Discovery of GeV gamma-ray emission from PSR B1259-63/LS 2883

The binary system PSR B1259-63/LS 2883 consists of a 47.8 ms radio pulsar that orbits the companion Be star with a period of 3.4 years in a highly eccentric orbit. The system is well sampled in radio, X-rays, and TeV gamma-rays, and shows orbital-phase-dependent variability in all observed frequencies. Here we report on the discovery of >100 MeV gamma-rays from PSR B1259-63/LS 2883 through the 2010 pariastron passage. Using data collected with the Large Area Telescope aboard Fermi from 33 days before pariastron to 75 days after pariastron, PSR B1259-63/LS 2883 is detected at a significance of 13.6 standard deviations. The gamma-ray light curve is highly variable over the above period, with changing photon index that correlates with gamma-ray flux. In particular, two major flares that occur after the pariastron passage were observed. The onset of gamma-ray emission occurs close to, but not at the same orbital phases as, the two disk passages that occur ~1 month before and ~1 month after the pariastron passage. The fact that the GeV orbital light curve is different from that of the X-ray and TeV light curves strongly suggests that GeV gamma-ray emission originates from a different component. We speculate that the observed GeV flares may be resulting from Doppler boosting effects.Comment: ApJ Letters, in press 7 pages, 4 figures (Fig. 4 added), 1 tabl

Radio/gamma-ray time delay in the parsec-scale cores of active galactic nuclei

We report the detection of a non-zero time delay between radio emission measured by the VLBA at 15.4 GHz and gamma-ray radiation (gamma-ray leads radio) registered by the Large Area Telescope (LAT) on board the Fermi Gamma-Ray Space Telescope for a sample of 183 radio and gamma-ray bright active galactic nuclei (AGNs). For the correlation analysis we used 100 MeV - 100 GeV gamma-ray photon fluxes, taken from monthly binned measurements from the first Fermi LAT catalog, and 15.4 GHz radio flux densities from the MOJAVE VLBA program. The correlation is most pronounced if the core flux density is used, strongly indicating that the gamma-ray emission is generated within the compact region of the 15 GHz VLBA core. Determining the Pearson's r and Kendall's tau correlation coefficients for different time lags, we find that for the majority of sources the radio/gamma-ray delay ranges from 1 to 8 months in the observer's frame and peaks at about 1.2 months in the source's frame. We interpret the primary source of the time delay to be synchrotron opacity in the nuclear region.Comment: 5 pages, 2 figures, 2 tables, accepted by the Astrophysical Journal Letters; minor corrections to the text are made; full table 1 in electronic form can be extracted from the preprint sourc

Cosmic-Ray Induced Diffuse Emissions from the Milky Way and Local Group Galaxies

Cosmic rays fill up the entire volume of galaxies, providing an important source of heating and ionisation of the interstellar medium, and may play a significant role in the regulation of star formation and galactic evolution. Diffuse emissions from radio to high-energy gamma rays (> 100 MeV) arising from various interactions between cosmic rays and the interstellar medium, interstellar radiation field, and magnetic field, are currently the best way to trace the intensities and spectra of cosmic rays in the Milky Way and other galaxies. In this contribution, I describe our recent work to model the full spectral energy distribution of galaxies like the Milky Way from radio to gamma-ray energies. The application to other galaxies, in particular the Magellanic Clouds and M31 that are detected in high-energy gamma-rays by the Fermi-LAT, is also discussed.Comment: Contribution to "The Spectral Energy Distribution of Galaxies" Proceedings IAU Symposium No. 284, 2011, eds. R.J. Tuffs & C.C.Popescu. 4 pages with 4 figure

Pulsar Outer-gap Electrodynamics: Hardening of Spectral Shape in the Trailing Peak in Gamma-ray Light Curve

