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Bow shocks generated by pulsars moving through weakly ionized interstellar medium (ISM) produce emission dominated by non-equilibrium atomic transitions. These bow shocks are primarily observed as H$_\alpha$ nebulae. We developed a package, named Shu, that calculates non-LTE intensity maps in more than 150 spectral lines, taking into account geometrical properties of the pulsars' motion and lines of sight. We argue here that atomic (CI, NI, OI) and ionic (SII, NII, OIII, NeIV) transitions can be used as complementary and sensitive probes of ISM. We perform self-consistent 2D relativistic hydrodynamic calculations of the bow shock structure and generate non-LTE emissivity maps, combining global dynamics of relativistic flows, and detailed calculations of the non-equilibrium ionization states. We find that though typically H$_\alpha$ emission is dominant, spectral fluxes in OIII, SII and NII may become comparable for relatively slowly moving pulsars. Overall, morphology of non-LTE emission, especially of the ionic species, is a sensitive probe of the density structures of the ISM., Comment: 20 pages, 14 figures, 4 tables. Submitted to PASA

This is a science white paper submitted to the Astro-2020 and Plasma-2020 Decadal Surveys. The paper describes the present status and emerging opportunities in Extreme Plasma Astrophysics -- a study of astrophysically-relevant plasma processes taking place under extreme conditions that necessitate taking into account relativistic, radiation, and QED effects., Comment: A science white paper submitted to the Astro-2020 and Plasma-2020 Decadal Surveys. 7 pages including cover page and references. Paper updated in late March 2019 to include a several additional co-authors and references, and a few small changes

THESEUS is a space mission concept aimed at exploiting Gamma-Ray Bursts for investigating the early Universe and at providing a substantial advancement of multi-messenger and time-domain astrophysics. These goals will be achieved through a unique combination of instruments allowing GRB and X-ray transient detection over a broad field of view (more than 1sr) with 0.5-1 arcmin localization, an energy band extending from several MeV down to 0.3 keV and high sensitivity to transient sources in the soft X-ray domain, as well as on-board prompt (few minutes) follow-up with a 0.7 m class IR telescope with both imaging and spectroscopic capabilities. THESEUS will be perfectly suited for addressing the main open issues in cosmology such as, e.g., star formation rate and metallicity evolution of the inter-stellar and intra-galactic medium up to redshift $\sim$10, signatures of Pop III stars, sources and physics of re-ionization, and the faint end of the galaxy luminosity function. In addition, it will provide unprecedented capability to monitor the X-ray variable sky, thus detecting, localizing, and identifying the electromagnetic counterparts to sources of gravitational radiation, which may be routinely detected in the late '20s / early '30s by next generation facilities like aLIGO/ aVirgo, eLISA, KAGRA, and Einstein Telescope. THESEUS will also provide powerful synergies with the next generation of multi-wavelength observatories (e.g., LSST, ELT, SKA, CTA, ATHENA)., Comment: Accepted for publication in Advances in Space Research. Partly based on the proposal submitted on October 2016 in response to the ESA Call for next M5 mission, with expanded and updated science sections

Rotation-powered pulsars and magnetars are two different observational manifestations of neutron stars: rotation powered pulsars are rapidly spinning objects that are mostly observed as pulsating radio sources, while magnetars, neutron stars with the highest known magnetic fields, often emit short-duration X-ray bursts. Here we report simultaneous observations of the high-magnetic-field radio pulsar PSR J1119-6127 at X-ray, with XMM-Newton & NuSTAR, and at radio energies with Parkes radio telescope, during a period of magnetar-like bursts. The rotationally powered radio emission shuts off coincident with the occurrence of multiple X-ray bursts, and recovers on a time scale of ~70 seconds. These observations of related radio and X-ray phenomena further solidify the connection between radio pulsars and magnetars, and suggest that the pair plasma produced in bursts can disrupt the acceleration mechanism of radio emitting particles., Comment: 8 pages, 4 figures. Submitted to ApJL

