Γ-ray Pulsars Research Articles

Superluminal γ-rays from the Crab pulsar

A tachyonic spectral fit to the pulsed γ-ray flux of the Crab is performed, based on the superluminal spectral densities generated by ultra-relativistic electrons. The γ-ray wideband of the pulsar can be reproduced by a tachyonic cascade spectrum, capable of generating the spectral curvature in double-logarithmic plots as well as the extended spectral plateau defined by COMPTEL and EGRET flux points in the MeV band. Estimates on the magnetospheric electron population generating the radiation are inferred from the spectral fit, such as power-law indices, electron temperature and the electronic source count.

Can Pulsars be used to Probe Quantum Gravity?

Several radio pulsars have now been shown to emit giant pulses that align in phase with high energy emission rather than with their integrated pulse components. We investigate whether timing of giant and γ-ray pulses can be used to improve limits on the energy scale of quantum gravity by placing bounds on the energy dependence of the speed of light. We find good sources for this experiment are putative Crab-like pulsars in M31, which should in principle be detectable by the current generation of radio telescopes. However, the detection of pulsed emission from their γ-ray counterparts seems unlikely in the near future.

Gamma-ray Mature Pulsars: Unidentified EGRET Sources, Possible TeV Sources and Radio Detectivity

We study the γ-ray emission from the pulsar magnetosphere based on outer gap models, and the TeV radiation from pulsar wind nebulae (PWNe) through inverse Compton scattering using a one-zone model. We carry out Monte Carlo simulations of γ-ray pulsars in the Galaxy and the Gould Belt, assuming values for the pulsar birth rate, initial position, proper motion velocity, period, and magnetic field distribution and evolution based on observational statistics. We select from the simulation a sample of 44 mature pulsars in the Galactic plane (|b|⩽5°) and a sample of 32 mature pulsars at high latitudes (|b|>5°) which could be detected by EGRET. About 12 mature pulsar could be observed in the radio band. The results showed that GeV radiation from the magnetosphere of mature pulsars can contribute to the high latitude unidentified EGRET sources. The TeV fluxes from the pulsar wind nebulae of our simulated sample produced through inverse Compton scattering by relativistic electrons on the cosmic microwave background and synchrotron seed photons are calculated. We suggest that strong EGRET sources may be potential TeV source candidates, and up to 15 sources could be detected by present and future three-generation TeV telescopes if they are mature pulsars.

A Comptonization Model for the Prompt Optical and Infrared Emission of GRB 041219A

Prompt optical emission from the γ-ray burst of GRB 041219A has been reported by Vestrand et al. There was a fast rise of optical emission simultaneous with the dominant γ-ray pulse, and a tight correlation with the prompt γ-ray emission has been displayed. These indicate that the prompt optical emission and γ-ray emission would naturally have a common origin. We propose that this optical component can be modeled by considering the Comptonization of γ-ray photons by an electron cloud. As a result of this mechanism, the arrival time of the optical photons is delayed compared with that of the γ-rays. We restrict that the lag time to be shorter than 10 s, within which the prompt optical emission is considered to vary simultaneously with the prompt γ-ray emission. Taking the observations of GRB 041219A into account, we derive the number density of the surrounding electron cloud required for Comptonization. The redshift of GRB 041219A is predicted to be z ≲ 0.073 as well.

Non-thermal pulsed X-ray emission from rotation-powered pulsars

We study the properties of the pulsed component of hard (>2 keV) X-ray emission from pulsars based on the new version of the outer-gap model proposed by Zhang et al. (2004). In this outer-gap model, high-energy photons emitted by relativistic charged particles produce e pairs through magnetic-pair production on their way from the outer gap to the neutron-star surface, and these pairs produce the bulk of pulsed X-rays by the synchrotron radiation. The X-ray luminosity of rotation-powered pulsars is a function not only of the period and magnetic field strength, but also of the magnetic inclination angle. Application of this model to the observed pulsed X-ray emission of normal pulsars by ASCA shows a better consistence. Further, the inclination angles of these pulsars are estimated using the observed X-ray data, and the predicted conversion efficiencies of high-energy γ-rays for seven confirmed γ-ray pulsars are consistent with the observed data.

