Demosthenes Kazanas

The Axisymmetric Pulsar Magnetosphere

We present, for the first time, the structure of the axisymmetric force-free magnetosphere of an aligned rotating magnetic dipole, in the case in which there exists a sufficiently large charge density (whose origin we do not question) to satisfy the ideal MHD condition, E B=0, everywhere. The unique distribution of electric current along the open magnetic field lines that is required for the solution to be continuous and smooth is obtained numerically. With the geometry of the field lines thus determined, we compute the dynamics of the associated MHD wind. The main result is that the relativistic outflow contained in the magnetosphere is not accelerated to the extremely relativistic energies required for the flow to generate gamma rays. We expect that our solution will be useful as the starting point for detailed studies of pulsar magnetospheres under more general conditions, namely, when the force-free and/or the ideal MHD condition, E B=0, are not valid in the entire magnetosphere. Based on our solution, we consider that the most likely positions of such an occurrence are the polar cap, the crossings of the zero space charge surface by open field lines, and the return current boundary, but not the light cylinder.

Exact Vacuum Solution to Conformal Weyl Gravity and Galactic Rotation Curves

The complete, exact exterior solution for a static, spherically symmetric source in locally conformal invariant Weyl gravity is presented. The solution includes the familiar exterior Schwarzschild solution as a special case and contains an extra gravitational potential term which grows linearly with distance. The obtained solution provides a potential explanation for observed galactic rotation curves without the need for dark matter. The solution also has some interesting implications for cosmology.

Thermal instability and photoionized X-ray reflection in accretion disks

We study the illumination of accretion disks in the vicinity of compact objects by an overlying X-ray source. Our approach differs from previous works of the subject in that we relax the simplifying assumption of constant gas density used in these studies; instead we determine the density from hydrostatic balance which is solved simultaneously with the ionization balance and the radiative transfer in a plane-parallel geometry. We calculate the temperature profile of the illuminated layer and the reprocessed X-ray spectra for a range of physical conditions, values of photon index Γ for the illuminating radiation, and the incident and viewing angles. In accordance with some earlier studies, we find that the self-consistent density determination makes evident the presence of a thermal ionization instability well known in the context of quasar emission line studies. The main effect of this instability is to prevent the illuminated gas from attaining temperatures at which the gas is unstable to thermal perturbations. Thus, in sharp contrast to the constant density calculations that predict a continuous and rather smooth variation of the gas temperature in the illuminated material, we find that the temperature profile consists of several well defined thermally stable layers. Transitions between these stable layers are very sharp and can be treated as discontinuities as far as the reprocessed spectra are concerned. In particular, the uppermost layers of the X-ray illuminated gas are found to be almost completely ionized and at the local Compton temperature (~107-108 K); at larger depths, the gas temperature drops abruptly to form a thin layer with T ~ 106 K, while at yet larger depths it decreases sharply to the disk effective temperature. For a given X-ray spectral index, this discontinuous temperature structure is governed by just one parameter, A, which characterizes the strength of the gravitational force relative to the incident X-ray flux. We find that most of the Fe Kα line emission and absorption edge are produced in the coolest, deepest layers, while the Fe atoms in the hottest, uppermost layers are generally almost fully ionized, hence making a negligible contribution to reprocessing features in the ~6.4-10 keV energy range. We also find that the Thomson depth of the top hot layers is pivotal in determining the fraction of the X-ray flux which penetrates to the deeper cooler layers, thereby affecting directly the strength of the Fe line, edge and reflection features. Due to the interplay of these effects, for Γ 2, the equivalent width (EW) of the Fe features decreases monotonically with the magnitude of the illuminating flux, while the line centroid energy remains at 6.4 keV. We provide a summary of the dependence of the reprocessing features in the X-ray reflected spectra on the gravity parameter A, the spectral index Γ, and other parameters of the problem. We emphasis that the results of our self-consistent calculations are both quantitatively and qualitatively different from those obtained using the constant density assumption. Therefore, we propose that future X-ray reflection calculations should always utilize hydrostatic balance in order to provide a reliable interpretation of X-ray spectra of active galactic nuclei and galactic black hole candidates.

