnetic field. We explore the nonlinear regime of the non-resonant mode using Particle-in-Cell (PIC) hybrid simulations, with kinetic ions and fluid electrons, and analyze the saturation mechanism for realistic CR and background plasma parameters. In the linear regime, the observed growth rates and wavelengths match the theoretical predictions; the nonlinear stage of the instability shows a strong reaction of both the background plasma and the CR particles, with the saturation level of the magnetic field varying with the CR parameters. The simulations with CR-to-background density ratios of n_CR/n_b=10^(-5) reveal the highest magnetic field saturation levels, with energy also being transferred to the background plasma and to the perpendicular velocity components of the CR particles. The results show that amplification factors >10 for the magnetic field can be achieved, and suggest that this instability is important for the generation of magnetic field turbulence, and for the acceleration of CR particles.

A current-driven instability in parallel, relativistic shocks

Recently, Bell (2004 Mon. Not. R. Astron. Soc. 353 550) has reanalysed the problem of wave excitation by cosmic rays propagating in the pre-cursor region of a supernova remnant shock front. He pointed out a strong, non-resonant, current-driven instability that had been overlooked in the kinetic treatments by Achterberg (1983 Astron. Astrophys. 119 274) and McKenzie and Völk (1982 Astron. Astrophys. 116 191), and suggested that it is responsible for substantial amplification of the ambient magnetic field. Magnetic field amplification is also an important issue in the problem of the formation and structure of relativistic shock fronts, particularly in relation to models of gamma-ray bursts. We have therefore generalized the linear analysis to apply to this case, assuming a relativistic background plasma and a monoenergetic, unidirectional incoming proton beam. We find essentially the same non-resonant instability observed by Bell and show that also, under GRB conditions, it grows much faster than the resonant waves. We quantify the extent to which thermal effects in the background plasma limit the maximum growth rate.

Technical Report: Radio Continuum and Polarization Surveys

The detection of radio emission from the Milky Way more than seventy years ago was a big surprise for the astronomical community, since there was no prediction for that [1]. It took more than ten years until it was commonly accepted that synchrotron radiation originating in interstellar space is the dominating emission process at radio frequencies up to a few GHz or wavelengths as short as a few cm. Relativistic electrons with energies up to a few GeV, which are mostly accelerated in the shock fronts of supernova remnants (SNRs), radiate in the galactic magnetic field, which has a typical strength of a few μGauss.

Green Bank Telescope Observations of IC 443: The Nature of OH (1720 MHz) Masers and OH Absorption

We present spectral line observations of the ground-state transitions of hydroxyl (OH) toward supernova remnant (SNR) IC 443 carried out with the Green Bank Telescope. Weak, extended OH (1720 MHz) maser emission with OH (1667, 1665, 1612 MHz) absorption is detected along the southern extent of the remnant where no bright compact maser sources have been previously observed. These newly detected SNR-type masers are coincident with well-known molecular clumps and a ridge of shocked H2 emission indicative of the SNR shock front interacting with the adjacent molecular cloud. Simultaneous observation of all four ground-state transitions of OH permits us to fit physical conditions of the shocked gas at the interaction site. A simple two-component model for the line profiles yields the physical parameters for detected regions of maser emission, including excitation temperature, OH column density, and filling factor. Observed line profiles suggest the shock is largely propagating toward the line of sight in the region of these newly identified weak masers. The implications of shock geometry and physical parameters in producing spatially extended OH maser emission in SNRs are explored. We also present VLA radio continuum observations at 330 MHz for comparison with OH line observations of the remnant.

Small-scale structure of non-thermal X-rays in historical SNRs

The shock fronts of supernova remnants (SNRs) are the most probable acceleration sites of high energy electrons (up to ∼100 TeV) with diffusive shock acceleration (DSA). However, the evolution of the acceleration in SNRs is still unclear. Using the Chandra archival data, we resolve the spatial structure of the shocked region of five historical SNRs (Cas A, Kepler, Tycho, SN 1006, and RCW 86) in non-thermal X-rays. The non-thermal emission concentrates on very thin and filamentary regions (“filaments”) in all SNRs, as already reported in the SN 1006 case. We also report on the time evolution of the scale length of the filaments and roll-off frequency of their spectra.

An Array of Cherenkov Telescopes Yields the First Resolved Image of a Celestial Gamma-Ray Source

Imaging with TeV photons provides the most direct evidence yet that cosmic rays originate in the shock fronts of supernova remnants.

SUPERNOVA-REMNANT SHOCK WAVES CLOSE UP

ABSTRACT The supernova remnant shock fronts generally observed at optical and UV wavelengths are radiative shocks, which convert most of the thermal energy of the shocked gas into radiation. Much fainter, pure Balmer line emission is produced by nonradiative shocks in partially neutral gas. Because this emission arises very close to the shock front itself, it provides several important diagnostics for the physical processes in high-Mach-number collisionless shocks.