Abrupt acceleration of a ‘cold’ ultrarelativistic wind from the Crab pulsar

On the VHE Spectrum and Formation of the Teraelectronvolt Pulsed Emission of the Crab Pulsar

In the present paper, a model for the pulsed γ-ray emission of the Crab pulsar from 0.01 GeV to 1 TeV in the context of synchrotron emission generated in the vicinity of a light cylinder is developed. The generation of such high energies through the synchrotron process requires the existence of very energetic plasma particles in pulsar magnetospheres. It is assumed that the emitting particles are ultra-relativistic primary beam electrons re-accelerated to very high energies due to the Landau damping process of a special type of parametrically driven Langmuir waves. This type of Langmuir wave carries energy released through the rotational slow-down of a pulsar and is very effective in supplying the resonant particles with energy from a natural reservoir. The model provides simultaneous generation of energetic γ-ray and low-frequency radio (0.1–1 GHz) emission in the same location of the pulsar magnetosphere. These two radiations processes are triggered by a single plasma process, namely excitation of the cyclotron instability. This provides a natural explanation for the observed coincidence of radio and γ-ray signals observed from the Crab pulsar.

The double-stranded RNA-dependent protein kinase inhibitor alleviates endoplasmic reticulum stress and alleviates sepsis-induced renal injury

The double-stranded RNA-dependent protein kinase (PKR) is involved in inflammatory cytokine expression and disease pathogenesis in many conditions. The aim of this study was to explore the role of PKR in sepsis-induced renal tissue injury. Six-week-old C57BL/6J mice received PKR inhibitor (imoxin) and Endoplasmic reticulum (ER) inducer (tunicamycin) 2 hr prior to induction of inflammation via cecal ligation and puncture (CLP). Renal tissues were collected 24 hr after the CLP treatment and protein expression were assessed. The expression of creatinine (Cre) and blood urea nitrogen (BUN) in serum and inflammation factor in tissues was detected by ELISA, and the apoptosis of renal tissue was detected by TUNEL staining. PKR inhibitors reduce the expression of sepsis-induced ER stress in renal tissue, as well as the pathological changes and renal impairment in renal tissue. PKR inhibitors reduce the expression of sepsis-induced inflammatory response and sepsis-induced apoptosis in renal tissue by ER stress. In conclusion, PKR inhibitor alleviates ER stress and alleviates sepsis-induced renal injury.

Peta-electron volt gamma-ray emission from the Crab Nebula

The Crab Nebula is a bright source of gamma rays powered by the Crab Pulsar's rotational energy through the formation and termination of a relativistic electron-positron wind. We report the detection of gamma rays from this source with energies from 5 × 10^-4 to 1.1 peta-electron volts with a spectrum showing gradual steepening over three energy decades. The ultrahigh-energy photons imply the presence of a peta-electron volt electron accelerator (a pevatron) in the nebula, with an acceleration rate exceeding 15% of the theoretical limit. We constrain the pevatron's size between 0.025 and 0.1 parsecs and the magnetic field to ≈110 microgauss. The production rate of peta-electron volt electrons, 2.5 × 10³⁶ ergs per second, constitutes 0.5% of the pulsar spin-down luminosity, although we cannot exclude a contribution of peta-electron volt protons to the production of the highest-energy gamma rays.

Parallel firehose instability in electron-positron plasmas

In a magnetized uniform plasma, firehose instability may arise as a result of pressure anisotropy of P_{||}>P_{⊥}, where P_{||} and P_{⊥} are the thermal pressure parallel and perpendicular to the magnetic field, respectively. In this paper, we examine the parallel firehose instability in electron-positron plasmas based on the particle simulations along with the linear fluid theory, which may give rise to the dispersion relation, instability criteria, and growth rate, etc., for comparisons with those calculated from the kinetic simulations. As for the firehose instability in electron-proton plasmas, the magnetic field grows rapidly and then decreases with oscillations. The nonlinear saturated state complies with the linear stability criterion, α=μ_{0}(P_{||}-P_{⊥})/B^{2}=1, derived from the fluid theory only for relatively smaller values of initial α or ω_{p}/ω_{c}, where ω_{p} and ω_{c} are the plasma and cyclotron frequencies, respectively. For relatively larger values of initial α and ω_{p}/ω_{c}, the saturated α values are smaller than 1 as a result of kinetic resonant effects. The dominant wave numbers are kc/ω_{p}<0.5 and the growth rates are in the range of 0.1-0.3ω_{c}, which are approximately consistent with the linear fluid theory for the same wavelengths. Both electrostatic and electromagnetic modes predicted by the linear fluid theory are identified in the kinetic simulations.

CONSTRAINING RELATIVISTIC BOW SHOCK PROPERTIES IN ROTATION-POWERED MILLISECOND PULSAR BINARIES

Multiwavelength followup of unidentified Fermi sources has vastly expanded the number of known galactic-field "black widow" and "redback" millisecond pulsar binaries. Focusing on their rotation-powered state, we interpret the radio to X-ray phenomenology in a consistent framework. We advocate the existence of two distinct modes differing in their intrabinary shock orientation, distinguished by the phase-centering of the double-peaked X-ray orbital modulation originating from mildly-relativistic Doppler boosting. By constructing a geometric model for radio eclipses, we constrain the shock geometry as functions of binary inclination and shock stand-off R0. We develop synthetic X-ray synchrotron orbital light curves and explore the model parameter space allowed by radio eclipse constraints applied on archetypal systems B1957+20 and J1023+0038. For B1957+20, from radio eclipses the stand-off is R0 ~ 0.15-0.3 fraction of binary separation from the companion center, depending on the orbit inclination. Constructed X-ray light curves for B1957+20 using these values are qualitatively consistent with those observed, and we find occultation of the shock by the companion as a minor influence, demanding significant Doppler factors to yield double peaks. For J1023+0038, radio eclipses imply R0 ≲ 0.4 while X-ray light curves suggest 0.1 ≲ R0 ≲ 0.3 (from the pulsar). Degeneracies in the model parameter space encourage further development to include transport considerations. Generically, the spatial variation along the shock of the underlying electron power-law index should yield energy-dependence in the shape of light curves motivating future X-ray phase-resolved spectroscopic studies to probe the unknown physics of pulsar winds and relativistic shock acceleration therein.

The surprising Crab pulsar and its nebula: a review

The Crab nebula and its pulsar (referred to together as 'the Crab') have historically played a central role in astrophysics. True to this legacy, several unique discoveries have been made recently. The Crab was found to emit gamma-ray pulsations up to energies of 400 GeV, beyond what was previously expected from pulsars. Strong gamma-ray flares, of durations of a few days, were discovered from within the nebula, while the source was previously expected to be stable in flux on these time scales. Here we review these intriguing and suggestive developments. In this context we give an overview of the observational properties of the Crab and our current understanding of pulsars and their nebulae.