Strong magnetic fields, synchrotron emission, and Compton scattering are omnipresent in compact celestial X-ray sources. Emissions in the X-ray energy band are consequently expected to be linearly ...polarized. X-ray polarimetry provides a unique diagnostic to study the location and fundamental mechanisms behind emission processes. The polarization of emissions from a bright celestial X-ray source, the Crab, is reported here for the first time in the hard X-ray band (~20-160 keV). The Crab is a complex system consisting of a central pulsar, a diffuse pulsar wind nebula, as well as structures in the inner nebula including a jet and torus. Measurements are made by a purpose-built and calibrated polarimeter, PoGO+. The polarization vector is found to be aligned with the spin axis of the pulsar for a polarization fraction, PF = (20.9 ± 5.0)%. This is higher than that of the optical diffuse nebula, implying a more compact emission site, though not as compact as, e.g., the synchrotron knot. Contrary to measurements at higher energies, no significant temporal evolution of phase-integrated polarisation parameters is observed. The polarization parameters for the pulsar itself are measured for the first time in the X-ray energy band and are consistent with observations at optical wavelengths.
The spatial extension of a γ-ray source is an essential ingredient to determine its spectral properties, as well as its potential multiwavelength counterpart. The capability to spatially resolve ...γ-ray sources is greatly improved by the newly delivered Fermi-Large Area Telescope (LAT) Pass 8 event-level analysis, which provides a greater acceptance and an improved point-spread function, two crucial factors for the detection of extended sources. Here, we present a complete search for extended sources located within 7° from the Galactic plane, using 6 yr of Fermi-LAT data above 10 GeV. We find 46 extended sources and provide their morphological and spectral characteristics. This constitutes the first catalog of hard Fermi-LAT extended sources, named the Fermi Galactic Extended Source Catalog, which allows a thorough study of the properties of the Galactic plane in the sub-TeV domain.
Abstract
The Large Area Telescope (LAT), the primary instrument for the Fermi Gamma-ray Space Telescope (Fermi) mission, is an imaging, wide field-of-view, high-energy gamma-ray telescope, covering ...the energy range from 30 MeV to more than 300 GeV. We describe the performance of the instrument at the 10 yr milestone. LAT performance remains well within the specifications defined during the planning phase, validating the design choices and supporting the compelling case to extend the duration of the Fermi mission. The details provided here will be useful when designing the next generation of high-energy gamma-ray observatories.
The Fermi-LAT experiment recently reported high precision measurements of the spectrum of cosmic-ray electrons-plus-positrons (CRE) between 20
GeV and 1
TeV. The spectrum shows no prominent spectral ...features, and is significantly harder than that inferred from several previous experiments. Here we discuss several interpretations of the Fermi results based either on a single large scale Galactic CRE component or by invoking additional electron–positron primary sources, e.g. nearby pulsars or particle dark matter annihilation. We show that while the reported Fermi-LAT data alone can be interpreted in terms of a single component scenario, when combined with other complementary experimental results, specifically the CRE spectrum measured by H.E.S.S. and especially the positron fraction reported by PAMELA between 1 and 100
GeV, that class of models fails to provide a consistent interpretation. Rather, we find that several combinations of parameters, involving both the pulsar and dark matter scenarios, allow a consistent description of those results. We also briefly discuss the possibility of discriminating between the pulsar and dark matter interpretations by looking for a possible anisotropy in the CRE flux.
Gamma-ray bursts (GRBs) are highly energetic explosions signaling the death of massive stars in distant galaxies. The Gamma-ray Burst Monitor and Large Area Telescope onboard the Fermi Observatory ...together record GRBs over a broad energy range spanning about 7 decades of gamma-ray energy. In September 2008, Fermi observed the exceptionally luminous GRB 080916C, with the largest apparent energy release yet measured. The high-energy gamma rays are observed to start later and persist longer than the lower energy photons. A simple spectral form fits the entire GRB spectrum, providing strong constraints on emission models. The known distance of the burst enables placing lower limits on the bulk Lorentz factor of the outflow and on the quantum gravity mass.