We investigate the dynamics of bow shock nebulae created by pulsars moving supersonically through a partially ionized interstellar medium. A fraction of interstellar neutral hydrogen atoms penetrating into the tail region of a pulsar wind will undergo photo-ionization due to the UV light emitted by the nebula, with the resulting mass loading dramatically changing the flow dynamics of the light leptonic pulsar wind. Using a quasi 1-D hydrodynamic model of relativistic flow we find that if a relatively small density of neutral hydrogen, as low as $10^{-4}$ cm$^{-3}$, penetrate inside the pulsar wind, this is sufficient to strongly affect the tail flow. Mass loading leads to the fast expansion of the pulsar wind tail, making the tail flow intrinsically non-stationary. The shapes predicted for the bow shock nebulae compare well with observations, both in H$\alpha$ and X-rays., Comment: 7 pages, 2 figures. Proceeding to the conference "High Energy Phenomena in Relativistic Outflow V", La Plata 2015, AAA Workshop Series 8, 2015

We study the transport of high-energy particles in pulsar wind nebulae (PWN) using three-dimensional MHD (see Porth et al. (2014) for details) and test-particle simulations, as well as a Fokker-Planck particle transport model. The latter includes radiative and adiabatic losses, diffusion, and advection on the background flow of the simulated MHD nebula. By combining the models, the spatial evolution of flux and photon index of the X-ray synchrotron emission is modelled for the three nebulae G21.5-0.9, the inner regions of Vela, and 3C 58, thereby allowing us to derive governing parameters: the magnetic field strength, average flow velocity and spatial diffusion coefficient. For comparison, the nebulae are also modelled with the semi-analytic Kennel & Coroniti (1984) model but the Porth et al. (2014) model generally yields better fits to the observational data. We find that high velocity fluctuations in the turbulent nebula (downstream of the termination shock) give rise to efficient diffusive transport of particles, with average P\'eclet number close to unity, indicating that both advection and diffusion play an important role in particle transport. We find that the diffusive transport coefficient of the order of $\sim2\times 10^{27} (L_{\rm s}/0.42\rm Ly) cm^{2}s^{-1}$ ($L_{\rm s}$ is the size of the termination shock) is independent of energy up to extreme particle Lorentz factors of $\gamma_{p}\sim10^{10}$., Comment: submitted to MNRAS

The relativistic double neutron star binary PSR J0737-3039 shows clear evidence of orbital phase-dependent wind-companion interaction, both in radio and X-rays. In this paper we present the results of timing analysis of PSR J0737-3039 performed during 2006 and 2011 XMM-Newton Large Programs that collected ~20,000 X-ray counts from the system. We detected pulsations from PSR J0737-3039A (PSR A) through the most accurate timing measurement obtained by XMM-Newton so far, the spin period error being of 2x10^-13 s. PSR A's pulse profile in X-rays is very stable despite significant relativistic spin precession that occurred within the time span of observations. This yields a constraint on the misalignment between the spin axis and the orbital momentum axis Delta_A ~6.6^{+1.3}_{-5.4} deg, consistent with estimates based on radio data. We confirmed pulsed emission from PSR J0737-3039B (PSR B) in X-rays even after its disappearance in radio. The unusual phenomenology of PSR B's X-ray emission includes orbital pulsed flux and profile variations as well as a loss of pulsar phase coherence on time scales of years. We hypothesize that this is due to the interaction of PSR A's wind with PSR B's magnetosphere and orbital-dependent penetration of the wind plasma onto PSR B closed field lines. Finally, the analysis of the full XMM-Newton dataset provided evidences of orbital flux variability (~7%) for the first time, involving a bow-shock scenario between PSR A's wind and PSR B's magnetosphere., Comment: Comments: 16 Pages, 6 Figures. Accepted for publication in Astrophysical Journal (Draft Version)

