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.     

A census of high-energy observations of Galactic supernova remnants

We present the first public database of high-energy observations of all known Galactic supernova remnants (SNRs). In Section 1 we introduce the rationale for this work motivated primarily by studying particle acceleration in SNRs, and which aims at bridging the already existing census of Galactic SNRs (primarily made at radio wavelengths) with the ever-growing but diverse observations of these objects at high-energies (in the X-ray and γ$\gamma$-ray regimes). In Section 2 we show how users can browse the database using a dedicated web front–end (http://www.physics.umanitoba.ca/snr/SNRcat). In Section 3 we give some basic statistics about the records we have collected so far, which provides a summary of our current view of Galactic SNRs. Finally, in Section 4, we discuss some possible extensions of this work. We believe that this catalogue will be useful to both observers and theorists, and timely with the synergy in radio/high-energy SNR studies as well as the upcoming new high-energy missions. A feedback form provided on the website will allow users to provide comments or input, thus helping us keep the database up-to-date with the latest observations.     

Einstein observations of supernova remnants

Recent observations of SNRs have produced X-ray images with resolution comparable to that routinely achieved at optical wavelengths. There has also been a great improvement in the quality of X-ray spectra. Since most of the energy radiated by SNRs appears as X-rays, these new data are crucial to studies of SNRs, the interaction of SNRs with the interstellar medium, and the SN explosion itself. Images show a variety of shapes ranging from shel-like remnants to those dominated by the influence of central objects which appear both as point sources and as centers of diffuse activity. Once the temperature and spatial distribution of X-ray emitting material is known, the mass of ejected material and the energy release of the SN explosion can sometimes be calculated. X-ray images and spectra of several remnants are shown, and some quantitative results are given.     

Propagation of cosmic-ray electrons in the Galaxy

We formulate the global propagation model of cosmic-ray electrons including the source region, which is currently considered to be supernova remnants (SNRs). The model is characterized by the escape rate of electrons from SNRs into the interstellar space. It becomes clear that the energy index of the escape rate influences the high energy side of the interstellar spectrum and makes it possible to explain the observed data up to 2 TeV in the case of source spectral index smaller than 2.2 that is expected from the radio spectrum in SNRs. The escape lifetime of electrons in SNRs is also discussed by using the ratio of the radio flux in two regions: SNRs and the Galaxy. The result shows the mean lifetime in SNRs of ∼10⁴ yr around 1 GeV, which corresponds to the SNR age in the Sedov phase.     

Cosmic ray acceleration by supernova shocks

We analyse the results of recent measurements of nonthermal emission from individual supernova remnants (SNRs) and their correspondence to the nonlinear kinetic theory of cosmic ray (CR) acceleration in SNRs. It is shown that the theory fits these data in a satisfactory way and provides the strong evidences for the efficient CR production in SNRs accompanied by significant magnetic field amplification. Magnetic field amplification leads to considerable increase of CR maximum energy so that the spectrum of CRs accelerated in SNRs is consistent with the requirements for the formation of Galactic CR spectrum up to the energy ∼10¹⁷ eV.     

Relationship of solar flare accelerated particles to solar energetic particles (SEPs) observed in the interplanetary medium

Observations of hard X-ray (HXR)/γ-ray continuum and γ-ray lines produced by energetic electrons and ions, respectively, colliding with the solar atmosphere, have shown that large solar flares can accelerate ions up to many GeV and electrons up to hundreds of MeV. Solar energetic particles (SEPs) are observed by spacecraft near 1 AU and by ground-based instrumentation to extend up to similar energies as in large SEP events, but it appears that a different acceleration process, one associated with fast coronal mass ejections is responsible. Much weaker SEP events are observed that are generally rich in electrons, ³He, and heavy elements. The energetic particles in these events appear to be similar to those accelerated in flares. The Ramaty high energy solar spectroscopic imager (RHESSI) mission provides high-resolution spectroscopy and imaging of flare HXRs and γ-rays. Such observations can provide information on the location, energy spectra, and composition of the flare accelerated energetic particles at the Sun. Here, preliminary comparisons of the RHESSI observations with observations of both energetic electron and ion near 1 AU are reviewed, and the implications for the particle acceleration and escape processes are discussed.     

Patterns of variability in γ-ray blazars

Preliminary results of a systematic study on the simultaneous optical-to-X-rays variability in blazars are presented. Data from Swift observations of four bright γ-ray blazars (3C 279, ON +231, S5 0716+71, PKS 2155−304) have been analyzed, compared, and discussed. Specifically, 3C 279 shows a variable flattening in the low energy part of the X-ray spectrum that appears to be confined in a specific X-ray vs optical/UV fluxes region. Some implications are shortly analyzed.     

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.     

