The statistics of gamma-ray pulsars

We use Monte Carlo methods to simulate the properties of a Galactic population of rotation-powered pulsars in the self-consistent outer gap model proposed by Zhang & Cheng, where the initial magnetic field and spatial and velocity distributions of the neutron stars at birth are obtained from the statistical results of radio pulsars. We obtain the distance, period, age, magnetic field, and γ-ray flux distributions of the γ-ray pulsars whose γ-ray and radio fluxes are above the detectable threshold fluxes. Furthermore, we simulate the properties of Geminga-like pulsars and obtain the possible parametric region and the number of the Geminga-like pulsars. In our simulations, the different beaming effects of radio and γ-ray beams are taken into account. We predict that there may be ∼11 γ-ray pulsars that are detectable at both radio and γ-ray energies and ∼ 37 Geminga-like pulsars in the Galactic plane (| b | < 5°) if the γ-ray beaming fraction is 0.67 and the birthrate of the neutron stars is 1 per century. Compared to the flux distribution, individual spectrum, and distance distribution obtained by assuming OB star associations of our simulated Geminga-like pulsars and the unidentified EGRET sources at | b | < 5°, we conclude that the majority of the unidentified γ-ray point sources near the Galactic plane may be Geminga-like pulsars. We also suggest that the most possible radio pulsars likely to be confirmed as γ-ray pulsars in future are those with LX/Ėsd ∼ 10-3. © 1998. The American Astronomical Society. All rights reserved.published_or_final_versio

Time delay of photons of different energies in multidimensional cosmological models

We consider the general expressions for the time delay of photons of different energies in the framework of multidimensional cosmological models. In models with compactified extra dimensions (Kaluza-Klein-type models), the main source of the photon time delay is the time variation of the electromagnetic coupling due to dimensional reduction, which induces an energy dependence of the speed of light. A similar relation between the fine-structure constant and the multidimensional gauge couplings also appears in models with large (non-compactified) extra dimensions. For photons of energies around 1 TeV propagating at cosmological distances in an expanding universe, the time delay could range from a few seconds in the case of Kaluza-Klein models to a few days for models with large extra dimensions. As a consequence of the multidimensional effects, the intrinsic time profiles at the emitter rest frame differ from the detected time profiles. The formalism developed in the present paper allows the transformation of the predicted light curves of various energy ranges of the emitter into the frame of the observer for comparison with observations. Therefore, the study of the energy and redshift dependence of the time delay of photons emitted by astrophysical sources at cosmological distances could discriminate between the different multidimensional models and/or quantum gravity effects.published_or_final_versio

Quiescent high-energy gamma-ray emission from soft gamma-ray repeaters

We present a model for the high-energy γ-ray emission from the outer gap of soft gamma-ray repeaters (SGRs) during their quiescent states. In this model, X-rays come from the stellar surface, but the emerging X-ray spectrum will have a power-law tail because of the multiple scattering at the cyclotron resonance in the magneto sphere, as pointed out by Thompson, Lyukitov, &Kulkarni. The outer gap is sustained by the collision between these X-rays with the high-energy photons produced in the outer gap through the photon-photon pair production. We have taken a magnetic dipole geometry into account in estimating the fractional size of the outer gap. The fractional size of the outer gap depends on the period, surface magnetic field, average X-ray energy, and the magnetic inclination angle of the neutron star. After the average fractional size outer gap is determined, the spectrum and luminosity of high-energy photons from the outer gap can be calculated. We apply this model to some SGRs, such as SGR 1806-20 and SGR 1900+14, and compare the expected integral fluxes with the sensitivities of EGRET, GLAST, MAGIC, and VERITAS. We predict that the integral flux of SGR 1900+14 may be greater than the sensitivity of GLAST, and especially that the integral flux for a large magnetic inclination angle (say 80°) may be greater than the sensitivities of GLAST and MAGIC. However, we predict that SGR 1806-20 would not be detected by GLAST because its distance is about 3 times of that of SGR 1900+14.published_or_final_versio

Gamma-ray production through inverse compton scattering with anisotropic photon field from accretion disk in AGNs

We give a detailed consideration of the photon spectrum produced by inverse Compton scattering off relativistic electrons in the anisotropic soft photon field from the accretion disk in jet model, assuming that the relativistic electrons and positrons are produced by collisions of relativistic protons with the soft photon and matter field. We considered soft photons from either the standard accretion disk or from the two-temperature accretion disk, respectively, which resulted in very different minimum Lorentz factors of the relativistic electrons and different scattered photon spectra. The scattered photon spectra seen from different angles and produced in various emission regions have been calculated for the different soft photon sources concerned and compared with results of other authors. The results of our calculations indicate that the instantaneous and stationary scattered photon spectra vary with the angle of scatter, the Lorentz factor of the jet, and the emission region of the γ-rays. Furthermore, whether X-rays and γ-rays can be produced cospatially depends upon the production mechanisms of the relativistic electrons. © 1997. The American Astronomical Society. All rights reserved.published_or_final_versio

