Improving the angular resolution of EGRET and new limits on supersymmetric dark matter near the galactic center

Using the EGRET data and an improved point source analysis, including an energy-dependent point spread function and an unbinned maximum likelihood technique, we have been able to place considerably lower limits on the γ-ray flux from the galactic center region. We also test this method on known sources, the Crab and Vela pulsars. In both cases, we find that our method improves the angular precision of EGRET data over the 3EG catalog.This new limit on γ-rays from the galactic center can be used to test models of annihilating supersymmetric dark matter and galactic halo profiles. We find that the present EGRET data can limit many supersymmetric models if the density of the galactic dark matter halo is cuspy or spiked toward the galactic center. We also discuss the ability of GLAST to test these models.     

Constraints on injection parameters of cosmic rays in the Galactic center from resolved HESS observations

In this study, we investigate how restrictive the γ-ray emission from the Galactic center region, as seen by HESS and other Cherenkov air shower arrays, is against various models for cosmic ray injection. We derive diffusion coefficients which fit the observed spatial scales of diffuse γ-ray emission from the extended emission associated with the molecular clouds SgrA, B and C. Using these diffusion coefficients, we then obtain a limit for time scale of assumed recent proton acceleration near the SMBH, as the spatial size of SgrA^∗ in VHE γ-rays has to be consistent with the observed unresolved HESS point source size at this position. The signal from this hadronic component may be mixed with the expected VHE inverse Compton emission from the nearby unresolved pulsar wind nebula.     

TeV gamma-ray observations of pulsar wind nebulae with the HESS detector

High energy stereoscopic system (HESS) is a recent operational detector dedicated to the observation of γ-rays in the very high energy range. Situated in Namibia, it is composed of four Imaging Atmospheric Cherenkov Telescopes and gives a significant improvement in sensitivity and in accuracy of the reconstructed γ-ray parameters. First results on observations of pulsar wind nebulae are reported here. The binary system PSR B1259-63/SS 2883 has been detected in 2004 around the periastron, showing clear flux variations. The pulsar wind nebula around PSR B1706-44 has been observed and upper limits on its flux have been derived.     

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.     

Imaging systems using modulation and coded aperture masks

The advent of improved γ-ray telescopes which incorporate high angular resolution imaging properties and adequate sensitivity will advance this branch of astronomy from the discovery phase to the exploratory phase. As in other fields, such as radio and X-ray astronomy, which have recently undergone this change, it will prove a fascinating era. The recent development of position sensitive γ-ray detection planes operated in conjunction with a suitable coded aperture mask have made γ-ray telescopes feasible which are capable of generating γ-ray images of the sky with a precision of 1 arc minute over the photon energy range 0.1 to 10 MeV. With a sensitivity of at least 1–10 milliCrab and scintillation standard spectral resolution not only can a large number of discrete γ-ray objects be identified and studied in detail but nuclear γ-ray line images of extended objects such as the Galactic Plane, Cloud Complexes, and supernovae remnants may be generated by this class of astronomical instrument.     

The double-pulsar binary J0737–3039

A recent multibeam pulsar survey of the outer Galactic plane at Parkes has discovered the first-known double-pulsar binary, a very compact double neutron star system in which both stars are observable as radio pulsars. In this review, we briefly describe the discovery and the studies which have been enabled by the unique properties of the system. These range from the most precise confirmation yet of the theory of general relativity, with the possibility of even more new tests and the measurement of second-order post-Newtonian effects, to studies of the magnetospheres and emission properties of the two pulsars. The discovery also results in a significant increase in the expected rate of occurrence of the mergers of double neutron star systems, and hence the rate of detection of such events by the new ground-based gravitational wave detectors.     

Binary and millisecond radio pulsars

Recent pulsar surveys, especially those using the multibeam receiver on the Parkes radio telescope, have more than doubled the number of known pulsars, bringing the current total to about 1720. Of these, 171 have millisecond periods, 126 are members of binary systems and 99 are associated with globular clusters. Binary pulsars may be divided into several distinct groups depending on their observed properties such as pulsar period and age, binary period, eccentricity and companion mass. These groups all have different evolutionary histories. Correlations between the various properties are discussed using the increased samples now available. The Parkes Pulsar Timing Array is a project which has the goal of using timing observations of millisecond pulsars to make a direct detection of gravitational waves. The parameters and present status of the project are briefly discussed.     

