Evidence for shock acceleration of high-energy electrons in the supernova remnant SN1006

Electron Slingshot Acceleration in Relativistic Preturbulent Shocks Explored via Emitted Photon Polarization

Transient electron dynamics near the interface of counterstreaming plasmas at the onset of a relativistic collisionless shock (RCS) is investigated using particle-in-cell simulations. We identify a slingshotlike injection process induced by the drifting electric field sustained by the flowing focus of backward-moving electrons, which is distinct from the well-known stochastic acceleration. The flowing focus signifies the plasma kinetic transition from a preturbulent laminar motion to a chaotic turbulence. We find a characteristic correlation between the electron dynamics in the slingshot acceleration and the photon emission features. In particular, the integrated radiation from the RCS exhibits a counterintuitive nonmonotonic dependence of the photon polarization degree on the photon energy, which originates from a polarization degradation of relatively high-energy photons emitted by the slingshot-injected electrons. Our results demonstrate the potential of photon polarization as an essential information source in exploring intricate transient dynamics in RCSs with relevance for Earth-based plasma and astrophysical scenarios.

Conditions of structural transition for collisionless electrostatic shock

Collisionless shock acceleration, which transfers localized particle energies to nonthermal energetic particles via electromagnetic potential, is ubiquitous in space plasma. We investigate dynamics of collisionless electrostatic shocks that appear at the interface of two plasma slabs with different pressures using one-dimensional particle-in-cell (PIC) simulations and find that the shock structure transforms to a double-layer structure at the high density gradient. The threshold condition of the structure transformation is identified as density ratio of the two plasma slabs Γ ∼40 regardless of the temperature ratio between them. We then update the collisionless shock model that takes into account density expansion effects caused by a rarefaction wave to improve the prediction of the critical Mach numbers. These critical Mach numbers are benchmarked by PIC simulations for a wide range of Γ. Furthermore, we introduce a semianalytical approach to forecast the shock velocity just from the initial conditions based on a concept of the accelerated fraction α.

Experimental evidence of early-time saturation of the ion-Weibel instability in counterstreaming plasmas of CH, Al, and Cu

The collisionless ion-Weibel instability is a leading candidate mechanism for the formation of collisionless shocks in many astrophysical systems, where the typical distance between particle collisions is much larger than the system size. Multiple laboratory experiments aimed at studying this process utilize laser-driven (I≳10^{15} W/cm^{2}), counterstreaming plasma flows (V≲2000 km/s) to create conditions unstable to Weibel-filamentation and growth. This technique intrinsically produces temporally varying plasma conditions at the midplane of the interaction where Weibel-driven B fields are generated and studied. Experiments discussed herein demonstrate robust formation of Weibel-driven B fields under multiple plasma conditions using CH, Al, and Cu plasmas. Linear theory based on benchmarked radiation-hydrodynamic FLASH calculations is compared with Fourier analyses of proton images taken ∼5-6 linear growth times into the evolution. The new analyses presented here indicate that the low-density, high-velocity plasma-conditions present during the first linear-growth time (∼300-500 ps) sets the spectral characteristics of Weibel filaments during the entire evolution. It is shown that the dominant wavelength (∼300μm) at saturation persists well into the nonlinear phase, consistent with theory under these experimental conditions. However, estimates of B-field strength, while difficult to determine accurately due to the path-integrated nature of proton imaging, are shown to be in the ∼10-30 T range, an order of magnitude above the expected saturation limit in homogenous plamas but consistent with enhanced B fields in the midplane due to temporally varying plasma conditions in experiments.

The supernova remnant SN 1006 as a Galactic particle accelerator

The origin of cosmic rays is a pivotal open issue of high-energy astrophysics. Supernova remnants are strong candidates to be the Galactic factory of cosmic rays, their blast waves being powerful particle accelerators. However, supernova remnants can power the observed flux of cosmic rays only if they transfer a significant fraction of their kinetic energy to the accelerated particles, but conclusive evidence for such efficient acceleration is still lacking. In this scenario, the shock energy channeled to cosmic rays should induce a higher post-shock density than that predicted by standard shock conditions. Here we show this effect, and probe its dependence on the orientation of the ambient magnetic field, by analyzing deep X-ray observations of the Galactic remnant of SN 1006. By comparing our results with state-of-the-art models, we conclude that SN 1006 is an efficient source of cosmic rays and obtain an observational support for the quasi-parallel acceleration mechanism.

