Magnetic photon splitting c ] cc, a quantum electrodynamics process that becomes important only in magnetic Ðelds approaching the quantum critical value, G, is investigated as a mecha-B cr \ 4.41 ] 1013 nism for attenuation of c-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 Ðelds it may dominate photon attenuation above pair threshold. We model photon-splitting attenuation and subsequent splitting cascades in c-ray pulsars, including the dipole Ðeld and curved spacetime geometry of the neutron star magnetosphere. We focus speciÐcally on PSR 1509[58, which has the highest surface magnetic Ðeld of all the c-ray pulsars G).\ 3 ] 1013 We Ðnd that splitting will not be important for most c-ray pulsars, i.e., those with either in B 0 [ 0.2B cr , competition with pair production attenuation in pair cascades, or in photon escape cuto †s in the spectrum. Photon splitting will be important for c-ray pulsars having where the splitting attenu-B 0 Z 0.3B cr , ation 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 Ðnd that photon splitting, or combined splitting and pair production, can explain the unusually low cuto † 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 D25¡.

We present results of a population synthesis of millisecond pulsars from the Galactic disk. Excluding globular clusters, we model the spatial distribution of millisecond pulsars by assuming their birth in the Galactic disk with a random kick velocity and evolve them to the present within the Galactic potential. We assume that normal and millisecond pulsars are standard candles described with a common radio luminosity model that invokes a new relationship between radio core and cone emission suggested by recent studies. In modeling the radio emission beams, we explore the relativistic effects of time delay, aberration and sweepback of the open field lines. While these effects are essential in understanding pulse profiles, the phase-averaged flux is adequately described without a relativistic model. We use a polar cap acceleration model for the γ-ray emission. We present the preliminary results of our recent study and the implications for observing millisecond pulsars with GLAST and AGILE.

We present results of a pulsar population synthesis study that incorporates a number of recent developments and some significant improvements over our previous study. We have included the results of the Parkes multibeam pulsar survey in our select group of nine radio surveys, doubling our sample of radio pulsars. More realistic geometries for the radio and -ray beams are included in our Monte Carlo computer code, which simulates the characteristics of the Galactic population of radio and -ray pulsars. We adopted with some modifications the radio-beam geometry of Arzoumanian, Chernoff, and Cordes. For the -ray beam, we have assumed the slot gap geometry described in the work of Muslimov and Harding. To account for the shape of the distribution of radio pulsars in theṖ-P diagram, we continue to find that decay of the magnetic field on a timescale of 2.8 Myr is needed. With all nine surveys, our model predicts that EGRET should have seen seven radio-quiet (below the sensitivity of these radio surveys) and 19 radio-loud -ray pulsars. AGILE (nominal sensitivity map) is expected to detect 13 radio-quiet and 37 radio-loud -ray pulsars, while GLAST, with greater sensitivity, is expected to detect 276 radio-quiet and 344 radio-loud -ray pulsars. When the Parkes multibeam pulsar survey is excluded, the ratio of radio-loud to radio-quiet -ray pulsars decreases, especially for GLAST. The decrease for EGRET is 45%, implying that some fraction of EGRET unidentified sources are radio-loud -ray pulsars. In the radio geometry adopted, short-period pulsars are core dominated. Unlike the EGRET -ray pulsars, our model predicts that when two -ray peaks appear in the pulse profile, a dominant radio core peak appears in between the -ray peaks. Our findings suggest that further improvements are required in describing both the radio and -ray geometries.

In more than four years of observation the Large Area Telescope on board the Fermi satellite has identified pulsed γ-ray emission from more than 80 young or middle-aged pulsars, in most cases providing light curves with high statistics. Fitting the observed profiles with geometrical models can provide estimates of the magnetic obliquity α and of the line of sight angle ζ, yielding estimates of the radiation beaming factor and radiated luminosity. Using different γ-ray emission geometries (Polar Cap, Slot Gap, Outer Gap, One Pole Caustic) and core plus cone geometries for the radio emission, we fit γ-ray light curves for 76 young or middle-aged pulsars and we jointly fit their γ-ray plus radio light curves when possible. We find that a joint radio plus γ-ray fit strategy is important to obtain (α, ζ) estimates that can explain simultaneously detectable radio and γ-ray emission: when the radio emission is available, the inclusion of the radio light curve in the fit leads to important changes in the (α, ζ) solutions. The most pronounced changes are observed for Outer Gap and One Pole Caustic models for which the γ-ray only fit leads to underestimated α or ζ when the solution is found to the left or to the right of the main α-ζ plane diagonal respectively. The intermediate-to-high altitude magnetosphere models, Slot Gap, Outer Gap, and One pole Caustic, are favoured in explaining the observations. We find no apparent evolution of α on a time scale of 10 6 years. For all emission geometries our derived γ-ray beaming factors are generally less than one and do not significantly evolve with the spin-down power. A more pronounced beaming factor vs. spin-down power correlation is observed for Slot Gap model and radio-quiet pulsars and for the Outer Gap model and radio-loud pulsars. The beaming factor distributions exhibit a large dispersion that is less pronounced for the Slot Gap case and that decreases from radio-quiet to radio-loud solutions. For all models, the correlation between γ-ray luminosity and spin-down power is consistent with a square root dependence. The γ-ray luminosities obtained by using the beaming factors estimated in the framework of each model do not exceed the spin-down power. This suggests that assuming a beaming factor of one for all objects, as done in other studies, likely overestimates the real values. The data show a relation between the pulsar spectral characteristics and the width of the accelerator gap. The relation obtained in the case of the Slot Gap model is consistent with the theoretical prediction.

We simulate the characteristics of the Galactic population of radio and c-ray pulsars using Monte Carlo techniques. At birth, neutron stars are spatially distributed in the Galactic disk, with supernovakick velocities, and randomly dispersed in age back to 109 yr. They are evolved in the Galactic gravitational potential to the present time. From a radio luminosity model, the radio Ñux is Ðltered through a selected set of radio-survey parameters. c-ray luminosities are assigned using the features of recent polar cap acceleration models invoking space-chargeÈlimited Ñow, and a pulsar death valley further attenuates the population of radio-loud pulsars. Assuming a simple emission geometry with aligned radio and c-ray beams of 1 sr solid angle, our model predicts that EGRET should have seen seven radio-loud and one radio-quiet c-ray pulsars. With much improved sensitivity, GLAST, on the other hand, is expected to observe 76 radio-loud and 74 radio-quiet c-ray pulsars, of which seven would be identiÐed as pulsed sources. We also explore the e †ect of magnetic Ðeld decay on the characteristics of the radio and c-ray pulsar populations. Including magnetic Ðeld decay on a timescale of 5 Myr improves agreement with the radio pulsar population and increases the predicted number of GL AST -detected pulsars to 90 radio-loud and 101 radio-quiet (nine pulsed) c-ray pulsars. The lower Ñux threshold allows GL AST to detect c-ray pulsars at larger distances than those observed by the radio surveys used in this study.