40 research outputs found
Cyclotron line formation in the magnetized atmospheres of compact stars: I. The transfer equations for polarized radiation
We find the forms of the transfer equations for polarized cyclotron radiation
in the atmospheres of compact stars, which are simple enough to allow practical
implementation and still preserve all important physical effects. We take into
account a frequency redistribution of radiation within the cyclotron line as
well as the relativistic and quantum-electrodynamic effects. Our analysis is
valid for the magnetic fields up to G and for temperatures well below
500keV.} We present and compare two forms of the radiation transfer equations.
The first form, for the intensities of ordinary and extraordinary modes, is
applicable for the compact stars with a moderate magnetic field strength up to
G for typical neutron star and up to G for magnetic white
dwarfs. The second form, for the Stokes parameters, is more complex, but
applicable even if a linear mode coupling takes place somewhere in the
scattering-dominated atmosphere. Analysing dispersion properties of a
magnetized plasma {in the latter case, we describe a range of parameters where
the linear mode coupling is possible and essential.Comment: 12 pages, 3 figures, MNRA
High-energy emission from off-axis relativistic jets
We analyze how the spectrum of synchrotron and inverse Compton radiation from
a narrow relativistic jet changes with the observation angle. It is shown that
diversity of acceleration mechanisms (in particular, taking the converter
mechanism (Derishev et al. 2003) into account) allows for numerous
modifications of the observed spectrum. In general, the off-axis emission in
GeV-TeV energy range appears to be brighter, has a much harder spectrum and a
much higher cut-off frequency compared to the values derived from Doppler
boosting considerations alone. The magnitude of these effects depends on the
details of particle acceleration mechanisms, what can be used to discriminate
between different models.
One of the implications is the possibility to explain high-latitude
unidentified EGRET sources as off-axis but otherwise typical relativistic-jet
sources, such as blazars. We also discuss the broadening of beam pattern in
application to bright transient jet sources, such as Gamma-Ray Bursts.Comment: 6 pages, Proceedings of the International Symposium "High Energy
Gamma-Ray Astronomy", 26-30 July 2004, Heidelberg, German
Off-axis emission from relativistic plasma flows
We show that there is no universal law describing how the spectra and
luminosity of synchrotron and inverse Compton radiation from relativistic jets
change with increasing observation angle. Instead, the physics of particle
acceleration leaves pronounced imprints in the observed spectra and allows for
a freedom in numerous modifications of them. The impact of these effects is the
largest for high-energy radiation and depends on the details of particle
acceleration mechanism(s), what can be used to discriminate between different
models. Generally, the beam patterns of relativistic jets in GeV-TeV spectral
domain are much wider than the inverse Lorentz factor. The off-axis emission in
this energy range appear to be brighter, have much harder spectra and a much
higher cut-off frequency compared to the values derived from Doppler boosting
considerations alone.
The implications include the possibility to explain high-latitude
unidentified EGRET sources as off-axis but otherwise typical relativistic-jet
sources, such as blazars, and the prediction of GeV-TeV afterglow from
transient jet sources, such as Gamma-Ray Bursts. We also discuss the phenomenon
of beam-pattern broadening in application to neutrino emission.Comment: Submitted to the Astrophysical Journa
Particle acceleration through multiple conversions from a charged into a neutral state and back
We propose a new way of quick and very efficient acceleration of protons
and/or electrons in relativistic bulk flows. The new mechanism takes advantage
of conversion of particles from the charged state (protons or
electrons/positrons) into neutral state (neutrons or photons) and back. In most
cases, the conversion is photon-induced and requires presence of intense
radiation fields, but the converter acceleration mechanism may also operate via
inelastic nucleon-nucleon collisions.
Like in the traditional model -- ``stochastic'' (or diffusive) acceleration,
-- the acceleration cycle in our scenario consists of escape of particles from
the relativistic flow followed by their return back after deflection from the
ambient magnetic field. The difference is that the charge-changing reactions,
which occur during the cycle, allow accelerated particles to increase their
energies in each cycle by a factor roughly equal to the bulk Lorentz factor
squared.
The emerging spectra of accelerated particles can be very hard and their
cut-off energy in some cases is larger than in the standard mechanism. This
drastically reduces the required energy budget of the sources of the
highest-energy particles observed in cosmic rays. Also, the proposed
acceleration mechanism may serve as an efficient means of transferring the
energy of bulk motion to gamma-radiation and, if the accelerated particles are
nucleons, routinely produces high-energy neutrinos at relative
efficiency.Comment: extended version, 10 pages, submitted to Phys. Rev.
