Observations by the Fermi Large Area Telescope of gamma-ray millisecond pulsar light curves imply copious pair production in their magnetospheres, and not exclusively in those of younger pulsars. Such pair cascades may be a primary source of Galactic electrons and positrons, contributing to the observed enhancement in positron flux above ~10 GeV. Fermi has also uncovered many new millisecond pulsars, impacting Galactic stellar population models. We investigate the contribution of Galactic millisecond pulsars to the flux of terrestrial cosmic-ray electrons and positrons. Our population synthesis code predicts the source properties of present-day millisecond pulsars. We simulate their pair spectra invoking an offset-dipole magnetic field. We also consider positrons and electrons that have been further accelerated to energies of several TeV by strong intrabinary shocks in black widow and redback systems. Since millisecond pulsars are not surrounded by pulsar wind nebulae or supernova shells, we assume that the pairs freely escape and undergo losses only in the intergalactic medium. We compute the transported pair spectra at Earth, following their diffusion and energy loss through the Galaxy. The predicted particle flux increases for non-zero offsets of the magnetic polar caps. Pair cascades from the magnetospheres of millisecond pulsars are only modest contributors around a few tens of GeV to the lepton fluxes measured by AMS-02, PAMELA, and Fermi, after which this component cuts off. The contribution by black widows and redbacks may, however, reach levels of a few tens of percent at tens of TeV, depending on model parameters.
We present results of a pulsar population synthesis of normal pulsars in the Galactic plane. Over the past several years, a program has been developed to simulate pulsar birth, evolution, and emission using Monte Carlo techniques. We model the spatial distribution of pulsars by assuming they are born with a random kick velocity and then evolved within the Galactic potential. We assume that pulsars are standard candles and invoke a new relationship between core and cone emission suggested by recent studies, which we also apply to millisecond pulsars. From our studies of radio pulsars that have clearly identifiable core and cone components, in which we fit the polarization sweep as well as the pulse profiles to constrain the viewing geometry, we develop a model describing the ratio of radio core‐to‐cone peak fluxes. In this model, short period pulsars are more cone‐dominated than in our previous studies. We use both a low and high altitude slot gap model for describing the gamma‐ray emission. We also include gamma‐ray emission from an outer‐gap model to compare the statistics of radio‐loud and radio‐quiet gamma‐ray pulsars on the same footing as pulsars from our slot gap, polar cap model. We present results of our recent study and the implications for observing these pulsars with GLAST and AGILE.
The extremely efficient process of resonant Compton upscattering by relativistic electrons in high magnetic fields is believed to be a leading emission mechanism of high field pulsars and magnetars in the production of intense X-ray radiation. New analytic developments for the Compton scattering cross section using Sokolov & Ternov (S&T) states with spin-dependent resonant widths are presented. These new results display significant numerical departures from both the traditional cross section using spin-averaged widths, and also from the spin-dependent cross section that employs the Johnson & Lippmann (J&L) basis states, thereby motivating the astrophysical deployment of this updated resonant Compton formulation. Useful approximate analytic forms for the cross section in the cyclotron resonance are developed for S&T basis states. These calculations are applied to an inner magnetospheric model of the hard X-ray spectral tails in magnetars, recently detected by RXTE and INTEGRAL. Relativistic electrons cool rapidly near the stellar surface in the presence of intense baths of thermal X-ray photons. We present resonant Compton cooling rates for electrons, and the resulting photon spectra at various magnetospheric locales, for magnetic fields above the quantum critical value. These demonstrate how this scattering mechanism has the potential to produce the characteristically flat spectral tails observed in magnetars.
From coincidence measurements between projectile-like fragments or heavy residues and their associated \ensuremath{\gamma} rays, the angular momentum transfers for a variety of incomplete fusion reactions of 180 and 310 MeV $^{16}\mathrm{O}$ with $^{154}\mathrm{Sm}$ have been derived. At the higher energy, the correlation between angular momentum transfer and linear momentum transfer has been obtained over the entire range of linear momentum transfer. A comparison of the data with calculations of both the sum-rule and geometric overlap models indicates that each makes reasonable predictions of the observed trend even though the assumptions of the models are quite different, and very different initial partial waves are predicted to contribute to particular reaction channels. This results primarily from prescriptions relating fractional mass transfer to fractional angular momentum transfer. The reconstruction of the initial partial wave distributions from correlated measurements of linear momentum and angular momentum transfers is addressed. Comparisons are also made with more recent model calculations which focus on nucleon-nucleon scattering as the mechanism of momentum transfer.
The total energy dissipated in central collisions has been measured for the system $^{32}\mathrm{S}$${+}^{58}$Ni at about 1 GeV incident energy. An event-by-event reconstruction of the atomic charge of the reaction products was performed by means of a 4\ensuremath{\pi} charged-particle detector. Two distinct classes of events were thus separated: one consistent with a ``conventional'' incomplete fusion-evaporation process; a second where three or more heavy fragments are produced. A subtraction of the evaporative component from the particle spectra at all angles allowed extraction of the excitation energy removed from the system by pre-equilibrium emission. The average excitation energies corresponding to the two different classes of events were determined. Comparisons with statistical model calculations as well as a multifragmentation model are presented.
Magnetic photon splitting γ → γγ, a quantum electrodynamics process that becomes important only in magnetic fields approaching the quantum critical value, Bcr = 4.41 × 1013 G, is investigated as a mechanism for attenuation of γ-rays emitted near the surface of strongly magnetized pulsars. Since splitting has no threshold, it can attenuate photons and degrade their energies below the threshold for one-photon pair production, and in high enough fields it may dominate photon attenuation above pair threshold. We model photon-splitting attenuation and subsequent splitting cascades in γ-ray pulsars, including the dipole field and curved spacetime geometry of the neutron star magnetosphere. We focus specifically on PSR 1509-58, which has the highest surface magnetic field of all the γ-ray pulsars (B0 = 3 × 1013 G). We find that splitting will not be important for most γ-ray pulsars, i.e., those with B0 ≲ 0.2Bcr, either in competition with pair production attenuation in pair cascades, or in photon escape cutoffs in the spectrum. Photon splitting will be important for γ-ray pulsars having B0 ≳ 0.3Bcr, where the splitting attenuation lengths and escape energies become comparable to or less than those for pair production. We compute Monte Carlo spectral models for PSR 1509-58, assuming that either a full photon-splitting cascade or a combination of splitting and pair production (depending on which splitting modes operate) attenuate a power-law input spectrum. We find that photon splitting, or combined splitting and pair production, can explain the unusually low cutoff energy (between 2 and 30 MeV) of PSR 1509-58, and that the model cascade spectra, which display strong polarization, are consistent with the observed spectral points and upper limits for polar cap emission at a range of magnetic colatitudes up to ~25°.