Results from the Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Observatory

The Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory covers the high energy gamma ray energy range, approximately 30 MeV to 30 GeV, with a sensitivity considerably greater than earlier high energy gamma-ray satellites. Thus far, 4 pulsars have been detected and their properties measured, including in 3 cases the energy spectrum as a function of phase. The details of the galactic plane are being mapped and a spectra of the center region has been obtained in good agreement with that expected from cosmic ray interactions. The Magellanic clouds have been examined with the Large Magellanic Cloud having been detected at a level consistent with it having a cosmic ray density compatible with quasi-stable equilibrium. Sixteen Active Galactic Nuclei (AGN's) have been seen thus far with a high degree of certainty including 12 quasars and 4 BL Lac objects, but no Seyferts. Time variation has been detected in some of these AGN's.     

Bow shocks around pulsars and neutron stars

Pulsar wind nebulae are now well-established as important probes both of neutron stars’ relativistic winds and of the surrounding interstellar medium. Amongst this diverse group of objects, pulsar bow shocks have long been regarded as an oddity, only seen around a handful of rapidly moving neutron stars. However, recent efforts at optical, radio and X-ray wavelengths have identified many new pulsar bow shocks, and these results have consequently motivated renewed theoretical efforts to model these systems. Here, I review the new results and ideas which have emerged on these spectacular systems, and explain how bow shocks and “Crab-like” nebulae now form a consistent picture within our understanding of pulsar winds.     

Low and medium energy gamma-ray astronomy — present status and future aspects

During the last few years quite some progress has been achieved in the field of low and medium energy gamma-ray astronomy below about 30 MeV. Gamma rays from the galactic center and anti-center region have been detected, which require a high interstellar electron flux in the 100 MeV range, if they are predominantly diffuse in nature. Though the Crab pulsar and its nebula are still the only galactic gamma-ray sources which definitely have been detected, some recently determined upper limits to the gamma-ray fluxes of other radio pulsars are close to the theoretically expected values. Active galaxies seem to have a maximum of luminosity in the range between several 100 keV and a few MeV and, therefore, are of special interest. First observational results have been reported on the Seyfert galaxies NGC 4151 and MCG 8-11-11, and the radio galaxy CenA. The nature of the diffuse cosmic gamma-ray component at low gamma-ray energies is not yet solved. Unresolved active galaxies are good candidates for its origin.Considering the present status of gamma ray astronomy the study of galactic sources like radio pulsars and the unidentified high energy gamma-ray sources, the Milky Way as a whole, active galaxies and the diffuse cosmic sky seem to be the prime targets for broad band observations below 30 MeV in the GRO area. An unexplored field like that of low energy gamma-ray astronomy, however, is always open for surprises.     

毫秒脉冲星计时的多星观测

在评价脉冲星时研究发展趋势的基础上 ,讨论了基于毫秒脉冲星自转的脉冲星时间的短期特征和长期稳定度。并对影响其稳定度的因素作了初步分析。 5 0m射电望远镜计时观测的目的之一就是综合脉冲星时研究。对于 5 0m望远镜计时观测的选星和由多星观测进行综合脉冲星时研究中的数据分析问题进行了讨论。     