It is known that cosmic rays could be accelerated by shock waves in supernova (SN) remnants. Here, the authors show that SN 1006 remnant is an efficient source of cosmic rays, providing observational support for the quasi-parallel acceleration mechanism.

Pulsar Wind Nebulae and Unidentified Galactic Very High Energy Sources

The riddle of the origin of Cosmic Rays (CR) has been an open question for over a century. Gamma ray observations above 100 MeV reveal the sites of cosmic ray acceleration to energies where they are unaffected by solar modulation; recent evidence supports the existence of hadronic acceleration in Supernova Remnants (SNR), as expected in the standard model of cosmic ray acceleration. Nevertheless, the results raise new questions, and no final answer has been provided thus far. Among the suggested possible alternative accelerators in the Very High Energy (VHE) gamma ray sky, pulsar wind nebulae (PWNe, which together with dark matter are the main candidates to explain the local positron excess as well) are the dominant population among known Galactic sources. However, the most numerous population in absolute terms is represented by unidentified sources (~50% of VHE gamma ray sources). The relationship between PWNe and unidentified sources seems very close; in fact, in a PWN, the lifetime of inverse Compton (IC) emitting electrons not only exceeds the lifetime of its progenitor pulsar, but also exceeds the age of the electrons that emit via synchrotron radiation. Therefore, during its evolution, a PWN can remain bright in IC such that its GeV-TeV gamma ray flux remains high for timescales much larger than the lifetimes of the pulsar and the X-ray PWN. In addition, the shell-type remnant of the supernova explosion in which the pulsar was formed has a much shorter lifetime than the electrons responsible for IC emission. Hence, understanding PWNe and VHE unidentified sources is a crucial piece of the solution to the riddle of the origin of cosmic rays. Both theoretical aspects (with particular emphasis on the ancient pulsar wind nebulae scenario) and their observational proofs are discussed in this paper. Specifically, the scientific cases of HESS J1616-508 and HESS J1813-126 are examined in detail.

Revealing a peculiar supernova remnant G106.3+2.7 as a petaelectronvolt proton accelerator with X-ray observations

Supernova remnants (SNRs) have long been considered as one of the most promising sources of Galactic cosmic rays. In the SNR paradigm, petaelectronvolt (PeV) proton acceleration may only be feasible at the early evolution stage, lasting a few hundred years, when the SNR shock speed is high. While evidence supporting the acceleration of PeV protons in young SNRs has yet to be discovered, X-ray synchrotron emission is an important indicator of fast shock. Here, we report the first discovery of X-ray synchrotron emission from the possibly middle-aged SNR G106.3+2.7, implying that this SNR is still an energetic particle accelerator despite its age. This discovery, along with the ambient environmental information, multiwavelength observation, and theoretical arguments, supports SNR G106.3+2.7 as a likely powerful proton PeV accelerator.

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Nonthermal X-ray emission is discovered from the SNR G106.3+2.7

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X-ray observations indicate a high velocity of the SNR shock in the tail region, which is expanding in a low-density hydrogen cavity

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The high shock velocity enables the acceleration of PeV protons, which are also needed to interpret the multiwavelength spectrum of the tail region

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SNR G106.3+2.7 is likely the long-sought source of Galactic PeV cosmic rays

Role of collisionality and radiative cooling in supersonic plasma jet collisions of different materials

Currently there is considerable interest in creating scalable laboratory plasmas to study the mechanisms behind the formation and evolution of astrophysical phenomena such as Herbig-Haro objects and supernova remnants. Laboratory-scaled experiments can provide a well diagnosed and repeatable supplement to direct observations of these extraterrestrial objects if they meet similarity criteria demonstrating that the same physics govern both systems. Here, we present a study on the role of collision and cooling rates on shock formation using colliding jets from opposed conical wire arrays on a compact pulsed-power driver. These diverse conditions were achieved by changing the wire material feeding the jets, since the ion-ion mean free path (λ_{mfp-ii}) and radiative cooling rates (P_{rad}) increase with atomic number. Low Z carbon flows produced smooth, temporally stable shocks. Weakly collisional, moderately cooled aluminum flows produced strong shocks that developed signs of thermal condensation instabilities and turbulence. Weakly collisional, strongly cooled copper flows collided to form thin shocks that developed inconsistently and fragmented. Effectively collisionless, strongly cooled tungsten flows interpenetrated, producing long axial density perturbations.