We investigate the dynamics of bow shock nebulae created by pulsars moving supersonically through a partially ionized interstellar medium. A fraction of interstellar neutral hydrogen atoms penetrating into the tail region of a pulsar wind will undergo photo-ionization due to the UV light emitted by the nebula, with the resulting mass loading dramatically changing the flow dynamics of the light leptonic pulsar wind. Using a quasi 1-D hydrodynamic model of both non-relativistic and relativistic flow, and focusing on scales much larger than the stand-off distance, we find that if a relatively small density of neutral hydrogen, as low as $10^{-4}\,\text{cm}^{-3}$, penetrate inside the pulsar wind, this is sufficient to strongly affect the tail flow. Mass loading leads to the fast expansion of the pulsar wind tail, making the tail flow intrinsically non-stationary. The shapes predicted for the bow shock nebulae compare well with observations, both in H$\alpha$ and X-rays., Comment: 18 pages, 10 figures. Accepted for publication in MNRAS

In this paper we describe a simple numerical approach which allows to study the structure of steady-state axisymmetric relativistic jets using one-dimensional time-dependent simulations. It is based on the fact that for narrow jets with v~c the steady-state equations of relativistic magnetohydrodynamics can be accurately approximated by the one-dimensional time-dependent equations after the substitution z=ct. Since only the time-dependent codes are now publicly available this is a valuable and efficient alternative to the development of a high-specialized code for the time-independent equations. The approach is also much cheaper and more robust compared to the relaxation method. We tested this technique against numerical and analytical solutions found in literature as well as solutions we obtained using the relaxation method and found it sufficiently accurate. In the process, we discovered the reason for the failure of the self-similar analytical model of the jet reconfinement in relatively flat atmospheres and elucidated the nature of radial oscillations of steady-state jets., Comment: published in the Journal of Computational Astrophysics and Cosmology http://www.comp-astrophys-cosmol.com/

This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of gamma-ray bursts. For a summary, we refer to the paper., Comment: White Paper in Support of the Mission Concept of the Large Observatory for X-ray Timing

We present the results of 71.6 hours of observations of the Geminga pulsar (PSR J0633+1746) with the VERITAS very-high-energy gamma-ray telescope array. Data taken with VERITAS between November 2007 and February 2013 were phase-folded using a Geminga pulsar timing solution derived from data recorded by the XMM-\emph{Newton} and \emph{Fermi}-LAT space telescopes. No significant pulsed emission above 100 GeV is observed, and we report upper limits at the 95% confidence level on the integral flux above 135 GeV (spectral analysis threshold) of 4.0$\times10^{-13}$ s$^{-1}$ cm$^{-2}$ and 1.7$\times10^{-13}$ s$^{-1}$ cm$^{-2}$ for the two principal peaks in the emission profile. These upper limits, placed in context with phase-resolved spectral energy distributions determined from five years of data from the \emph{Fermi}-LAT, constrain possible hardening of the Geminga pulsar emission spectra above $\sim$50 GeV., Comment: 7 pages, 2 figures, 2 tables. Accepted for publication in ApJ

In March 2013, a flaring episode from the Crab Nebula lasting ~2 weeks was detected by the Fermi-LAT (Large Area Telescope on board the Fermi Gamma-ray Space Telescope). VERITAS provides simultaneous observations throughout this period. During the flare, the Fermi-LAT detected a 20-fold increase in flux above the average synchrotron flux >100 MeV seen from the Crab Nebula. Simultaneous measurements with VERITAS are consistent with the non-variable long-term average Crab Nebula flux at TeV energies. Assuming a linear correlation between the very-high-energy flux change >1 TeV and the flux change seen in the Fermi-LAT band >100 MeV during the period of simultaneous observations, the linear correlation factor can be constrained to be at most 8.6 * 10^-3 with 95% confidence., Comment: 4 figures, 1 table, published in The Astrophysical Journal Letters

We present solutions for Hall equilibria applicable to neutron star crusts. Such magnetic configurations satisfy a Grad-Shafranov-type equation, which is solved analytically and numerically. The solutions presented cover a variety of configurations, from purely poloidal fields connected to an external dipole to poloidal-toroidal fields connected to an external vacuum field, or fully confined within the star. We find that a dipole external field should be supported by a uniformly rotating electron fluid. The energy of the toroidal magnetic field is generally found to be a few percent of the total magnetic field energy for the fields with an external component. We discuss the evolution due to Ohmic dissipation which leads to slowing down of the electron fluid. We also find that the transition from an MHD equilibrium to a state governed by Hall effect, generates spontaneously an additional toroidal field in regions where the electron fraction changes., Comment: 12 pages, 9 figures, accepted by MNRAS