MHD interaction of pulsar wind nebulae with SNRs and with the ISM

In the late 1960s the discovery of the Crab pulsar in its associated supernova remnant, launched a new field in supernova remnant research: the study of pulsar-driven or plerionic supernova remnants. In these type of remnants, the relativistic wind emitted by the pulsar, blows a pulsar wind nebula into the interior of its supernova remnant. Now, more then forty years after the discovery of the Crab pulsar, there are more then fifty plerionic supernova remnants known, due to the ever-increasing capacity of observational facilities. These observational studies reveal a Zoo of complex morphologies over a wide range of frequencies, indicating the significance of the interaction between a pulsar wind nebula with its surrounding supernova remnant. A pulsar which gained a kick velocity at birth, will ultimately break outside of its remnant, after which the pulsar wind nebula interacts directly with the interstellar medium. In general these pulsar wind nebulae are bounded by a bow shock, due to the supersonic motion of the pulsar. There are a few examples known of these pulsar-powered bow shocks, a number which is slowly increasing.I will review our current understanding of the different evolutionary stages of a pulsar wind nebula as it is interacting with its associated supernova remnant. Therefore, I will discuss both analytical and more recent numerical (M)HD models. The four main stages of a pulsar wind nebula are: the supersonic expansion stage, the reverse shock interaction stage, the subsonic expansion stage and ultimately the stage when the head of the bubble is bounded by a bow shock, due to the supersonic motion of the pulsar. Ultimately this pulsar wind nebula bow shock will break through its associated remnant, after which the pulsar-powered bow shock will interact directly with the interstellar medium. I will discuss recent numerical models from these type of pulsar wind nebulae and their morphology.     

SIGMA observations of hard X-ray and soft gamma-ray emission from X-ray binaries

After more than two years of operation, the imaging γ-ray SIGMA telescope has accumulated several days of observation toward well known X-ray binaries. Four bright sources falling in this category have been detected so far: The pulsar GX 1+4 near the center of our galaxy, the stellar wind accreting system 4U 1700-377, and the black hole candidates Cygnus X-1 and GX 339-4. Moreover, SIGMA have observed three transients sources, which turned out to be also hard X-ray sources : The burster KS 1731-260, Tra X-1, and the Musca Nova. The properties of these systems in the SIGMA domain will be reviewed and a spectral distinction between black holes and neutron stars will be sketched.     

Possible radio JETS in GX 1+4

GX 1+4 is an X-ray pulsar with a nominal period of 2 or 4 min. and has the highest of 0.22 yr⁻¹ among the X-ray pulsars. The source is identified with an optical counterpart V2116 Oph, however no binary period has been reported so far. There are several features of the source like , variable pulse shape, period history and the spectral changes which do not fit well with our present understanding of the source.

The radio observation of the galactic center region were made using Molonglo observatory Synthesis Telescope operating at a frequency of 843 MHz. A 12 hr observation was made with the field center at the position coordinates of the optical counterpart. No significant radio emission was seen corresponding to the center position.

The synthesis map however, showed the presence of two regions of radio enhancements almost equidistant from the optical source in the diametrically opposite directions/1/. The data is interpreted as due to synchrotron emission produced by the expanding jets of GX 1+4, similar to the radio jets observed in galaxies. Radio jets have been inferred for three galactic X-ray binary sources namely, Sco X-1, Cyg X-3, and SS 433. However, for all these sources no X-ray pulsation has been detected so far. The present observations if confirmed will be the first case of radio jets in a stellar system containing an X-ray pulsar.     

High energy emission from pulsars: Outer gap scenario

I will give a brief review of the recent development in the emission models of isolated, rapidly rotating neutron stars, focusing on the γ-ray radiation mechanism in their outer magnetospheres. By examining the Poisson equation for the electrostatic potential, I show that an active particle accelerator must extend from the vicinity of the neutron star surface to the vicinity of light cylinder. Furthermore, combining the Poisson equation with the Boltzmann equations for electrons/positrons and γ-rays, and assuming that the gap trans-field thickness is large compared to the longitudinal width, I demonstrate that the energy distribution of ultra-relativistic particles cannot be described by a power-law but by a quasi-monoenergetic distribution at the terminal Lorentz factor. The particles are accelerated in the gap and escape from it with large Lorentz factors. Is is shown that such energetic particles migrating outside of the gap contribute significantly to the γ-ray luminosity and reproduce the observed soft γ-ray spectrum between 100 MeV and 3 GeV for the Vela pulsar.     