Observational Evidence for Stochastic Shock Drift Acceleration of Electrons at the Earth's Bow Shock

The first-order Fermi acceleration of electrons requires an injection of electrons into a mildly relativistic energy range. However, the mechanism of injection has remained a puzzle both in theory and observation. We present direct evidence for a novel stochastic shock drift acceleration theory for the injection obtained with Magnetospheric Multiscale observations at the Earth's bow shock. The theoretical model can explain electron acceleration to mildly relativistic energies at high-speed astrophysical shocks, which may provide a solution to the long-standing issue of electron injection.

Collisionless Shocks Driven by Supersonic Plasma Flows with Self-Generated Magnetic Fields

Collisionless shocks are ubiquitous in the Universe as a consequence of supersonic plasma flows sweeping through interstellar and intergalactic media. These shocks are the cause of many observed astrophysical phenomena, but details of shock structure and behavior remain controversial because of the lack of ways to study them experimentally. Laboratory experiments reported here, with astrophysically relevant plasma parameters, demonstrate for the first time the formation of a quasiperpendicular magnetized collisionless shock. In the upstream it is fringed by a filamented turbulent region, a rudiment for a secondary Weibel-driven shock. This turbulent structure is found responsible for electron acceleration to energies exceeding the average energy by two orders of magnitude.

Detection of TeV Gamma Rays from SN1006

In spite of the recent progress of high energy gamma-ray astronomy, there still remains quite unclear and important problem about the origin of cosmic rays. Supernova remnants (SNRs) are the favoured site for cosmic rays up to 1016 eV, as they satisfy the requirements such as an energy input rate. But direct supporting evidence is sparse. Recently intense non-thermal X-ray emission from the rims of the Type Ia SNR SN1006 (G327.6+14.6) has been observed by ASCA (Koyama et al. 1995)and ROSAT (Willingale et al. 1996), which is considered, by attributing the emission to synchrotron radiation, to be strong evidence of shock acceleration of high energy electrons up to ~100 TeV. If so, TeV gamma rays would also be expected from inverse Compton scattering (IC) of low energy photons (mostly attributable to the 2.7 K cosmic background photons) by these electrons. By assuming the magnetic field strength (B) in the emission region of the SNR, several theorists (Pohl 1996; Mastichiadis 1996; Mastichiadis & de Jager 1996; Yoshida & Yanagita 1997) calculated the expected spectra of TeV gamma rays using the observed radio/X-ray spectra. Observation of TeV gamma rays would thus provide not only the further direct evidence of the existence of very high energy electrons but also the another important information such as the strength of the magnetic field and diffusion coefficient of the shock acceleration. With this motivation, SN1006 was observed by the CANGAROO imaging air Cerenkov telescope in 1996 March and June, also 1997 March and April.

The microphysics of collisionless shock waves

Collisionless shocks, that is shocks mediated by electromagnetic processes, are customary in space physics and in astrophysics. They are to be found in a great variety of objects and environments: magnetospheric and heliospheric shocks, supernova remnants, pulsar winds and their nebulæ, active galactic nuclei, gamma-ray bursts and clusters of galaxies shock waves. Collisionless shock microphysics enters at different stages of shock formation, shock dynamics and particle energization and/or acceleration. It turns out that the shock phenomenon is a multi-scale non-linear problem in time and space. It is complexified by the impact due to high-energy cosmic rays in astrophysical environments. This review adresses the physics of shock formation, shock dynamics and particle acceleration based on a close examination of available multi-wavelength or in situ observations, analytical and numerical developments. A particular emphasis is made on the different instabilities triggered during the shock formation and in association with particle acceleration processes with regards to the properties of the background upstream medium. It appears that among the most important parameters the background magnetic field through the magnetization and its obliquity is the dominant one. The shock velocity that can reach relativistic speeds has also a strong impact over the development of the micro-instabilities and the fate of particle acceleration. Recent developments of laboratory shock experiments has started to bring some new insights in the physics of space plasma and astrophysical shock waves. A special section is dedicated to new laser plasma experiments probing shock physics.