The spectral characteristics of the pulsed gamma-ray emission from outer-magnetospheric particle accelerators are investigated. Either positrons or electrons are accelerated outwards by the magnetic-field-aligned electric field to emit gamma-rays via curvature process. Since the particles move along relatively straight paths in the trailing side of a rotating magnetosphere, they attain higher Lorentz factors to emit more energetic gamma-rays than those in the leading side. It is first demonstrated that the cutoff energy of the curvature radiation evolves with the rotation phase owing to the variation of the curvature radii of the particle paths and maximizes at a slightly later phase of the trailing peak in the gamma-ray light curve.Comment: 5 pages, 5 figures, ApJ Letters in pres

Search for variable gamma-ray emission from the Galactic plane in the Fermi data

High-energy gamma-ray emission from the Galactic plane above ~100 MeV is composed of three main contributions: diffuse emission from cosmic ray interactions in the interstellar medium, emission from extended sources, such as supernova remnants and pulsar wind nebulae, and emission from isolated compact source populations. The diffuse emission and emission from the extended sources provide the dominant contribution to the flux almost everywhere in the inner Galaxy, preventing the detection of isolated compact sources. In spite of this difficulty, compact sources in the Galactic plane can be singled out based on the variability properties of their gamma-ray emission. Our aim is to find sources in the Fermi data that show long-term variability. We performed a systematic study of the emission variability from the Galactic plane, by constructing the variability maps. We find that emission from several directions along the Galactic plane is significantly variable on a time scale of months. These directions include, in addition to known variable Galactic sources and background blazars, the Galactic ridge region at positive Galactic longitudes and several regions containing young pulsars. We argue that variability on the time scale of months may be common to pulsars, originating from the inner parts of pulsar wind nebulae, similarly to what is observed in the Crab pulsar.Comment: 4 pages, 4 figures, accepted to Astronomy & Astrophysic

Fermi-LAT Detection of the Young SuperNova Remnant Tycho

After almost three years of data taking in sky survey mode, the \emph{Fermi}-LAT has detected $\gamma$-ray emission toward the Tycho's Supernova Remnant (SNR). The Tycho SNR is among the youngest remnants in the Galaxy, originating from a Type Ia Supernova in AD 1572. The $\gamma$-ray integral flux from 400 MeV up to 100 GeV has been measured to be (3.5$\pm1.1_{stat}\pm0.7_{syst}$)$\times10^{-9}$ cm$^{-2}$s$^{-1}$ with a photon index of 2.3$\pm0.2_{stat}\pm0.1_{syst}$

Cosmic Gamma-Ray Background from Star-Forming Galaxies

The origin of the extragalactic gamma-ray background is a pressing cosmological mystery. The Fermi Gamma-Ray Space Telescope has recently measured the intensity and spectrum of this background; both are substantially different from previous measurements. We present a novel calculation of the gamma-ray background from normal star-forming galaxies. Contrary to long-standing expectations, we find that numerous but individually faint normal galaxies may comprise the bulk of the Fermi signal, rather than rare but intrinsically bright active galaxies. This result has wide-ranging implications, including: the possibility to probe the cosmic star formation history with gamma rays; the ability to infer the cosmological evolution of cosmic rays and galactic magnetic fields; and an increased likelihood of identifying subdominant components from rare sources (e.g., dark matter clumps) through their large anisotropy.Comment: 5 pages, 1 figure. Matches version published in ApJL. Sharper focus on normal (vs starburst) galaxies. Basic conclusions unchange

The Effect of Blazar Spectral Breaks on the Blazar Contribution to the Extragalactic Gamma-ray Background

