Magnetic Field Amplification by a Plasma Cavitation Instability in Relativistic Shock Precursors

Plasma streaming instabilities play an important role in magnetic field amplification and particle acceleration in relativistic shocks and their environments. However, in the far shock precursor region where accelerated particles constitute a highly relativistic and dilute beam, streaming instabilities typically become inefficient and operate at very small scales when compared to the gyroradii of the beam particles. We report on a plasma cavitation instability that is driven by dilute relativistic beams and can increase both the magnetic field strength and coherence scale by orders of magnitude to reach near-equipartition values with the beam energy density. This instability grows after the development of the Weibel instability and is associated with the asymmetric response of background leptons and ions to the beam current. The resulting net inductive electric field drives a strong energy asymmetry between positively and negatively charged beam species. Large-scale particle-in-cell simulations are used to verify analytical predictions for the growth and saturation level of the instability and indicate that it is robust over a wide range of conditions, including those associated with pair-loaded plasmas. These results can have important implications for the magnetization and structure of shocks in gamma-ray bursts, and more generally for magnetic field amplification and asymmetric scattering of relativistic charged particles in plasma astrophysical environments.

Forbidden Line Emission from Type Ia Supernova Remnants Containing Balmer-dominated Shells

Balmer-dominated shells in supernova remnants (SNRs) are produced by collisionless shocks advancing into a partially neutral medium and are most frequently associated with Type Ia supernovae. We have analyzed Hubble Space Telescope (HST) images and Very Large Telescope (VLT)/Multi-Unit Spectroscopic Explorer (MUSE) or AAT/Wide Field Integral Spectrograph observations of five Type Ia SNRs containing Balmer-dominated shells in the LMC: 0509–67.5, 0519–69.0, N103B, DEM L71, and 0548–70.4. Contrary to expectations, we find bright forbidden-line emission from small dense knots embedded in four of these SNRs. The electron densities in some knots are higher than 104 cm−3. The size and density of these knots are not characteristic for interstellar medium—they most likely originate from a circumstellar medium ejected by the SN progenitor. Physical property variations of dense knots in the SNRs appear to reflect an evolutionary effect. The recombination timescales for high densities are short, and HST images of N103B taken 3.5 yr apart already show brightness changes in some knots. VLT/MUSE observations detect [Fe xiv] line emission from reverse shocks into SN ejecta as well as forward shocks into the dense knots. Faint [O iii] line emission is also detected from the Balmer shell in 0519–69.0, N103B, and DEM L71. We exclude the postshock origin because the [O iii] line is narrow. For the preshock origin, we considered three possibilities: photoionization precursor, cosmic-ray precursor, and neutral precursor. We conclude that the [O iii] emission arises from oxygen that has been photoionized by [He ii] λ304 photons and is then collisionally excited in a shock precursor heated mainly by cosmic rays.

Stars and gas in the most metal-poor galaxies – I. COS and MUSE observations of SBS 0335−052E

ABSTRACT Among the nearest most metal-poor starburst-dwarf galaxies known, SBS 0335−052E is the most luminous in integrated nebular He ii λ4686 emission. This makes it a unique target to test spectral synthesis models and spectral interpretation tools of the kind that will be used to interpret future rest-frame UV observations of primeval galaxies. Previous attempts to reproduce its He ii λ4686 luminosity found that X-ray sources, shocks, and single Wolf–Rayet stars are not main contributors to the He ii-ionizing budget; and that only metal-free single rotating stars or binary stars with a top-heavy IMF and an unphysically low metallicity can reproduce it. We present new UV (COS) and optical (MUSE) spectra that integrate the light of four super star clusters in SBS 0335−052E. Nebular He ii, [C iii], C iii], C iv, and O iii] UV emission lines with equivalent widths between 1.7 and 5 Å and a C iv λλ1548, 1551 P-Cygni like profile are detected. Recent extremely metal-poor shock + precursor models and binary models fail to reproduce the observed optical emission-line ratios. We use different sets of UV and optical observables to test models of constant star formation with single non-rotating stars that account for very massive stars as blueshifted O v λ1371 absorption is present. Simultaneously fitting the fluxes of all high-ionization UV lines requires an unphysically low metallicity. Fitting the P-Cygni like + nebular components of C iv λλ1548, 1551 does not constrain the stellar metallicity and time since the beginning of star formation. We obtain 12+log(O/H)$\, = 7.45\pm 0.04$ and log(C/O)$\, = -0.45^{+0.03}_{-0.04}$ for the galaxy. Model testing would benefit from higher spatial resolution UV and optical spectroscopy of the galaxy.

Gamma-ray emission from pulsar binaries

Pulsar winds, containing charged particles, waves and a net (phase-averaged) magnetic field, are thought to fuel the high-energy emission from several gamma-ray binaries. They terminate where the ram pressure matches that of the surroundings - which, in binaries, is provided by the wind of the companion. Before termination, pulsed emission can be produced by inverse Compton scattering of photons from the companion by particles in the waves. After termination, both the bulk kinetic energy of the particles and the Poynting flux in the waves are dissipated into an energetic particle population embedded in the surviving phase-averaged magnetic field. Pulsed emission is no longer possible, but a substantial flux of unpulsed high-energy photons can be produced. I will present results showing that the physical conditions at the termination shock can be divided into two regimes: a high density one, where current sheets in the wind are first compressed by an MHD shock and subsequently dissipate by reconnection, and a low density one, where the wind can first convert into an electromagnetic wave in the shock precursor, which then damps and merges into the wind nebula. The shocks surrounding isolated pulsars fall into the low-density category, but those around pulsars in binary systems, may transit from one regime to the other according to binary phase. The implications of the shock-structure dichotomy for these objects will be discussed.