We present Hall equilibrium solutions for neutron stars crusts containing toroidal and poloidal magnetic field. Some simple cases are solved analytically while more complicated configurations are found numerically through a Gauss-Seidel elliptic partial differential equation solver., Comment: Conference Proceedings, Magnetic Fields in the Universe IV (2013)

We present the results of a joint observational campaign between the Green Bank radio telescope and the VERITAS gamma-ray telescope, which searched for a correlation between the emission of very-high-energy (VHE) gamma rays ($E_{\gamma} >$ 150 GeV) and Giant Radio Pulses (GRPs) from the Crab pulsar at 8.9 GHz. A total of 15366 GRPs were recorded during 11.6 hours of simultaneous observations, which were made across four nights in December 2008 and in November and December 2009. We searched for an enhancement of the pulsed gamma-ray emission within time windows placed around the arrival time of the GRP events. In total, 8 different time windows with durations ranging from 0.033 ms to 72 s were positioned at three different locations relative to the GRP to search for enhanced gamma-ray emission which lagged, led, or was concurrent with, the GRP event. Further, we performed separate searches on main pulse GRPs and interpulse GRPs and on the most energetic GRPs in our data sample. No significant enhancement of pulsed VHE emission was found in any of the preformed searches. We set upper limits of 5-10 times the average VHE flux of the Crab pulsar on the flux simultaneous with interpulse GRPs on single-rotation-period time scales. On $\sim$8-second time scales around interpulse GRPs, we set an upper limit of 2-3 times the average VHE flux. Within the framework of recent models for pulsed VHE emission from the Crab pulsar, the expected VHE-GRP emission correlations are below the derived limits., Comment: 12 pages, 12 figures, To appear in The Astrophysical Journal

In the double pulsar system PSR J0737-3039A/B the strong wind produced by pulsar A distorts the magnetosphere of pulsar B. The influence of these distortions on the orbital-dependent emission properties of pulsar B can be used to determine the location of the coherent radio emission generation region in the pulsar magnetosphere. Using a model of the wind-distorted magnetosphere of pulsar B and the well defined geometrical parameters of the system, we determine the minimum emission height to be ~ 20 neutron star radii in the two bright orbital longitude regions. We can determine the maximum emission height by accounting for the amount of deflection of the polar field line with respect to the magnetic axis using the analytical magnetic reconnection model of Dungey and the semi-empirical numerical model of Tsyganenko. Both of these models estimate the maximum emission height to be ~ 2500 neutron star radii. The minimum and maximum emission heights we calculate are consistent with those estimated for normal isolated pulsars., Comment: 29 pages, 14 figures, Accepted by ApJ on 3 March 2012

The double pulsar PSR J0737-3039A/B displays short, 30 s eclipses that arise around conjunction when the radio waves emitted by pulsar A are absorbed as they propagate through the magnetosphere of its companion pulsar B. These eclipses offer a unique opportunity to probe directly the magnetospheric structure and the plasma properties of pulsar B. We have performed a comprehensive analysis of the eclipse phenomenology using multi-frequency radio observations obtained with the Green Bank Telescope. We have characterized the periodic flux modulations previously discovered at 820 MHz by McLaughlin et al., and investigated the radio frequency dependence of the duration and depth of the eclipses. Based on their weak radio frequency evolution, we conclude that the plasma in pulsar B's magnetosphere requires a large multiplicity factor (~ 10^5). We also found that, as expected, flux modulations are present at all radio frequencies in which eclipses can be detected. Their complex behavior is consistent with the confinement of the absorbing plasma in the dipolar magnetic field of pulsar B as suggested by Lyutikov & Thompson and such a geometric connection explains that the observed periodicity is harmonically related to pulsar B's spin frequency. We observe that the eclipses require a sharp transition region beyond which the plasma density drops off abruptly. Such a region defines a plasmasphere which would be well inside the magnetospheric boundary of an undisturbed pulsar. It is also two times smaller than the expected standoff radius calculated using the balance of the wind pressure from pulsar A and the nominally estimated magnetic pressure of pulsar B., Comment: 9 pages, 7 figures, 3 tables, ApJ in press

