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  • 1
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    A systematic synchro-curvature modelling of pulsar {gamma}-ray spectra unveils hidden trends (2015)
    Oxford University Press
    Details
    Publication Date: 2015-09-27
    Description: -ray radiation from pulsars is usually thought to be mostly produced by the synchro-curvature (SC) losses of accelerated particles. Here, we present a systematic study of all currently reported, good-quality Fermi -LAT pulsar spectral data. We do so by applying a model which follows the particle dynamics and consistently computes the emission of SC radiation. By fitting observational data on a case by case basis, we are able to obtain constraints about the parallel electric field, the typical length-scale over which particles emit the bulk of the detected radiation, and the number of involved particles. The model copes well with data of several dozens of millisecond and young pulsars. By correlating the inferred model parameters with the observed timing properties, some trends are discovered. First, a non-negligible part of the radiation comes from the loss of perpendicular momentum soon after pair creation. Second, the electric field strongly correlates with both the inverse of the emission length-scale and the magnetic field at light cylinder, thus ruling out models with high-energy photon production close to the surface. These correlations unify young and millisecond pulsars under the same physical scenario, and predict that magnetars are intrinsically -ray quiet via synchro-curvature processes, since magnetospheric particles are not accelerated enough to emit a detectable -ray flux.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 2
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    Modelling of the {gamma}-ray pulsed spectra of Geminga, Crab, and Vela with synchro-curvature radiation (2015)
    Oxford University Press
    Details
    Publication Date: 2015-04-12
    Description: -ray spectra of pulsars have been mostly studied in a phenomenological way, by fitting them to a cut-off power-law function. Here, we analyse a model where pulsed emission comes from synchro-curvature processes in a gap. We calculate the variation of kinetic energy of magnetospheric particles along the gap and the associated radiated spectra, considering an effective particle distribution. We fit the phase-averaged and phase-resolved Fermi -LAT (Large Area Telescope) spectra of the three brightest -ray pulsars: Geminga, Crab, and Vela, and constrain the three free parameters we leave free in the model. Our best-fitting models well reproduce the observed data, apart from residuals above a few GeV in some cases, range for which the inverse Compton scattering likely becomes the dominant mechanism. In any case, the flat slope at low energy ( GeV) seen by Fermi -LAT both in the phase-averaged and phase-resolved spectra of most pulsars, including the ones we studied, requires that most of the detected radiation below ~GeV is produced during the beginning of the particle trajectories, when radiation mostly come from the loss of perpendicular momentum.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 3
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    Compact formulae, dynamics and radiation of charged particles under synchro-curvature losses (2014)
    Oxford University Press
    Details
    Publication Date: 2014-12-21
    Description: We consider the fundamental problem of charged particles moving along and around a curved magnetic field line, revising the synchro-curvature radiation formulae introduced by Cheng & Zhang. We provide more compact expressions to evaluate the spectrum emitted by a single particle, identifying the key parameter that controls the transition between the curvature-dominated and the synchrotron-dominated regime. This parameter depends on the local radius of curvature of the magnetic field line, the gyration radius, and the pitch angle. We numerically solve the equations of motion for the emitting particle by considering self-consistently the radiative losses, and provide the radiated spectrum produced by a particle when an electric acceleration is balanced by its radiative losses, as it is assumed to happen in the outer gaps of pulsar's magnetospheres. We compute the average spectrum radiated throughout the particle trajectory finding that the slope of the spectrum before the peak depends on the location and size of the emission region. We show how this effect could then lead to a variety of synchro-curvature spectra. Our results reinforce the idea that the purely synchrotron or curvature losses are, in general, inadequate to describe the radiative reaction on the particle motion, and the spectrum of emitted photons. Finally, we discuss the applicability of these calculations to different astrophysical scenarios.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 4
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    An assessment of the pulsar outer gap model - II. Implications for the predicted {gamma}-ray spectra (2015)
    Oxford University Press
    Details
    Publication Date: 2015-01-14
