X-ray transients are among the most enigmatic objects in the cosmic sky. In recent years, the unpredictability and underlying nature of their transient behavior has prompted many studies. While significant progress has been made in this field, a more complete understanding of such events is often hampered by the delay in the rapid follow-up of any transient event. An efficient way to mitigate this constraint would be to devise a way for near real-time detection of such transient phenomena. The Advanced Telescope for High-Energy Astrophysics/Wide Field Imager (Athena/WFI), with its 40 ′ × 40 ′ field of view and large effective area, will detect a large number of x-ray variable or transient objects daily. We discuss an algorithm for the rapid onboard or ground-based detection of x-ray transients with WFI. We present a feasibility test of the algorithm using simulated Athena WFI data and show that a fairly simple algorithm can effectively detect transient and variable sources in typical Athena WFI observations.
The world's premier X-ray astronomical observatories, Chandra and XMM-Newton, have been operating for about 20 years. The next flagship X-ray observatory launched will be ESA's Athena mission. We discuss planned US contributions to the Athena Wide Field Imager instrument, which encompass transient source detection, background characterization and reduction, and detector electronics design and testing, in addition to scientific contributions.
The Science Products Module (SPM), a US contribution to the Athena Wide Field Imager, is a highly capable secondary CPU that performs special processing on the science data stream. The SPM will have access to both accepted X-ray events and those that were rejected by the on-board event recognition processing. It will include two software modules. The Transient Analysis Module will perform on-board processing of the science images to identify and characterize variability of the prime target and/or detection of serendipitous transient X-ray sources in the field of view. The Background Analysis Module will perform more sophisticated flagging of potential background events as well as improved background characterization, making use of data that are not telemetered to the ground, to provide improved background maps and spectra. We present the preliminary design of the SPM hardware as well as a brief overview of the software algorithms under development.
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. JANUS 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.
JANUS 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.
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 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 X-Ray Telescope (XRT) on board the Swift 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 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 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 Swift 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 Swift. 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 Swift 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 Swift 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 Swift 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 Swift mission.
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 XRT is a sensitive, autonomous X-ray imaging spectrometer onboard the Swift Gamma-Ray Burst Observatory. The unique observing capabilities of the XRT allow it to autonomously refine the Swift BAT positions (~1-4' uncertainty) to better than 2.5 arcsec in XRT detector coordinates, within 5 seconds of target acquisition by the Swift 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.