114 research outputs found
Full simulation of the open field lines above pulsar's polar cap - Part I
We have programmed a full simulation of the open field lines above the polar
cap of a magnetized pulsar, using time dependent Particle In Cell (PIC)
numerical code (Birdsall and Langdon 1991). We consider the case of free
ejection of electrons from the star surface, a Space Charge Limited Flow (SCLF)
model. We report here the first results of the simulation. Electrons are
accelerating along the open field lines to the flat space-time SCLF maximum
Lorentz factor prediction, with oscillation pattern. Than we add the General
Relativistic Frame Dragging correction that provide the particles the high
Lorentz factor (10^6 - 10^7) needed to initiate pair production. The electrons
accelerate according to the analytic expressions given in Muslimov and Tsygan
1992, and Muslimov and Harding 1997, with oscillation pattern.
Electron-positron pair production is now being programmed, using the cross
sections appears in the literature, and Monte-Carlo code. After completing this
stage, we will automatically get: a) the positron return current (thus we could
calculate the polar cap heating and the X-ray emission). b). The photons and
electrons observed spectrum (photons and electrons that escape the
magnetosphere after the cascade). c). The pulsar death line (pulsars with not
enough pair production). d). The PFF height (pair formation front location).
Those results will be report in a different paper.Comment: 7 pages, 6 figure
Automatic Dish Name Extraction from User-generated Content Using LLM
Extraction of dish names from user-provided content such as food photographs and captions, restaurant reviews, and other free-form text is a challenging task. Rule-based approaches are difficult to maintain and improve. Pattern matching against a predefined dictionary often suffers from low recall. Conventional machine learning models require large amounts of labeled data to perform named entity recognition (e.g., to recognize dish names) which is often costly and does not scale well across multiple languages and countries. This disclosure describes the use of a multimodal large language model to automatically extract dish names from user-generated content such as food photographs and associated free-form text such as tags, captions, etc. Dish name extraction from the user-provided tags can be formulated as an open vocabulary dish name entity recognition and discovery task, which fits naturally with the framework of pre-trained LLMs, and leverages the model capability in handling multilingual, multicultural text understanding
Gamma-ray emission from rotation-powered pulsars
Using a simplified model of cascade pair creation over pulsar polar caps
presented in two previous papers, we investigate the expected gamma-ray output
from pulsars' low altitude particle acceleration and pair creation regions. We
divide pulsars into several categories, based on which mechanism truncates the
particle acceleration off the polar cap, and give estimates for the expected
luminosity of each category.
We find that inverse Compton scattering above the pulsar polar cap provides
the primary gamma rays which initiate the pair cascades in most pulsars. This
reduces the expected -ray luminosity below previous estimates which
assumed curvature gamma ray emission was the dominant initiator of pair
creation in all pulsars.Comment: 10 pages, 5 figures, to be published in Ap
Altitude Limits for Rotating Vector Model Fitting of Pulsar Polarization
Traditional pulsar polarization sweep analysis starts from the point dipole
rotating vector model (RVM) approximation. If augmented by a measurement of the
sweep phase shift, one obtains an estimate of the emission altitude
(Blaskiewicz, Cordes, & Wasserman). However, a more realistic treatment of
field line sweepback and finite altitude effects shows that this estimate
breaks down at modest altitude ~ 0.1R_{LC}. Such radio emission altitudes turn
out to be relevant to the young energetic and millisecond pulsars that dominate
the \gamma-ray population. We quantify the breakdown height as a function of
viewing geometry and provide simple fitting formulae that allow observers to
correct RVM-based height estimates, preserving reasonable accuracy to R ~
0.3R_{LC}. We discuss briefly other observables that can check and improve
height estimates
Adjustment of the electric charge and current in pulsar magnetospheres
We present a simple numerical model of the plasma flow within the open field
line tube in the pulsar magnetosphere. We study how the plasma screens the
rotationally induced electric field and maintains the electric current demanded
by the global structure of the magnetosphere. We show that even though bulk of
the plasma moves outwards with relativistic velocities, a small fraction of
particles is continuously redirected back forming reverse plasma flows. The
density and composition (positrons or electrons, or both) of these reverse
flows are determined by the distribution of the Goldreich-Julian charge density
along the tube and by the global magnetospheric current. These reverse flows
could significantly affect the process of the pair plasma production in the
polar cap accelerator. Our simulations also show that formation of the reverse
flows is accompanied by the generation of long wavelength plasma oscillations,
which could be converted, via the induced scattering on the bulk plasma flow,
into the observed radio emission.Comment: 24 pages, 11 figure
Pair Multiplicities and Pulsar Death
Through a simple model of particle acceleration and pair creation above the
polar caps of rotation-powered pulsars, we calculate the height of the
pair-formation front (PFF) and the dominant photon emission mechanism for the
pulsars in the Princeton catalog. We find that for most low- and moderate-field
pulsars, the height of the pair formation front and the final Lorentz factor of
the primary beam is set by nonresonant inverse Compton scattering (NRICS), in
the Klein-Nishina limit. NRICS is capable of creating pairs over a wide range
of pulsar parameters without invoking a magnetic field more complicated than a
centered dipole, although we still require a reduced radius of curvature for
most millisecond pulsars. For short-period pulsars, the dominant process is
curvature radiation, while for extremely high-field pulsars, it is resonant
inverse Compton scattering (RICS). The dividing point between NRICS dominance
and curvature dominance is very temperature-dependent; large numbers of pulsars
dominated by NRICS at a stellar temperature of K are dominated by
curvature at K. We apply these results to pulsar death-line calculations
and to the issue of particle injection into the Crab Nebula.Comment: 14 pages, 7 figures, to appear in Ap
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