Simulations of collisionless perpendicular shocks in partially ionized plasmas

Perpendicular collisionless shocks propagating into partially ionized plasmas are investigated by two-dimensional hybrid particle simulations. It is shown that some neutral particles leak into the upstream region from the downstream region, the leaking neutral particles become pickup ions in the upstream region and modify the shock structure, the pickup ions are preferentially accelerated, and plasma instabilities are excited by the pickup ions in the upstream and downstream regions.

Time-resolved characterization of the formation of a collisionless shock

We report on the temporally and spatially resolved detection of the precursory stages that lead to the formation of an unmagnetized, supercritical collisionless shock in a laser-driven laboratory experiment. The measured evolution of the electrostatic potential associated with the shock unveils the transition from a current free double layer into a symmetric shock structure, stabilized by ion reflection at the shock front. Supported by a matching particle-in-cell simulation and theoretical considerations, we suggest that this process is analogous to ion reflection at supercritical collisionless shocks in supernova remnants.

Generation of a purely electrostatic collisionless shock during the expansion of a dense plasma through a rarefied medium

A two-dimensional numerical study of the expansion of a dense plasma through a more rarefied one is reported. The electrostatic ion-acoustic shock, which is generated during the expansion, accelerates the electrons of the rarefied plasma inducing a superthermal population which reduces electron thermal anisotropy. The Weibel instability is therefore not triggered and no self-generated magnetic fields are observed, in contrast with published theoretical results dealing with plasma expansion into vacuum. The shock front develops a filamentary structure which is interpreted as the consequence of the electrostatic ion-ion instability, consistently with published analytical models and experimental results.

A critical Mach number for electron injection in collisionless shocks

Electron acceleration in collisionless shocks with arbitrary magnetic field orientations is discussed. It is shown that the injection of thermal electrons into the diffusive shock acceleration process is achieved by an electron beam with a loss cone in velocity space that is reflected back upstream from the shock through the shock drift acceleration mechanism. The electron beam is able to excite whistler waves which can scatter the energetic electrons themselves when the Alfvén Mach number of the shock is sufficiently high. A critical Mach number for the electron injection is obtained as a function of upstream parameters. The application to supernova remnant shocks is discussed.

Observation of collisionless shocks in laser-plasma experiments

The propagation in a rarefied plasma (n(e) < or approximately 10(15) cm(-3)) of collisionless shock waves and ion-acoustic solitons, excited following the interaction of a long (tauL approximately 470 ps) and intense (I approximately 10(15) W cm(-2)) laser pulse with solid targets, has been investigated via proton probing techniques. The shocks' structures and related electric field distributions were reconstructed with high spatial and temporal resolution. The experimental results were interpreted within the framework of the nonlinear wave description based on the Korteweg-de Vries-Burgers equation.

Modern Tests of Lorentz Invariance

Motivated by ideas about quantum gravity, a tremendous amount of effort over the past decade has gone into testing Lorentz invariance in various regimes. This review summarizes both the theoretical frameworks for tests of Lorentz invariance and experimental advances that have made new high precision tests possible. The current constraints on Lorentz violating effects from both terrestrial experiments and astrophysical observations are presented.

Role of gamma delta T cells in inflammatory bowel disease

gammadelta T cells have previously been shown to play a protective role in various animal models of chronic inflammation (e.g., experimental autoimmune encephalomyelitis, collagen-induced arthritis, and non-obese diabetes). This immunoregulatory potential is exerted by synthesizing various anti-inflammatory cytokines and growth factors (e.g., transforming growth factor-beta). As the normal balance between inflammatory and regulatory cytokines is perturbed in inflammatory bowel disease (IBD) a protective effect of gammadelta T cells seems likely. This notion is supported by our finding of increased mortality of rats with 2,4,6-trinitrobenzene sulfonic acid-induced colitis following gammadelta T cell depletion. In contrast, no effect was observed after depletion of gammadelta T cells in a Crohn's disease animal model with terminal ileitis (TNF(DeltaARE) mice). Therefore, future studies must further define where in the intestinal immune system gammadelta T cells exert their protective function and how this can be used in the treatment of IBD.