Centaurus A: Multiwavelength observations of the nearest active galaxy from radio to gamma-rays

Centaurus A (Cen A, NGC 5128) is the nearest active galaxy and, notably, the viewing angle with respect to the jet axis is very large (> 70°). A first contemporaneous OSSE, COMPTEL, and EGRET spectrum obtained in October 1991 covers an energy range from 50 keV up to 1 GeV. This γ-ray broad-band spectrum was taken when Cen A was in an intermediate emission state as defined by the BATSE X-ray light-curve. The first simultaneous multiwavelength spectrum from radio to γ-rays was measured in July 1995 when Cen A was in a low emission state (the prevailing state for the last 7 years). The different spatial and temporal resolution in the different frequency regimes produces problems in the construction and interpretation of the multiwavelength spectra. These are addressed in this paper. The detection of emission > 1 MeV makes the inclusion of such high-energy emission into models for the spectral energy distribution mandatory.     

New evidence for strong non-thermal effects in Tycho’s supernova remnant

For the case of Tycho’s supernova remnant (SNR), we present the relation between the blast wave and contact discontinuity radii calculated within the nonlinear kinetic theory of cosmic ray (CR) acceleration in SNRs. It is demonstrated that these radii are confirmed by recently published Chandra measurements which show that the observed contact discontinuity radius is very close to the shock radius. Therefore a consistent explanation of these observations can be given in terms of efficient CR acceleration which makes the medium more compressible.     

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.     

多头锗空间γ谱仪探测效率的提高

在γ谱线天文观测中采用多头锗晶体作为γ谱仪中的中心探测器,以提高探测器的观测灵敏度。本文以一个九头高纯锗探测器系统为实例,用蒙特卡洛模拟方法,分析了入射光子在各晶体中的能损分配和各类效率的比例,给出在几种晶体输出幅度组合相加情况下,探测的全能峰效率可提高的份额。并用较完整的模型对单个晶体的模拟结果与实测结果进行了详细讨论。      

New insights into SNR evolution revealed by the discovery of recombining plasmas

We report the discovery of recombining plasmas in three supernova remnants (SNRs) with the Suzaku X-ray astronomy satellite. During SNR’s evolution, the expanding supernova ejecta and the ambient matter are compressed and heated by the reverse and forward shocks to form an X-ray emitting hot plasma. Since ionization proceeds slowly compared to shock heating, most young or middle-aged SNRs have ionizing (underionized) plasmas. Owing to high sensitivity of Suzaku, however, we have detected radiative recombination continua (RRCs) from the SNRs IC 443, W49B, and G359.1–0.5. The presence of the strong RRC is the definitive evidence that the plasma is recombining (overionized). As a possible origin of the overionization, an interaction between the ejecta and dense circumstellar matter is proposed; the highly ionized gas was made at the initial phase of the SNR evolution in dense regions, and subsequent rapid adiabatic expansion caused sudden cooling of the electrons. The analysis on the full X-ray band spectrum of IC 443, which is newly presented in this paper, provides a consistent picture with this scenario. We also comment on the implications from the fact that all the SNRs having recombining plasmas are correlated with the mixed-morphology class.     

Viewing radiation signatures of solar energetic particles in interplanetary space

A current serious limitation on the studies of solar energetic particle (SEP) events is that their properties in the inner heliosphere are studied only through in situ spacecraft observations. Our understanding of spatial distributions and temporal variations of SEP events has come through statistical studies of many such events over several solar cycles. In contrast, flare SEPs in the solar corona can be imaged through their radiative and collisional interactions with solar fields and particles. We suggest that the heliospheric SEPs may also interact with heliospheric particles and fields to produce signatures which can be remotely observed and imaged. A challenge with any such candidate signature is to separate it from that of flare SEPs. The optimum case for imaging high-energy (E > 100 MeV) heliospheric protons may be the emission of π⁰-decay γ-rays following proton collisions with solar wind (SW) ions. In the case of E > 1 MeV electrons, gyrosynchrotron radio emission may be the most readily detectible remote signal. In both cases we may already have observed one or two such events. Another radiative signature from nonthermal particles may be resonant transition radiation, which has likely already been observed from solar flare electrons. We discuss energetic neutrons as another possible remote signature, but we rule out γ-ray line and 0.511 MeV positron annihilation emission as observable signatures of heliospheric energetic ions. We are already acquiring global signatures of large inner-heliospheric SW density features and of heliosheath interactions between the SW and interstellar neutral ions. By finding an appropriate observable signature of remote heliospheric SEPs, we could supplement the in situ observations with global maps of energetic SEP events to provide a comprehensive view of SEP events.     

High energy radiation from the direction of the galactic black hole Sgr A

X-ray observations indicate that the Galactic black hole Sgr A^∗ is inactive now, however, we suggest that Sgr A^∗ can become active when a captured star is tidally disrupted and matter is accreted into the black hole. Consequently the Galactic black hole could be a powerful source of relativistic protons with a characteristic energy ∼10⁵² erg per capture. The diffuse GeV and TeV γ-rays emitted in the direction of the Galactic Center (GC) are the direct consequences of p–p collisions of such relativistic protons ejected by very recent capture events occurred ?10⁵ yr ago. On the other hand, the extended electron-positron annihilation line emission observed from GC is a phenomenon related to a large population of thermalized positrons, which are produced, cooled down and accumulated through hundreds of past capture events during a period of ∼10⁷ yr. In addition to explaining GeV, TeV and 511 keV annihilation emissions we also estimate the photon flux of several MeV resulting from in-flight annihilation process.     

