J. B. Stephen

Coded aperture imaging in X- and gamma-ray astronomy

Coded aperture imaging in high energy astronomy represents an important technical advance in instrumentation over the full energy range from X- to γ-rays and is playing a unique role in those spectral ranges where other techniques become ineffective or impracticable due to limitations connected to the physics of interactions of photons with matter. The theory underlying this method of indirect imaging is of strong relevance both in design optimization of new instruments and in the data analysis process. The coded aperture imaging method is herein reviewed with emphasis on topics of mainly practical interest along with a description of already developed and forthcoming implementations.

The Ibis-Picsit detector onboard integral

PICsIT is the high-energy detector layer of the IBIS Imager, composed of 4096 CsI(Tl) scintillator detectors 8:4 8:4 300 mm in size with PhotoDiode readout. The detector operates in the 175 keV 20.4 MeV range and its data generation modes make it possible to collect information from single events and multiple coincident events. PICsIT is surrounded by the active BGO VETO and is located about 3 metres below the coded mask. The entire PICsIT plane is physically divided into 8 modules and logically divided into smaller units. The overall performance of the plane is directly related to the behaviour of each individual pixel, including its electronics, the system interconnection logic, and interaction with the other sub-systems. Pixels and electronic parameters were monitored constantly during instrument assembly. The following report describes PICsIT design and contains a summary of on-ground test results.

IBIS/PICsIT in-flight performances

PICsIT (Pixellated Imaging CaeSium Iodide Telescope) is the high energy detector of the IBIS telescope on-board the INTEGRAL satellite. PICsIT operates in the gamma-ray energy range between 175 keV and 10 MeV, with a typical energy resolution of 10% at 1 MeV, and an angular resolution of 12 arcmin within a100 square degree field of view, with the possibility to locate intense point sources in the MeV region at the few arcmin level. PICsIT is based upon a modular array of 4096 independent CsI(Tl) pixels,0.70 cm 2 in cross-section and 3 cm thick. In this work, the PICsIT on-board data handling and science operative modes are described. This work presents the in-flight performances in terms of background count spectra, sensitivity limit, and imaging capabilities.

BeppoSAX Average Spectra of Seyfert Galaxies

We have studied the average 3-200 keV spectra of Seyfert galaxies of type 1 and 2, using data obtained with BeppoSAX. The average Seyfert 1 spectrum is well fitted by a power-law continuum with photon spectral index Γ ~ 1.9, a Compton reflection component R ~ 0.6-1 (depending on the inclination angle between the line of sight and the reflecting material), and a high-energy cutoff at around 200 keV; there is also an iron line at 6.4 keV characterized by an equivalent width of 120 eV. Seyfert 2 galaxies, on the other hand, show stronger neutral absorption [NH = × 1022 atoms cm-2], as expected, but are also characterized by an X-ray power law that is substantially harder (Γ ~ 1.75) and with a cutoff at lower energies (Ec ~ 130 keV); the iron line parameters are instead substantially similar to those measured in type 1 objects. There are only two possible solutions to this problem: to assume more reflection in Seyfert 2 galaxies than observed in Seyfert 1 galaxies or more complex absorption than estimated in the first instance. The first possibility is ruled out by the Seyfert 2 to Seyfert 1 ratio, while the second provides an average Seyfert 2 intrinsic spectrum very similar to that of the Seyfert 1. The extra absorber is likely an artifact due to summing spectra with different amounts of absorption, although we cannot exclude its presence in at least some individual sources. Our result argues strongly for a very similar central engine in both types of galaxies, as expected under the unified theory.

Exploring the spectral properties of faint hard X-ray sources with XMM Newton

We present a spectroscopic study of 41 hard X-ray sources detected serendipitously with high significance (>5 in the 2-10 keV band) in seven EPIC performance/verification phase observations. The large collecting area of EPIC allows us to explore the spectral properties of these faint hard X-ray sources with 2< F2 10< 80 10 14 erg cm 2 s 1 even though the length of the exposures are modest (20 ks). Optical identifications are available for 21 sources of our sample. Using a simple power law plus Galactic absorption model we find an average value of the photon index 1.6-1.7, broadly consistent with recent measurements made at similar fluxes with ASCA and with Chandra stacked spectral analyses. We find that 31 out of 41 sources are well fitted by this simple model and only eight sources require absorption in excess of the Galactic value. Interestingly enough, one third of these absorbed sources are broad line objects, though with moderate column densities. Two sources in the sample are X-ray bright optically quiet galaxies and show flat X-ray spectra. Comparing our observational results with those expected from standard synthesis models of the cosmic X-ray background (CXB) we find a fraction of unabsorbed to absorbed sources larger than predicted by theoretical models at our completeness limit of F2 10 5 10 14 erg cm 2 s 1 .T he results presented here illustrate well how wide-angle surveys performed with EPIC on board XMM-Newton allow population studies of interesting and unusual sources to be made as well as enabling constraints to be placed on some input parameters for synthesis models of the CXB.