Supernova--remnant origin of cosmic rays?

It is thought that Galactic cosmic-ray nuclei are gradually accelerated to high energies (up to about 300 TeV per nucleon, where 1 TeV is 10(12) eV) in the expanding shock waves connected with the remnants of powerful supernova explosions. However, this conjecture has eluded direct observational confirmation since it was first proposed in 1953 (ref. 3). Enomoto et al. claim to have finally found definitive evidence that corroborates this model, proposing that very-high-energy, TeV-range, gamma-rays from the supernova remnant (SNR) RX J1713.7-3946 are due to the interactions of energetic nuclei in this region. Here we argue that their claim is not supported by the existing multiwavelength spectrum of this source. The search for the origin(s) of Galactic cosmic-ray nuclei may be closing in on the long-suspected supernova-remnant sources, but it is not yet over.

Predominance of type-2 immune response in idiopathic membranous nephropathy. Cytoplasmic cytokine analysis

BACKGROUND

The Th1/Th2 paradigm is proving increasingly useful in the understanding of infectious diseases and many autoimmune diseases. Th1 cells predominantly produce interleukin-2 (IL-2) and interferon-gamma (IFN-gamma) and are instrumental in initiating delayed-type hypersensitivity and activating macrophages. Th2 cells secrete other cytokines, such as IL-4, IL-5, IL-10 and IL-13 that trigger B-cell activation and immunoglobulin synthesis. It has been shown that in patients with membranous nephropathy, there may be a predominance of Th2, because of the presence of IgG, particularly IgG4, which belongs to a subclass of the type-2 immune response, and complement deposits in glomeruli. In this study, we investigated the immunoresponse of helper T cells, i.e. Th predominance in patients with idiopathic membranous nephropathy.

METHODS

We used flow cytometry to assess the levels of circulating Th cells in patients with idiopathic membranous nephropathy (n = 8) and in normal individuals (n = 23) based on the expression of intracellular type-1 and type-2 cytokines. Because the production of each of these cytokines has a specific time course, we observed the cytokine synthesis at 3, 6, 9 and 12 h after stimulation.

RESULTS

The percentages of IL-2+/CD4+ cells from patients with idiopathic membranous nephropathy were significantly lower than those from normal individuals at 6, 9 and 12 h, with the difference becoming more significant over time. IFN-gamma+/CD4+ cells and IL-4+/CD4+ cells were not significantly different between the two groups. In patients with idiopathic membranous nephropathy, the percentages of IL-10+/CD4+ cells were significantly higher than those in normal individuals at each point in time.

CONCLUSION

Increased IL-10-producing Th cells may lead to suppression of delayed-type hypersensitivity and activate suppressor cells and IgG4 synthesis, resulting in idiopathic membranous nephropathy.

The acceleration of cosmic-ray protons in the supernova remnant RX J1713.7-3946

Protons with energies up to approximately 10(15) eV are the main component of cosmic rays, but evidence for the specific locations where they could have been accelerated to these energies has been lacking. Electrons are known to be accelerated to cosmic-ray energies in supernova remnants, and the shock waves associated with such remnants, when they hit the surrounding interstellar medium, could also provide the energy to accelerate protons. The signature of such a process would be the decay of pions (pi(0)), which are generated when the protons collide with atoms and molecules in an interstellar cloud: pion decay results in gamma-rays with a particular spectral-energy distribution. Here we report the observation of cascade showers of optical photons resulting from gamma-rays at energies of approximately 10(12) eV hitting Earth's upper atmosphere, in the direction of the supernova remnant RX J1713.7-3946. The spectrum is a good match to that predicted by pion decay, and cannot be explained by other mechanisms.