Observational constraints on the radio and γ-ray emission regions of PSR B1055−52

Observational constraints on the radio and γ-ray emission regions of PSR B1055-52 seen through our line of sight are presented by analysing the position angle curves of radio linear polarization and fitting the observed pulse widths of radio and γ-ray pulses and the phase offsets between them. Aberration, retardation and magnetic field sweep back effects that can cause additional phase offset between the emissions from different locations are taken into account. The following conclusions are obtained. (i) The radio main pulse and γ-ray pulses are emitted from the same pole, while the radio interpulse is emitted from the opposite pole. (ii) The interpulse emission region locates on the open field lines much closer to the magnetic axis than those of the main pulse, and the emission altitudes are higher than those of the main pulse. (iii) At each pole, there are probably two groups of field lines where radio emission is generated, of which the outer one consists of open field lines near (or including) the last open field lines and the inner one consists of open field lines from very near the magnetic pole to approximately the midway between the magnetic axis and the last open field lines. (iv) The γ-ray pulse comes from inner open field lines rather than from the last open field lines, and the emission altitudes are beyond the null charge surface.

A starquake model for the Vela pulsar

The measured values of the glitch healing parameter, Q, of the Vela pulsar are found to be inconsistent with the starquake mechanism for glitch generation in various neutron star (NS) models, based upon the parametrized equations of state (EOSs) of dense nuclear matter. Such models correspond to an unrealistic mass range ≤0.5 M ⊙ for the pulsar, if the observational constraints of the fractional moment of inertia of the core component (I core /I total ≤ 0.2) which is called the glitch healing parameter, Q, according to the starquake model, are imposed on these models. However, we show that these observational constraints yield a realistic mass range for NS models, corresponding to a core given by the stiffest equation of state, dP/dE = 1 (in geometrized units), and the envelope is characterized by the well-known EOS of an adiabatic polytrope (d In P/d In p = Γ 1 ), if the continuity of the adiabatic speed of sound (υ = dP/dE), together with the pressure (P), the energy density (E) and the two metric parameters (v and λ), is assured at the core-envelope boundary of the models and this boundary is worked out on the basis of the 'compatibility criterion' for hydrostatic equilibrium. The models yield a stable sequence of NS masses in the range 1.758 ≤ M ≤ 2.2 M ⊙ , corresponding to the glitch healing parameter range 0 < Q < 0.197, for a choice of the 'transition density' E b = 1.342 × 10 15 g cm -3 at the core-envelope boundary. The maximum stable value of 2.2 M ⊙ in this sequence, in fact, corresponds to the lowest possible upper bound on NS masses calculated in the literature, on the basis of modem EOSs for NS matter. The models yield the surface redshift z R ≃ 0.6913 and mass M ≃ 2.153 M ⊙ for the 'central' weighted mean value, Q = 0.12 ± 0.07, of the glitch healing parameter of the Vela pulsar. This value of mass can increase slightly up to M ≃ 2.196 M ⊙ , whereas the surface redshift can increase up to the value z R ≃ 0.7568 [which represents an ultracompact object (zR ≥ 0.73)], if the observational constraint of the upper weighted mean value of Q ≃ 0.19 is imposed on these models. However, for the lower weighted mean value of Q ≃ 0.05, the mass and surface redshift can decrease to the values of z R ≃ 0.6066 and M ≃ 2.052 M ⊙ respectively. These results set the lower bound on the energy of gravitationally redshifted radiation in the rather narrow range of 0.291-0.302 MeV. The observation of the lower bound on the energy of a γ-ray pulse at about 0.30 MeV from the Vela pulsar in 1984 is in excellent agreement with this result, provided that this energy can be interpreted as the energy of gravitationally redshifted electron-positron annihilation radiation from the surface of the star.

A study on emission geometry and the separation of peaks in the γ-ray pulse profiles of pulsar

In the inner annular gap (IAG) model the γ-ray radiation sources are suggested to be located close to the null charge surface (NCS). A method to explore two geometric parameters ( κ, λ) of the IAG model is given in this paper. Pulsar population statistics are proposed to test the radiation locations of γ-ray pulsars. Within the IAG model, predictions for the GLAST observation of γ-ray pulsars are also made.

