Structuring Metadata for the Cherenkov Telescope Array

The landscape of ground-based gamma-ray astronomy is changing drastically with the perspective of the Cherenkov Telescope Array (CTA) composed of more than 100 Cherenkov telescopes. For the first time in this energy domain, CTA will be operated as an observatory open to the astronomy community. In this context, a structured high level data model is being developed to describe a CTA observation. The data model includes different classes of metadata on the project definition, the configuration of the instrument, the ambient conditions, the data acquisition and the data processing. This last part relies on the Provenance Data Model developed within the International Virtual Observatory Alliance (IVOA), for which CTA is one of the main use cases. The CTA data model should also be compatible with the Virtual Observatory (VO) for data diffusion. We have thus developed a web-based data diffusion prototype to test this requirement and ensure the compliance

The Cherenkov Telescope Array Observatory workflow management system

The Cherenkov Telescope Array Observatory (CTAO) is the next generation ground-based observatory for gamma-ray astronomy at very high energies. It is expected to produce about 2 PB of raw data each year and to manage a global data volume which will grow through the years to reach about 100 PB in 2030. In addition, CTAO will require a high computing capacity for data processing and Monte Carlo simulations, of the order of hundreds of millions of CPU HS06 hours per year. To meet these requirements, CTAO will adopt a distributed computing model using 4 academic data centers, and will use the DIRAC framework as its workload management system. In the past ten years, to optimize the instrument design and study its performances, CTAO has used the European Grid Infrastructure (EGI) to run massive Monte Carlo campaigns. In order to handle these campaigns and to automatize simulation and data processing workflows, we have developed a production system prototype based on DIRAC. Recently, we have also developed a user interface allowing for the configuration and submission of complex workflows. In this contribution we present the production system prototype, its user interface for workflow management as well as its application to CTAO workflows

An X-ray Polarimeter for HXMT Mission

The development of micropixel gas detectors, capable to image tracks produced in a gas by photoelectrons, makes possible to perform polarimetry of X-ray celestial sources in the focus of grazing incidence X-ray telescopes. HXMT is a mission by the Chinese Space Agency aimed to survey the Hard X-ray Sky with Phoswich detectors, by exploitation of the direct demodulation technique. Since a fraction of the HXMT time will be spent on dedicated pointing of particular sources, it could host, with moderate additional resources a pair of X-ray telescopes, each with a photoelectric X-ray polarimeter in the focal plane. We present the design of the telescopes and the focal plane instrumentation and discuss the performance of this instrument to detect the degree and angle of linear polarization of some representative sources. Notwithstanding the limited resources the proposed instrument can represent a breakthrough in X-ray Polarimetry.Comment: 10 pages, 9 figure

Raman LIDARs for the atmospheric calibrationalong the line-of-sight of CTA

The Cherenkov Telescope Array (CTA) is the next generation ground-based observatory forgamma-ray astronomy at very-high energies. Employing more than 100 Imaging AtmosphericCherenkov Telescopes in the northern and southern hemispheres, it was designed to reach un-precedented sensitivity and energy resolution. Understanding and correcting for systematic bi-ases on the absolute energy scale and instrument response functions will be a crucial issue forthe performance of CTA. The LUPM group and the Spanish/Italian/Slovenian collaboration arecurrently building two Raman LIDAR prototypes for the online atmospheric calibration alongthe line-of-sight of the CTA. Requirements for such a solution include the ability to characterizeaerosol extinction at two wavelengths to distances of 30 km with an accuracy better than 5%,within time scales of about a minute, steering capabilities and close interaction with the CTAarray control and data acquisition system as well as other auxiliary instruments. Our Raman LI-DARs have design features that make them different from those used in atmospheric science andare characterized by large collecting mirrors (∼2.5 m2), liquid light-guides that collect the light atthe focal plane and transport it to the readout system, reduced acquisition time and highly preciseRaman spectrometers. The Raman LIDARs will participate in a cross-calibration and character-ization campaign of the atmosphere at the CTA North site at La Palma, together with other sitecharacterization instruments. After a one-year test period there, an in-depth evaluation of the so-lutions adopted by the two projects will lead to a final Raman LIDAR design proposal for bothCTA sites

XIPE: the X-ray Imaging Polarimetry Explorer

X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and temporal variability measurements and to imaging, allows a wealth of physical phenomena in astrophysics to be studied. X-ray polarimetry investigates the acceleration process, for example, including those typical of magnetic reconnection in solar flares, but also emission in the strong magnetic fields of neutron stars and white dwarfs. It detects scattering in asymmetric structures such as accretion disks and columns, and in the so-called molecular torus and ionization cones. In addition, it allows fundamental physics in regimes of gravity and of magnetic field intensity not accessible to experiments on the Earth to be probed. Finally, models that describe fundamental interactions (e.g. quantum gravity and the extension of the Standard Model) can be tested. We describe in this paper the X-ray Imaging Polarimetry Explorer (XIPE), proposed in June 2012 to the first ESA call for a small mission with a launch in 2017 but not selected. XIPE is composed of two out of the three existing JET-X telescopes with two Gas Pixel Detectors (GPD) filled with a He-DME mixture at their focus and two additional GPDs filled with pressurized Ar-DME facing the sun. The Minimum Detectable Polarization is 14 % at 1 mCrab in 10E5 s (2-10 keV) and 0.6 % for an X10 class flare. The Half Energy Width, measured at PANTER X-ray test facility (MPE, Germany) with JET-X optics is 24 arcsec. XIPE takes advantage of a low-earth equatorial orbit with Malindi as down-link station and of a Mission Operation Center (MOC) at INPE (Brazil).Comment: 49 pages, 14 figures, 6 tables. Paper published in Experimental Astronomy http://link.springer.com/journal/1068

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