    Description: One of the most important predictions of any gap model for pulsar magnetospheres is the predicted -ray spectra. In the outer gap model, the properties of the synchro-curvature radiation are sensitive to many parameters, whose realistic ranges have been studied in detail in an accompanying paper. There we demonstrated that the uncertainty in the radius of curvature, the magnetic field geometry, and the X-ray surface flux may affect by orders of magnitude the predicted flux and spectral peak in the -ray regime. Here, we present a systematic, numerical study of the impact of the different parameters on the particle dynamics along the gap and calculate the emitted synchro-curvature radiation along the trajectory. By integrating the emitted radiation along the gap and convolving it with a parametrized particle distribution, we discuss how the comparison with the wealth of Fermi -LAT data can be used to constrain the applicability of the model. The resulting spectra show very different energy peaks, fluxes, and shapes, qualitatively matching the great variety of the observed Fermi -LAT pulsars. In particular, if we see a large fraction of photons emitted from the initial part of the trajectory, we show that the spectra will be flatter at the low-energy Fermi -LAT regime (100 MeV–1 GeV). This provides a solution for such observed flat spectra, while still maintain synchro-curvature radiation as the origin of these photons.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 5
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    An assessment of the pulsar outer gap model - I. Assumptions, uncertainties, and implications on the gap size and the accelerating field (2015)
    Oxford University Press
    Details
    Publication Date: 2015-01-14
    Description: The popular outer gap model of magnetospheric emission from pulsars has been widely applied to explain the properties observed in -rays. However, its quantitative predictions rely on a number of approximations and assumptions that are usually overlooked. Here we examine them, reviewing the main ingredients entering the model, evaluating their range of uncertainties. Usually, in the quantitative applications of the model, key parameters like the radius of curvature and the energies of the interacting photons are taken to be a fixed, single value. Instead, here we explore their realistic ranges, and the impact of these on the consistency of the model itself. We conclude that the popular evaluation of the transfield size of the gap as a function of period and period derivative, is unreliable and affected by a huge dispersion. Last, the exploration of the possible values for the radius of curvature, the local magnetic field and other quantities deserve more attention for quantitative applications of the outer gap model, like the calculation of -ray spectra, which is the subject of an accompanying paper.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 6
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    Compact formulae, dynamics and radiation of charged particles under synchro-curvature losses (2014)
    Oxford University Press
    Details
    Publication Date: 2014-12-29
    Description: We consider the fundamental problem of charged particles moving along and around a curved magnetic field line, revising the synchro-curvature radiation formulae introduced by Cheng & Zhang. We provide more compact expressions to evaluate the spectrum emitted by a single particle, identifying the key parameter that controls the transition between the curvature-dominated and the synchrotron-dominated regime. This parameter depends on the local radius of curvature of the magnetic field line, the gyration radius, and the pitch angle. We numerically solve the equations of motion for the emitting particle by considering self-consistently the radiative losses, and provide the radiated spectrum produced by a particle when an electric acceleration is balanced by its radiative losses, as it is assumed to happen in the outer gaps of pulsar's magnetospheres. We compute the average spectrum radiated throughout the particle trajectory finding that the slope of the spectrum before the peak depends on the location and size of the emission region. We show how this effect could then lead to a variety of synchro-curvature spectra. Our results reinforce the idea that the purely synchrotron or curvature losses are, in general, inadequate to describe the radiative reaction on the particle motion, and the spectrum of emitted photons. Finally, we discuss the applicability of these calculations to different astrophysical scenarios.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 7
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    A systematic synchro-curvature modelling of pulsar {gamma}-ray spectra unveils hidden trends (2015)
    Oxford University Press
    Details
    Publication Date: 2015-09-05
    Description: -ray radiation from pulsars is usually thought to be mostly produced by the synchro-curvature (SC) losses of accelerated particles. Here, we present a systematic study of all currently reported, good-quality Fermi -LAT pulsar spectral data. We do so by applying a model which follows the particle dynamics and consistently computes the emission of SC radiation. By fitting observational data on a case by case basis, we are able to obtain constraints about the parallel electric field, the typical length-scale over which particles emit the bulk of the detected radiation, and the number of involved particles. The model copes well with data of several dozens of millisecond and young pulsars. By correlating the inferred model parameters with the observed timing properties, some trends are discovered. First, a non-negligible part of the radiation comes from the loss of perpendicular momentum soon after pair creation. Second, the electric field strongly correlates with both the inverse of the emission length-scale and the magnetic field at light cylinder, thus ruling out models with high-energy photon production close to the surface. These correlations unify young and millisecond pulsars under the same physical scenario, and predict that magnetars are intrinsically -ray quiet via synchro-curvature processes, since magnetospheric particles are not accelerated enough to emit a detectable -ray flux.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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