In this paper, we present a detailed hydrodynamical study of the properties of the flow produced by the collision of a pulsar wind with the surrounding in a binary system. This work is the first attempt to simulate interaction of the ultrarelativistic flow (pulsar wind) with the non‐relativistic stellar wind. Obtained results show that the wind collision could result in the formation of an ‘unclosed’ (at spatial scales comparable to the binary system size) pulsar wind termination shock even when the stellar wind ram pressure exceeds significantly the pulsar wind kinetic pressure. Moreover, the post‐shock flow propagates in a rather narrow region, with very high bulk Lorentz factor (γ∼ 100). This flow acceleration is related to adiabatic losses which are purely hydrodynamical effects. Interestingly, in this particular case, no magnetic field is required for formation of the ultrarelativistic bulk outflow. The obtained results provide a new interpretation for the orbital variability of radio, X‐ray and gamma‐ray signals detected from binary pulsar system PSR B1259−63/SS2883.

The inverse Compton (IC) scattering of ultrarelativistic electrons accelerated at the pulsar wind termination shock is generally believed to be responsible for TeV gamma‐ray signal recently reported from the binary system PSR B1259−63/SS2883. While this process can explain the energy spectrum of the observed TeV emission, the gamma‐ray fluxes detected by the Array of Imaging Atmospheric Cherenkov Telescopes (HESS) at different epochs do not agree with the published theoretical predictions of the TeV light curve. The main objective of this paper is to show that the HESS results can be explained, under certain reasonable assumptions concerning the cooling of relativistic electrons, by IC scenarios of gamma‐ray production in PSR B1259−63. In this paper we study evolution of the energy spectra of relativistic electrons under different assumptions about the acceleration and energy‐loss rates of electrons, and the impact of these processes on the light curve of IC gamma‐rays. We demonstrate that the observed TeV light curve can be explained (i) by adiabatic losses which dominate over the entire trajectory of the pulsar with a significant increase towards the periastron or (ii) by the ‘early’ (sub‐TeV) cut‐offs in the energy spectra of electrons due to the enhanced rate of Compton losses close to the periastron. The first four data points obtained just after periastron comprise an exception – possibly due to interaction with the Be star disc, which introduces additional physics not included in the presented model. The calculated spectral and temporal characteristics of the TeV radiation provide conclusive tests to distinguish between these two working hypotheses. The Compton deceleration of the electron–positron pulsar wind contributes to the decrease of the non‐thermal power released in the accelerated electrons after the wind termination, and thus to the reduction of the IC and synchrotron components of radiation close to the periastron. Although this effect alone cannot explain the observed TeV and X‐ray light curves, the Comptonization of the cold ultrarelativistic wind leads to the formation of gamma‐radiation with a specific line‐type energy spectrum. While the HESS data already constrain the Lorentz factor of the wind, Γ≤ 106 (for the most likely orbit inclination angle i= 35°, and assuming an isotropic pulsar wind), future observations of this object with GLAST should allow a deep probe of the wind Lorentz factor in the range between 104 and 106.

We propose a new model for the description of ultra-short flares from TeV blazars by compact magnetized condensations (blobs), produced when red giant stars cross the jet close to the central black hole. Our study includes a simple dynamical model for the evolution of the envelope lost by the star in the jet, and its high energy nonthermal emission through different leptonic and hadronic radiation mechanisms. We show that the fragmented envelope of the star can be accelerated to Lorentz factors up to 100 and radiate effectively the available energy in gamma-rays predominantly through proton synchrotron radiation or external inverse Compton scattering of electrons. The model can readily explain the minute-scale TeV flares on top of longer (typical time-scales of days) gamma-ray variability as observed from the blazar PKS 2155−304. In the framework of the proposed scenario, the key parameters of the source are robustly constrained. In the case of proton synchrotron origin of the emission a mass of the central black hole of M BH ≈ 10 8 M ⊙ , a total jet power of L j ≈ 2 × 10 47 erg s

Pulsars are thought to eject electron-positron winds that energize the surrounding environment, with the formation of a pulsar wind nebula. The pulsar wind originates close to the light cylinder, the surface at which the pulsar co-rotation velocity equals the speed of light, and carries away much of the rotational energy lost by the pulsar. Initially the wind is dominated by electromagnetic energy (Poynting flux) but later this is converted to the kinetic energy of bulk motion. It is unclear exactly where this takes place and to what speed the wind is accelerated. Although some preferred models imply a gradual acceleration over the entire distance from the magnetosphere to the point at which the wind terminates, a rapid acceleration close to the light cylinder cannot be excluded. Here we report that the recent observations of pulsed, very high-energy γ-ray emission from the Crab pulsar are explained by the presence of a cold (in the sense of the low energy of the electrons in the frame of the moving plasma) ultrarelativistic wind dominated by kinetic energy. The conversion of the Poynting flux to kinetic energy should take place abruptly in the narrow cylindrical zone of radius between 20 and 50 light-cylinder radii centred on the axis of rotation of the pulsar, and should accelerate the wind to a Lorentz factor of (0.5-1.0) × 10(6). Although the ultrarelativistic nature of the wind does support the general model of pulsars, the requirement of the very high acceleration of the wind in a narrow zone not far from the light cylinder challenges current models.