Origin of the hard x-ray emission from the Galactic plane

The Galactic plane is a strong emitter of hard x-rays (2 to 10 kiloelectron volts), and the emission forms a narrow continuous ridge. The currently known hard x-ray sources are far too few to explain the ridge x-ray emission, and the fundamental question of whether the ridge emission is ultimately resolved into numerous dimmer discrete sources or truly diffuse emission has not yet been settled. In order to obtain a decisive answer, using the Chandra X-ray Observatory, we carried out the deepest hard x-ray survey of a Galactic plane region that is devoid of known x-ray point sources. We detected at least 36 new hard x-ray point sources in addition to strong diffuse emission within a 17' by 17' field of view. The surface density of the point sources is comparable to that at high Galactic latitudes after the effects of Galactic absorption are considered. Therefore, most of these point sources are probably extragalactic, presumably active galaxies seen through the Galactic disk. The Galactic ridge hard x-ray emission is diffuse, which indicates omnipresence within the Galactic plane of a hot plasma, the energy density of which is more than one order of magnitude higher than any other substance in the interstellar space.

gammadelta T cells as regulators of airway hyperresponsiveness

Airway responsiveness (AR) is determined by complex mechanisms reflecting lung responses to airborne stimuli. Murine studies have identified a number of potential factors modulating AR and thus have contributed to the current understanding of these mechanisms. In allergic inflammation, immune cells, in particular alphabeta T cells, have emerged as important contributors to increased AR. We have found that in contrast to alphabeta T cells, gammadelta T cells can have a negative regulatory effect on AR. Here, we review the current studies on gammadelta T cells in allergic inflammation and discuss their role in modulating AR. We propose that gammadelta T cells exhibit different immune properties depending on the type of stimulus and inflammation. These differential immune properties appear to be associated with specific gammadelta T cell subsets, which control AR to airborne stimuli. In particular, our recent data indicate that the Vgamma4(+) T cell subset acts as an important negative regulator of AR and contributes to maintaining normal lung function in mice.

The role of alpha-galactosylceramide-activated Valpha14 natural killer T cells in the regulation of Th2 cell differentiation

Valpha14 natural killer T (NKT) cells produce large amounts of both IL-4 and IFN-gamma upon stimulation with a ligand, alpha-galactosylceramide (alpha-GalCer), and play a crucial role in various immune responses, including allergic diseases. Interestingly, Valpha14 NKT cells are not essential for the induction of IgE responses but rather induce suppression of specific IgE production upon activation. The suppression in the IgE production is not detected either in Valpha14 NKT cell-deficient mice or in IFN-gamma-deficient mice. Thus, activated Valpha14 NKT cells are likely to exert a potent suppressive activity on Th2 cell differentiation and subsequent IgE production by producing a large amount of IFN-gamma. In marked contrast, little regulatory effect of IL-4 produced by Valpha14 NKT cells on Th2 cell differentiation is suggested.

Pulmonary defence mechanisms

Lung defence involves a wide array of mechanisms needed to remove inhaled particles and organisms. The various innate immune processes which take place either in the central or in the more distant airways are reviewed. The recruitment and the development of an adaptive immunity following the innate response are described. This entails the production of secretory immunoglobulins in the airways and either the influx of polymorphonuclear neutrophils in the alveoli to phagocytose more or less opsonized organisms or the activation of the T lymphocyte response to improve the removal of intracellular pathogens.

Cosmic gamma-ray background from structure formation in the intergalactic medium

The Universe is filled with a diffuse background of gamma-ray radiation, the origin of which remains one of the unsolved puzzles of cosmology. Less than one-quarter of the gamma-ray flux can be attributed to unresolved discrete sources, such as active galactic nuclei; the remainder appears to constitute a truly diffuse background. Here we show that the shock waves induced by gravity in the gas of the intergalactic medium, during the formation of large-scale structures like filaments and sheets of galaxies, produce a population of highly relativistic electrons. These electrons scatter a small fraction of the cosmic microwave background photons in the local Universe up to gamma-ray energies, thereby providing the gamma-ray background. The predicted diffuse flux agrees with the observed background across more than four orders of magnitude in photon energy, and the model predicts that the gamma-ray background, though generated locally, is isotropic to better than five per cent on angular scales larger than a degree. Moreover, the agreement between the predicted and observed background fluxes implies a mean cosmological density of baryons that is consistent with Big Bang nucleosynthesis.

