A population of gamma-ray emitting globular clusters seen with the Fermi Large Area Telescope

Globular clusters with their large populations of millisecond pulsars (MSPs) are believed to be potential emitters of high-energy gamma-ray emission. Our goal is to constrain the millisecond pulsar populations in globular clusters from analysis of gamma-ray observations. We use 546 days of continuous sky-survey observations obtained with the Large Area Telescope aboard the Fermi Gamma-ray Space Telescope to study the gamma-ray emission towards 13 globular clusters. Steady point-like high-energy gamma-ray emission has been significantly detected towards 8 globular clusters. Five of them (47 Tucanae, Omega Cen, NGC 6388, Terzan 5, and M 28) show hard spectral power indices $(0.7 < \Gamma <1.4)$ and clear evidence for an exponential cut-off in the range 1.0-2.6 GeV, which is the characteristic signature of magnetospheric emission from MSPs. Three of them (M 62, NGC 6440 and NGC 6652) also show hard spectral indices $(1.0 < \Gamma < 1.7)$, however the presence of an exponential cut-off can not be unambiguously established. Three of them (Omega Cen, NGC 6388, NGC 6652) have no known radio or X-ray MSPs yet still exhibit MSP spectral properties. From the observed gamma-ray luminosities, we estimate the total number of MSPs that is expected to be present in these globular clusters. We show that our estimates of the MSP population correlate with the stellar encounter rate and we estimate 2600-4700 MSPs in Galactic globular clusters, commensurate with previous estimates. The observation of high-energy gamma-ray emission from a globular cluster thus provides a reliable independent method to assess their millisecond pulsar populations that can be used to make constraints on the original neutron star X-ray binary population, essential for understanding the importance of binary systems in slowing the inevitable core collapse of globular clusters.Comment: Accepted for publication in A&A. Corresponding authors: J. Kn\"odlseder, N. Webb, B. Pancraz

Fermi Large Area Telescope Constraints on the Gamma-ray Opacity of the Universe

The Extragalactic Background Light (EBL) includes photons with wavelengths from ultraviolet to infrared, which are effective at attenuating gamma rays with energy above ~10 GeV during propagation from sources at cosmological distances. This results in a redshift- and energy-dependent attenuation of the gamma-ray flux of extragalactic sources such as blazars and Gamma-Ray Bursts (GRBs). The Large Area Telescope onboard Fermi detects a sample of gamma-ray blazars with redshift up to z~3, and GRBs with redshift up to z~4.3. Using photons above 10 GeV collected by Fermi over more than one year of observations for these sources, we investigate the effect of gamma-ray flux attenuation by the EBL. We place upper limits on the gamma-ray opacity of the Universe at various energies and redshifts, and compare this with predictions from well-known EBL models. We find that an EBL intensity in the optical-ultraviolet wavelengths as great as predicted by the "baseline" model of Stecker et al. (2006) can be ruled out with high confidence.Comment: 42 pages, 12 figures, accepted version (24 Aug.2010) for publication in ApJ; Contact authors: A. Bouvier, A. Chen, S. Raino, S. Razzaque, A. Reimer, L.C. Reye

