arXiv subjects

Ioannis Contopoulos

Publications and source records attributed to Ioannis Contopoulos.

At least 37 records · Page 2Linked to original sources

arXiv2018

Current closure in the pulsar magnetosphere

Current closure in the pulsar magnetosphere holds the key to its structure. We must determine not only the global electric circuit, but also the source of its electric charge carriers. We address this issue with the minimum number of assumptions: a) The magnetosphere is everywhere ideal and force-free, except above the polar cap and in some finite part of the current sheet; and b) pairs are produced above the polar cap with multiplicity kappa. We show that a thin region of width delta ~ r_pc/2 kappa << r_pc along the rim of the polar cap provides all the charges that are needed in the equatorial and separatrix electric current sheet. These charges are transferred to the current sheet in a narrow dissipation zone just outside the magnetospheric Y-point. The maximum accelerating potential in this region is equal to the potential drop in the thin polar cap rim, which is approximately equal to 1/kappa times the potential drop from the center to the edge of the polar cap. The dissipated electromagnetic energy is approximately equal to 0.5/kappa times the total pulsar spindown energy loss. Our framework allows to calculate the high energy emission in terms of the pair multiplicity.

astro-ph.HE↗

arXiv2018

Variable Nature of Magnetically-Driven Ultra-Fast Outflows

Among a number of active galactic nuclei (AGNs) that drive ionized outflows in X-rays, a low-redshift (z = 0.184) quasar, PDS 456, is long known to exhibit one of the exemplary ultra-fast outflows (UFOs). However, the physical process of acceleration mechanism is yet to be definitively constrained. In this work, we model the variations of the Fe K UFO properties in PDS 456 over many epochs in X-ray observations in the context of magnetohydrodynamic (MHD) accretion-disk winds employed in our earlier studies of similar X-ray absorbers. We applied the model to the 2013/2014 XMM-Newton/NuSTAR spectra to determine the UFO's condition; namely, velocity, ionization parameter, column density and equivalent width (EW). Under some provisions on the dependence of X-ray luminosity on the accretion rate applicable to near-Eddington state, our photoionization calculations, coupled to a 2.5-dimensional MHD-driven wind model, can further reproduce the observed correlations of the UFO velocity and the anti-correlation of its EW with X-ray strength of PDS 456. This work demonstrates that UFOs, even without radiative pressure, can be driven as an extreme case purely by magnetic interaction while also producing the observed spectrum and correlations.

astro-ph.HE↗

arXiv2017

Magnetized Disk-Winds in NGC 3783

We analyze a 900-ks stacked Chandra/HETG spectrum of NGC 3783 in the context of magnetically-driven accretion-disk wind models in an effort to provide tight constraints on the global conditions of the underlying absorbers. Motivated by the earlier measurements of its absorption measure distribution (AMD) indicating X-ray-absorbing ionic columns that decrease slowly with decreasing ionization parameter, we employ 2D magnetohydrodynamic (MHD) disk-wind models to describe the global outflow. We compute its photoionization structure along with the wind kinematic properties allowing us to further calculate in a self-consistent fashion the shapes of the major X-ray absorption lines. With the wind radial density profile determined by the AMD, the profiles of the ensemble of the observed absorption features are determined by the two global parameters of the MHD wind; i.e. disk inclination θ_obs and wind density normalization n_o. Considering the most significant absorption features in the (~1.8A-20A) range, we show that the MHD-wind is best described by n(r)~6.9e11(r/ro)^-1.15 [cm^-3] and θ_obs=44deg. We argue that winds launched by X-ray heating, radiation pressure or even MHD winds but with steeper radial density profiles are strongly disfavored by data. Considering the properties of Fe K band absorption features (i.e. Fe xxv and Fe xxvi), while typically prominent in the AGN X-ray spectra, they appear to be weak in NGC 3783. For the specific parameters of our model obtained by fitting the AMD and the rest of absorption features, these features are found to be weak in agreement with observation.

astro-ph.HE↗

arXiv2017

The immediate environment of an astrophysical black hole

In view of the upcoming observations with the Event Horizon Telescope (EHT), we present our thoughts on the immediate environment of an astrophysical black hole. We are concerned that two approximations used in general relativistic magnetohydrodynamic numerical simulations, namely numerical density floors implemented near the base of the black hole jet, and a magnetic field that comes from large distances, may mislead our interpretation of the observations. We predict that three physical processes will manifest themselves in EHT observations, namely dynamic pair formation just above the horizon, electromagnetic energy dissipation along the boundary of the black hole jet, and a region of weak magnetic field separating the black hole jet from the disk wind.