Compilation of papers contributed by the VERITAS Collaboration to the 32nd International Cosmic Ray Conference, held 11-18 August 2011 in Beijing, China., Comment: 1 html page. ICRC 2011, Beijing, China. A full list of authors and collaborating institutions can be found at http://veritas.sao.arizona.edu/conferences/authors?icrc2011

The VERITAS telescope array has been operating smoothly since 2007, and has detected gamma-ray emission above 100 GeV from 40 astrophysical sources. These include blazars, pulsar wind nebulae, supernova remnants, gamma-ray binary systems, a starburst galaxy, a radio galaxy, the Crab pulsar, and gamma-ray sources whose origin remains unidentified. In 2009, the array was reconfigured, greatly improving the sensitivity. We summarize the current status of the observatory, describe some of the scientific highlights since 2009, and outline plans for the future., Comment: Presented at the 32nd ICRC, Beijing, 2011

We present an analytical class of equilibrium solutions for the structure of relativistic sheared and rotating magnetized jets that contain no boundary current sheets. We demonstrate the overall dynamical stability of these solutions and, most importantly, a better numerical resistive stability than the commonly employed force-free structures which inevitably require the presence of dissipative surface currents. The jet is volumetrically confined by the external pressure, with no pressure gradient on the surface. We calculate the expected observed properties of such jets. Given the simplicity of these solution we suggest them as useful initial conditions for relativistic jet simulations., Comment: 13 pages, 13 figures, Accepted by MNRAS

We report the detection of pulsed gamma rays from the Crab pulsar at energies above 100 Gigaelectronvolts (GeV) with the VERITAS array of atmospheric Cherenkov telescopes. The detection cannot be explained on the basis of current pulsar models. The photon spectrum of pulsed emission between 100 Megaelectronvolts (MeV) and 400 GeV is described by a broken power law that is statistically preferred over a power law with an exponential cutoff. It is unlikely that the observation can be explained by invoking curvature radiation as the origin of the observed gamma rays above 100 GeV. Our findings require that these gamma rays be produced more than 10 stellar radii from the neutron star., Comment: 17 pages, corresponding authors: Nepomuk Otte nepomuk.otte@gmail.com; Andrew McCann mccann@hep.physics.mcgill.ca; Martin Schroedter schroedter@veritas.sao.arizona.edu

To constrain the giant pulse (GP) emission mechanism and test the model of Lyutikov (2007) for GP emission, we have carried out a campaign of simultaneous observations of the Crab pulsar at gamma-ray (Fermi) and radio (Green Bank Telescope) wavelengths. Over 10 hours of simultaneous observations we obtained a sample of 2.1x10^4 giant pulses, observed at a radio frequency of 9 GHz, and 77 Fermi photons, with energies between 100 MeV and 5 GeV. The majority of GPs came from the interpulse (IP) phase window. We found no change in the GP generation rate within 10-120 s windows at lags of up to +-40 min of observed gamma-ray photons. The 95% upper limit for a gamma-ray flux enhancement in pulsed emission phase window around all GPs is 4 times the average pulsed gamma-ray flux from the Crab. For the subset of IP GPs, the enhancement upper limit, within the IP emission window, is 12 times the average pulsed gamma-ray flux. These results suggest that GPs, at least high-frequency IP GPs, are due to changes in coherence of radio emission rather than an overall increase in the magnetospheric particle density., Comment: 9 pages, 6 figures; to appear in The Astrophysical Journal, February 2011