Spectral redistribution of gyroresonant photons in magnetized atmospheres of isolated compact stars
Aims. We analyze the spectral redistribution of gyroresonant photons in the course of radiation transfer through magnetized plasma atmospheres of isolated compact stars.
Methods. We use analytical estimate and Monte Carlo simulations to prove that this redistribution crucially influences the spectral line formation for atmospheric parameters typical of neutron stars and white dwarfs.
Results. We point out the importance of the frequency redistribution of the gyroresonant photons to the process of radiation transfer and analyze its main effects in atmospheres of isolated compact stars with strong magnetic fields, where multiple scattering dominates over the absorption of photons. We estimate analytically and numerically the rate of this redistribution and show that photons’ escape from the line center, which in this case is one-dimensional (1D) in origin, is a very pronounced effect despite being strongly inhibited with respect to three-dimensional (3D) photon redistribution, which takes place in the case of atomic or ion spectral lines. The escape of photons from the cyclotron line greatly affects both the line’s profile and the characteristic optical depth, from where the outgoing radiation originates. Through this, the spectral redistribution of gyroresonant photons changes the radiation pressure on the atmospheric plasma, what makes it one of the key phenomena need to be included in studies of cyclotron-driven winds
Physical parameters and emission mechanism in Gamma-Ray Bursts
Detailed information on the physical parameters in the sources of
cosmological Gamma-Ray Bursts (GRBs) is obtained from few plausible assumptions
consistent with observations. Model-independent requirements posed by these
assumptions on the emission mechanism in GRBs are formulated. It is found that
the observed radiation in sub-MeV energy range is generated by the synchrotron
emission mechanism, though about ten per cent of the total GRB energy should be
converted via the inverse Compton process into ultra-hard spectral domain
(above 100 GeV). We estimate the magnetic field strength in the emitting
region, the Lorentz factor of accelerated electrons, and the typical energy of
IC photons.
We show that there is a "line-of-death" relation for GRBs and derive from
this relation the lower limits on both GRB duration and GRB variability
timescale. The upper limit on the Lorentz factor of GRB fireballs is also
found. We demonstrate that steady-state electron distribution consistent with
the Compton losses may produce different spectral indices, e.g., 3/4 as opposed
to the figure 1/2 widely discussed in the literature. It is suggested that the
changes in the decline rate observed in the lightcurves of several GRB
afterglows may be due to the time evolution of spectral break, which appears in
the synchrotron emission generated by steady-state self-consistent electron
distribution.Comment: Journal reference added, introduction extended, minor changes in
notation
Cooperative recombination of electron-hole pairs in semiconductor quantum wells under quantizing magnetic fields
Journals published by the American Physical Society can be found at http://journals.aps.org/We present results of detailed investigations of light emission from semiconductor multiple quantum wells at low temperatures and high magnetic fields excited by intense femtosecond laser pulses. The intensity and linewidth as well as the directional and statistical properties of photoemission strongly depended on the magnetic field strength and pump laser fluence. We also investigated the effects of spot size, temperature, excitation geometry, and excitation pulse width on the emission properties. The results suggest that the initially incoherent photoexcited electron-hole pairs spontaneously form a macroscopic coherent state upon relaxation into the low-lying magnetoexcitonic states, followed by the emission of a superfluorescent burst of radiation. We have developed a theoretical model for superfluorescent emission from semiconductor quantum wells, which successfully explained the observed characteristics
Comparative Analysis of the Dynamical Spectra of a Polarization of an Active Medium and an Electromagnetic Field in the Superradiant Heterolasers
The complicated pulsed generation regimes of a CW-pumped superradiant semiconductor laser are analyzed via the dynamical spectra of the dipole optical oscillations of active centers. This novel approach appears to be more informative than the standard analysis of the dynamical spectra of laser emission if a dipole relaxation rate is less than a cavity relaxation rate. The advantages of the method are demonstrated for a number of superradiant lasing regimes on the basis of the numerical solution to 1D Maxwell–Bloch equations for a two-level active medium in a low-Q cavity within one-dimensional approximation
Cooperative Recombination of a Quantized High-Density Electron-Hole Plasma
We investigate photoluminescence from a high-density electron-hole plasma in
semiconductor quantum wells created via intense femtosecond excitation in a
strong perpendicular magnetic field, a fully-quantized and tunable system. At a
critical magnetic field strength and excitation fluence, we observe a clear
transition in the band-edge photoluminescence from omnidirectional output to a
randomly directed but highly collimated beam. In addition, changes in the
linewidth, carrier density, and magnetic field scaling of the PL spectral
features correlate precisely with the onset of random directionality,
indicative of cooperative recombination from a high density population of free
carriers in a semiconductor environment