astro-ph.HE↗

arXiv2017

Magnetically Advected Winds

Observations of X-ray absorption lines in magnetically driven disk winds around black hole binaries and active galactic nuclei yield a universal radial density profile rho proportional to r^{-1.2} in the wind. This is in disagreement with the standard Blandford & Payne profile rho_BP proportional to r^{-1.5} expected when the magnetic field is neither advected nor diffusing through the accretion disk. In order to account for this discrepancy, we establish a new paradigm for magnetically driven astrophysical winds according to which the large scale ordered magnetic field that threads the disk is continuously generated by the Cosmic Battery around the inner edge of the disk and continuously diffuses outward. We obtain self-similar solutions of such magnetically advected winds (MAW) and discuss their observational ramifications.

astro-ph.HE↗

arXiv2017

Numerical simulations of the Cosmic Battery in accretion flows around astrophysical black holes

We implement the KORAL code to perform two sets of very long general relativistic radiation magnetohydrodynamic simulations of an axisymmetric optically thin magnetized flow around a non-rotating black hole: one with a new term in the electromagnetic field tensor due to the radiation pressure felt by the plasma electrons on the comoving frame of the electron-proton plasma, and one without. The source of the radiation is the accretion flow itself. Without the new term, the system evolves to a standard accretion flow due to the development of the magneto-rotational instability (MRI). With the new term, however, the system eventually evolves to a magnetically arrested state (MAD) in which a large scale jet-like magnetic field threads the black hole horizon. Our results confirm the secular action of the Cosmic Battery in accretion flows around astrophysical black holes.

astro-ph.HE↗

arXiv2017

Magnetic Origin of Black Hole Winds Across the Mass Scale

Black hole accretion disks appear to produce invariably plasma outflows that result in blue-shifted absorption features in their spectra. The X-ray absorption-line properties of these outflows are quite diverse, ranging in velocity from non-relativistic ($\sim 300$ km/sec) to sub-relativistic ($\sim 0.1c$ where $c$ is the speed of light) and a similarly broad range in the ionization states of the wind plasma. We report here that semi-analytic, self-similar magnetohydrodynamic (MHD) wind models that have successfully accounted for the X-ray absorber properties of supermassive black holes, also fit well the high-resolution X-ray spectrum of the accreting stellar-mass black hole, GRO J1655-40. This provides an explicit theoretical argument of their MHD origin (aligned with earlier observational claims) and supports the notion of a universal magnetic structure of the observed winds across all known black hole sizes.

astro-ph.HE↗

arXiv2016

Are there two types of pulsars?

In order to investigate the importance of dissipation in the pulsar magnetosphere we decided to combine Force-Free with Aristotelian Electrodynamics. We obtain solutions that are ideal (non-dissipative) everywhere except in an equatorial current sheet where Poynting flux from both hemispheres converges and is dissipated into particle acceleration and radiation. We find significant dissipative losses (up to about 50% of the pulsar spindown luminosity), similar to what is found in global PIC simulations in which particles are provided only on the stellar surface. We conclude that there might indeed exist two types of pulsars, strongly dissipative ones with particle injection only from the stellar surface, and ideal (weakly dissipative) ones with particle injection in the outer magnetosphere and in particular at the Y-point.

astro-ph.HE↗

arXiv2016

The Magnetic Rayleigh-Taylor Instability in Astrophysical Disks

This is our first study of the magnetic Rayleigh-Taylor instability at the inner edge of an astrophysical disk around a central back hole. We derive the equations governing small-amplitude oscillations in general relativistic ideal magnetodydrodynamics and obtain a criterion for the onset of the instability. We suggest that static disk configurations where magnetic field is held by the disk material are unstable around a Schwarzschild black hole. On the other hand, we find that such configurations are stabilized by the spacetime rotation around a Kerr black hole. We obtain a crude estimate of the maximum amount of poloidal magnetic flux that can be accumulated around the center, and suggest that it is proportional to the black hole spin. Finally, we discuss the astrophysical implications of our result for the theoretical and observational estimations of the black hole jet power.