The oblique geometry of pulsar wind termination shock ensures that the Doppler beaming has a strong impact on the shock emission. We illustrate this using recent relativistic MHD simulations of the Crab Nebula and also show that the observed size, shape, and distance from the pulsar of the Crab Nebula inner knot are consistent with its interpretation as a Doppler-boosted emission from the termination shock. If the electrons responsible for the synchrotron gamma-rays are accelerated only at the termination shock then their short life-time ensures that these gamma-rays originate close to the shock and are also strongly effected by the Doppler beaming. As the result, bulk of the observed synchrotron gamma-rays of the Crab Nebula around 100 MeV may come from its inner knot. This hypothesis is consistent with the observed optical flux of the inner knot provided its optical-gamma spectral index is the same as the injection spectral index found in the Kennel & Coroniti model of the nebula spectrum. The observed variability of synchrotron gamma-ray emission can be caused by the instability of the termination shock discovered in recent numerical simulations. Given the small size of the knot, it is possible that the September 2010 gamma-ray flare of the Crab Nebula also came from the knot, though the actual mechanism remains unclear. The model predicts correlation of the temporal variability of the synchrotron gamma-ray flux in the Fermi and AGILE windows with the variability of the unpulsed optical flux from within 1 arcsec of the Crab pulsar., Comment: submitted to MNRAS, typos corrected, new references added, additional issues discussed

To constrain the giant pulse (GP) emission mechanism and test the model of Lyutikov (2007) of GP emission, we are carrying out a campaign of simultaneous observations of the Crab pulsar between gamma-rays (Fermi) and radio wavelengths. The correlation between times of arrival of radio GPs and high-energy photons, whether it exists or not, will allow us to choose between different origins of GP emission and further constrain the emission physics. Our foremost goal was testing whether radio GPs are due to changes in the coherence of the radio emission mechanism, variations in the pair creation rate in the pulsar magnetosphere, or changes in the beaming direction. Accomplishing this goal requires an enormous number of simultaneous radio GPs and gamma-photons. Thus, we organized a radio observations campaign using the 42-ft telescope at the Jodrell Bank Observatory (UK), the 140-ft telescope, and the 100-m Robert C. Byrd Green Bank Telescope (GBT) at the Green Bank Observatory (WV). While the observations with the two first ones are ongoing, here we present the preliminary results of 20 hrs of observations with the GBT at the high frequency of 8.9 GHz. These particular observations were aimed to probe the model of GP emission by Lyutikov (2007) which predicts that GPs at frequencies > 4 GHz should be accompanied by gamma-ray photons of energies of 1-100 GeV., Comment: 2009 Fermi Symposium, eConf Proceedings C091122

We present a systematic fit of a model of resonant cyclotron scattering (RCS) to the X and soft gamma-ray data of four magnetars, including anomalous X-ray pulsars, and soft gamma repeaters. In this scenario, non-thermal magnetar spectra in the soft X-rays result from resonant cyclotron scattering of the thermal surface emission by hot magnetospheric plasma. We find that this model can successfully account for the soft X-ray emission of magnetars, while an additional component is still required to model the hard X-ray persistent magnetar emission recently discovered by INTEGRAL. The latter is an important component in terms of magnetars' luminosity, and cannot be neglected when modelling the soft X-ray part of the spectrum., Comment: 12 pages, 4 figures, presented at the 7th INTEGRAL Workshop "An INTEGRAL View of Compact Objects", 8-11 September 2008, to be published by PoS

In this White Paper, we emphasize the need for and the important role of plasma astrophysics in the studies of formation, evolution of, and feedback by Active Galaxies. We make three specific recommendations: 1) We need to significantly increase the resolution of VLA, perhaps by building an EVLA-II at a modest cost. This will provide the angular resolution to study jets at kpc scales, where, for example, detailed Faraday rotation diagnosis can be done at 1GHz transverse to jets; 2) We need to build coordinated programs among NSF, NASA, and DOE to support laboratory plasma experiments (including liquid metal) that are designed to study key astrophysical processes, such as magneto-rotational instability (origin of angular momentum transport), dynamo (origin of magnetic fields), jet launching and stability. Experiments allowing access to relativistic plasma regime (perhaps by intense lasers and magnetic fields) will be very helpful for understanding the stability and dissipation physics of jets from Supermassive Black Holes; 3) Again through the coordinated support among the three Agencies, we need to invest in developing comprehensive theory and advanced simulation tools to study the accretion disks and jets in relativistic plasma physics regime, especially in connecting large scale fluid scale phenomena with relativistic kinetic dissipation physics through which multi-wavelength radiation is produced., Comment: a White Paper submitted to GCT and FCP panels of Astro2010