Solar proton events recorded in the stratosphere during cosmic ray balloon observations in 1957–2008

Long-term balloon observations have been performed by the Lebedev Physical Institute since 1957 up to the present time. The observations are taken several times a week at the polar and mid latitudes and allow us to study dynamics of galactic and solar cosmic ray as well as secondary particle fluxes in the atmosphere and in the near-Earth space. Solar energetic particles (120) – mostly protons – (SEP) events with >100 MeV proton intensity above 1 cm⁻² s⁻¹ s⁻¹ were recorded during 1958–2006. Before the advent of the SEP monitoring on spacecraft these results constituted the only homogeneous series of >100 MeV SEP events. The SEP intensities and energy spectra inferred from the Lebedev Physical Institute observations are consistent with the results taken in the adjacent energy intervals by the spacecraft and neutron monitors. Joint consideration of the SEP events series recorded by balloons and by neutron monitors during solar cycles 20–23 makes it possible to restore the probable number of events in solar cycle 19, which was not properly covered by observations. Some correlation was found between duration of SEP event production in a solar cycle and sunspot cycle characteristics.     

Highly significant detection of solar neutrons on 2005 September 7

We have successfully detected solar neutrons at ground level in association with the X17.0 solar flare that occurred on 2005 September 7. Observations were made with the solar neutron telescopes and neutron monitors located in Bolivia and Mexico. In this flare, large fluxes of hard X-rays and γ-rays were observed by the GEOTAIL and the INTEGRAL satellites. The INTEGRAL observations include the 4.4 MeV line γ-rays of ¹²C. The data suggest that solar neutrons were produced at the same time as these hard electromagnetic radiations. We have however found an apparent discrepancy between the observed and the expected time profiles. This fact suggests a possible extended neutron emission.     

Nitric oxide density enhancements due to solar flares

A differential emission measure technique is used to determine flare spectra using solar observations from the soft X-ray instruments aboard the Thermosphere Ionosphere Mesosphere Energetics Dynamics and Solar Radiation and Climate Experiment satellites. We examine the effect of the solar flare soft X-ray energy input on the nitric oxide (NO) density in the lower thermosphere. The retrieved spectrum of the 28 October 2003 X18 flare is input to a photochemical thermospheric NO model to calculate the predicted flare NO enhancements. Model results are compared to Student Nitric Oxide Explorer Ultraviolet Spectrometer observations of this flare. We present results of this comparison and show that the model and data are in agreement. In addition, the NO density enhancements due to several flares are studied. We present results that show large solar flares can deposit the same amount of 0.1–2 and 0.1–7 nm energy to the thermosphere during a relatively short time as the Sun normally deposits in one day. The NO column density nearly doubles when the daily integrated energy above 5 J m⁻² is doubled.     

Space observations of comets during solar flares: A possible explanation for comet brightness outbursts

Problems connected with mechanisms for comet brightness outbursts as well as for gamma-ray bursts remain open. Meantime, calculations show that irradiation of a certain class of comet nuclei, having high specific electric resistance, by intense fluxes of energetic protons and positively charged ions with kinetic energies more than 1 MeV/nucleon, ejected from the Sun during strong solar flares, can produce a macroscopic high-voltage electric double layer with positive charge in the subsurface zone of the nucleus, during irradiation times of the order of 10–100 h at heliocentric distances around 1–10 AU. The maximum electric energy accumulated in such layer will be restricted by the electric discharge potential of the layer material. For comet nuclei with typical radii of the order of 1–10 km the accumulated energy of such natural electric capacitor is comparable to the energy of large comet outbursts that are estimated on the basis of ground based optical observations. The impulse gamma and X-ray radiation together with optical burst from the comet nucleus during solar flares, anticipated due to high-voltage electric discharge, may serve as an indicator of realization of the processes above considered. Multi-wavelength observations of comets and pseudo-asteroids of cometary origin, having brightness correlation with solar activity, using ground based optical telescopes as well as space gamma and X-ray observatories, during strong solar flares, are very interesting for the physics of comets as well as for high energy astrophysics.     

Prospects for observing dark-matter remnants with GLAST

The satellite-based experiment, GLAST (Gamma-ray Large Area Space Telescope), is under construction and is planned to measure the cosmic γ-ray flux in the energy range 20 MeV to >300 GeV, with supporting measurements for γ-ray bursts from 10 keV to 25 MeV. With its launch in 2007, GLAST will open a new and important window on a wide variety of high-energy phenomena, including exotic relics from the Big Bang. Among these may be the decay/annihilation products of the hypothesized super symmetric image of the known particles. Single-photon energy thresholds for channels leading to such final states have been excluded in a model-dependent manner by accelerator searches to energies greater than 50 GeV. The ability of GLAST to set limits on this important component of cosmological evolution is presented along with an update on the present status of this mission.