Long-Lag, Wide-Pulse Gamma-Ray Bursts

Currently, the best available probe of the early phase of gamma-ray burst (GRB) jet attributes is the prompt gamma-ray emission, in which several intrinsic and extrinsic variables determine GRB pulse evolution. Bright, usually complex bursts have many narrow pulses that are difficult to model due to overlap. However, the relatively simple, long spectral lag, wide-pulse bursts may have simpler physics and are easier to model. In this work we analyze the temporal and spectral behavior of wide pulses in 24 long-lag bursts, using a pulse model with two shape parameters—width and asymmetry—and the Band spectral model with three shape parameters. We find that pulses in long-lag bursts are distinguished both temporally and spectrally from those in bright bursts: the pulses in long spectral lag bursts are few in number and ~100 times wider (tens of seconds), have systematically lower peaks in νF(ν), and have harder low-energy spectra and softer high-energy spectra. We find that these five pulse descriptors are essentially uncorrelated for our long-lag sample, suggesting that at least ~5 parameters are needed to model burst temporal and spectral behavior. However, pulse width is strongly correlated with spectral lag; hence, these two parameters may be viewed as mutual surrogates. We infer that accurate formulations for estimating GRB luminosity and total energy will depend on several gamma-ray attributes, at least for long-lag bursts. The prevalence of long-lag bursts near the BATSE trigger threshold, their predominantly low νF(ν) spectral peaks, and relatively steep upper power-law spectral indices indicate that Swift will detect many such bursts.

Temporal and Spectral Properties of Comptonized Radiation and Its Applications

We have found relations between the temporal and spectral properties of radiation Comptonized in an extended atmosphere associated with compact accreting sources. We demonstrate that the Nuctuation power spectrum density (PSD) imposes constraints on the atmosphere scale and pro-le. Furthermore, we indicate that the slope and low-frequency break of the PSD are related to the Thomson depth, of the q 0 , atmosphere and the radius of its physical size, respectively. Since the energy spectrum of the escaping radiation also depends on (and the electron temperature the relation between spectral and tem- q 0 kT e ), poral properties follows. This relation allows, for the -rst time, an estimate of the accreting matter Thomson depth, independent of arguments involving Comptonization. We present -gures for the q 0 , light curves and PSDs of di†erent energy bands, the photon energy spectra, and the phase lags as func- tions of the variability frequency. The temporal properties of the high (soft) and low (hard) state of black hole sources are discussed in this context. Subject headings: accretion, accretion disks E black hole physics E radiation mechanisms: nonthermal E stars: neutron

Magnetar Spin-Down

We examine the effects of a relativistic wind on the spin-down of a neutron star and apply our results to the study of soft gamma repeaters (SGRs), which are thought to be neutron stars with magnetic fields greater than 1014 G. We derive a spin-down formula that includes torques from both dipole radiation and episodic or continuous particle winds. We find that if SGR 1806-20 puts out a continuous particle wind of 1037 ergs s-1, then the pulsar age is consistent with that of the supernova remnant, but the derived surface dipole magnetic field is only 3x1013 G, in the range of normal radio pulsars. If instead the particle wind flows are episodic with small duty cycle, then the observed period derivatives imply magnetar-strength fields, while still allowing characteristic ages within a factor of 2 of the estimated supernova remnant age. Close monitoring of the periods of SGRs will allow us to establish or place limits on the wind duty cycle and thus the magnetic field and age of the neutron star.