Multi-wavelength studies at radio, infrared, optical, X-ray, and TeV wavelengths have discovered probable counterparts to many Galactic sources of GeV emission detected by EGRET. These include pulsar wind nebulae, high mass X-ray binaries, and mixed morphology supernova remnants. Here we provide an overview of the observational properties of Galactic sources which emit across 19 orders of magnitude in energy. We also present new observations of several sources., Comment: 4 pages, 5 figures, Proceedings of the The 4th Heidelberg International Symposium on High Energy Gamma-Ray Astronomy, eds. Aharonian, Hofmann, Rieger

The X-ray spectra of the magnetar candidates are customarily fitted with an empirical, two component model: an absorbed blackbody and a power-law. However, the physical interpretation of these two spectral components is rarely discussed. It has been recently proposed that the presence of a hot plasma in the magnetosphere of highly magnetized neutron stars might distort, through efficient resonant cyclotron scattering, the thermal emission from the neutron star surface, resulting in the production of non-thermal spectra. Here we discuss the Resonant Cyclotron Scattering (RCS) model, and present its XSPEC implementation, as well as preliminary results of its application to Anomalous X-ray Pulsars and Soft Gamma-ray Repeaters., Comment: 5 pages, 5 color figures; Astrophysics & Space Science, in press ("Isolated Neutron Stars"; London, UK)

Many astrophysical sources of high energy emission, such as black hole magnetospheres, superstrongly magnetized neutron stars (magnetars), and probably relativistic jets in Active Galactic Nuclei and Gamma Ray Bursts involve relativistically magnetically dominated plasma. In such plasma the energy density of magnetic field greatly exceeds the thermal and the rest mass energy density of particles. Therefore the magnetic field is the main reservoir of energy and its dissipation may power the bursting emission from these sources, in close analogy to Solar flares. One of the principal dissipative instabilities that may lead to release of magnetic energy is the tearing instability. In this paper we study, both analytically and numerically, the development of tearing instability in relativistically magnetically-dominated plasma using the framework of resistive magnetodynamics. We confirm and elucidate the previously obtained result on the growth rate of the tearing mode: the shortest growth time is the same as in the case of classical non-relativistic MHD, namely $\tau =\sqrt{\tau_a \tau_d}$ where $\tau_a$ is the \Alfven crossing time and $\tau_d$ is the resistive time of a current layer., Comment: Submitted to MNRAS, few typos corrected

We report the first X-ray detection of the radio pulsar PSR B1757-24 using the Chandra X-ray Observatory. The image reveals point-source emission at the pulsar position, consistent with being magnetospheric emission from the pulsar. In addition, we detect a faint tail extending nearly 20" east of the pulsar, in the same direction and with comparable morphology to the pulsar's well-studied radio tail. The X-ray tail is unlikely to be emission left behind following the passage of the pulsar, but rather is probably from synchrotron-emitting pulsar wind particles having flow velocity ~7000 km/s. Assuming the point-source X-ray emission is magnetospheric, the observed X-ray tail represents only ~0.01% of the pulsar's spin-down luminosity, significantly lower than the analogous efficiencies of most known X-ray nebulae surrounding rotation-powered pulsars., Comment: 4 pages, incl 2 figures; To appear in "Neutron Stars in Supernova Remnants" (ASP Conference Proceedings), eds P. O. Slane and B. M. Gaensler

