100 research outputs found
Particle Acceleration in Dissipative Pulsar Magnetospheres
Pulsar magnetospheres represent unipolar inductor-type electrical circuits at which an EM potential across the polar cap (due to the rotation of their magnetic field) drives currents that run in and out of the polar cap and close at infinity. An estimate ofthe magnitude of this current can be obtained by dividing the potential induced across the polar cap V approx = B(sub O) R(sub O)(Omega R(sub O)/c)(exp 2) by the impedance of free space Z approx eq 4 pi/c; the resulting polar cap current density is close to where is the Goldreich-Julian (GJ) charge density. This argument suggests that even at current densities close to the GJ one, pulsar magnetospheres have a significant component of electric field , parallel to the magnetic field, a condition necessary for particle acceleration and the production of radiation. We present the magnetic and electric field structures as well as the currents, charge densities, spin down rates and potential drops along the magnetic field lines of pulsar magnetospheres which do not obey the ideal MHD condition . By relating the current density along the poloidal field lines to the parallel electric field via a kind of Ohm's law we study the structure of these magnetospheres as a function of the conductivity . We find that for sigma 11 OmegaS. Finally, we present dissipative magnetospheric solutions with spatially variable that supports various microphysical properties and are compatible with the observations
A test of the power law relationship between gamma-ray burst pulse width ratio and energy expected in fireballs or uniform jets
Recently, under the assumption that the Doppler effect of the
relativistically expanding fireball surface is important, Qin et al. showed
that in most cases the power law relationship between the pulse width and
energy of gamma-ray bursts (GRBs)would exist in a certain energy range. We
check this prediction with two GRB samples which contain well identified
pulses. A power law anti-correlation between the full pulse width and energy
and a power law correlation between the pulse width ratio and energy are seen
in the light curves of the majority (around 65%) of bursts of the two samples
within the energy range of BATSE, suggesting that these bursts are likely to
arise from the emission associated with the shocks occurred on a
relativistically expanding fireball surface. For the rest of the bursts, the
relationships between these quantities were not predicted previously. We
propose to consider other spectral evolutionary patterns or other radiation
mechanisms such as a varying synchrotron or Comptonized spectrum to check if
the observed relationships for these rest bursts can also be accounted for by
the Doppler model. In addition, we find that the upper limits of the width
ratio for the two samples do not exceed 0.9, in agrement with what predicted
previously by the Doppler model. The plateau/power law/plateau and the peaked
features predicted and detected previously by Qin et al. are generally
observed, with the exceptions being noticed only in a few cases. According to
the distinct values of two power law indices of FWHM and ratio and energy, we
divide the bursts into three subsets which are located in different areas of
the two indices plane. We suspect that different locations of the two indices
might correspond to different mechanisms.Comment: 16 pages, 7 figures, MNRAS accepte
High Energy Neutrinos from Quasars
We review and clarify the assumptions of our basic model for neutrino
production in the cores of quasars, as well as those modifications to the model
subsequently made by other workers. We also present a revised estimate of the
neutrino background flux and spectrum obtained using more recent empirical
studies of quasars and their evolution. We compare our results with other
thoeretical calculations and experimental upper limits on the AGN neutrino
background flux. We also estimate possible neutrino fluxes from the jets of
blazars detected recently by the EGRET experiment on the Compton Gamma Ray
Observatory. We discuss the theoretical implications of these estimates.Comment: 14 pg., ps file (includes figures), To be published in Space Science
Review
Predictions for Fourier-resolved X-ray spectroscopy from the model of magnetic flare avalanches above an accretion disc with hot ionized skin
The magnetic flare avalanches model of Poutanen & Fabian for X-ray
variability of accreting black holes is combined with computations of vertical
structure of illuminated accretion discs in hydrostatic equilibrium. The latter
predict the existence of a hot ionized skin, due to the thermal instability of
X-ray illuminated plasma. The presence of such ionized skin, with properties
dependent on disc radius, introduces a dependence of the emitted X-ray spectrum
on the position on the disc. If the position is related to the time scale of
the flares, the X-ray energy spectra (both the primary continuum and the
reprocessed component) gain an additional dependence on Fourier frequency,
beside that resulting from spectral evolution during a flare. We compute the
Fourier frequency resolved spectra in this model and demonstrate that the
presence of the hot skin introduces trends opposite to those observed in black
hole binaries. Furthermore, the flare profile is strongly constrained, if the
Fourier frequency dependence due to spectral evolution is to agree with
observations.Comment: 10 pages, MNRAS, in pres
Relativistic spine jets from Schwarzschild black holes: "Application to AGN radioloud sources"
The two types of Fanaroff-Riley radio loud galaxies, FRI and FRII, exhibit
strong jets but with different properties. These differences may be associated
to the central engine and/or the external medium. Aims: The AGN classification
FRI and FRII can be linked to the rate of electromagnetic Poynting flux
extraction from the inner corona of the central engine by the jet. The
collimation results from the distribution of the total electromagnetic energy
across the jet, as compared to the corresponding distribution of the thermal
and gravitational energies. We use exact solutions of the fully relativistic
magnetohydrodynamical (GRMHD) equations obtained by a nonlinear separation of
the variables to study outflows from a Schwarzschild black hole corona. A
