Richard M. Ambrosi

Readout modes and automated operation of the Swift X-ray Telescope

The Swift X-ray Telescope (XRT) is designed to make astrometric, spectroscopic, and photometric observations of X-ray emission from Gamma-ray Bursts and their afterglows in the energy band 0.2-10 keV. In order to provide rapid-response, automated observations of these randomly occurring objects without ground intervention, the XRT must be able to observe objects covering some seven orders of magnitude in flux, extracting the maximum possible science from each one. This requires a variety of readout modes designed to optimise the information collected in response to shifting scientific priorities as the flux from the burst diminishes. The XRT will support four major readout modes: imaging, two timing modes and photon-counting, with several sub-modes. We describe in detail the readout modes of the XRT. We describe the flux ranges over which each mode will operate, the automated mode switching that will occur and the methods used for collection of bias information for this instrument. We also discuss the data products produced from each mode.

SWIFT XRT Point Spread Function measured at the Panter end-to-end tests

The SWIFT X-ray Telescope (XRT) is designed to make astrometric, spectroscopic and photometric observations of the X-ray emission from Gamma-ray bursts and their afterglows, in the energy band 0.2 - 10 keV. Here we report the results of the analysis of SWIFT XRT Point Spread Function (PSF) as measured during the end-to-end calibration campaign at the Panter X-Ray beam line facility. The analysis comprises the study of the PSF both on-axis and off-axis. We compare the laboratory results with the expectations from the ray-tracing software and from the mirror module tested as a single unit. We show that the measured HEW meets the mission scientific requirements. On the basis of the calibration data we build an analytical model which is able to reproduce the PSF as a function of the energy and the position within the detector.

The effect of proton damage on the X-ray spectral response of MOS CCDs for the Swift X-ray telescope

The effect of non-ionising energy loss of protons in charge-coupled devices is to displace silicon atoms and any dopant materials present from their lattice positions to form lattice defects which in turn can trap electrons (IEEE Trans. Nucl. Sci. NS-40 (1993) 1628). A CCD operating as a photon counter for X-ray spectroscopy relies on the efficient transfer of charge from one region to another. The number of defects produced will reduce the charge transfer efficiency and hence degrade the spectral resolution of the energy distribution of interest (Jet-X Project Document: JET-X(94) UL-230 WP2220 (1994)). The Swift X-ray telescope will be equipped with a single EPIC MOS CCD22 as developed for the XMM project SPIE 3445 (1998) 13. It is the aim of this study to determine the effect of the radiation environment on the performance of the CCD and its impact on the scientific objective of the X-ray telescope, to probe the X-ray afterglow of gamma-ray bursts.

Factors affecting image formation in accelerator-based fast neutron radiography

Abstract Fast neutrons can be generated with accelerators via various reactions. The reactions 7 Li(p,n) 7 Be and D(d,n) 3 He were employed for this study to generate neutrons at various energies. Radiographs of different materials have been produced and analysed in terms of contrast and resolution. Monte Carlo methods have been used to evaluate the physical factors determining the image quality in these practical situations. In each case the theoretical and experimental values were compared. A series of gamma-ray radiographs generated using a Co-60 source provided an experimental benchmark for comparison with fast neutron radiographs. The neutron radiographs were generated as a function of fast neutron producing reaction (neutron energy spectrum), scintillator type and thickness, charged particle energy, imaging geometry and sample material. Both a CCD camera operating at room temperature and cooled CCD were used in the imaging process.

A Monte Carlo study of the effect of neutron scattering in a fast neutron radiography facility

Abstract Monte Carlo random sampling methods offer a powerful tool for the investigation of how neutron scattering can affect image formation in a fast neutron radiography facility. A laboratory developed for accelerator based fast neutron radiography contains various apparatus which may scatter neutrons. These include vacuum pumps, the accelerator tube, target used for generating the fast neutrons and the laboratory walls and ceiling composed of high density concrete bricks used to shield the outside environment from high energy neutrons. The Monte Carlo package MCNP-4A has been used to investigate the effect of scattering on image formation for a particular neutron source and geometry. In this study the known variation of neutron source strength and energy spectrum as a function of scattering angle has been included in the modelling process in order to produce an accurate assessment of the effect of scattered neutrons in any fast neutron radiography facility.

