The Shape of Cas A

Based on optical, IR and X-ray studies of Cas A, we propose a geometry for the remnant based on a "jet-induced" scenario with significant systematic departures from axial symmetry. In this model, the main jet axis is oriented in the direction of strong blue-shifted motion at an angle of 110 - 120 degrees East of North and about 40 - 50 degrees to the East of the line of sight. Normal to this axis would be an expanding torus as predicted by jet-induced models. In the proposed geometry, iron-peak elements in the main jet-like flow could appear "beyond" the portions of the remnant rich in silicon by projection effects, not the effect of mixing. In the context of the proposed geometry, the displacement of the compact object from the kinematic center of the remnant at a position angle of ~169 degrees can be accommodated if the motion of the compact object is near to, but slightly off from, the direction of the main "jet" axis by of order 30 degrees. In this model, the classical NE "jet," the SW "counter-jet" and other protrusions, particularly the "hole" in the North, are non-asymmetric flows approximately in the equatorial plane, e.g., out through the perimeter of the expanding torus, rather than being associated with the main jet. We explore the spoke-like flow in the equatorial plane in terms of Rayleigh-Taylor, Richtmyer-Meshkov and Kelvin-Helmholz instabilities and illustrate these instabilities with a jet-induced simulation.Comment: 25 pages, 4 figures. Accepted for publication in the Astrophysical Journa

Polarization in the inner region of Pulsar Wind Nebulae

We present here the first effort to compute synthetic synchrotron polarization maps of Pulsar Wind Nebulae (PWNe). Our goal is to highlight how polarization can be used as an additional diagnostic tool for the flow structure in the inner regions of these nebulae. Recent numerical simulations suggest the presence of flow velocities ~0.5 c in the surroundings of the termination shock, where most of the high energy emission comes from. We construct polarization maps taking into account relativistic effects like Doppler boosting and position angle swing. The effect of different bulk velocities is clarified with the help of a toy-model consisting of a uniformly emitting torus. We also present a map based on recent numerical simulations of the entire nebula and compare it with presently available data. The comparison with upcoming high resolution observations could provide new insight into the inner structure of the nebula and put constraints on the geometrical properties of the magnetic field.Comment: Accepted for publication on A&A, 6 pages, 2 figure

General relativistic radiative transfer: formulation and emission from structured tori around black holes

We construct a general relativistic radiative transfer (RT) formulation, applicable to particles with or without mass in astrophysical settings. Derived from first principles, the formulation is manifestly covariant. Absorption and emission, as well as relativistic, geometrical and optical depth effects are treated self-consistently. The RT formulation can handle 3D geometrical settings and structured objects with variations and gradients in the optical depths across the objects and along the line-of-sight. The presence of mass causes the intensity variation along the particle bundle ray to be reduced by an aberration factor. We apply the formulation and demonstrate RT calculations for emission from accretion tori around rotating black holes, considering two cases: idealised optically thick tori that have a sharply defined emission boundary surface, and structured tori that allow variations in the absorption coefficient and emissivity within the tori. Intensity images and emission spectra of these tori are calculated. Geometrical effects, such as lensing-induced self-occulation and multiple-image contribution are far more significant in accretion tori than geometrically thin accretion disks. Optically thin accretion tori emission line profiles are distinguishable from the profiles of lines from optically thick accretion tori and optically thick geometrically thin accretion disks. Line profiles of optically thin accretion tori have a weaker dependence on viewing inclination angle than those of the optically thick accretion tori or accretion disks, especially at high viewing inclination angles. Limb effects are present in accretion tori with finite optical depths. Finally, in accretion flows onto relativistic compact objects, gravitationally induced line resonance can occur. This resonance occurs easily in 3D flows, but not in 2D flows, such as a thin accretion disk around a black hole.Comment: 13 pages, 10 figures, Accepted for publication in Astronomy and Astrophysic

AGN obscuration from winds: from dusty infrared-driven to warm and X-ray photoionized

