1,034 research outputs found
Gamma-ray Flares and VLBI Outbursts of Blazars
A model is developed for the time dependent electromagnetic - radio to
gamma-ray - emission of active galactic nuclei, specifically, the blazars,
based on the acceleration and creation of leptons at a propagating
discontinuity or {\it front} of a Poynting flux jet. The front corresponds to a
discrete relativistic jet component as observed with
very-long-baseline-interferometry (VLBI). Equations are derived for the number,
momentum, and energy of particles in the front taking into account synchrotron,
synchrotron-self-Compton (SSC), and inverse-Compton processes as well as
photon-photon pair production. The apparent synchrotron, SSC, and
inverse-Compton luminosities as functions of time are determined. Predictions
of the model are compared with observations in the gamma, optical and radio
bands. The delay between the high-energy gamma-ray flare and the onset of the
radio is explained by self-absorption and/or free-free absorption by external
plasma. Two types of gamma-ray flares are predicted depending on pair creation
in the front.Comment: 11 pages, submitted to ApJ. 10 figures can be obtained from R.
Lovelace by sending postal address to [email protected]
Boundary Between Stable and Unstable Regimes of Accretion
We investigated the boundary between stable and unstable regimes of accretion
and its dependence on different parameters. Simulations were performed using a
"cubed sphere" code with high grid resolution (244 grid points in the azimuthal
direction), which is twice as high as that used in our earlier studies. We
chose a very low viscosity value, with alpha-parameter alpha=0.02. We observed
from the simulations that the boundary strongly depends on the ratio between
magnetospheric radius r_m (where the magnetic stress in the magnetosphere
matches the matter stress in the disk) and corotation radius r_cor (where the
Keplerian velocity in the disk is equal to the angular velocity of the star).
For a small misalignment angle of the dipole field, Theta=5 degrees, accretion
is unstable if r_cor/r_m>1.35, and is stable otherwise. In cases of a larger
misalignment angle of the dipole, Theta=20 degrees, instability occurs at
slightly larger values, r_cor/r_m>1.41.Comment: 4 pages, 4 figures, conference proceedings: "Physics at the
Magnetospheric Boundary", Geneva, Switzerland, 25-28 June, 201
Jets and Disk-Winds from Pulsar Magnetospheres
We discuss axisymmetric force-free pulsar magnetospheres with magnetically
collimated jets and a disk-wind obtained by numerical solution of the pulsar
equation.
This solution represents an alternative to the quasi-spherical wind solutions
where a major part of the current flow is in a current sheet which is unstable
to magnetic field annihilation.Comment: 6 figures, accepted for publication in the Ap
Three Disk Oscillation Modes of Rotating Magnetized Neutron Stars
We discuss three specific modes of accretion disks around rotating magnetized
neutron stars which may explain the separations of the kilo Hertz quasi
periodic oscillations (QPO) seen in low mass X-ray binaries. The existence of
these modes requires that there be a maximum in the angular velocity of the
accreting material, and that the fluid is in stable, nearly circular motion
near this maximum rather than moving rapidly towards the star or out of the
disk plane into funnel flows. It is presently not known if these conditions
occur, but we are exploring this with 3D magnetohydrodynamic simulations and
will report the results elsewhere. The first mode is a corotation mode which is
radially trapped in the vicinity of the maximum of the disk rotation rate and
is unstable. The second mode, relevant to relatively slowly rotating stars, is
a magnetically driven eccentric () oscillation of the disk excited at a
Lindblad radius in the vicinity of the maximum of the disk rotation. The third
mode, relevant to rapidly rotating stars, is a magnetically coupled eccentric
() and an axisymmetric () radial disk perturbation which has an inner
Lindblad radius also in the vicinity of the maximum of the disk rotation. We
suggest that the first mode is associated with the upper QPO frequency,
, the second with the lower QPO frequency, , and
the third with the lower QPO frequency, , where
is the star's rotation rate.Comment: 6 pages, 2 figure
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