Gamma-ray bursts (GRBs) provide extremely luminous background light sources that can be used to study the
high redshift universe out to z ~ 12. Identification of high-z GRBs has been difficult to date because no good
high-z indicators have been found in the prompt or afterglow emission of GRBs, so ground-based spectroscopic
observations are required. <i>JANUS</i> is an Explorer mission that incorporates a GRB locator and a near-IR
telescope with low resolution spectroscopic capability so that it can measure the redshifts of GRBs immediately
after their discovery. It is expected to discover 50 GRBs with z > 5 as well as hundreds of high redshift quasars.
<i>JANUS</i> will facilitate study of the reionization phase, star formation, and galaxy formation in the very early
universe. Here we discuss the mission design and status.
We are exploiting the Swift X-ray Telescope (XRT) deepest GRB follow-up observations to study the cosmic
X-Ray Background (XRB) population in the 0.2-10 keV energy band. We present some preliminary results of a
serendipitous survey performed on 221 fields observed with exposure longer than 10 ks. We show that the XRT is
a profitable instrument for surveys and that it is particularly suitable for the search and observation of extended
objects like clusters of galaxies. We used the brightest serendipitous sources and the longest observations to test
the XRT optics performance and the background characteristics all over the field of view, in different energy
bands during the first 2.5 years of fully operational mission.
The Swift X-ray Telescope (XRT) is a CCD based X-ray telescope designed for localization, spectroscopy and long term
light curve monitoring of Gamma-Ray Bursts and their X-ray afterglows. Since the launch of Swift in November 2004,
the XRT has undergone significant evolution in the way it is operated. Shortly after launch there was a failure of the
CCD thermo-electric cooling system, which led to the XRT team being required to devise a method of keeping the CCD
temperature below −50C utilizing only passive cooling by minimizing the exposure of the XRT radiator to the Earth. We
present in this paper an update on how the modeling of this passive cooling method has improved in first ~1000 days
since the method was devised, and the success rate of this method in day-to-day planning. We also discuss the changes
to the operational modes and onboard software of the XRT. These changes include improved rapid data product
generation in order to improve speed of rapid Gamma-Ray Burst response and localization to the community; changes to
the way XRT observation modes are chosen in order to better fine tune data acquisition to a particular science goal;
reduction of "mode switching" caused by the contamination of the CCD by Earth light or high temperature effects.
The X-ray telescope (XRT) on board the Swift Gamma Ray Burst Explorer has successfully operated since the spacecraft
launch on 20 November 2004, automatically locating GRB afterglows, measuring their spectra and lightcurves and
performing observations of high-energy sources. In this work we investigate the properties of the instrumental
background, focusing on its dynamic behavior on both long and short timescales. The operational temperature of the
CCD is the main factor that influences the XRT background level. After the failure of the Swift active on-board
temperature control system, the XRT detector now operates at a temperature range between -75C and -45C thanks to a
passive cooling Heat Rejection System. We report on the long-term effects on the background caused by radiation,
consisting mainly of proton irradiation in Swift's low Earth orbit and on the short-term effects of transits through the
South Atlantic Anomaly (SAA), which expose the detector to periods of intense proton flux. We have determined the
fraction of the detector background that is due to the internal, instrumental background and the part that is due to
unresolved astrophysical sources (the cosmic X-ray background) by investigating the degree of vignetting of the
measured background and comparing it to the expected value from calibration data.
The Swift X-ray Telescope (XRT) focal plane camera is a front-illuminated MOS CCD, providing a spectral response kernel of 144 eV FWHM at 6.5 keV. We describe the CCD calibration program based on celestial and on-board calibration sources, relevant in-flight experiences, and developments in the CCD response model. We illustrate how the revised response model describes the calibration sources well. Loss of temperature control motivated a laboratory program to re-optimize the CCD substrate voltage, we describe the small changes in the CCD response that would result from use of a substrate voltage of 6V.
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 0.2-10 keV energy band. Here we report the initial results of the analysis of Swift XRT effective area as measured both on-axis and off-axis during the in-flight calibration phase using the laboratory results and ray-tracing simulations as a starting point. Our analysis includes the study of the effective area at a range of energies, for different event grade selection and operating modes using two astronomical sources characterized by different intrinsic spectra.
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.
Swift was successfully launched on 2004 November 20. Here we report the results of the analysis of Swift XRT Point Spread Function (PSF) as measured in the first four months of the mission during the instrument calibration phase.
