Accreting X-ray pulsars are among the best observed objects of X-ray astronomy with a rich data set of observational phenomena in the spectral and timing domain. While the general picture for these sources is well established, the detailed physics be
hind the observed phenomena are often subject of debate. We present recent observational, theoretical and modeling results for the structure and dynamics of the accretion column in these systems. Our results indicate the presence of different accretion regimes and possible explanations for observed variations of spectral features with luminosity.
Recently observed minute timescale variability of blazar emission at TeV energies has imposed severe constraints on jet models and TeV emission mechanisms. We focus on a robust jet instability to explain this variability. As a consequence of the bulk
outflow of the jet plasma, the pressure is likely to be anisotropic, with the parallel pressure $P_{||}$ in the forward jet direction exceeding the perpendicular pressure $P_{perp}$. Under these circumstances, the jet is susceptible to the firehose instability, which can cause disruptions in the large scale jet structure and result in variability of the observed radiation. For a realistic range of parameters, we find that the growth timescale of the firehose instability is $approx$ a few minutes, in good agreement with the observed TeV variability timescales for Mrk 501 (Albert et al. 2007) and PKS 2155-304 (Aharonian et al. 2007).
Highly magnetized pulsars accreting matter in a binary system are bright sources in the X-ray band (0.1-100 keV). Despite the early comprehension of the basic emission mechanism, their spectral energy distribution is generally described by phenomenol
ogical or simplified models. We propose a study of the spectral emission from the high mass X-ray binary pulsar 4U 0115+634 by means of thermal and bulk Comptonization models based on the physical properties of such objects. For this purpose, we analyze the BeppoSAX data in the energy range 0.7-100 keV of the 1999 giant outburst, 12 days after the maximum. We model the spectral energy distribution of the system using a two-component continuum. At higher energy, above ~7 keV, the emission is due to thermal and bulk Comptonization of the seed photons produced by cyclotron cooling of the accretion column, and at lower energy, the emission is due to thermal Comptonization of a blackbody source in a diffuse halo close to the stellar surface. From the best fit parameters, we argue that the cyclotron emission is produced ~1.7 km above the stellar surface, and escapes from the column near its base, where the absorption features are generated by the interaction with the magnetic field in a surrounding halo. We find that in 4U 0115+634, the observed spectrum is dominated by reprocessed cyclotron radiation, whereas in other bright sources with stronger magnetic fields such as Her X-1, the spectrum is dominated by reprocessed bremsstrahlung.
We aim to illustrate the role of hot protons in enhancing the magnetorotational instability (MRI) via the ``hybrid viscosity, which is due to the redirection of protons interacting with static magnetic field perturbations, and to establish that it is
the only relevant mechanism in this situation. It has recently been shown by Balbus cite{PBM1} and Islam & Balbus cite{PBM11} using a fluid approach that viscous momentum transport is key to the development of the MRI in accretion disks for a wide range of parameters. However, their results do not apply in hot, advection-dominated disks, which are collisionless. We develop a fluid picture using the hybrid viscosity mechanism, that applies in the collisionless limit. We demonstrate that viscous effects arising from this mechanism can significantly enhance the growth of the MRI as long as the plasma $beta gapprox 80$. Our results facilitate for the first time a direct comparison between the MHD and quasi-kinetic treatments of the magnetoviscous instability in hot, collisionless disks.
Accretion-powered X-ray pulsars are among the most luminous X-ray sources in the Galaxy. However, despite decades of theoretical and observational work since their discovery, no satisfactory model for the formation of the observed X-ray spectra has e
merged. In this paper, we report on a self-consistent calculation of the spectrum emerging from a pulsar accretion column that includes an explicit treatment of the bulk and thermal Comptonization occurring in the radiation-dominated shocks that form in the accretion flows. Using a rigorous eigenfunction expansion method, we obtain a closed-form expression for the Greens function describing the upscattering of monochromatic radiation injected into the column. The Greens function is convolved with bremsstrahlung, cyclotron, and blackbody source terms to calculate the emergent photon spectrum. We show that energization of photons in the shock naturally produces an X-ray spectrum with a relatively flat continuum and a high-energy exponential cutoff. Finally, we demonstrate that our model yields good agreement with the spectra of the bright pulsar Her X-1 and the low luminosity pulsar X Per.