分时段实时观测脉冲星的单探测器导航方法

传统的X射线脉冲星导航系统需要同时观测3~4颗脉冲星,有效载荷的质量和功耗极大。针对航天器环地飞行中受地球遮挡、探测器可探测范围等因素影响,导致脉冲星并非所有时刻均可见的现象,提出了单探测器分时段实时观测脉冲星的导航方法。根据航天器的实际飞行情况,系统分析探测器在不同探测范围下对脉冲星实时的可见性状况,从每时段的可见星中选取单星进行导航。仿真表明,该方法可大大提高单探测器的导航性能,导航位置误差达到337m,同时有效地减小了导航系统的重量,为X射线脉冲星导航的工程实现提供了参考依据。     

Possible consequences of a disk around B0656+14 on e–e and near-1 Hz gravitational wave production

Nearby pulsars B0656+14 and Geminga were proposed in the literature as the main sources of cosmic-ray positrons observed near Earth above 10 GeV. B0656+14 has comparable distance from Earth, similar magnetic field and period of Geminga. However, observations in the R and I bands indicate the presence of a disk of approximately 10⁻⁴ M_⊙ around B0656+14. Radio and pulsed γ-ray flux observations from this pulsar are also consistent with supernova fallback material and disk entering the light cylinder and partially quenching the development of electromagnetic showers in the magnetosphere. If this is the case, B0656+14 has unlikely given any contribution to e⁺ and e⁻ observed near Earth. Absolute flux measurements and the level of anisotropy in the high energy electron and positron arrival directions above 50 GeV will help in revealing if none, one of both nearby pulsars are sources of these particles observed near Earth.     

Optical, X-ray and γ-ray observations of compact objects in globular clusters

In the past three years, a new era of study of globular clusters has begun with multiwavelength observations from the current generation of astronomical telescopes in space. We review the recent results obtained from our studies of compact binaries and x-ray sources in globulars with ROSAT and HST as well as our balloon-borne hard x-ray telescope EXITE and ground-based observations (CTIO). With ROSAT, we have obtained the most sensitive high resolution soft x-ray images of clusters which show multiple low luminosity sources in cluster cores that are likely indicative of the long-sought population of cataclysmic variables (CVs). We have obtained deep H images of two clusters with HST and found CV candidates for 3 of the ROSAT sources in the core of NGC 6397. New CTIO imaging and spectroscopy of two ‘dim source’ fields in ω-Cen are also described. With EXITE we carried out the first hard x-ray imaging observations of the cluster 47 Tuc; such studies can ultimately limit the populations of millisecond pulsars and pulsar emission mechanisms. A long ROSAT exposure on 47 Tuc also shows probable cluster diffuse emission, possibly due to hot gas from ablating millisecond pulsars. Multiwavelength studies of globular clusters may provide new constraints on problems as diverse as the origin of CVs and LMXBs and the origin of hot gas in globulars.     

Gamma-ray astronomy and cosmic ray origin problem

We analyse the results of the nonlinear kinetic theory of cosmic ray (CR) acceleration in supernova remnants (SNRs) in order to describe their relevant properties: the remnant dynamics and the characteristics of nonthermal emission produced by CRs. It is shown that the theory fits the existing data in a satisfactory way and that the magnetic field in SNRs is significantly amplified due to efficient acceleration of the nuclear CR component. From the fact that magnetic field amplification occurs in all the young SNRs for which relevant data exist, and given the strong theoretical connection between magnetic field amplification and efficiently accelerating the nuclear CRs, we tentatively conclude that the Galactic SNRs are the source population of the Galactic CRs. Due to high interior magnetic fields in young SNRs the π⁰-decay γ-rays generated by the nuclear CR component as a rule dominate over γ-rays generated by the electron CR component, and the calculated γ-ray flux fits existing data.     

Two years of INTEGRAL result on X-ray binaries

INTEGRAL is the ESA lead International Gamma-Ray Astrophysical Laboratory, successfully launched the 17th October 2002 from Baikonur with a Proton vehicle. In view of the high sensitivity of the two γ-ray instruments IBIS and SPI and their capability to provide at the same time image, spectra and time profiles of all the sources in their wide field of view, a key project was approved as “Core Programme” to obtain deep observations of the Galactic Centre (GCDE) and to exploit regular scan of the whole Galaxy Plane since the beginning of the mission. This paper will briefly review the main astrophysical results obtained in the field of high energy Galactic sources with the INTEGRAL/IBIS γ-ray Imager onboard INTEGRAL, and make a comparison with the previous scenario depicted by the BeppoSAX and RXTE results.