BL Lac identification for the ultraluminous X-ray source observed in the direction of NGC 4698

We report the identification of the optical and radio counterparts of the ultraluminous X-ray (ULX) source XMMU J124825.9+083020 (NGC 4698-ULX1). The optical spectrum taken with the VLT yields a redshift of z= 0:43, which implies that the ULX is not associated with the nearby galaxy NGC 4698. The spectral energy distribution calculated from the available data indicates that the source is likely to be a BL Lac object. The possible synchrotron peak at X-ray energies suggests that this source may be a-ray emitter.

A position sensitive detector for a gamma-ray imaging telescope

Abstract Recent developments in the field of position sensitive scintillation detectors have made possible the development of an astronomical gamma-ray telescope which is capable of generating images of the sky with a resolution of about 10 arc min. The position sensitive gamma-ray detector from which the focal plane of the telescope may be constructed is described in this paper. The instrument will operate in the photon energy range 100 keV to 10 MeV. The results of laboratory tests on the positional and energy resolution of incident gamma-rays is compared to the relevant theory and the expected performance evaluated via Monte Carlo simulation.

An evaluation of the possible use of CdTe microdetectors for astrophysical, biomedical and industrial imaging

Abstract Various imaging techniques in fields as diverse as astrophysical research, biomedical diagnostics and industrial tomography are closely related to the development of γ-ray detectors with improved spectral and imaging performances. From this point of view, cadmium telluride solid state detectors are very promising due to their capability to operate at room temperature, their high stopping power and their possibility of miniaturization. The necessity to have contemporaneously good efficiency and high spatial resolution demands that these microdetectors are produced in a novel manner, where the polarization of the applied electric field is perpendicular to the direction of the incoming radiation. In this way, it is possible to achieve a good charge collection efficiency and hence a high energy resolution, together with a high absorption thickness while at the same time having a spatial resolution commensurate with the detector size of about 2×2 mm 2 . The results of measurements regarding the energy resolution of various sizes of μdetector, ranging from 2×2×2 mm 3 to 2.5×2.5×20 mm 3 , are presented and discussed.

Timing response of CdTe detectors

Abstract Semiconductor CdTe detectors are gaining wide acceptance in many applications where X- and γ-ray measurements are necessary, such as in astrophysical research, medical imaging and industrial radiography. Good timing response is critical both in applications like positron emission tomography, where fast coincidence capabilities are required, and in single photon counting when a high counting rate is needed. The typical configuration employed, where the direction of the impinging radiation beam is parallel to the collecting electric field, has one well known drawback: an increase in active layer, necessary in order to reach a satisfactory absorption efficiency for the detection of high energy photons, leads to a longer transport path for the charge carriers generated. As a consequence, there is a degradation in energy resolution and a broadening in time response. In the present paper, measurements of the timing response for an unusual configuration of CdTe detectors are presented. In this configuration, which we call a PTF (planar transverse field) detector, the collecting electric field and hence the transport direction of carriers is transverse to the direction of the incoming photons and so detection thickness and transport length are independent. In this way the absorption layer can be increased without impairing the timing performance. The measurements described herein have been performed using a PTF detector having dimensions of 2.5 × 2.5 × 20 mm 3 , in order to have a good efficiency for annihilation γ-ray photons.

In-flight calibrations of IBIS/PICsIT

PICsIT (Pixellated Imaging CaeSium Iodide Telescope) is the high energy detector of the IBIS telescope on-board the INTEGRAL satellite. It consists of 4096 independent detection units,0.7 cm 2 in cross-section, operating in the energy range between 175 keV and 10 MeV. The intrinsically low signal to noise ratio in the gamma-ray astronomy domain implies very long observations, lasting 10 5 -10 6 s. Moreover, the image formation principle on which PICsIT works is that of coded imaging in which the entire detection plane contributes to each decoded sky pixel. For these two main reasons, the monitoring, and possible correction, of the spatial and temporal non-uniformity of pixel performances, expecially in terms of gain and energy resolution, is of paramount importance. The IBIS on-board 22 Na calibration source allows the calibration of each pixel at an accuracy of<0.5% by integrating the data from a few revolutions at constant temperature. The two calibration lines, at 511 and 1275 keV, allow also the measurement and monitoring of the PICsIT energy resolution which proves to be very stable at19% and9% (FWHM) respectively, and consistent with the values expected analytical predictions checked against pre-launch tests.