High-energy gamma-ray emission from anomalous X-ray pulsars

We study the high-energy gamma-ray radiation from the outer magnetospheres of the anomalous X-ray pulsars. For pulsars with superstrong magnetic fields (B > 1014 G), the magnetic field in the region far away from the neutron star surface drops below the quantum critical value, and high-energy gammaray emission can be emitted. The electrons/positrons produced by collisions between high-energy photons from the outer gap and the soft X-rays resulting from the stellar surface are accelerated inside the outer gap and emit the high-energy gamma rays through curvature radiation. The gamma-ray spectrum and luminosity in this model can be estimated once the pulsar parameters such as period, surface magnetic field, and surface temperature are given. We apply this model to some anomalous X-ray pulsars such as 4U 0142 + 615, 1E 1048.1-5937, RX J170849-4009, 1E 1841-045, and 1E 2259 + 586 and suggest that their gamma-ray fluxes could be detected by the Gamma-Ray Large-Area Space Telescope. © 2001. The American Astronomical Society. All rights reserved.published_or_final_versio

High-energy radiation from rapidly spinning pulsars with thick outer gaps

We propose a self-consistent mechanism to estimate the size of the acceleration region in the outer magnetosphere of pulsars (outer gap) and calculate the high-energy radiation produced by the synchrocurvature mechanism from the outer gap. We find that a power-law energy distribution of the accelerated particles can be obtained if the outer gap is thick enough that E · B̂ inside the gap can be approximately proportional to (Ω.r/c)1/2B(r). We apply our model to explain X-rays and γ-rays from Geminga and PSR B1055-52, whose outer gaps may occupy ∼70% of the outer magnetosphere region. If the radius of curvature near the light cylinder of the medium outer gap is larger than the dipolar structure, then perhaps this model may also apply to PSR B1951 + 32, PSR B1706-44, and others. © 1997. The American Astronomical Society. All rights reserved.published_or_final_versio

On the bimodal magnetic field distribution of binary pulsars

We combine the idea of movement of magnetic flux tubes in the stellar interiors with the analysis of crustal physics of accreting neutron stars in low- and high-mass X-ray binaries to explain the bimodal magnetic field distribution of binary pulsars. We propose that this distribution may result from different crustal properties of neutron stars with different accretion rates and amounts of accreted matter. In addition, we may provide an explanation for why the magnetic field strengths of millisecond pulsars with low-mass companions saturate at ∼3 × 10 8 G when accreted mass exceeds ∼0.5 M ⊙. © 1997. The American Astronomical Society. All rights reserved.published_or_final_versio

Are soft γ-ray repeaters strange stars?

The soft γ-ray repeaters (SGRs) are proposed to result from young, magnetized strange stars with superconducting cores. As such a strange star spins down, the quantized vortex lines move outward and drag the magnetic flux tubes because of the strong coupling between them. Since the terminations of the tubes interact with the stellar crust, the dragged tubes can produce sufficient tension to crack the crust and pull parts of the broken platelet into the quark core. The deconfinement of crustal matter into strange quark matter will release energy. The model burst energy, duration, time interval, spectrum, and the persistent x-ray emission from SGRs are shown to be in agreement with observed results.published_or_final_versio

Conversion of neutron stars to strange stars as a possible origin of γ-ray bursts

We propose that some neutron stars in low-mass x-ray binaries can accrete sufficient mass to undergo a phase transition to become strange stars. The energy released per conversion event satisfies the requirements of cosmological y-ray bursts, and the Lorentz factor of the resultant expanding fireball may exceed 5 × 103 because the strange star has very low baryon contamination. The model burst rate is consistent with observations.published_or_final_versio

Multicomponent X-ray emissions from regions near or on the pulsar surface

We present a model of X-ray emission from rotation-powered pulsars, which in general consist of one nonthermal component, two hard thermal components, and one soft thermal component. The nonthermal X-rays come from synchrotron radiation of e± pairs created in the strong magnetic field near the neutron star surface by curvature photons emitted by charged particles on their way from the outer gap to the neutron star surface. The first hard thermal X-ray component results from polar-cap heating by the return current in the polar gap. The second hard thermal X-ray component results from polar-cap heating by the return particles from the outer gap. Because of cyclotron resonance scattering, most of the hard thermal X-rays will be effectively reflected back to the stellar surface and eventually reemitted as soft thermal X-rays. However, some of the hard thermal X-rays can still escape along the open magnetic field lines, where the e+/e- pair density is low. Furthermore, the characteristic blackbody temperatures of the two hard X-ray components emitted from the polar-cap area inside the polar gap and the polar-cap area defined by the footprints of the outer-gap magnetic field lines are strongly affected by the surface magnetic field, which can be much larger than the dipolar field. In fact, the strong surface magnetic field can explain why the effective blackbody radiation area is nearly 2 orders of magnitude larger than that deduced from the dipolar field for young pulsars (2 orders of magnitude less for old pulsars). Our model indicates how several possible X-ray components may be observed, depending on the magnetic inclination angle and viewing angle. Using the expected X-ray luminosity and spectra, we explain the observed X-ray spectra from pulsars such as Geminga, PSR B1055-52, PSR B0656+14, and PSR B1929+10.published_or_final_versio