The analysis includes the study of the PSF of different point-like sources both on-axis and off-axis with different spectral properties. We compare the in-flight data with the expectations from the on-ground calibration. On the basis of the calibration data we built an analytical model to reproduce the PSF as a function of the energy and the source position within the detector which can be applied in the PSF correction calculation for any extraction region geometry.
The X-ray telescope (XRT) on board Swift, launched on 2004 Nov 20, is performing astrometric, spectroscopic and photometric observations of the X-ray emission from Gamma-ray burst afterglows in the energy band 0.2-10 keV. In this paper, we describe the results of the in-flight calibration relative to the XRT timing resolution and absolute timing capabilities. The timing calibration has been performed comparing the main pulse phases of the Crab profile obtained from several XRT observations in Low Rate Photodiode and Windowed Timing mode with those from contemporaneous RXTE observations. The XRT absolute timing is well reproduced with an accuracy of 200 μs for the Low Rate Photodiode and 300 μs for the Windowed Timing mode.
The XRT is a sensitive, autonomous X-ray imaging spectrometer onboard the <i>Swift</i> Gamma-Ray Burst Observatory. The unique observing capabilities of the XRT allow it to autonomously refine the <i>Swift</i> BAT positions (~1-4' uncertainty) to better than 2.5 arcsec in XRT detector coordinates, within 5 seconds of target acquisition by the <i>Swift</i> Observatory for typical bursts, and to measure the flux, spectrum, and light curve of GRBs and afterglows over a wide dynamic range covering more than seven orders of magnitude in flux (62 Crab to < 1 mCrab). The results of the rapid positioning capability of the XRT are presented here for both known sources and newly discovered GRBs, demonstrating the ability to automatically utilise one of two integration times according to the burst brightness, and to correct the position for alignment offsets caused by the fast pointing performance and variable thermal environment of the satellite as measured by the Telescope Alignment Monitor. The onboard results are compared to the positions obtained by groundbased follow-up. After obtaining the position, the XRT switches between four CCD readout modes, automatically optimising the scientific return from the source depending on the flux of the GRB. Typical data products are presented here.
The Swift X-ray Telescope (XRT) is a CCD based X-ray telescope designed for localization, spectroscopy and long term light curve monitoring of Gamma-Ray Bursts and their X-ray afterglows. Shortly after launch there was a failure of the thermo-electric cooler on the XRT CCD. Due to this the <i>Swift</i> XRT Team had the unexpected challenge of ensuring that the CCD temperature stayed below -50C utilizing only passive cooling through a radiator mounted on the side of the <i>Swift</i>. Here we show that the temperature of the XRT CCD is correlated with the average elevation of the Earth above the XRT radiator, which is in turn related to the targets that <i>Swift</i> observes in an orbit. In order to maximize passive cooling of the XRT CCD, the XRT team devised several novel methods for ensuring that the XRT radiator's exposure to the Earth was minimized to ensure efficient cooling. These methods include: picking targets on the sky for <i>Swift</i> to point at which are known to put the spacecraft into a good orientation for maximizing XRT cooling; biasing the spacecraft roll angle to point the XRT radiator away from the Earth as much as possible; utilizing time in the SAA, in which all of the instruments on-board <i>Swift</i> are non-operational, to point at "cold targets"; and restricting observing time on "warm" targets to only the periods at which the spacecraft is in a favorable orientation for cooling. By doing this at the observation planning stage we have been able to minimize the heating of the CCD and maintain the XRT as a fully operational scientific instrument, without compromising the science goals of the <i>Swift</i> mission.
The Swift Gamma Ray Burst Explorer, chosen in October 1999 as NASA's next MIDEX mission, is now scheduled for launch in October 2004. SWIFT carries three complementary instruments. The Burst Alert Telescope (BAT) identifies gamma-ray bursts (GRBs) and determines their location on the sky to within a few arc-minutes. Rapid slew by the fast-acting SWIFT spacecraft points the two narrow field instruments, an X-ray Telescope (XRT) and an Ultraviolet/Optical Telescope (UVOT), to within the BAT error circle within 70 seconds of a BAT detection. The XRT can determine burst locations to within 5 arc-seconds and measure X-ray spectra and photon flux, whilst the UVOT has a sensitivity down to 24th magnitude and sub arc-second positional accuracy in the optical/uv band. The three instruments combine to make a powerful multi-wavelength observatory with the capability for rapid determination of GRB positions to arc-second accuracy within a minute or so of their discovery, and the ability to measure light-curves and red-shifts of the bursts and after-glows. The paper summarises the mission's readiness for October's launch and operations.