astro-ph.HE↗

arXiv2016

Dominance of outflowing electric currents on decaparsec to kiloparsec scales in extragalactic jets

Helical magnetic fields embedded in the jets of active galactic nuclei (AGNs) are required by the broad range of theoretical models that advocate for electromagnetic launching of the jets. In most models, the direction of the magnetic field is random, but if the axial field is generated by a Cosmic Battery generated by current in the direction of rotation in the accretion disk, there is a correlation between the directions of the spin of the AGN accretion disk and of the axial field, which leads to a specific direction for the axial electric current, azimuthal magnetic field, and the resulting observed transverse Faraday-rotation (FR) gradient across the jet, due to the systematic change in the line-of-sight magnetic field. We consider new observational evidence for the presence of a nested helical magnetic-field structure such as would be brought about by the operation of the Cosmic Battery, and make predictions about the expected behavior of transverse FR gradients observed on decaparsec and kiloparsec scales. The collected results can be understood if the dominant azimuthal field on parsec scales corresponds to an axial electric current flowing inward along the jet, whereas the (weaker) dominant azimuthal field on kiloparsec scales corresponds to a outward-flowing current in the outer sheath of the jet and/or an extended disk wind.

astro-ph.HE↗

arXiv2016

The Equatorial Current Sheet and other interesting features of the Pulsar Magnetosphere

We want to understand what drives magnetospheric dissipation in the equatorial current sheet. Numerical simulations have limitations and, unless we have a clear a priori understanding of the physical processes involved, their results can be misleading. We argue that the canonical pulsar magnetosphere is strongly dissipative and that a large fraction (up to 30-40% in an aligned rotator) of the spindown luminosity is redirected towards the equator where it is dissipated into particle acceleration and emission of radiation. We show that this is due to the failure of the equatorial electric current to cross the Y-point at the tip of the corotating zone.

astro-ph.HE↗

arXiv2015

The rapid decay phase of the afterglow as the signature of the Blandford-Znajek mechanism

Gamma-ray bursts (GRBs) are believed to be powered by the electromagnetic extraction of spin energy from a black hole endowed with a magnetic field supported by electric currents in a surrounding disk (Blandford & Znajek 1977). A generic feature of this mechanism is that, under certain fairly general assumptions, the energy loss rate decays exponentially. In this work, we are looking precisely for such exponential decay in the lightcurves of long duration GRBs observed with the XRT instrument on the Swift satellite. We found out that almost 30 % of XRT lightcurves show such behavior before they reach the afterglow plateau. According to Blandford & Znajek, the duration of the burst depends on the magnetic flux accumulated on the event horizon. This allows us to estimate the surface magnetic field of a possible progenitor. Our estimations are consistent with magnetic fields observed in Wolf-Rayet stars.

astro-ph.HE↗

arXiv2015

Are Ultra Long Gamma Ray Bursts powered by black holes spinning down?

Gamma-ray bursts (GRBs) are violent explosions, coming from cosmological distances. They are detected in gamma-rays (also X-rays, UV, optical, radio) almost every day, and have typical durations of a few seconds to a few minutes. Some GRBs have been reported with extraordinary duration of 10^4 sec, the so-called Ultra Long GRBs. It has been debated whether these form a new distinct class of events or whether they are similar to long GRBs. According to Blandford & Znajek (1977), the spin energy of a rotating black hole can be extracted electromagnetically, should the hole be endowed with a magnetic field supported by electric currents in a surrounding disk. We argue that this can be the case for the central engines of GRBs and we show that the duration of the burst depends on the magnetic flux accumulated on the event horizon of the black hole. We thus estimate the surface magnetic field of a possible progenitor star, and we conclude that an Ultra Long GRB may originate from a progenitor star with a relatively low magnetic field.