MAGNETOHYDRODYNAMIC ACCRETION DISK WINDS AS X-RAY ABSORBERS IN ACTIVE GALACTIC NUCLEI

We present the two-dimensional (2D) ionization structure of self-similar magnetohydrodynamic (MHD) winds off accretion disks around irradiated by a central X-ray point source. Based on earlier observational clues and theoretical arguments, we focus our attention on a subset of these winds, namely those with radial density dependence n(r) ∝ 1/r (r is the spherical radial coordinate). We employ the photoionization code XSTAR to compute the ionic abundances of a large number of ions of different elements and then compile their line-of-sight (LOS) absorption columns. We focus our attention on the distribution of the column density of the various ions as a function of the ionization parameter ξ (or equivalently r) and the angle θ. Particular attention is paid to the absorption measure distribution (AMD), namely their Hydrogen-equivalent column per logarithmic ξ interval, dNH/d log ξ, which provides a measure of the winds’ radial density profiles. For the chosen density profile n(r) ∝ 1/r the AMD is found to be independent of ξ, in good agreement with its behavior inferred from the X-ray spectra of several active galactic nuclei (AGNs). For the specific wind structure and X-ray spectrum we also compute detailed absorption line profiles for a number of ions to obtain their LOS velocities, v ∼ 100−300 km s (at log ξ ∼ 2−3) for Fexvii and v ∼ 1, 000 − 4, 000 km s (at log ξ ∼ 4 − 5) for Fexxv, in good agreement with the observation. Our models describe the X-ray absorption properties of these winds with only two parameters, namely the mass-accretion rate ṁ and LOS angle θ. The probability of obscuration of the X-ray ionizing source in these winds decreases with increasing ṁ and increases steeply with the LOS inclination angle θ. As such, we concur with previous authors that these Email: Keigo.Fukumura@nasa.gov University of Maryland, Baltimore County (UMBC/CRESST), Baltimore, MD 21250 Astrophysics Science Division, NASA/Goddard Space Flight Center, Greenbelt, MD 20771 Research Center for Astronomy, Academy of Athens, Athens 11527, Greece Department of Physics, Technion, Haifa 32000, Israel Senior NPP Fellow

A Cosmic Battery

We show that the Poynting-Robertson drag effect in an optically thin advection-dominated accretion flow around active gravitating objects generates strong azimuthal electric currents that give rise to astrophysically significant magnetic fields. Although the mechanism is most effective in accreting compact objects, it also seems very promising as a way to account for the generation of stellar dipolar fields during the late protostellar collapse phase, when the star approaches the main sequence.

Newtonian limit of conformal gravity and the lack of necessity of the second order Poisson equation

We study the interior structure of a locally conformal invariant fourth order theory of gravity in the presence of a static, spherically symmetric gravitational source. We find, quite remarkably, that the associated dynamics is determined exactly and without any approximation at all by a simple fourth order Poisson equation which thus describes both the strong and weak field limits of the theory in this static case. We present the solutions to this fourth order equation and find that we are able to recover all of the standard Newton-Euler gravitational phenomenology in the weak gravity limit, to thus establish the observational viability of the weak field limit of the fourth order theory. Additionally, we make a critical analysis of the second order Poisson equation, and find that the currently available experimental evidence for its validity is not as clearcut and definitive as is commonly believed, with there not apparently being any conclusive observational support for it at all either on the very largest distance scales far outside of fundamental sources, or on the very smallest ones within their interiors. Our study enables us to deduce that even though the familiar second order Poisson gravitational equation may be sufficient to yield Newtons Law of Gravity it is not in fact necessary.

Towards resolving the crab sigma-problem: a linear accelerator?

Using the exact solution of the axisymmetric pulsar magnetosphere derived in a previous publication and the conservation laws of the associated MHD flow, we show that the Lorentz factor of the outflowing plasma increases linearly with distance from the light cylinder. Therefore, the ratio of the Poynting to particle energy flux, generically referred to as σ, decreases inversely proportional to distance from a large value (typically 104) near the light cylinder to σ 1 at a transition distance Rtrans. Beyond this distance, the inertial effects of the outflowing plasma become important, and the magnetic field geometry must deviate from the almost monopolar form it attains between Rlc and Rtrans. We anticipate that this is achieved by collimation of the poloidal field lines toward the rotation axis, ensuring that the magnetic field pressure in the equatorial region will fall off faster than 1/R2 (R being the cylindrical radius). This leads both to a value σ = σs 1 at the nebular reverse shock at distance Rs (Rs Rtrans) and to a component of the flow perpendicular to the equatorial component, as required by observation. The presence of the strong shock at R = Rs allows for the efficient conversion of kinetic energy into radiation. We speculate that the Crab pulsar is unique in requiring σs 3 × 10-3 because of its small translational velocity, which allows for the shock distance Rs to grow to values Rtrans.