The <i>Swift</i> 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 first results of the analysis of <i>Swift</i> XRT effective area at five different energies as measured during the end-to-end calibration campaign at the Panter X-ray beam line facility. The analysis comprises the study of the effective area both on-axis and off-axis for different event grade selection. We compare the laboratory results with the expectations and show that the measured effective area meets the mission scientific requirements.
The Swift Gamma-Ray Explorer is designed to make prompt multiwavelength observations of Gamma-Ray Bursts (GRBs) and GRB Afterglows. The X-ray Telescope (XRT) provides key capabilities that permit Swift to determine GRB positions with a few arcseconds accuracy within 100 seconds of the burst onset. The <i>XRT</i> utilizes a superb mirror set built for <i>JET-X</i> and a state-of-the-art <i>XMM/EPIC MOS</i> CCD detector to provide a sensitive broad-band (0.2-10 keV) X-ray imager with effective area of 135 cm<sup>2</sup> at 1.5 keV, field of view of 23.6 x 23.6 arcminutes, and angular resolution of 18 arcseconds (HEW). The detection sensitivity is 2x10<sup>-14</sup> erg/cm<sup>2</sup>/s in 10<sup>4</sup> seconds. The instrument is designed to provide automated source detection and position reporting within 5 seconds of target acquisition. It can also measure redshifts of GRBs for bursts with Fe line emission or other spectral features. The <i>XRT</i> will operate in an auto-exposure mode, adjusting the CCD readout mode automatically to optimize the science return for each frame as the source fades. The <i>XRT</i> will measure spectra and lightcurves of the GRB afterglow beginning about a minute after the burst and will follow each burst for days as it fades from view.
The <i>SWIFT</i> 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 <i>SWIFT</i> 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 UV/optical telescope (UVOT) is one of three instruments flying aboard the Swift Gamma-ray Observatory. It is designed to capture the early (~1 minute) UV and optical photons from the afterglow of gamma-ray bursts as well as long term observations of these afterglows. This is accomplished through the use of UV and optical broadband filters and grisms. The UVOT has a modified Ritchey-Chretien design with micro-channel plate intensified charged-coupled device detectors that provide sub-arcsecond imaging. Unlike most UV/optical telescopes the UVOT can operate in a photon-counting mode as well as an imaging mode. We discuss some of the science to be pursued by the UVOT and the overall design of the instrument.
Swift is a multi-wavelength observatory designed for the autonomous detection and immediate follow-up of gamma-ray bursts and their afterglows. Following launch in early 2004, Swift's Burst Alert Telescope (BAT) will detect 100s of GRBs per year, and autonomously maneuver sensitive UV/optical and X-ray telescopes onto the burst within 10 to 75 seconds. GRB and X-ray positions and UV/optical finding chart will be rapidly distributed thorugh the GCN to promote ground-based observations. Afterglows will be monitored by Swift for days to weeks. All data will be converted into standard FITS formats and rapidly made available to the community from data centers in the US, Italy, and the UK.
The <i>Swift</i> 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.
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.
The ACIS instrument has been operating for three years in orbit, producing high quality scientific data on a wide variety of X-ray emitting astronomical objects. Except for a brief period at the very beginning of the mission when the CCDs were exposed to the radiation environment of the Outer van Allen Belts which resulted in substantial radiation damage to the front illuminated CCDs, the instrument has operated nearly flawlessly. The following report presents a description of the instrument, the current status of the instrument calibration and a few highlights of the scientific results obtained from the Guaranteed Observer Time.
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. The XRT has a variety of readout modes which it automatically selects in order to observe objects covering 7 orders of magnitude in flux and to extract the maximum possible science from each one, in response to the flux from the burst diminishing. The primary goal of the XRT is to locate the position of the Gamma-Ray Burst to 1 arcsec and to transmit this position to the UVOT and the ground within 100 seconds of the initial observation of the burst. We describe in detail the use of imaging mode and a centroid algorithm to determine the position of the Gamma-Ray Burst with sub-pixel accuracy.