astro-ph.HE↗

arXiv2015

Magnetically-Driven Accretion-Disk Winds and Ultra-Fast Outflows in PG1211+143

We present a study of X-ray ionization of magnetohydrodynamic (MHD) accretion-disk winds in an effort to constrain the physics underlying the highly-ionized ultra-fast outflows (UFOs) inferred by X-ray absorbers often detected in various sub-classes of Seyfert active galactic nuclei (AGNs). Our primary focus is to show that magnetically-driven outflows are indeed physically plausible candidates for the observed outflows accounting for the AGN absorption properties of the present X-ray spectroscopic observations. Employing a stratified MHD wind launched across the entire AGN accretion disk, we calculate its X-ray ionization and the ensuing X-ray absorption line spectra. Assuming an appropriate ionizing AGN spectrum, we apply our MHD winds to model the absorption features in an {\it XMM-Newton}/EPIC spectrum of the narrow-line Seyfert, \pg. We find, through identifying the detected features with Fe K$α$ transitions, that the absorber has a characteristic ionization parameter of $\log (ξ_c [erg~cm~s$^{-1}$]) \simeq 5-6$ and a column density on the order of $N_H \simeq 10^{23}$ cm$^{-2}$, outflowing at a characteristic velocity of $v_c/c \simeq 0.1-0.2$ (where $c$ is the speed of light). The best-fit model favors its radial location at $r_c \simeq 200 R_o$ ($R_o$ is the black hole innermost stable circular orbit), with an inner wind truncation radius at $R_{\rm t} \simeq 30 R_o$. The overall K-shell feature in the data is suggested to be dominated by \fexxv\ with very little contribution from \fexxvi\ and weakly-ionized iron, which is in a good agreement with a series of earlier analysis of the UFOs in various AGNs including \pg.

astro-ph.HE↗

arXiv2015

The Cosmic Battery in Astrophysical Accretion Disks

The aberrated radiation pressure at the inner edge of the accretion disk around an astrophysical black hole imparts a relative azimuthal velocity on the electrons with respect to the ions which gives rise to a ring electric current that generates large scale poloidal magnetic field loops. This is the Cosmic Battery established by Contopoulos and Kazanas in 1998. In the present work we perform realistic numerical simulations of this important astrophysical mechanism in advection-dominated accretion flows-ADAF. We confirm the original prediction that the inner parts of the loops are continuously advected toward the central black hole and contribute to the growth of the large scale magnetic field, whereas the outer parts of the loops are continuously diffusing outward through the turbulent accretion flow. This process of inward advection of the axial field and outward diffusion of the return field proceeds all the way to equipartition, thus generating astrophysically significant magnetic fields on astrophysically relevant timescales. We confirm that there exists a critical value of the magnetic Prandtl number between unity and 10 in the outer disk above which the Cosmic Battery mechanism is suppressed.

astro-ph.HE↗

arXiv2015

Radiation from charged particles on eccentric orbits in a dipolar magnetic field around a Schwarzschild black hole

We obtain an approximate solution for the motion of a charged particle around a Schwarzschild black hole immersed in a weak dipolar magnetic field. We focus on eccentric bound orbits in the equatorial plane of the Schwarzschild black hole and derive an analytic expression for the spectral distribution of the electromagnetic emission from a charged particle on such an orbit. Two sets of harmonic contributions appear, with specific frequency spacing. The expression can be written in compact form, if it is truncated up to the lowest order harmonic contributions.

gr-qc↗

arXiv2015

Black hole spin down in GRB observations and Cosmology

According to Blandford & Znajek (1977), energy can be extracted electromagnetically from a rotating black hole, should the latter be endowed with a magnetic field supported by electric currents in a surrounding disk. We show that exact models of black hole magnetospheres produce Poynting flux that decreases almost exponentially with time. We went through the Swift BAT-XRT lightcurves and identified a subclass of GRBs that exhibits a clear exponential decay over more than three orders of magnitude in flux (EDOHS GRBs). We estimate the energy given-off in the X-rays and discuss a possible correlation between the peak brightness of the X-ray prompt emission and its decay time. We investigate a possible application of this result in high redshift Cosmology.

astro-ph.HE↗

arXiv2014

Accretion disk radiation dynamics and the cosmic battery

We investigate the dynamics of radiation in an astrophysical accretion disk around a Kerr black hole. The source of the radiation is the accretion disk itself, and not a central object as in previous studies of the Poynting-Robertson effect. We generate numerical sky maps from photon trajectories that originate on the surface of the disk as seen from the inner edge of the disk at the position of the innermost stable circular orbit (ISCO). We investigate several accretion disk morphologies with a Shakura-Sunyaev surface temperature distribution. Finally, we calculate the electromotive source of the Cosmic Battery mechanism around the inner edge of the accretion disk and obtain characteristic timescales for the generation of astrophysical magnetic fields.

astro-ph.HE↗