The axisymmetric 3-D MHD outflow of a cold plasma from a magnetized and rotating astrophysical object is numerically simulated with the purpose of investigating the outflow's magnetocentrifugal acceleration and eventual collimation. Gravity and thermal pressure are neglected while a split-monopole is used to describe the initial magnetic field configuration. It is found that the stationary final state depends critically on a single parameter alpha expressing the ratio of the corotating speed at the Alfven distance to the initial flow speed along the initial monopole-like magnetic fieldlines. Several angular velocity laws have been used for relativistic and nonrelativistic outflows. The acceleration of the flow is most effective at the equatorial plane and the terminal flow speed depends linearly on alpha. Significant flow collimation is found in nonrelativistic efficient magnetic rotators corresponding to relatively larger than 1 values of alpha while very weak collimation occurs in inefficient magnetic rotators with values of alpha smaller than about 1. Part of the flow around the rotation and magnetic axis is cylindrically collimated while the remaining part obtains radial asymptotics. The transverse radius of the jet is inversely proportional to alpha while the density in the jet grows linearly with alpha. For alpha greater than about 5 the magnitude of the flow in the jet remains below the fast MHD wave speed everywhere. In relativistic outflows, no collimation is found in the supersonic region for parameters typical for radio pulsars. All above results verify the main conclusions of general theoretical studies on the magnetic acceleration and collimation of outflows from magnetic rotators and extend previous numerical simulations to large stellar distances.Comment: 15 pages, 13 figures. Accepted for publication, MNRA

We show that the relativistic wind of the Crab pulsar, which is commonly thought to be invisible in the region upstream of the termination shock at rrS∼0.1 pc, in fact could be directly observed through its inverse Compton (IC) γ‐ray emission. This radiation is caused by illumination of the wind by low‐frequency photons emitted by the pulsar, and consists of two, pulsed and unpulsed, components associated with the non‐thermal (pulsed) and thermal (unpulsed) low‐energy radiation of the pulsar, respectively. These two components of γ‐radiation have distinct spectral characteristics, which depend essentially on the site of formation of the kinetic‐energy‐dominated wind, as well as on the Lorentz factor and the geometry of propagation of the wind. Thus, the search for such specific radiation components in the spectrum of the Crab Nebula can provide unique information about the unshocked pulsar wind that is not accessible at other wavelengths. In particular, we show that the comparison of the calculated flux of the unpulsed IC emission with the measured γ‐ray flux of the Crab Nebula excludes the possibility of formation of a kinetic‐energy‐dominated wind within 5 light‐cylinder radii of the pulsar, Rw5RL. The analysis of the pulsed IC emission, calculated under reasonable assumptions concerning the production site and angular distribution of the optical pulsed radiation, yields even tighter restrictions, namely Rw30RL.

We argue that the bright flare of the binary pulsar PSR B1259−63/LS2883 detected by the Fermi Large Area Telescope (LAT), is due to the inverse Compton (IC) scattering of the unshocked electron-positron pulsar wind with a Lorentz factor Γ 0 ≈ 10 4 . The combination of two effects both linked to the circumstellar disk (CD), is a key element in the proposed model. The first effect is related to the impact of the surrounding medium on the termination of the pulsar wind. Inside the disk, the "early" termination of the wind results in suppression of its gamma-ray luminosity. When the pulsar escapes the disk, the conditions for termination of the wind undergo significant changes. This would lead to a dramatic increase of the pulsar wind zone, and thus to the proportional increase of the gamma-ray flux. On the other hand, if the parts of the CD disturbed by the pulsar can supply infrared photons of density high enough for efficient Comptonization of the wind, almost the entire kinetic energy of the pulsar wind

Abstract.A stationary self-consistent outflow of a magnetised relativistic plasma from a rotating object with an initially monopole-like magnetic field is investigated in the ideal MHD approximation under the condition σ/U 2 0 > 1, where σ is the ratio of the Poynting flux over the mass energy flux at the equator and the surface of the star, with U0 = γ0v0/c and γ0 the initial four-velocity and Lorentz factor of the plasma. The mechanism of the magnetocentrifugal acceleration and self-collimation of the relativistic plasma is investigated. A jet-like relativistic flow along the axis of rotation is found in the steady-state solution under the condition σ/U 2 0 > 1 with properties predicted analytically. The amount of the collimated matter in the jet is rather small in comparison to the total mass flux in the wind. An explanation for the weak self-collimation of relativistic winds is given.