We present calculations of AGN winds at ~parsec scales, along with the associated obscuration. We take into account the pressure of infrared radiation on dust grains and the interaction of X-rays from a central black hole with hot and cold plasma. Infrared radiation (IR) is incorporated in radiation-hydrodynamic simulations adopting the flux-limited diffusion approximation. We find that in the range of X-ray luminosities L=0.05 - 0.6 L_edd, the Compton-thick part of the flow (aka torus) has an opening angle of approximately 72-75 degrees regardless of the luminosity. At L > 0.1 L_edd the outflowing dusty wind provides the obscuration with IR pressure playing a major role. The global flow consists of two phases: the cold flow at inclinations \theta > 70 degrees and a hot, ionized wind of lower density at lower inclinations. The dynamical pressure of the hot wind is important in shaping the denser IR supported flow. At luminosities <0.1 L_edd episodes of outflow are followed by extended periods when the wind switches to slow accretion.Comment: accepted for publication in Ap

The X-ray spectrum of a disk illuminated by ions

The X-ray spectrum from a cool disk embedded in an ion supported torus is computed. The interaction of the hot ions with the disk increases the hard X-ray luminosity of the system}. A surface layer of the disk is heated by the protons from the torus. The Comptonized spectrum produced by this layer has a shape that depends only weakly on the incident energy flux and the distance from the accreting compact object. It consists of a `blue bump' of unComptonized soft photons and a flat high energy tail, reminiscent of the observed spectra. The hard tail becomes flatter as the thermalization depth in the cool disk is increased. Further evidence for ion illumination are the Li abundance in the secondaries of low mass X-ray binaries and the 450 keV lines sometimes seen in black-hole transient spectra.Comment: 7p, to appear in Monthly Notice

Characterization of the optical and X-ray properties of the northwestern wisps in the Crab Nebula

We have studied the wisps to the north-west of the Crab pulsar as part of a multi-wavelength campaign in the visible and in X-rays. Optical observations were obtained using the Nordic Optical Telescope in La Palma and X-ray observations were made with the Chandra X-ray Observatory. The observing campaign took place from 2010 October until 2012 September. About once per year we observe wisps forming and peeling off from (or near) the region commonly associated with the termination shock of the pulsar wind. We find that the exact locations of the northwestern wisps in the optical and in X-rays are similar but not coincident, with X-ray wisps preferentially located closer to the pulsar. This suggests that the optical and X-ray wisps are not produced by the same particle distribution. Our measurements and their implications are interpreted in terms of a Doppler-boosted ring model that has its origin in magne- tohydrodynamic (MHD) modelling. While the Doppler boosting factors inferred from the X-ray wisps are consistent with current MHD simulations of pulsar wind nebulae (PWN), the optical boosting factors are not, and typically exceed values from MHD simulations by about a factor of 3.Comment: 11 pages, 12 figure

Modeling the effect of small-scale magnetic turbulence on the X-ray properties of Pulsar Wind Nebulae

Pulsar Wind Nebulae (PWNe) constitute an ideal astrophysical environment to test our current understanding of relativistic plasma processes. It is well known that magnetic fields play a crucial role in their dynamics and emission properties. At present, one of the main issues concerns the level of magnetic turbulence present in these systems, which in the absence of space resolved X-ray polarization measures cannot be directly constrained. In this work we investigate, for the first time using simulated synchrotron maps, the effect of a small scale fluctuating component of the magnetic field on the emission properties in X-ray. We illustrate how to include the effects of a turbulent component in standard emission models for PWNe, and which consequences are expected in terms of net emissivity and depolarization, showing that the X-ray surface brightness maps can provide already some rough constraints. We then apply our analysis to the Crab and Vela nebulae and, by comparing our model with Chandra and Vela data, we found that the typical energies in the turbulent component of the magnetic field are about 1.5 to 3 times the one in the ordered field.Comment: 9 pages, 8 figures, accepted for publication in MNRA