The Swift MIDEX mission is the first-of-its-kind observatory for multi-wavelength transient astronomy. The goal of the mission is to ascertain the origin of gamma-ray bursts and to utilize these bursts to probe the early universe. The Ultra- Violet/Optical Telescope (UVOT) is one of three telescopes flying aboard Swift. The UVOT is a working 'copy' of the Optical Monitor on the X-ray Multi-mirror Mission (XMM- Newton). It is a Ritchey-Chretien telescope with microchannel plate intensified charged-coupled devices (MICs) that provide sub-arcsecond imaging. These MICs are photon counting devices, capable of detecting very low signal levels. When flown above the atmosphere, the UVOT will have the equivalent sensitivity of a 4 m telescope on the ground, reaching a limiting magnitude of 24 for a 1000 second observation in the white light filter. A rotating filter wheel contains sensitive photometric broadband UV and visual filters for determining photometric redshifts. The filter wheel also contains UV and visual grisms for performing low-resolution spectroscopy.
The Swift Gamma Ray Burst Explorer will be launched in 2003 to observe hundreds of gamma-ray bursts per year and study their X-ray and optical afterglows, using a multiwavelength complement of three instruments: a wide-field Burst Alert Telescope (BAT), an X-Ray Telescope (XRT), and a UV/Optical Telescope (UVOT). The XRT is designed to study X-ray counterparts of the gamma-ray bursts and their afterglows, beginning 20 - 70 s from the time of the burst, and continuing for days or weeks. The XRT utilizes a superb mirror set built for JET-X and a state-of-the-art XMM/EPIC CCD detector to provide a sensitive broad-band (0.2 - 10 keV) X-ray imager with effective area of 110 cm<SUP>2</SUP> at 1.5 keV, field of view of 23.6 X 23.6 arcminutes, and angular resolution of 15 arcseconds (HEW). The sensitivity is 2 X 10<SUP>-14</SUP> erg/cm<SUP>2</SUP>s in 10<SUP>4</SUP> seconds. The telescope electronics are designed to provide automated source detection and position reporting, with a position good to 2.5 arcseconds transmitted to the ground within 100 seconds of the burst detection. The XRT will operate in an auto-exposure mode, adjusting the CCD readout mode automatically to optimize the science return for each frame as the source fades. The XRT will measure spectra and lightcurves of the GRB afterglow beginning within about a minute after the burst and will follow each burst until it fades from view, typically monitoring 2 - 3 'old' bursts at a time while waiting for a new burst to be detected.
We present preliminary results from observations of supernova remnants by the Chandra X-ray Observatory. The data include imaging spectroscopy from objects observed with both GTO and GO data. The high spatial resolution of Chandra is revealing a wealth of small-scale structure in these remnants. Specifically, we have resolved the remnant of SN1987A, and have discovered fine-scale structure in N103B and G292.0+1.8.
The optical chain of the spectroscopic x-ray telescopes aboard the Constellation-X spacecraft employs a reflective grating spectrometer to provide high resolution spectra for multiple spectra as a slitless spectrometer in the spectral feature rich, soft x-ray band. As a part of the spectroscopic readout array, we provide a zero-order camera that images the sky in the soft band inaccessible to the microcalorimeters. Technological enhancements required for producing the RGS instruments are described, along with prototype development progress, fabrication and testing results.
Acquisition of ground calibration data from the AXAF CCD Imaging Spectrometer, one of two focal plane instruments on NASA's Advanced X-ray Astrophysics Facility, was completed in 1997. Here we summarize results of the detector level calibration effort. Our calibration program has included measurements of CCD response to undispersed synchrotron radiation, measurements of x-ray absorption fine structure, and of sub-pixel structure in the detector. Errors in the energy scale are at the level of a few tenths of one percent, and detection efficiency errors are no large than a few percent. We have also obtained new insights into the mechanisms by which the CCD gate structure and channel stops influence the CCD spectral redistribution function.
The flight AXAF CCD Imaging Spectrometer (ACIS) and the High-Resolution Mirror Assembly (HRMA) telescope were extensively calibrated at NASA MSFC's X-Ray Calibration Facility (XRCF). We present result based on HRMA/ACIS characteristics that were obtained with the following tests: (a) Point-Spread Function (PSF) tests measuring the core and wings of the PSF on-axis and at several off-axis positions, at the point of ideal focus. (b) Effective-Area (EA) test measuring the total effective collecting area over the PSF at many energies. We investigate the dependence of the effective area and energy response of ACIS/HRMA as a function of several ACIS parameters. (c) Count-rate- linearity (pile-up) test measuring the effect of increasing the mean incident rat of photons per pixel on the PSF, and EA, source centroid, and photon detection. The goal of the calibration is to provide accurate estimates of the in-orbit performance of the ACIS/HRMA instrument and to enable translation of in-orbit measurements to absolute values of the incident x-ray flux and physical models of the source emission.