Measuring Low Activities of Fission-Product Xenon Isotopes Using the β-γ Coincidence Method

γ-spectrometer facility operating in the β-γ coincidence mode is able to detect ultralow activities of 131mXe, 133mXe, 133Xe, and 135Xe isotopes. A gas sample is introduced into the measuring chamber that is a thin-walled aluminum cylinder with its inner surface coated with organic scintillator to detect β particles and conversion electrons. The chamber is placed in the well of a NaI(Tl) crystal detector of X and γ rays. The resultant γ-ray spectrum is formed by extracting X- and γ-ray pulses coincident in time with pulses of β particles and conversion electrons. The radiation background level is thereby reduced by two orders of magnitude in comparison with measurements in the standard mode (without coincidences), which leads to a significant increase in the spectrometer sensitivity. The design for the measuring system used in the АР ИК С-01 complex of equipment is described, along with the determination of its efficiency and its energy calibration. Data on the sensitivity achieved are presented.

Effect of Gamma-Ray Beaming on the Fluxes of Gamma-Ray Pulsars

We study the effect of γ-ray beaming on γ-ray emission ofthe pulsars in a self-sustained outer gap model. In this model, averagedγ-ray flux is a function of period, magnetic field, magneticinclination angle and solid angle of γ-ray beaming for aγ-ray pulsar. We generate a sample of γ-ray pulsars withtheir ages less than 106 years by using the Monte Carlo method, andthen study the γ-ray beaming effect. The comparison ofdistributions of periods, magnetic fields, distances, γ-rayenergy fluxes and period derivatives of the simulated γ-raypulsars with those of observed γ-ray pulsars by the detectorEGRET shows that γ-ray beaming has an important role on thedetection of γ-ray pulsars. Furthermore, possible γ-raypulsars observed by the detector GLAST are predicted.

Influence of non‐stoichiometry and doping on scintillating response of PbWO 4 crystals

Systematic scintillation response studies dealing with photoelectron yield (N p h e l s (E)), energy resolution (En.Res.(E)) and radio-luminescence (RL) measurements were accomplished on non-stoichiometric and/or (Ba, Mo) doped PbWO 4 single crystals. We used a new experimental set-up with a Hybrid Photomultiplier (HPMT) to measure γ-ray pulse height spectra of some lines in the energy range 500 - 1300 keV. Scintillation responses of PbWO 4 crystals were compared with that of a BGO crystal of the same shape and dimensions. PbWO 4 crystals, which show mainly blue emission, (λ p 410 nm) provide N p h e l s (E) between 2.4 - 4.4 % of BGO. In non-stoichiometric PbWO 4 ( 49 % of PbO in the melt) green emission (λ p 500 nm) dominates and N p h e l s (E) reaches 8 % of BGO. Non-stoichiometric crystals doped with Mo show the highest N p h e l s (E) (almost 10 % of that of BGO) while En.Res. at 662 keV reaches 20 %.

Tachyonic synchrotron radiation from γ-ray pulsars

Superluminal radiation emitted by electrons orbiting in strong magnetic fields is investigated. We show that electrons gyrating in the surface fields of rotation-powered neutron stars can radiate superluminal quanta (tachyons) via the synchrotron mechanism. The tachyonic luminosity of γ-ray pulsars is inferred from COMPTEL and EGRET observations, and so is the magnetospheric electron population generating this radiation. In the surface fields, electromagnetic synchrotron radiation in the γ-ray band is suppressed by a quantum cutoff, but not so tachyonic γ-radiation. This provides an exceptional opportunity to search for tachyon radiation, unspoiled by electromagnetic emission. Estimates of the superluminal power radiated and the tachyonic count rates are obtained for each of the seven established γ-ray pulsars, the Crab and Vela pulsars, as well as PSR B1706−44, Geminga, PSR B1055−52, B1951+32, and B1509−58. Detection mechanisms such as tachyonic ionization and Compton scattering are analyzed with regard to superluminal γ-rays.

