Catherine E. Grant

Chandra Spectra of the Soft X-Ray Diffuse Background

We present an exploratory Chandra ACIS-S3 study of the diffuse component of the cosmic X-ray background (CXB) in the 0.3–7 keV band for four directions at high Galactic latitudes, with emphasis on details of the ACIS instrumental background modeling. Observations of the dark Moon are used to model the detector background. A comparison of the Moon data and the data obtained with ACIS stowed outside the focal area showed that the dark Moon does not emit significantly in our band. Point sources down to 3 � 10 � 16 ergs s � 1 cm � 2 in the 0.5–2 keV band are excluded in our two deepest observations. We estimate the contribution of fainter, undetected sources to be less than 20% of the remaining CXB flux in this band in all four pointings. In the 0.3–1 keV band, the diffuse signal varies strongly from field to field and contributes between 55% and 90% of the total CXB signal. It is dominated by emission lines that can be modeled by a kT ¼ 0:1 0:4 keV plasma. In particular, the two fields located away from bright Galactic features show a prominent line blend at E � 580 eV (O vii+O viii) and a possible line feature at E � 300 eV. The two pointings toward the North Polar Spur exhibit a brighter O blend and additional bright lines at 730–830 eV (Fe xvii). We measure the total 1–2 keV flux of 1:0 1:2 � 0:2 ðÞ �10 � 15 ergs s � 1 cm � 2 arcmin � 2 (mostly resolved) and the 2–7 keV flux of 4:0 4:5 � 1:5 ðÞ �10 � 15 ergs s � 1 cm � 2 arcmin � 2 .A tE > 2 keV, the diffuse emission is consistent with zero, to an accuracy limited by the short Moon exposure and systematic uncertainties of the S3 background. Assuming Galactic or local origin of the line emission, we put an upper limit of � 3 � 10 � 15 ergs s � 1 cm � 2 arcmin � 2 on the 0.3–1 keV extragalactic diffuse flux. Subject headings: intergalactic medium — ISM: general — methods: data analysis — X-rays: diffuse background — X-rays: ISM

X-ray CCD Calibration for the AXAF CCD Imaging Spectrometer

Acquisition of ground calibration data from the AXAF CCD Imaging Spectrometer, one of two focal plane instruments on NASAs 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.

Radiation damage in the Chandra x-ray CCDs

Front side illuminated CCDs comprising focal plane of Chandra X-ray telescope have suffered some radiation damage in the beginning of the mission. Measurements of CTI and dark current at different temperatures led us to conclusion that the type of damage is inconsistent with the much studied type of damage created by protons with energies higher than 10 MeV. Intensive ground based investigation showed that irradiation of CCD with low energy protons (about 100 keV) results in the device characteristics very similar to the ones of the flight chips (very low dark current, the shape of the CTI temperature dependence). We were able to reliably determine that only image section of the flight chips was damaged and therefore only fast transfer from image to frame store section was affected. We have developed several techniques in order to determine the parameters of the electron traps introduced into the transfer channel of the irradiated device.

An Overview of the Performance of the Chandra X-ray Observatory

The Chandra X-ray Observatory is the X-ray component of NASAs Great Observatory Program which includes the recently launched Spitzer Infrared Telescope, the Hubble Space Telescope (HST) for observations in the visible, and the Compton Gamma-Ray Observatory (CGRO) which, after providing years of useful data has reentered the atmosphere. All these facilities provide, or provided, scientific data to the international astronomical community in response to peer-reviewed proposals for their use. The Chandra X-ray Observatory was the result of the efforts of many academic, commercial, and government organizations primarily in the United States but also in Europe. NASAs Marshall Space Flight Center (MSFC) manages the project and provides project science; Northrop Grumman Space Technology (NGST – formerly TRW) served as prime contractor responsible for providing the spacecraft, the telescope, and assembling and testing the observatory; and the Smithsonian Astrophysical Observatory (SAO) provides technical support and is responsible for ground operations including the Chandra X-ray Center (CXC). Telescope and instrument teams at SAO, the Massachusetts Institute of Technology (MIT), the Pennsylvania State University (PSU), the Space Research Institute of the Netherlands (SRON), the Max-Planck Institüt für extraterrestrische Physik (MPE), and the University of Kiel also provide technical support to the Chandra Project. We present here a detailed description of the hardware, its on-orbit performance, and a brief overview of some of the remarkable discoveries that illustrate that performance.

Detection of X-rays from galaxy groups associated with the gravitationally lensed systems PG 1115+080 and B1422+231

Gravitational lenses that produce multiple images of background quasars can be an invaluable cosmological tool. Deriving cosmological parameters, however, requires modeling the potential of the lens itself. It has been estimated that up to a quarter of lensing galaxies are associated with a group or cluster that perturbs the gravitational potential. Detection of X-ray emission from the group or cluster can be used to better model the lens. We report on the first detection in X-rays of the group associated with the lensing system PG 1115+080 and the first X-ray image of the group associated with the system B1422+231. We find a temperature and rest-frame luminosity of 0.8 keV and 7 × 1042 ergs s-1, respectively, for PG 1115+080 and 1.0 keV and 8 × 1042 ergs s-1, respectively, for B1422+231. We compare the spatial and spectral characteristics of the X-ray emission with the properties of the group galaxies, with lens models, and with the general properties of groups at lower redshift.