The coincidence of multiple x-rays in a detection cell within one CCD frame - an effect known as pile-up - is a significant source of spectral contamination for bright sources observed with the AXAF CCD Imaging Spectrometer (ACIS). Investigation of algorithms for correcting contaminated spectra is underway. In case where such algorithms fail, the observer may wish to discard events for the core of the AXAF point spread function (PSF), computing spectra using only events from the PSF wings, where pile-up effects are reduced. This work investigates the effectiveness of this technique for an isolated on-axis point source, using event lists produced by a Monte Carlo simulation of the ACIS instrument.
The Advanced X-ray Astrophysics Facility (AXAF) ground calibration program, easily the most extensive in the history of high energy astrophysics, requires careful attention to the verification of its validity for on-orbit operations of the observatory. The purpose of the Flight Contamination Monitor (FCM) is to verify the transfer of the AXAF absolute flux scale calibration from ground to on-orbit operations and to measure or bound any changes in molecular contamination on the AXAF mirrors. This paper reports the current status of the analysis of FCM measurements taken during ground calibration. The FCM measurements during the AXAF activation phase will be the first look at the on-orbit AXAF performance.
The ACIS flight instrument was recently extensively calibrated at the X-ray Calibration Facility at MSFC. For the analysis of a subset of the ACIS calibration data we have employed an automated even filtering software package developed at PSU. We present result describing the dependence of the Effective Area and Energy Response of ACIS/HRMA as a function of grade selection, split event threshold, CCD and CCD amplifier, and off-axis angle. The main goal of this study is to facilitate the selection of the appropriate ACIS parameters that optimize a desired feature in an observed ACIS spectrum and may also guide observers in selecting the appropriate CCD for the observation. Optimizing a particular feature in an ACIS observation in general may require making a trade-off between effect area and energy response of AXAF/ACIS. The present analysis will facilitate the selection of the appropriate grades, CCD, CCD amplifier and spilt even thresholds needed to attain the optimal point.As an illustration of the effectiveness of this approach we present several case studies of typical astrophysical source spectra in which we enhance a particular scientific feature in the observed ACIS spectrum by the appropriate selection of ACIS parameters.
The prelaunch calibration of AXAF encompasses many aspects of the telescope. In principle, all that is needed is the complete point response function. This is, however, a function of energy, off-axis angle of the source, and operating mode of the facility. No single measurement would yield the entire result. Also, any calibration made prior to launch will be affected by changes in conditions after launch, such as the change from one g to zero g. The reflectivity of the mirror and perhaps even the detectors can change as well, for example by addition or removal of small amounts of material deposited on their surfaces. In this paper, we give a broad view of the issues in performing such a calibration, and discuss how they are being addressed in prelaunch preparation of AXAF. As our title indicates, we concentrate here on the total throughput of the observatory. This can be thought of as the integral of the point response function, i.e. the encircled energy, out to the largest practical solid angle for an observation. Since there is no standard x-ray source in the sky whose flux is well known to the approximately 1% accuracy we are trying to achieve, we must do this calibration on the ground. We also must provide a means for monitoring any possible changes in this calibration from prelaunch until on-orbit operation can transfer the calibration to a celestial x-ray source whose emission is stable. In the paper, we analyze the elements of the absolute throughput calibration, which we call the effective area. We review the requirements for calibrations of components or subsystems of the AXAF facility, including the mirror, detectors, and gratings. We show how it is necessary to have an absolute calibrated detection system available during the prelaunch calibrations to measure the flux in the x-ray beam used for calibrating AXAF. We show how it is necessary to calibrate this ground-based detection system at standard man-made x-ray sources, such as electron storage rings. We present the status of all these calibrations, with indications of the measurements remaining to be done, even though the measurements on the AXAF flight optics and detectors will have been completed by the time this paper is presented. We evaluate progress toward the goal of making 1% measurements of the absolute x-ray flux from astrophysical sources, so that comparisons can be made with their emission at other wavelengths, in support of observations such as the Sunyaev-Zeldovitch effect, which can give absolute distance measurements independent of the traditional distance measuring techniques in astronomy.