The impact of low energy proton damage on the operational characteristics of EPIC-MOS CCDs

Abstract The University of Tubingen 3.5 MeV Van de Graaff accelerator facility was used to investigate the effect of low-energy protons on the performance of the European Photon Imaging Camera, metal-oxide-semiconductor, charge-coupled devices (CCDs). Two CCDs were irradiated in different parts of their detecting areas using different proton spectra and dose rates. Iron-55 was the calibration source in all cases and was used to measure any increases in charge transfer inefficiency (CTI) and spectral resolution of the CCDs. Additional changes in the CCD bright pixel table and changes in the low X-ray energy response of the device were examined. The Monte Carlo code Stopping Range of Ions in Matter was used to model the effect of a 10 MeV equivalent fluence of protons interacting with the CCD. Since the non-ionising energy loss function could not be applied effectively at such low proton energies. From the 10 MeV values, the expected CTI degradation could be calculated and then compared to the measured CTI changes.

Swift x-ray telescope (XRT)

The Swift Gamma-Ray Burst Explorer will be launched late in 2003 to make prompt multiwavelength observations of Gamma-Ray Bursts and Afterglows. The X-ray Telescope (XRT) provides key capabilities that permit Swift to determine GRB positions with several arcsecond accuracy within 100 seconds of the burst onset. The XRT is designed to observe GRB afterglows covering over seven orders of magnitude in flux in the 0.2-10 keV band, with completely autonomous operation. GRB positions are determined within seconds of target acquisition, and accurate positions are sent to the ground for distribution over the GCN. The XRT can also measure redshifts of GRBs for bursts with Fe line emission or other spectral features.

Spectral re-distribution and surface loss effects in Swift XRT (XMM-Newton EPIC) MOS CCDs

In the course of testing and selecting the EPIC MOS CCDs for the XMM-Newton observatory, the authors developed a Monte-Carlo model of the CCD response. Among other things, this model was used to investigate surface loss effects evident at low energies. By fitting laboratory data, these losses were characterised as a simple function of X-ray interaction depth and this result enabled the spectral re-distribution itself to be modelled as a simple analytical function. Subsequently, this analytical function has been used to generate the response matrix for the EPIC MOS instruments and will now be employed to model the spectral re-distribution for the Swift XRT CCD.

Effects of micrometeoroid and space debris impacts in grazing incidence telescopes

CCD detectors in the focal plane cameras of grazing incidence X-ray telescopes on the XMM-Newton and SWIFT satellites have encountered damage which has been attributed to impacts by external particles. The apparent mechanism is one whereby interplanetary micrometeoroid particles or space debris have been ingested by the grazing incidence mirrors and scattered down the telescope tube on to the CCD detectors in the focal plane. At the time of writing, there have been 5 such events detected in total by the three XMM telescopes during five years of operations and one event detected by the SWIFT X-ray Telescope (XRT) during one year in orbit. Significantly, no events of this type have been reported for Chandra. Modelling and analysis of scattering of small particles from grazing incidence mirrors allows us to explain the different impact rates seen by these three satellites. Furthermore, using the ESA MASTER2005 micrometeoroid and space debris impacts flux model, impact rates have been derived from consideration of Swifts orbit, pointing history and the dust and debris particle environment. This modelling can be used to determine whether risk mitigation strategies are required for the continuing operation of SWIFT and other operating observatories, and also provides a basis for predicting particle impact rates for grazing incidence telescopes on future missions such as XEUS, Constellation-X and others.

The spectroscopic performance of the Swift X-ray Telescope for gamma-ray burst afterglow studies

The Swift X-ray Telescope is a powerful instrument for measuring the X-ray spectral properties of GRB afterglows. The spectroscopic capabilities are obtained through the energy resolving properties of the X-ray CCD imager in the focal plane of the X-ray Telescope. A range of CCD operating modes allow GRB afterglows to be followed over 5 orders of brightness as the afterglow decays. The spectroscopic response in each mode has been determined as part of the XRT calibration program and is being incorporated into the XRT instrument response matrices. These responses are being used to simulate GRB spectra as part of the pre-launch mission planning for Swift.