Flight spectral response of the ACIS instrument

We discuss the flight calibration of the spectral response of the Advanced CCD Imaging Spectrometer (ACIS) on-board the Chandra X-ray Observatory (CXO). The spectral resolution and sensitivity of the ACIS instrument have both been evolving over the course of the mission. The spectral resolution of the frontside-illuminated (FI) CCDs changed dramatically in the first month of the mission due to radiation damage. Since that time, the spectral resolution of the FI CCDs and the Backside-illuminated (BI) CCDs have evolved gradually with time. We demonstrate the efficacy of charge-transfer inefficiency (CTI) correction algorithms which recover some of the lost performance. The detection efficiency of the ACIS instrument has been declining throughout the mission, presumably due to a layer of contamination building up on the filter and/or CCDs. We present a characterization of the energy dependence of the excess absorption and demonstrate software which models the time dependence of the absorption from energies of 0.4 keV and up. The spectral redistribution function and the detection efficiency are well-characterized at energies from 1.5 to 8.0~keV primarily due to the existence of strong lines in the ACIS calibration source in that energy range. The calibration at energies below 1.5 keV is challenging because of the lack of strong lines in the calibration source and also because of the inherent non-linear dependence with energy of the CTI and the absorption by the contamination layer. We have been using data from celestial sources with relatively simple spectra to determine the quality of the calibration below 1.5 keV. We have used observations of 1E0102.2-7219 (the brightest supernova remnant in the SMC), PKS2155-304 (a bright blazar), and the pulsar PSR~0656+14 (nearby pulsar with a soft spectrum), since the spectra of these objects have been well-characterized by the gratings on the CXO. The analysis of these observations demonstrate that the CTI correction recovers a significant fraction of the spectral resolution of the FI CCDs and the models of the time-dependent absorption result in consistent measurements of the flux at low energies for data from a BI (S3) CCD.

A charge transfer inefficiency correction model for the Chandra Advanced CCD Imaging Spectrometer

Soon after launch, the Advanced CCD Imaging Spectrometer (ACIS), one of the focal plane instruments on the Chandra X-ray Observatory, suffered radiation damage from exposure to soft protons during passages through the Earths radiation belts. The primary effect of the damage was to increase the charge transfer inefficiency (CTI) of the eight front illuminated CCDs by more than two orders of magnitude. The ACIS instrument team is continuing to study the properties of the damage with an emphasis on developing techniques to mitigate CTI and spectral resolution degradation. We will discuss the characteristics of the damage, the detector and the particle background and how they conspire to degrade the instrument performance. We have developed a model for ACIS CTI which can be used to correct each event and regain some of the lost performance. The correction uses a map of the electron trap distribution, a parameterization of the energy dependent charge loss and the fraction of the lost charge re-emitted into the trailing pixel to correct the pixels in the event island. This model has been implemented in the standard Chandra data processing pipeline. Some of the correction algorithm was inspired by the earlier work on ACIS CTI correction by Townsley and collaborators. The details of the CTI model and how each parameter improves performance will be discussed, as well as the limitations and the possibilities for future improvement.

Joint AXAF high-resolution mirror assembly and AXAF CCD Imaging Spectrometer calibration at the MSFC X-Ray Calibration Facility

The flight AXAF CCD Imaging Spectrometer (ACIS) and the High-Resolution Mirror Assembly (HRMA) telescope were extensively calibrated at NASA MSFCs 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.

Long-term trends in radiation damage of Chandra x-ray CCDs

Soon after launch, the Advanced CCD Imaging Spectrometer (ACIS), one of the focal plane instruments on the Chandra X-ray Observatory, suffered radiation damage from exposure to soft protons during passages through the Earths radiation belts. Current operations require ACIS to be protected during radiation belt passages to prevent this type of damage, but there remains a much slower and more gradual increase. We present the history of ACIS charge transfer inefficiency (CTI), and other measures of radiation damage, from January 2000 through June 2005. The rate of CTI increase is low, of order 1e-6 per year, with no indication of step-function increases due to specific solar events. Based on the time history and CCD location of the CTI increase, we speculate on the nature of the damaging particles.

Modeling Contamination Migration on the Chandra X-Ray Observatory - III

During its first 14 years of operation, the cold (about -60°C) optical blocking filter of the Advanced CCD Imaging Spectrometer (ACIS), aboard the Chandra X-ray Observatory, has accumulated a growing layer of molecular contamination that attenuates low-energy x rays. Over the past few years, the accumulation rate, spatial distribution, and composition have changed. This evolution has motivated further analysis of contamination migration within and near the ACIS cavity. To this end, the current study employs a higher-fidelity geometric model of the ACIS cavity, detailed thermal modeling based upon temperature data, and a refined model of the molecular transport.