The cosmic unresolved background instrument using CCDs (CUBIC) was scheduled for launch on the Argentine/U.S. SAC-B satellite in October 1996. This instrument is designed to perform moderate resolution nondispersive x-ray spectroscopy of the diffuse x-ray background over the band 0.2 - 10.0 keV using state-of-the-art photon-counting CCDs. The instrument is optimized for spectroscopy of diffuse emission with a field of view approximately 5 degrees multiplied by 5 degrees below 1 keV and 10 degrees multiplied by 10 degrees above 3 keV. Here we discuss the present state of analysis of our preflight calibration data and present preliminary operational plans.
The AXAF CCD imaging spectrometer (ACIS) consists of ten CCDs arranged in two arrays, one for imaging astronomical fields and one to be used in conjunction with transmission gratings for spectroscopy of astrophysical sources. ACIS uses Lexan/aluminum meshless films placed above these two CCD arrays to filter by mapping their soft x-ray transmission on fine spatial scales, so that the filter response can be removed from the CCD data and a more accurate estimate of the true sky recovered. We measured engineering and flight versions of these filters at the University of Wisconsin Synchrotron Radiation Center between June 1995 and April 1996. For all data, better than one percent accuracy in transmission as a function of energy was maintained over the entire filter area. The resulting transmission maps reveal spatial non-uniformities in the filters of about 0.5 percent to 2 percent. These transmission maps provide the finest spatial calibration ever achieved on such filters.
Measurements of the transmission properties of the AXAF CCD imaging spectrometer (ACIS) UV/optical blocking filters were performed at the National Synchrotron Light Source at Brookhaven Laboratories. The X-ray transmissions of two Al:Si/LEXAN/Al:Si three layer filters were measured between 260 and 3000 eV. The main purpose of the calibration was to determine a model transmission function with an accuracy of better than 1 percent. We present results from fits of model transmission functions to the measured x-ray transmission data. Detailed fine energy scans above the Al-K and C-K absorption edges revealed the presence of fine oscillations of the x-ray transmission. These features are most likely extended x-ray absorption fine structures (EXAFS). The amplitude of the EXAFS oscillations above the Al absorption edge is about 5 percent of the mean value of the x-ray transmission. EXAFS theory predicts a temperature dependence on the amplitude of the EXAFS oscillations. This dependence arises from the fact that thermal vibrations of the atoms in a solid produce a phase mismatch of the backscattered electron wave function. Since the ACIS filters will be at a much lower temperature on orbit we provide a prediction of the EXAFS component for the expected on orbit temperature of the ACIS filters.
We present the results of a comparison of data processing algorithms to be used with space- borne x-ray CCD cameras such as those aboard ASCA, CUBIC and AXAF. The goal is to optimize efficiency and accuracy based upon the capabilities and limitations of the on-board processors. We examine the two main components of processing: determination of the bias (or zero) -level, and event recognition. An algorithm to generate a pixel-by-pixel bias by on-board processing is developed and tested. The on-board bias frame is compared to a bias created from a standard laboratory pixel-by-pixel averaging of dark frames. We show that an accurate pixel-by-pixel bias frame can be created with an on-board algorithm in as few as 15 frames. We show that a bias frame created from that algorithm performs as well as meanframes created in the laboratory. On-board algorithms that handle bias determination and event selection simultaneously are also developed. We show that several types of these algorithms successfully process the CCD data, although the algorithm should be chosen according to the specific capabilities of the processors. The procedures were evaluated by examining event quality and single/split event ratios, and more importantly by the determination of spectral energy resolution (e.g., the FWHM of <SUP>55</SUP>Fe). The algorithms were compared and evaluated for laboratory data from several different cameras and types of CCD devices.
The cosmic unresolved background instrument using CCDs (CUBIC) is currently scheduled for launch on the Argentine/US SAC-B satellite late this year. This instrument is designed to perform moderate resolution nondispersive x-ray spectroscopy of the diffuse x-ray background over the band 0.2 - 10.0 keV using state-of-the-art photon-counting CCDs. The instrument is optimized for spectroscopy of diffuse emission with a field of view of 5 degrees by 5 degrees below 1 keV and 10 degrees by 10 degrees above 3 keV. Observations will typically last 1 - 3 days, and will obtain high quality CCD spectra of the diffuse background, nearby superbubbles and supernova remnants, and the brightest x-ray point sources. This paper gives an overview of the instrument design and CCD detectors.