strong correlation is found between the jet features and the energetic
distribution of the plasma of the inner corona which may be related to the
efficiency of the magnetic rotator. It is shown that observations of FRI and
FRII jets may be partially constrained by our model for spine jets. The
deceleration observed in FRI jets may be associated with a low magnetic
efficiency of the central magnetic rotator and an important thermal confinement
by the hot surrounding medium. Conversely, the strongly collimated and
accelerated FRII outflows may be self collimated by their own magnetic field
because of the high efficiency of the central magnetic rotator.Comment: Accepted for publication in the A&
Inflationary Initial Conditions Consistent with Causality
The initial condition problem of inflation is examined from the perspective
of both spacetime embedding and scalar field dynamics. The spacetime embedding
problem is solved for arbitrary initial spatial curvature Omega, which
generalizes previous works that primarily treat the flat case Omega=1. Scalar
field dynamics that is consistent with the embedding constraints are examined,
with the additional treatment of damping effects. The effects of
inhomogeneities on the embedding problem also are considered. A category of
initial conditions are identified that are not acausal and can develop into an
inflationary regime.Comment: 9 pages, 3 figures. Minor changes, matches version to appear in
Physical Review
High-Energy Polarimetry - a new window to probe extreme physics in AGN jets
The constantly improving sensitivity of ground-based and space-borne
observatories has made possible the detection of high-energy emission (X-rays
and gamma-rays) from several thousands of extragalactic sources. Enormous
progress has been made in measuring the continuum flux enabling us to perform
imaging, spectral and timing studies. An important remaining challenge for
high-energy astronomy is measuring polarization. The capability to measure
polarization is being realized currently at X-ray energies (e.g. with IXPE),
and sensitive gamma-ray telescopes capable of measuring polarization, such as
AMEGO, AdEPT, e-ASTROGAM, etc., are being developed. These future gamma-ray
telescopes will probe the radiation mechanisms and magnetic fields of
relativistic jets from active galactic nuclei at spatial scales much smaller
than the angular resolution achieved with continuum observations of the
instrument. In this white paper, we discuss the scientific potentials of
high-energy polarimetry, especially gamma-ray polarimetry, including the
theoretical implications, and observational technology advances being made. In
particular, we will explore the primary scientific opportunities and wealth of
information expected from synergy of multi-wavelength polarimetry that will be
brought to multi-messenger astronomy.Comment: submitted to Astro2020 (Astronomy and Astrophysics Decadal Survey
Modelling spectral and timing properties of accreting black holes: the hybrid hot flow paradigm
The general picture that emerged by the end of 1990s from a large set of
optical and X-ray, spectral and timing data was that the X-rays are produced in
the innermost hot part of the accretion flow, while the optical/infrared (OIR)
emission is mainly produced by the irradiated outer thin accretion disc. Recent
multiwavelength observations of Galactic black hole transients show that the
situation is not so simple. Fast variability in the OIR band, OIR excesses
above the thermal emission and a complicated interplay between the X-ray and
the OIR light curves imply that the OIR emitting region is much more compact.
One of the popular hypotheses is that the jet contributes to the OIR emission
and even is responsible for the bulk of the X-rays. However, this scenario is
largely ad hoc and is in contradiction with many previously established facts.
Alternatively, the hot accretion flow, known to be consistent with the X-ray
spectral and timing data, is also a viable candidate to produce the OIR
radiation. The hot-flow scenario naturally explains the power-law like OIR
spectra, fast OIR variability and its complex relation to the X-rays if the hot
flow contains non-thermal electrons (even in energetically negligible
quantities), which are required by the presence of the MeV tail in Cyg X-1. The
presence of non-thermal electrons also lowers the equilibrium electron
temperature in the hot flow model to <100 keV, making it more consistent with
observations. Here we argue that any viable model should simultaneously explain
a large set of spectral and timing data and show that the hybrid
(thermal/non-thermal) hot flow model satisfies most of the constraints.Comment: 26 pages, 13 figures. To be published in the Space Science Reviews
and as hard cover in the Space Sciences Series of ISSI - The Physics of
Accretion on to Black Holes (Springer Publisher
Dark Energy and Gravity
I review the problem of dark energy focusing on the cosmological constant as
the candidate and discuss its implications for the nature of gravity. Part 1
briefly overviews the currently popular `concordance cosmology' and summarises
the evidence for dark energy. It also provides the observational and
theoretical arguments in favour of the cosmological constant as the candidate
and emphasises why no other approach really solves the conceptual problems
usually attributed to the cosmological constant. Part 2 describes some of the
approaches to understand the nature of the cosmological constant and attempts
to extract the key ingredients which must be present in any viable solution. I
argue that (i)the cosmological constant problem cannot be satisfactorily solved
until gravitational action is made invariant under the shift of the matter
lagrangian by a constant and (ii) this cannot happen if the metric is the
dynamical variable. Hence the cosmological constant problem essentially has to
do with our (mis)understanding of the nature of gravity. Part 3 discusses an
alternative perspective on gravity in which the action is explicitly invariant
under the above transformation. Extremizing this action leads to an equation
determining the background geometry which gives Einstein's theory at the lowest
order with Lanczos-Lovelock type corrections. (Condensed abstract).Comment: Invited Review for a special Gen.Rel.Grav. issue on Dark Energy,
edited by G.F.R.Ellis, R.Maartens and H.Nicolai; revtex; 22 pages; 2 figure
- …