We present the angular distribution of gamma rays produced by proton-proton interactions in parameterized formulae to facilitate calculations in astrophysical environments. The parameterization is ...derived from Monte Carlo simulations of the up-to-date proton-proton interaction model and its extension by Kamae and coworkers. This model includes the logarithmically rising inelastic cross section, the diffraction dissociation process, and the Feynman scaling violation. The extension adds two baryon resonance contributions: one representing the (1232) and the other representing multiple resonances around 1600 MeV/c(2). We demonstrate the use of the formulae by calculating the predicted gamma-ray spectrum for two different cases: the first is a pencil beam of protons following a power law, and the second is a fanned proton jet with a Gaussian intensity profile impinging on the surrounding material. In both cases we find the predicted gamma-ray spectrum to be dependent on the viewing angle.
Abstract
The linear polarization fraction (PF) and angle of the hard X-ray emission from the Crab provide unique insight into high-energy radiation mechanisms, complementing the usual imaging, ...timing, and spectroscopic approaches. Results have recently been presented by two missions operating in partially overlapping energy bands, PoGO+ (18–160 keV) and AstroSat CZTI (100–380 keV). We previously reported PoGO+ results on the polarization parameters integrated across the light curve and for the entire nebula-dominated off-pulse region. We now introduce finer phase binning, in light of the AstroSat CZTI claim that the PF varies across the off-pulse region. Since both missions are operating in a regime where errors on the reconstructed polarization parameters are non-Gaussian, we adopt a Bayesian approach to compare results from each mission. We find no statistically significant variation in off-pulse polarization parameters, neither when considering the mission data separately nor when they are combined. This supports expectations from standard high-energy emission models.
We have measured the linear polarization of hard X-ray emission from the Crab in a previously unexplored energy interval, 20–120 keV. The introduction of two new observational parameters, the ...polarization fraction and angle stands to disentangle geometrical and physical effects, thereby providing information on the pulsar wind geometry and magnetic field environment. Measurements are conducted using the PoGOLite Pathfinder – a balloon-borne polarimeter. Polarization is determined by measuring the azimuthal Compton scattering angle of incident X-rays in an array of plastic scintillators housed in an anticoincidence well. The polarimetric response has been characterized prior to flight using both polarized and unpolarized calibration sources. We address possible systematic effects through observations of a background field. The measured polarization fraction for the integrated Crab light curve is
$18.4^{+9.8}_{-10.6}$
per cent, corresponding to an upper limit (99 per cent credibility) of 42.4 per cent, for a polarization angle of (149.2 ± 16.0)°.
Background: Thromboxane A2 receptor (TXA2R) abnormality appears to dominantly disturb platelet function. Objectives: To reveal a molecular genetic defect in a patient with TXA2R abnormality and ...investigate the mechanism for the impaired response to TXA2. Patient: The proband (OSP‐2, PT) was a 7‐year‐old Japanese girl, suffering from repeated mucocutaneous bleeding. Methods and results: U46619 (2.5 and 10 μm)‐induced platelet aggregation was remarkably impaired in the proband and her father. Immunoblots showed that TXA2R expression levels in their platelets were approximately 50% of controls, and nucleotide sequence analysis revealed that they were heterozygous for a novel mutation, c.167dupG in the TXA2R cDNA. Expression studies using Chinese hamster ovary (CHO) cells indicated that the mutation is responsible for the expression defect in TXA2R. We then examined αIIbβ3 activation by employing an initial velocity analysis and revealed that U46619 failed to induce a sustained αIIbβ3 and Rap1B activation in the proband. In addition, platelet secretion as monitored by P‐selectin expression was markedly impaired in response to U46619 but not to ADP. The interaction between secreted ADP and P2Y12 has been shown to play a critical role in the sustained αIIbβ3 activation (Kamae et al. J Thromb Haemost 2006; 4: 1379). As expected, small amounts of exogenous ADP (0.5 μm) partially restored the sustained αIIbβ3 activation induced by U46619. Conclusion: Our present data strongly suggest that the impaired platelet activation in response to U46619 in the heterozygous subject for the TXA2R mutation is, at least in part, as a result of the decrease in ADP secretion.
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