Sensitivität des ANTARES Neutrinoteleskops für Gammastrahlungsausbrüche

The ANTARES neutrino telescope is currently being installed in the Mediterranean Sea with the principal objective of searching for galactic and extragalactic sources of high-energy cosmic neutrinos and neutrinos from dark matter annihilation. This thesis is dedicated to the detection of neutrinos from gamma-ray bursts (GRBs) with the ANTARES detector. The "hindsight method" is presented as an off-line method for the identification of GRB-neutrinos, which can be applied a posteriori as it uses GRB coordinates recorded beforehand by gamma-ray satellites in orbit, together with ANTARES event data previously collected in normal sampling mode, thus complementing the existing alert-based method. To predict its sensitivity, the neutrino flux from individual gamma-ray bursts which have been observed with the BATSE detector is estimated. The neutrino emission is calculated in two ways within the framework of the GRB fireball model with internal shock waves: the first model assumes that 10% of the gamma-luminosity is emitted in the form of neutrinos, while the second model is more elaborate in the sense that it directly normalises the individual neutrino flux to the measured luminosity of the GRB in question. By summarising the individual neutrino spectra, the average GRB-neutrino flux is calculated for different types of GRBs (long bursts, short bursts and X-ray flashes). For the calculation of the expected rate of GRB-neutrinos in ANTARES the key parameters of the telescope such as effective area and angular resolution are determined from the modified muon reconstruction algorithm. Given the unexpectedly high rate of optical noise in the deep sea at the ANTARES site, probably due to the increase in bioluminescent activity since the first site evaluation measurements, this thesis has also been devoted to the development of a reconstruction algorithm which is robust with respect to high-level and varying background noise and retains most of the efficiency familiar from low noise levels, while at the same time maintaining the purity of the reconstructed events. Background studies taking into account both atmospheric neutrinos and muons alike suggest that one neutrino detected in coincidence with the time and angular window of a recorded GRB during an observation period of approximately 5 years would already be significant to a level of 3 sigma. This measurement of a single GRB-neutrino would already constitute a benchmark observation, since it would confirm the acceleration of hadrons to at least the measured energy and thereby provide a clue to the origin of high-energy cosmic rays. In the regrettable case of the absence of a GRB-neutrino signal a 5-year limit could be derived corresponding to E² Phi = 3.8 10^-9 GeV/(cm² s sr) if a flux corresponding to the second GRB flux model is expected.Das ANTARES Neutrinoteleskop wird derzeit am Boden des Mittelmeers aufgebaut, mit dem Ziel, nach galaktischen und extragalaktischen Quellen von hochenergetischen kosmischen Neutrinos zu suchen und nach Neutrinos, die bei der Annihilation von dunkler Materie entstehen. Diese Arbeit ist der Detektion von Neutrinos aus Gammastrahlungsausbrüchen (sogenannte Gamma-Ray Bursts, GRBs) mit dem ANTARES Detektor gewidmet. Die "Hindsight Methode" wird als eine Offline-Methode zur Identifikation von GRB-Neutrinos vorgestellt, die nachträglich angewendet werden kann, da sie lediglich die GRB-Koordinaten, die bereits von Gamma-Satelliten im Orbit aufgezeichnet wurden, zusammen mit den ANTARES Daten, die ebenfalls vorher im Standard-Aufnahmeverfahren gewonnen wurden benutzt, und somit die existierende Methode, die einen dedizierten Online-Trigger basierend auf Satellitendaten benutzt, ergänzt. Um die Sensitivität vorherzusagen wurde der Neutrinofluss von einzelnen Gamma-Ray Bursts abgeschätzt, die mit dem BATSE Detektor beobachtet wurden. Die Neutrinoemission wird auf zwei Arten berechnet im Rahmen des GRB Feuerballmodells mit internen Schockfronten: Das erste Modell nimmt an, dass 10% der Gamma-Luminosität in Form von Neutrinos ausgesandt wird, während das zweite Modell etwas raffinierter ist, im dem Sinne, dass es den individuellen Neutrinofluss direkt an die gemessene Luminosität des betreffenden GRBs normiert. Durch Aufsummieren der einzelnen Neutrinospektren wir der mittlere GRB-Neutrinofluss für unterschiedliche Arten von GRBs bestimmt (lange Bursts, kurze Bursts und Röntgenblitze). Für die Berechnung der zu erwartenden GRB-Neutrinorate in ANTARES wurden die Schlüsselparameter des Teleskops, wie die effektive Fläche und die Winkelauflösung für den modifizierten Myon-Rekonstruktionsalgorithmus bestimmt. Angesichts der unerwartet hohen optischen Rauschrate in der Tiefsee am ANTARES Standort, wahrscheinlich aufgrund von erhöhter Biolumineszenzaktivität seit den ersten Standortbewertungsmessungen, ist es ebenso ein Anliegen dieser Arbeit, einen Rekonstruktionsalgorithmus zu entwickeln, der robust ist gegenüber hohen und variablen Untergrundraten und gleichzeitig sowohl die Effizienz behält, die man von niedrigen Rauschraten kennt, als auch die Reinheit der rekonstruierten Ereignisse. Untergrundstudien, die sowohl die atmosphärischen Neutrinos als auch die Myonen berücksichtigen, deuten darauf hin, dass bereits ein Neutrino, das in Koinzidenz mit dem zeitlichen und räumlichen Fenster eines aufgezeichneten GRBs detektiert wird innerhalb eines Zeitraum von ca. 5 Jahren, zu 3 sigma signifikant sein könnte. Diese Messung eines einzelnen GRB-Neutrinos wäre eine Meilenstein-Beobachtung, da sie die Beschleunigung von Hadronen bis zu mindestens der gemessenen Energie bestätigen würde und damit einen Anhaltspunkt für den Ursprung der kosmischen Strahlung bieten würde. Für den Fall, dass kein GRB-Neutrinosignal beobachtet würde, könnte ein 5-Jahres Limit gesetzt werden: E² Phi = 3.8 10^-9 GeV/(cm² s sr) wenn der Neutrinofluss dem zweiten GRB Flussmodell entspräche

Search for gamma-ray emission from magnetars with the Fermi Large Area Telescope

We report on the search for 0.1-10 GeV emission from magnetars in 17 months of Fermi Large Area Telescope (LAT) observations. No significant evidence for gamma-ray emission from any of the currently known magnetars is found. The most stringent upper limits to date on their persistent emission in the Fermi energy range are estimated between ~10–12and10–10 erg s–1 cm–2, depending on the source. We also searched for gamma-ray pulsations and possible outbursts, also with no significant detection. The upper limits derived support the presence of a cutoff at an energy below a few MeV in the persistent emission of magnetars. They also show the likely need for a revision of current models of outer-gap emission from strongly magnetized pulsars, which, in some realizations, predict detectable GeV emission from magnetars at flux levels exceeding the upper limits identified here using the Fermi-LAT observations.Peer reviewe