Gerry K. Skinner

THE SWIFT/BAT HIGH-LATITUDE SURVEY: FIRST RESULTS

We present preliminary results from the first 3 months of the Swift Burst Alert Telescope (BAT) high Galactic latitude survey in the 14–195 keV band. The survey reaches a flux of ∼10 11 ergs cm 2 s 1 and has ∼2 .7 (90% confidence) positional uncertainties for the faintest sources. This represents the most sensitive survey to date in this energy band. These data confirm the conjectures that a high-energy–selected active galactic nucleus (AGN) sample would have very different properties from those selected in other bands and that it represents a “true” sample of the AGN population. We have identified 86% of the 66 high-latitude sources. Twelve are Galactictype sources, and 44 can be identified with previously known AGNs. All but five of the AGNs have archival X-ray spectra, enabling us to estimate the line-of-sight column densities and other spectral properties. Both of the objects are blazars. The median redshift of the others (excluding radio-loud objects) is 0.012. We z 1 0.11 find that the column density distribution of these AGNs is bimodal, with 64% of the nonblazar sources having column densities cm 2 . None of the sources with (cgs units) show high column densities, 22 N ≥ 10 log L 1 43.5 HX and very few of the lower LX sources have low column densities. Based on these data, we expect the final BAT catalog to have 1200 AGNs and reach fluxes of less than ∼10 11 ergs cm 2 s 1 over the entire sky.

The sky distribution of positronium annihilation continuum emission measured with SPI/INTEGRAL

We present a measurement of the sky distribution of positronium (Ps) annihilation continuum emission obtained with the SPI spectrometer on board ESA’s INTEGRAL observatory. The only sky region from which significant Ps continuum emission is detected is the Galactic bulge. The Ps continuum emission is circularly symmetric about the Galactic centre, with an extension of about 8 ◦ FWHM. Within measurement uncertainties, the sky distribution of the Ps continuum emission is consistent with that found by us for the 511 keV electron-positron annihilation line using SPI. Assuming that 511 keV line and Ps continuum emission follow the same spatial distribution, we derive a Ps fraction of 0.92±0.09. These results strengthen our conclusions regarding the origin of positrons in our Galaxy based on observations of the 511 keV line. In particular, they suggest that the main source of Galactic positrons is associated with an old stellar population, such as Type Ia supernovae, classical novae, or low-mass X-ray binaries. Light dark matter is a possible alternative source of positrons.

INTEGRAL/SPI ground calibration

Three calibration campaigns of the spectrometer SPI have been performed before launch in order to determine the instrument characteristics, such as the effective detection area, the spectral resolution and the angular resolution. Absolute determination of the effective area has been obtained from simulations and measurements. At 1 MeV, the effective area is 65 cm^2 for a point source on the optical axis, the spectral resolution ~2.3 keV. The angular resolution is better than 2.5 deg and the source separation capability about 1 deg. Some temperature dependant parameters will require permanent in-flight calibration.

INTEGRAL spectrometer SPI’s GRB detection capabilities : GRBs detected inside SPI’s FoV and with the anticoincidence system ACS

The spectrometer SPI, one of the two main instruments of the INTEGRAL spacecraft, oers significant gamma- ray burst detection capabilities. In its 35 (full width) field of view SPI is able to localise gamma-ray bursts at a mean rate of0.8/month. With its large anticoincidence shield of 512 kg of BGO crystals SPI is able to detect gamma-ray bursts quasi omni-directionally with a very high sensitivity. Burst alerts of the anticoincidence shield are distributed by the INTEGRAL Burst Alert System. In the first 8 months of the mission about 0.8/day gamma-ray burst candidates and 0.3/day gamma-ray burst positions were obtained with the anticoincidence shield by interplanetary network triangulations with other spacecrafts.

Diffractive-refractive optics for high energy astronomy - II. Variations on the theme

In a companion paper diffractive-refractive optics components such as Fresnel Zone Plates and their derivatives have been proposed as a basis for telescope systems for X-ray and gamma-ray astronomy with high sensitivity and superb angular resolution. A wide family of configurations is possible and the first paper concentrated on simple systems for gamma-ray energies. The main problems arise from the very long focal lengths involved (~10 6 km) and from chromatic aberration in the focussing system. Ideas are presented here that could in some circumstances allow the focal length to be reduced by many orders of magnitude. In addition it is shown how lenses which are to first order achromatic might be constructed. Finally, the possibility of using similar optical components for X-ray and gamma-ray interferometry is discussed.

GRB 021125 : The first GRB imaged by INTEGRAL

In the late afternoon of November 25th, 2002 a gamma-ray burst (GRB) was detected in the partially coded field of view (about 7:3 from the centre) of the imager IBIS on board the INTEGRAL satellite. The instruments on-board INTEGRAL allowed, for the first time, the observation of the prompt gamma-ray emission over a broad energy band from 15 to 500 keV. GRB 021125 lasted24 s with a mean flux of 5.0 photons cm 2 s 1 in the 20-500 keV energy band, and a fluence of 4:810 5 erg cm 2 in the same energy band. Here we report the analysis of the data from the imager IBIS and the spectrometer

Performance of CLAIRE, the first balloon-borne γ-ray lens telescope

CLAIRE is a balloon-borne telescope dedicated to validating the concept of a crystal diffraction lens for nuclear astrophysics. For the first time, focusing γ-rays enters into the domain of the high energy astrophysics. This represents a breakthrough in γ-ray instrumentation, and will allow unprecedent sensitivities. CLAIREs first flights occurred on June 15 2000 and on June 14 2001. Here we present its performance during the two flights in terms of pointing accuracy, background noise and estimated efficiency of the lens.

Calibration of the spectrometer aboard the INTEGRAL satellite

SPI, the Spectrometer on board the ESA INTEGRAL satellite, to be launched in October 2002, will study the gamma-ray sky in the 20 keV to 8 MeV energy band with a spectral resolution of 2 keV for photons of 1 MeV, thanks to its 19 germanium detectors spanning an active area of 500 cm2. A coded mask imaging technique provides a 2° angular resolution. The 16° field of view is defined by an active BGO veto shield, furthermore used for background rejection. In April 2001 the flight model of SPI underwent a one-month calibration campaign at CEA in Bruyères le Châtel using low intensity radioactive sources and the CEA accelerator for homogeneity measurements and high intensity radioactive sources for imaging performance measurements. After integration of all scientific payloads (the spectrometer SPI, the imager IBIS and the monitors JEM-X and OMC) on the INTEGRAL satellite, a cross-calibration campaign has been performed at the ESA center in Noordwijk. A set of sources has been placed in the field of view of the different instruments in order to compare their performances and determine their mutual influence. We report on the scientific goals of this calibration activity, and present the measurements performed as well as some preliminary results.

Imaging with the coded aperture gamma-ray spectrometer SPI aboard INTEGRAL

ESAs INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) will be launched in October 2002. Its two main instruments are the imager IBIS and the spectrometer SPI. Both emply coded apertures to obtain directional information on the incoming radiation. SPIs detection plane consists of 19 hexagonal Ge detectors, its coded aperture has 63 tungsten-alloy elements of 30 mm thickness.