Pulsar Wind Nebulae in Egret Error Boxes

A remarkable number of pulsar wind nebulae (PWN) are coincident with EGRET γ-ray sources. X-ray and radio imaging studies of unidentified EGRET sources have resulted in the discovery of at least six new pulsar wind nebulae (PWN). Stationary PWN (SPWN) appear to be associated with steady EGRET sources with hard spectra, typical for γ-ray pulsars. Their toroidal morphologies can help determine the geometry of the pulsar which is useful for constraining models of pulsed γ-ray emission. Rapidly moving PWN (RPWN) with more cometary morphologies seem to be associated with variable EGRET sources in regions where the ambient medium is dense compared to what is typical for the ISM.

Chandra and RXTE studies of the X-ray/γ-ray millisecond pulsar PSR J0218+4232

We report on high-resolution spatial and timing observations of the millisecond pulsar PSR J0218+4232 performed with the Chandra X-ray Observatory (CXO) and the Rossi X-ray Timing Explorer (RXTE). With these observations we were able to study: (a) the possible spatial extent at X-ray energies of the DC source coincident with PSR J0218+4232 in detail (CXO); (b) the relative phasing between the X-ray, radio and γ-ray profiles (CXO and RXTE); and (c) the spectral properties at energies beyond 10 keV (RXTE). We found no indications for extended emission at X-ray energies down to ∼1″ scales and confirmed the presence of a point-like DC-component. The two non-thermal pulses in the X-ray profile are found to be aligned with two of the three pulses visible at radio-frequencies and more importantly with the two γ-ray pulses seen in the EGRET 100–1000 MeV pulse profile. The latter reduces now the random occurrence probability for the detected γ-ray signal to ∼10 −6, which corresponds to a 4.9 σ detection significance. This strengthens the credibility of our earlier claimed detection of pulsed high-energy γ-ray emission from this millisecond pulsar. The pulsed RXTE spectrum appears to extend to ∼20 keV and can be described by a power-law with a photon index of 1.14, slightly softer than measured by BeppoSAX and XMM for energies below 10 keV. We compare the high-energy characteristics of PSR J0218+4232 with those of two other X-ray emitting millisecond pulsars PSR B1821-24 and PSR B1937+21.

Pulsar populations and unidentified γ-ray sources

The EGRET telescope on the Compton Gamma-ray Observatory detected over 200 sources and the majority of these are still unidentified. At least three subpopulations of EGRET sources have been associated with the Galaxy: bright sources lying along the Galactic plane, weaker sources spatially correlated with the Gould Belt and a high-latitude, halo population. Many of these sources may be pulsars and there are more than two-dozen radio pulsars in or near EGRET source error boxes, most of them recently discovered in the Parkes Multibeam Survey. We present results from several population synthesis studies of pulsars in the Galaxy, which predict the number of pulsars detected by γ-ray and radio surveys assuming models for the high-energy emission beam and its relation to the radio beam. Future γ-ray pulsar detections by AGILE and GLAST together with the recent large rise in the radio pulsar population will give greatly improved statistics. The relative numbers of radio and γ-ray pulsars detected in the plane and in the Gould Belt populations will provide an important discriminator between models of high-energy emission and will profoundly affect the numbers and birth rate of pulsars in the Galaxy.

Prospects for high energy studies of pulsars with the AGILE γ-ray telescope

AGILE is a small γ-ray astronomy satellite, with good spatial resolution, excellent timing capabilities and an unprecedented large field of view (∼1/5 of the sky). It will be the next mission dedicated to high energy astrophysics in the range 30 MeV–50 GeV and will be launched in 2005. Pulsars are a major topic of investigation of AGILE and, besides studying the small sample of known objects, AGILE will offer the first possibility of detecting several young and energetic radio pulsars that have been discovered since the end of the CGRO mission. We provide an estimate of the expected number of detectable γ-ray pulsars and present AGILE capabilities for timing analysis with small counting statistics, based on the analysis of data from simulations, from the EGRET archive and from radio pulsar catalogs.

