Probing millisecond pulsar emission geometry using light curves from the Fermi Large Area Telescope

An interesting new high-energy pulsar sub-population is emerging following early discoveries of gamma-ray millisecond pulsars (MSPs) by the Fermi Large Area Telescope (LAT). We present results from 3D emission modeling, including the Special Relativistic effects of aberration and time-of-flight delays and also rotational sweepback of B-field lines, in the geometric context of polar cap (PC), outer gap (OG), and two-pole caustic (TPC) pulsar models. In contrast to the general belief that these very old, rapidly-rotating neutron stars (NSs) should have largely pair-starved magnetospheres due to the absence of significant pair production, we find that most of the light curves are best fit by TPC and OG models, which indicates the presence of narrow accelerating gaps limited by robust pair production -- even in these pulsars with very low spin-down luminosities. The gamma-ray pulse shapes and relative phase lags with respect to the radio pulses point to high-altitude emission being dominant for all geometries. We also find exclusive differentiation of the current gamma-ray MSP population into two MSP sub-classes: light curve shapes and lags across wavebands impose either pair-starved PC (PSPC) or TPC / OG-type geometries. In the first case, the radio pulse has a small lag with respect to the single gamma-ray pulse, while the (first) gamma-ray peak usually trails the radio by a large phase offset in the latter case. Finally, we find that the flux correction factor as a function of magnetic inclination and observer angles is typically of order unity for all models. Our calculation of light curves and flux correction factor for the case of MSPs is therefore complementary to the "ATLAS paper" of Watters et al. for younger pulsars.Comment: 51 pages, 23 figures, 3 tables; low-resolution figures; accepted for publication by Ap

Pair production rates in mildly relativistic, magnetized plasmas

Electron-positron pairs may be produced by either one or two photons in the presence of a strong magnetic field. In magnetized plasmas with temperatures kT approximately sq mc, both of these processes may be important and could be competitive. The rates of one-photon and two-photon pair production by photons with Maxwellian, thermal bremsstrahlung, thermal synchrotron and power law spectra are calculated as a function of temperature or power law index and field strength. This allows a comparison of the two rates and a determination of the conditions under which each process may be a significant source of pairs in astrophysical plasmas. It is found that for photon densities n(gamma) or = 10 to the 25th power/cu cm and magnetic field strengths B or = 10 to the 12th power G, one-photon pair production dominates at kT approximately sq mc for a Maxwellian, at kT approximately 2 sq mc for a thermal bremsstrahlung spectrum, at all temperatures for a thermal synchrotron spectrum, and for power law spectra with indices s approximately 4

The role of beam geometry in population statistics and pulse profiles of radio and gamma-ray pulsars

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 multi-beam pulsar survey in our select group of nine radio surveys, doubling our sample of radio pulsars. We adopted with some modifications the radio beam geometry of Arzoumanian, Chernoff & Cordes (2002). For the $\gamma$-ray beam, we have assumed the slot gap geometry described in the work of Muslimov & Harding (2003). To account for the shape of the distribution of radio pulsars in the $\dot P-P$ 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 7 radio-quiet (below the sensitivity of these radio surveys) and 19 radio-loud $\gamma$-ray pulsars. AGILE (nominal sensitivity map) is expected to detect 13 radio-quiet and 37 radio-loud $\gamma$-ray pulsars, while GLAST, with greater sensitivity is expected to detect 276 radio-quiet and 344 radio-loud $\gamma$-ray pulsars. When the Parkes multi-beam pulsar survey is excluded, the ratio of radio-loud to radio-quiet $\gamma$-ray pulsars decreases, especially for GLAST. The decrease for EGRET is 45%, implying that some fraction of EGRET unidentified sources are radio-loud $\gamma$-ray pulsars. In the radio geometry adopted, short period pulsars are core dominated. Unlike the EGRET $\gamma$-ray pulsars, our model predicts that when two $\gamma$-ray peaks appear in the pulse profile, a dominant radio core peak appears in between the $\gamma$-ray peaks. Our findings suggest that further improvements are required in describing both the radio and $\gamma$-ray geometries.Comment: 39 pages, 13 eps figures, accepted for publication in ApJ, April 1, 200

Promoting positive communication environments: a service evaluation.

Purpose The purpose of this paper is to present an evaluation of a programme of training and support provided to staff, which aimed to encourage supported communication environments for people with learning disabilities. Design/methodology/approach Training, monitoring and support for communication, specifically augmentative and alternative communication (AAC) strategies, was provided by speech and language therapy staff to two residential services over 46 weeks. Staff and service user communications were observed pre- and post-intervention. Findings In one provision there was an increase in service user initiations and the use of some AAC strategies by support staff. In the other provision there was no change in service user initiations and a decrease in the range of AAC strategies used. It appears that some forms for AAC remain challenging for staff to implement. Originality/value This evaluation explores ways of using specialist support services to improve communication environments for people with learning difficulties. Possible reasons for differences in the outcome of the intervention are discussed. Future research into the types of communication interactions experienced by people with learning disabilities across the range of communication styles may be useful so that support staff can be better helped to provide sustained and enriched communication environments

High Energy Neutrinos and Photons from Curvature Pions in Magnetars

We discuss the relevance of the curvature radiation of pions in strongly magnetized pulsars or magnetars, and their implications for the production of TeV energy neutrinos detectable by cubic kilometer scale detectors, as well as high energy photons.Comment: 19 pages, 4 figures, to appear in JCA

Resonant Inverse Compton Scattering Spectra from Highly-magnetized Neutron Stars

Hard, non-thermal, persistent pulsed X-ray emission extending between 10 keV and $\sim 150$ keV has been observed in nearly ten magnetars. For inner-magnetospheric models of such emission, resonant inverse Compton scattering of soft thermal photons by ultra-relativistic charges is the most efficient production mechanism. We present angle-dependent upscattering spectra and pulsed intensity maps for uncooled, relativistic electrons injected in inner regions of magnetar magnetospheres, calculated using collisional integrals over field loops. Our computations employ a new formulation of the QED Compton scattering cross section in strong magnetic fields that is physically correct for treating important spin-dependent effects in the cyclotron resonance, thereby producing correct photon spectra. The spectral cut-off energies are sensitive to the choices of observer viewing geometry, electron Lorentz factor, and scattering kinematics. We find that electrons with energies $\lesssim 15$ MeV will emit most of their radiation below 250 keV, consistent with inferred turnovers for magnetar hard X-ray tails. More energetic electrons still emit mostly below 1 MeV, except for viewing perspectives sampling field line tangents. Pulse profiles may be singly- or doubly-peaked dependent upon viewing geometry, emission locale, and observed energy band. Magnetic pair production and photon splitting will attenuate spectra to hard X-ray energies, suppressing signals in the Fermi-LAT band. The resonant Compton spectra are strongly polarized, suggesting that hard X-ray polarimetry instruments such as X-Calibur, or a future Compton telescope, can prove central to constraining model geometry and physics.Comment: 43 pages, 12 figures; accepted for publication in ApJ; v3 fixes typos and updates some reference