We present high-resolution radio and X-ray studies of the composite supernova remnant G11.2-0.3. Using archival VLA data, we perform radio spectral tomography to measure for the first time the spectrum of the shell and plerion separately. We compare the radio morphology of each component to that observed in the hard and soft Chandra X-ray images. We measure the X-ray spectra of the shell and the emission in the interior and discuss the hypothesis that soft X-ray emission interior to the shell is the result of the expanding pulsar wind shocking with the supernova ejecta. We also see evidence for spatial variability in the hard X-ray emission near the pulsar, which we discuss in terms of ion mediated relativistic shocks., Comment: 4 pages, 2 figures, To appear in "Neutron Stars in Supernova Remnants" (ASP Conference Proceedings), eds P. O. Slane and B. M. Gaensler

In the framework of the magnetar model for the Soft Gamma Repeaters and Anomalous X-ray Pulsars, we consider the structure of neutron star magnetospheres threaded by large-scale electrical currents. We construct self-similar, force-free equilibria under the assumption of axisymmetry and a power law dependence of magnetic field on radius, ${\bf B} \propto r^{-(2+p)}$. A large-scale twist of the field lines softens the radial dependence to $p < 1$, thereby accelerating the spindown torque with respect to a vacuum dipole. A magnetosphere with a strong twist ($B_\phi/B_\theta = O(1)$ at the equator) has an optical depth $\sim 1$ to resonant cyclotron scattering, independent of frequency (radius), surface magnetic field strength, or the charge/mass ratio of the scattering charge. We investigate the effects of the resonant Compton scattering by the charge carriers (both electrons and ions) on the emergent X-ray spectra and pulse profiles., Comment: 4 pages, 2 figures, PSR/SNR Workshop ``Neutron Stars in Supernova Remnants''

(ABBREVIATED) We consider the structure of neutron star magnetospheres threaded by large-scale electrical currents, and the effect of resonant Compton scattering by the charge carriers (both electrons and ions) on the emergent X-ray spectra and pulse profiles. In the magnetar model for the SGRs and AXPs, these currents are maintained by magnetic stresses acting deep inside the star. We construct self-similar, force-free equilibria of the current-carrying magnetosphere with a power-law dependence of magnetic field on radius, B ~ r^(-2-p), and show that a large-scale twist softens the radial dependence to p < 1. The spindown torque acting on the star is thereby increased in comparison with a vacuum dipole. We comment on the strength of the surface magnetic field in the SGR and AXP sources, and the implications of this model for the narrow measured distribution of spin periods. A magnetosphere with a strong twist, B_\phi/B_\theta = O(1) at the equator, has an optical depth ~ 1 to resonant cyclotron scattering, independent of frequency (radius), surface magnetic field strength, or charge/mass ratio of the scattering charge. When electrons and ions supply the current, the stellar surface is also heated by the impacting charges at a rate comparable to the observed X-ray output of the SGR and AXP sources, if B_{dipole} ~ 10^{14} G. Redistribution of the emerging X-ray flux at the ion and electron cyclotron resonances will significantly modify the emerging pulse profile and, through the Doppler effect, generate a non-thermal tail to the X-ray spectrum. The sudden change in the pulse profile of SGR 1900+14 after the 27 August 1998 giant flare is related to an enhanced optical depth to electron cyclotron scattering, resulting from a sudden twist imparted to the external magnetic field., Comment: 31 January 2002, minor revisions, new section 5.4.2

We report the first X-ray detection of the radio pulsar PSR B1757-24 using the Chandra X-ray Observatory. We detect point-source emission at the pulsar position plus a faint tail extending nearly 20" east of the pulsar, in the same direction and with comparable morphology to the radio tail. Assuming the point-source X-ray emission is magnetospheric, the observed X-ray tail represents only ~0.01% of the pulsar's spin-down luminosity. This is significantly lower than the analogous efficiencies of most known X-ray nebulae surrounding rotation-powered pulsars. Assuming a non-thermal spectrum for the tail photons, we show that the tail is unlikely to be emission left behind following the passage of the pulsar, but rather is probably from synchrotron-emitting pulsar wind particles having flow velocity ~7000 km/s. We also show that there must be a significant break in the tail synchrotron spectrum between the radio and X-ray bands that is intrinsic to the particle spectrum. No emission is detected from the shell supernova remnant G5.4-1.2. The upper limits on remnant emission are unconstraining., Comment: Updated Journal ref, corrected typo for flow velocity