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 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) 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.
We present science highlights and performance from the Swift X-ray Telescope (XRT), which was launched on November
20, 2004. The XRT covers the 0.2-10 keV band, and spends most of its time observing gamma-ray burst (GRB)
afterglows, though it has also performed observations of many other objects. By mid-August 2007, the XRT had observed
over 220 GRB afterglows, detecting about 96% of them. The XRT positions enable followup ground-based optical
observations, with roughly 60% of the afterglows detected at optical or near IR wavelengths. Redshifts are measured
for 33% of X-ray afterglows. Science highlights include the discovery of flaring behavior at quite late times, with
implications for GRB central engines; localization of short GRBs, leading to observational support for compact merger
progenitors for this class of bursts; a mysterious plateau phase to GRB afterglows; as well as many other interesting
observations such as X-ray emission from comets, novae, galactic transients, and other objects.
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 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 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 X-Ray Telescope (XRT) on board the <i>Swift</i> satellite is a sensitive imaging spectrometer utilizing a MAT-22 CCD at the Focal plane. The system was designed to operate the CCD at -100 °C +/- 1 °C for the duration of the mission. Due to a failure of the temperature control sub-system, the CCD operates under variable thermal conditions dictated by the view factor of the radiator- heatpipe sub-system to the Earth and sun. A temperature variation of up to 5° C is seen during a single orbit due to the satellite transition from sun light into eclipse and the full operational regime of the instrument ranges from temperatures of -75°C to -45°C due to the persistent heating/cooling effects of satellite orientation to the sun and earth. To maintain the highest quality data products possible from the XRT data stream, a recalibration of the XRT is required to account for this variable thermal environment. We present the methodology for and results from a temperature dependent analysis of on-orbit XRT data, collected during the Swift commissioning phase, used to produce gain, bias and warm pixel calibration products. We also discuss the quality of XRT science products capable with these temperature dependent calibration files and future plans for updates to these calibration products.
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 <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 <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 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 CCDs on the Chandra X-ray Observatory are sensitive to radiation damage, particularly from low-energy protons scattering off the telescope's mirrors onto the focal plane. In its highly elliptical orbit, Chandra passes through a spatially and temporally varying radiation environment, ranging from the radiation belts to the solar wind. Translating the Advanced CCD Imaging Spectrometer (ACIS) out of the focal position during radiation-belt passages has prevented loss of scientific utility. However, carefully managing the radiation damage during the remainder of the orbit, without unnecessarily sacrificing observing time, is essential to optimizing the scientific value of this exceptional observatory throughout its planned 10-year mission. In working toward this optimization, the Chandra team developed and applied a radiation-management strategy. This strategy includes autonomous instrument safing triggered by the on-board radiation monitor, as well as monitoring, alerts, and intervention based upon real-time space environment data from NOAA and NASA spacecraft. Furthermore, because Chandra often spends much of its orbit out of the solar wind (in the Earth's outer magnetosphere and magnetosheath), the team developed the Chandra Radiation Model to describe the complete low-energy-proton environment. Management of the radiation damage has thus far succeeded in limiting degradation of the charge-transfer inefficiency (CTI) to less than 3.5(10<sup>-6</sup>) and 1.3(10<sup>-6</sup>) per year for the front-illuminated and back-illuminated CCDs, respectively. This rate of degradation is acceptable for maintaining the scientific viability of all ACIS CCDs for more than ten years.
For the last 20 months, the Chandra X-Ray Observatory (Weisskopf et. al. 2000) has been producing X-ray images of the universe in stunning detail. This is due in large part to the excellent post-facto pointing aspect determination for Chandra (Aldcroft et. al. 2000). This aspect determination performance is achieved using elliptical gaussian centroiding techniques. Application of point spread function (PSF) fitting using a true PSF model for the Aspect Camera Assembly (ACA) on Chandra could improve this performance. We have investigated the use of an ACA PSF model in the post-facto centroiding of stars and fiducial lights imaged by the ACA. We will present the methodologies explored for use in determining a model for the ACA PSF and discuss the results of a comparison of PSF fit centroiding and the current method of elliptical gaussian centroiding as they apply to post-facto aspect reconstruction. The first method of recovering the ACA PSF uses a raytrace model of the ACA to generate simulated stellar PSFs. In this method, the MACOS raytracing software package is used to describe each element of the Chandra aspect optical system. The second method investigated is the so called shift and add method whereby we build a high resolution image of the PSF by combining several thousand low resolution images of a single star collected by the ACA while tracking during normal science observations. The programmed dither of the spacecraft slowly sweeps the stellar image across the ACA focal plane, and the many slightly offset images are used to effectively increase the resolution of the resultant image of the star to a fraction of an ACA pixel. In each method, a library of PSF images is built at regularly gridded intervals across the ACA focal plane. This library is then used to interpolate a PSF at any desired position on the focal plane. We have used each method to reprocess the aspect solution of a set of archived Chandra observation and compare the results to one another and to the delivered post-facto aspect solution, currently derived using elliptical gaussian centroiding of ACA star images. Finally, we will present a summary of Chandra's aspect performance achieved to date, and discuss the effect of incorporating a PSF model into the post-facto aspect determination software.