We present data from a charge-coupled device (CCD), collaboratively designed by PSU/JPL/Loral, which incorporates several novel features that make it well suited for soft X-ray spectroscopy. It is a three-phase, front-side illuminated device with 1024x1024 pixels. Each pixel is 18 microns by 18 microns.The device has four output amplifiers: two conventional floating diffusion amplifiers (FDAs) and two floating gate amplifiers (FGAs). The FGA non-destructively samples the output charge, allowing the charge in each pixel to be measured multiple times. The readnoise of a given pixel is reduced as the square root of the number of readouts, allowing one to reduce the amplifier noise of these devices to well below the 1/f knee. We have been able to achieve sub-electron readnoise performance with the floating gate amplifier (0.9 e+-) rms with 16 reads per pixel). Using the FGA, the measured energy resolution at 5.9 keV is 120 eV (FWHM). The CCD also has a thin poly gate structure to maximize soft X-ray quantum efficiency. Two-thirds of the active area of the chip is covered only by an insulating layer (1000 angstrom) and a thin poly silicon electrode (400 angstrom). This design enhances the soft X-ray quantum efficiency, but retains the excellent charge transfer efficiency and soft X-ray charge collection efficiency of front-side illuminated devices. The measured energy resolution at 277 eV is 38 eV (FWHM) with a measured quantum efficiency of 15%. We also show that this device performs well below 100 eV, as demonstrated by the detection of Al L fluorescence at 72 eV with a measured FWHM of 16 eV.
CUBIC, the Cosmic Unresolved X-ray Background Instrument Using CCDs, is designed to make moderate resolution X-ray spectral measurements at spatial scales of a few degrees. While the energy range is nominally 200 eV - 10 keV, the CCDs have been designed to maximize the soft X-ray performance by using novel structures. The CUBIC CCDs, fabricated by Loral Fairchild, are 1024 X 1024 pixels in size, with 18 micrometers X 18 micrometers pixels. The CCDs use a new `thin poly' gate structure designed to maximize low energy quantum efficiency, while still retaining the advantages of front-side illumination and the high Charge Transfer Efficiency of a three-phase device. Being front-side illuminated, the design avoids the surface stability problems of backside illuminated devices. Fabrication of the first lot of CCDs and test structures has been completed, and we report laboratory camera testing of the CCDs at Penn State.
The recent restructuring of the AXAF program has necessitated a review of the design of the ACIS instrument. In this paper we report on the current status of these design activities. We concentrate on changes to the baseline CCD and its impact on aspects such as the operating modes. Also we review changes to the mechanical design with respect to the passive cooling scheme facilitated by the change to a highly eccentric deep earth orbit.
We present a description of Monte Carlo simulations of the pulse height energy response of CCD x-ray detectors. Effects of event charge splitting between pixels are accounted for, and the variation of energy resolution and detection efficiency with event selection criteria are discussed. We show that there are important implications for background rejection efficiency, on-board calibration, and ground-based data reduction. In orbit energy resolution may become degraded by traps created by radiation damage. We present an analysis of trapping and emission time scales, which allows us to predict the energy resolution for a wide range of device operating conditions.
The Cosmic Unresolved X-ray Background InsLrumenl using CCDs (CUBIC ) is designed to obtain spectral observations of the Diffuse X-ray Background (DXRB) with moderate spectral resolution (E/E 10—60) over the energy range 0.2 — 10 keV using mechanically collimated CCDs. It will be launched on the NASA/Argentine minisat SA C-B in December 1994. At this time, it is the only planned satellite payload devoted to the study of the spectrum of the DXRB. Observations will consist of 1—2 day pointed exposures of each target direction, resulting in a series of high quality spectra. Over the anticipated 3 year lifetime of the satellite, CUBIC will be able to study up to 50% of the sky with 5° x 5° spatial resolution for the subkilovolt Galactic diffuse background, and with 1O x 1O spatial resolution for the extragalactic diffuse background above 2 keV. CUBIC will obtain high quality non-dispersive spectra of soft X-ray emission from the interstellar medium, supernova remnants, and some bright sources, and will make a sensitive search for line emission or other features in the extragalactic cosmic X-ray background from 2 — 10 keY