Chandra Observations of the Crab-like Supernova Remnant G21.5-0.9

Chandra observations of the Crab-like supernova remnant G21.5-0.9 reveal a compact central core and spectral variations indicative of synchrotron burn-off of higher energy electrons in the inner nebula. The central core is slightly extended, perhaps indicating the presence of an inner wind-shock nebula surrounding the pulsar. No pulsations are observed from the central region, yielding an upper limit of approximately 40% for the pulsed fraction. A faint outer shell may be the first evidence of the expanding ejecta and blast wave formed in the initial explosion, indicating a composite nature for G21.5-0.9.

Nucleosynthesis and Mixing in Cassiopeia A

We present results from the first light observations of the Cassiopeia A supernova remnant (SNR) by the Chandra X-Ray Observatory. Based on representative spectra from four selected regions, we investigate the processes of nucleosynthesis and mixing in Cas A. We make the first unequivocal identification of iron-rich ejecta produced by explosive silicon burning in a young Galactic SNR. Elsewhere in the remnant, we see silicon-rich ejecta from explosive oxygen burning. The Fe-rich ejecta lie outside the Si-rich material, indicating that bulk motions were extensive and energetic enough in Cas A to cause a spatial inversion of a significant portion of the supernova core. It is likely that this inversion was caused by "Fe"-rich ejecta emerging in plumes from the rising bubbles in the neutrino-driven convection layer during the supernova explosion. In addition, the radioactive decay energy from 56Ni may have contributed to the subsequent evolution of the material. We have also discovered faint, well-defined filaments with featureless X-ray spectra that are possibly sites of cosmic-ray acceleration in Cas A.

Expansion and immunological study of human tumor infiltrating gamma/delta T lymphocytes in vitro

Goffa/delta T cells have stimulated a lot of interest because of their unique features in antigen recognition and cytotoxicities to many autologous and/or allogeneic tumor cells. We have developed a novel method to selectively expand larger amounts of human tumor-infiltrating gamma/delta T lymphocytes (gamma/delta TILs) ex vivo by immobilized pan- anti-TCRgamma/delta monoclonal antibody in the presence of exogenous IL-2. The expanded gamma/delta TILs mainly expressed CD45RO and HLA-DR molecules and did not express CD4. CD8+ gamma/delta TILs accounted for 19% of gamma/delta TILs. The expression of CD25 molecule on expanded gamma/delta T cells was inducible and downregulated following a time course. The Vdelta1 and Vdelta2 subsets amount to 37 and 58%, respectively. The expanded gamma/delta TILs show an IL-2-dependent proliferation, MHC class I-unrestricted and TCRgamma/delta-related cytotoxicities to two MHC class I+ and two MHC class I+ allogeneic tumor cell lines in vitro.

The Th1/Th2 paradigm and experimental murine leishmaniasis

In recent years, the Th1/Th2 concept has become of prime importance in the understanding of heterogeneous responses of the immune system and implications thereof for infectious and autoimmune diseases. Originally established on the basis of different cytokines produced by T cell clones, it is now known that the Th1/Th2 concept really defines totally different immune pathways that affect most if not all cells of the immune system. Murine experimental leishmaniasis was the first model to confirm the relevance of the Th1/Th2 concept in vivo. Herein, we summarize the current knowledge on the characteristics and generation of Th1 and Th2 cells, as well as on recent advances of the application of this concept to murine cutaneous leishmaniasis.

Possible production of high-energy gamma rays from proton acceleration in the extragalactic radio source markarian 501

The active galaxy Markarian 501 was discovered with air-Cerenkov telescopes at photon energies of 10 tera-electron volts. Such high energies may indicate that the gamma rays from Markarian 501 are due to the acceleration of protons rather than electrons. Furthermore, the observed absence of gamma ray attenuation due to electron-positron pair production in collisions with cosmic infrared photons implies a limit of 2 to 4 nanowatts per square meter per steradian for the energy flux of an extragalactic infrared radiation background at a wavelength of 25 micrometers. This limit provides important clues about the epoch of galaxy formation.