The spectral shapes of the contributions of different classes of unresolved gamma-ray emitters can provide insight into their relative contributions to the extragalactic gamma-ray background (EGB) and the natures of their spectra at GeV energies. We calculate the spectral shapes of the contributions to the EGB arising from BL Lacertae type objects (BL Lacs) and flat-spectrum radio quasars (FSRQs) assuming blazar spectra can be described as broken power laws. We fit the resulting total blazar spectral shape to the Fermi Large Area Telescope measurements of the EGB, finding that the best-fit shape reproduces well the shape of the Fermi EGB for various break scenarios. We conclude that a scenario in which the contribution of blazars is dominant cannot be excluded on spectral grounds alone, even if spectral breaks are shown to be common among Fermi blazars. We also find that while the observation of a featureless (within uncertainties) power-law EGB spectrum by Fermi does not necessarily imply a single class of contributing unresolved sources with featureless individual spectra, such an observation and the collective spectra of the separate contributing populations determine the ratios of their contributions. As such, a comparison with studies including blazar gamma-ray luminosity functions could have profound implications for the blazar contribution to the EGB, blazar evolution, and blazar gamma-ray spectra and emission.Comment: 8 pages, emulateapj format; 5 figures; accepted for publication in Ap

Mineral resources governance, the UK petroleum fiscal regime: a historical evaluation

After 40 years of oil investments, the UK is now a mature oil province. During these 40 years or so, the UK Government has changed the type of governance it uses to manage its petroleum resources. This paper introduces the theoretical background to two models of mineral resource governance: the so-called proprietorial and nonproprietorial regimes. It investigates the adoption of these two models by the UK Government and their effect on the overall tax take from the UK's petroleum resources. The analysis tracks the changes in the UK petroleum taxation system since establishment up until 2010. It assesses how these tax changes have affected the overall petroleum marginal tax rate. The study concludes that the UK Government adopted a proprietorial type of mineral governance during the period 1975-1982, before changing to a non-proprietorial regime in the period 1983-2000. Since 2000 it has begun to move back towards a proprietorial style of governance. This change is still in its early stages, however; the evidence shows that although there has been an increase in fiscal revenues, this increase has been small

National human rights institutions and access to justice: the role and practice of the human rights commission of Ethiopia in advancing access to justice

National human rights institutions (NHRIs) are quasi-judicial bodies set up specifically for the purpose of promoting and protecting human rights. Among their core functions included are the sensitization of and research and debate on human rights, and investigation of complaints involving human rights issues. As part of their mandate, NHRIs should promote and facilitate complainant’s access to rights and remedies, including the ones offered by themselves and remedies that exist elsewhere, such as courts of law. Provision of free legal assistance and dissemination of information on human rights by NHRIs helps the poor to access justice, removing financial and information obstacles hampering them from doing so. Set up in 2000 pursuant to the FDRE Constitution, the Human Rights Commission of Ethiopia has been trying to promote and protect human rights. One of its modest achievements is its attempt to promote human rights through sensitization campaigns and to reach out to the poor via legal aid centres so that such people could access justice. In that regard, the Commission has been undertaking various training and education activities. Also, it has been providing legal counsel and aid to poor litigants despite the fact that the enabling statute makes no explicit mention of such scheme. In the protection realm, the Commission has been investigating complaints lodged to it, helping citizens to get remedies in rather limited cases. In this piece attempt is made to review the Commission’s endeavour in promoting and protecting human rights and thereby cast light on its contribution in advancing access to justice. It examines the Commission’s practice based on promotional as well as protection activities undertaken by the institution since it commenced discharging its function in 2005. Among questions to be investigated in the study included are the following. In the face of the bulk of population that are unaware of the existence and/or the nature of laws and the means to vindicate them and given the fact that there are few effective methods to create and build awareness, or to provide legal services, what measures have been taken by the Commission, as a complementary body, to fill the void? Given the wide-spread problems facing vulnerable groups to access justice in the nation, has the institution tried to reach out to such group? Does it manage to put in place effective strategies that streamline vulnerable group in its operational procedures? The examination of these questions relies on the review of the Commission’s promotional strategies and tools and documents produced for such purpose, investigation of enabling legislation, relevant literature, and interview. Before delving into examining the Commission’s role to enhance access to justice, it is important to offer background information on the establishment process, structure, mandate, and enforcement powers of the institution