438 research outputs found
Inverse Compton model of pulsar high energy emission
We reproduce the broadband spectrum of Crab pulsar, from UV to very high
energy gamma-rays - nearly ten decades in energy, within the framework of the
cyclotron-self-Compton model. Emission is produced by two counter-streaming
beams within the outer gaps, at distances above 20 NS radii. The outward
moving beam produces UV--ray photons via Doppler-booster cyclotron emission,
and GeV photons by Compton scattering the cyclotron photons produced by the
inward going beam. The scattering occurs in the deep Klein-Nishina regime,
whereby the IC component provides a direct measurement of particle distribution
within the magnetosphere. The required plasma multiplicity is high, , but is consistent with the average particle flux injected into the
pulsar wind nebula.
The importance of Compton scattering in the Klein-Nishina regime also implies
the importance of pair production in the outer gaps. We suggest that outer gaps
are important sources of pairs in pulsar magnetospheres.
Cyclotron motion of particles in the pulsar magnetosphere may be excited due
to coherent emission of radio waves by streaming particles at the anomalous
cyclotron resonance. Thus, a whole range of Crab non-thermal emission, from
coherent radio waves to very high energy -rays - nearly eighteen
decades in energy - may be a manifestation of inter-dependent radiation
processes.
The present model, together with the observational evidence in favor of the
IC scattering (Lyutikov et al. 2012; Lyutikov 2012), demonstrates that the
inverse Compton scattering can be the dominant high energy emission mechanism
in majority of pulsars.Comment: 20 pages, 4 figure
Mass-loading of bow shock pulsar wind nebulae
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 cm, 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 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,
201
Magnetic draping of merging cores and radio bubbles in clusters of galaxies
Sharp fronts observed by Chandra satellite between dense cool cluster cores
moving with near-sonic velocity through the hotter intergalactic gas, require
strong suppression of thermal conductivity across the boundary. This may be due
to magnetic fields tangential to the contact surface separating two plasma
components. We point out that a super-Alfvenic motion of a plasma cloud (a core
of a merging galaxy) through a weakly magnetized intercluster medium leads to
"magnetic draping", formation of a thin, strongly magnetized boundary layer
with a tangential magnetic field. For supersonic cloud motion, M_s > 1,
magnetic field inside the layer reaches near-equipartition values with thermal
pressure. Typical thickness of the layer is L /M_A^2 << L, where L is the size
of the obstacle (plasma cloud) moving with Alfven Mach number M_A >> 1. To a
various degree, magnetic draping occurs both for sub- and supersonic flows,
random and ordered magnetic fields and it does not require plasma
compressibility. The strongly magnetized layer will thermally isolate the two
media and may contribute to the Kelvin-Helmholtz stability of the interface.
Similar effects occur for radio bubbles, quasi-spherical expanding cavities
blown up by AGN jets; in this case the thickness of the external magnetized
layer is smaller, L /M_A^3 << L.Comment: 16 pages, 2 figure
GRBs from unstable Poynting dominated outflows
Poynting flux driven outflows from magnetized rotators are a plausible
explanation for gamma-ray burst engines. We suggest a new possibility for how
such outflows might transfer energy into radiating particles. We argue that the
Poynting flux drives non-linearly unstable large amplitude electromagnetic
waves (LAEMW) which ``break'' at radii cm where the MHD
approximation becomes inapplicable. In the ``foaming'' (relativisticly
reconnecting) regions formed during the wave breaks the random electric fields
stochastically accelerate particles to ultrarelativistic energies which then
radiate in turbulent electromagnetic fields. The typical energy of the emitted
photons is a fraction of the fundamental Compton energy with plus additional boosting due to the bulk motion
of the medium. The emission properties are similar to synchrotron radiation,
with a typical cooling time sec. During the wave break, the
plasma is also bulk accelerated in the outward radial direction and at larger
radii can produce afterglows due to the interactions with external medium. The
near equipartition fields required by afterglow models maybe due to magnetic
field regeneration in the outflowing plasma (similarly to the field generation
by LAEMW of laser-plasma interactions) and mixing with the upstream plasma.Comment: 15 pages, 1 figur
Dynamics of relativistic reconnection
The dynamics of the steady-state Sweet--Parker-type reconnection is analyzed
in relativistic regime when energy density in the inflowing region is dominated
by magnetic field. The structure of reconnection layer (its thickness, inflow
and outflow velocities) depends on the ratio of two large dimensionless
parameters of the problem - magnetization parameter (the ratio
of the magnetic to particle energy-densities in the inflowing region) and the
Lundquist number . The inflow velocity may be relativistic (for ) or non-relativistic (for ), while the outflowing plasma
is moving always relativisticly. For extremely magnetized plasmas with , the inflow four-velocity becomes of the order of the \Alfven
four-velocity.Comment: 22 pages, 4 figures, submitted to Ap
Resolving doppler-factor crisis in active galactic nuclei: Non-steady magnetized outflows
Magnetically driven non-stationary acceleration of jets in active galactic nuclei results in the leading parts of the flow being accelerated to much higher Lorentz factors than in the case of steady-state acceleration with the same parameters. The higher Doppler-boosted parts of the flow may dominate the high-energy emission of blazar jets. We suggest that highly variable GeV and TeV emission in blazars is produced by the faster moving leading edges of highly magnetized non-stationary ejection blobs, while the radio data trace the slower-moving bulk flow. Thus, the radio and gamma-ray emission regions have different, but correlated, Doppler factors. High-energy emission is generated, typically within the optically thick core, in the outer parts of the broad-line emission region, avoiding the radiative drag on the faster parts of the flow. The radio emission should correlate with the gamma-ray emission, delayed with frequency-dependent time lag of the order of weeks to months. Model predictions compare favorably with the latest Fermi gamma-ray and MOJAVE radio very long baseline interferometry results
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