PSR J2021+3651: a new γ-ray pulsar candidate

The COS B high energy γ-ray source 2CG 075+00, also known as GeV J2020+3658 or 3EG J2021+3716, has avoided identification with a low energy counterpart for over 20 years. We present a likely identification with the discovery and subsequent timing of a young and energetic pulsar, PSR J2021+3651, with the Wideband Arecibo Pulsar Processor at the Arecibo Observatory. PSR J2021+3651 has a rotation period P∼104 ms and P ̇ ∼9.6×10 −14 , implying a characteristic age τ c∼17 kyr and a spin-down luminosity E ̇ ∼3.4×10 36 erg s −1. The pulsar is also coincident with the ASCA source AX J2021.1+3651. The implied luminosity of the associated X-ray source suggests the X-ray emission is dominated by a pulsar wind nebula unresolved by ASCA. The pulsar’s unexpectedly high dispersion measure (DM ∼ 371 pc cm −3) and the d⩾10 kpc DM distance pose a new question: is PSR J2021+3651 an extremely efficient γ-ray pulsar at the edge of the Galaxy? This is a question for AGILE and GLAST to answer.

Preliminary research results for generation and application of high power ion beams on FLASH II accelerator

Preliminary results for the generation and application of the high power ion beam(HPIB) on the FLASH Ⅱ accelerator are reported. The structure and principle of the pinch reflex ion beam diode are introduced. The equation of parapotential flow is corrected for the reduction of diode A-K gap due to the motion of cathode and anode plasma. The HPIB peak current of ～160kA is obtained with a peak energy of ～500keV. Experimental investigations of generating 6—7MeV quasi-monoenergetic pulsed γ-rays with high power ion(proton) beams striking 19F target are presented. In addition, the results of the thermal-mechanical effects on the material irradiated with HPIB, which are applied to the simulation of 1keV black body radiation x-rays, are also discussed.

Absolute timing with IBIS, SPI and JEM-X aboard INTEGRAL

We have verified the absolute timing capabilities of the high-energy instruments aboard INTEGRAL, i.e. the imager IBIS, the spectrometer SPI and the X-ray monitor JEM-X. Calibration observations of the Crab, contemporaneous with the Rossi X-ray Timing Explorer (RXTE), have been used to measure the absolute phase of the main pulse of the Crab pulse profile using the same Jodrell Bank radio ephemeris. The three INTEGRAL instruments and RXTE give within the statistical and systematic uncertainties consistent results: The X-ray main pulse is leading the radio pulse by . Also the shapes of the X-ray pulse profiles as measured by the different instruments are fully consistent with each other. In addition, we present the first measurement of the absolute phase of the main pulse at γ-ray energies above 30 MeV using data from the EGRET instrument aboard the Compton Gamma-Ray Observatory: The γ-ray main pulse is leading the radio one by , consistent with the value for the X-ray main pulse. Comparing absolute arrival times at multiple frequencies gives important constraints to models explaining the production of non-thermal emission in magnetospheres of rotation powered neutron stars.

Extended Acceleration in Slot Gaps and Pulsar High‐Energy Emission

We revise the physics of primary electron acceleration in the slot (SG) above the pulsar polar caps (PCs), a regime originally proposed by Arons and Scharlemann (1979) in their electrodynamic model of pulsar PCs. We employ the standard definition of the SG as a pair-free space between the last open field lines and the boundary of the pair plasma column which is expected to develop above the bulk of the PC. The rationale for our revision is that the proper treatment of primary acceleration within the pulsar SGs should take into account the effect of the narrow geometry of the gap on the electrodynamics within the gap and also to include the effect of inertial frame dragging on the particle acceleration. We show that the accelerating electric field within the gap, being significantly boosted by the effect of frame dragging, becomes reduced because of the gap geometry by a factor proportional to the square of the SG width. The combination of the effects of frame dragging and geometrical screening in the gap region naturally gives rise to a regime of extended acceleration, that is not limited to favorably curved field lines as in earlier models, and the possibility of multiple-pair production by curvature photons at very high altitudes, up to several stellar radii. We present our estimates of the characteristic SG thickness across the PC, energetics of primaries accelerated within the gap, high-energy bolometric luminosities emitted from the high altitudes in the gaps, and maximum heating luminosities produced by positrons returning from the elevated pair fronts. The estimated theoretical high-energy luminosities are in good agreement with the corresponding empirical relationships for gamma-ray pulsars. We illustrate the results of our modeling of the pair cascades and gamma-ray emission from the high altitudes in the SG for the Crab pulsar. The combination of the frame-dragging field and high-altitude SG emission enables both acceleration at the smaller inclination angles and a larger emission beam, both necessary to produce widely-spaced double-peaked profiles.