We are developing Wolter-I X-ray optics for use in SmallSat missions. These optics are being designed for telescope focal lengths on the order of 0.5 - 1 m, much shorter than typical Astrophysics missions. The various parameters of the optics module: diameter, length, number nested shells, coatings, etc., depend partly on the spacecraft bus but the final design is driven by the science and instrument requirements of the mission (effective area, resolution, and energy band of interest). Ray trace software was developed and used to project the performance of several optics configurations, which, designed for SmallSat missions, meet the instrument requirements for the SmallSat Exosphere Explorer of hot Jupiters (SEEJ) . Results of this modeling is presented.
The first detected exoplanets found were "hot Jupiters"; these are large Jupiter-like planets in close orbits with their host star. The stars in these so-called "hot Jupiter systems" can have significant X-ray emission and the X-ray flux likely changes the evolution of the overall star-planetary system in at least two ways: (1) the intense high energy flux alters the structure of the upper atmosphere of the planet - in some cases leading to significant mass loss; (2) the angular momentum and magnetic field of the planet induces even more activity on the star, enhancing its X-rays, which are then subsequently absorbed by the planet. If the alignment of the systems is appropriate, the planet will transit the host star. The resulting drop in flux from the star allows us to measure the distribution of the low-density planetary atmosphere. We describe a science mission concept for a SmallSat Exosphere Explorer of hot Jupiters (SEEJ; pronounced "siege"). SEEJ will monitor the X-ray emission of nearby X-ray bright stars with transiting hot Jupiters in order to measure the lowest density portion of exoplanet atmospheres and the coronae of the exoplanet hosts. SEEJ will use revolutionary Miniature X-ray Optics (MiXO) and CMOS X-ray detectors to obtain sufficient collecting area and high sensitivity in a low mass, small volume and low-cost package. SEEJ will observe scores of transits occurring on select systems to make detailed measurements of the transit depth and shape which can be compared to out-of-transit behavior of the target system. The depth and duration of the flux change will allow us to characterize the exospheres of multiple hot Jupiters in a single year. In addition, the long baselines (covering multiple stellar rotation periods) from the transit data will allow us to characterize the temperature, flux and flare rates of the exoplanet hosts at an unprecedented level. This, in turn, will provide valuable constraints for models of atmospheric loss. In this contribution we outline the science of SEEJ and focus on the enabling technologies Miniature X-ray Optics and CMOS X-ray detectors.
We describe a process for cross-calibrating the effective areas of X-ray telescopes that observe common targets. The targets are not assumed to be "standard candles" in the classic sense, in that the only constraint placed on the source flux is that it is the same for all instruments. We apply a technique developed by Chen et al. (submitted to J. Amer. Stat. Association) that involves a popular statistical method called shrinkage estimation, which effectively reduces the noise in disparate measurements by combining information across common observations. We can then determine effective area correction factors for each instrument that brings all observatories into the best agreement, consistent with prior knowledge of their effective areas. We have preliminary values that characterize systematic uncertainties in effective areas for almost all operational (and some past) X-ray astronomy instruments in bands covering factors of two in photon energy from 0.15 keV to 300 keV. We demonstrate the method with several data sets from Chandra and XMM-Newton.
Unlike statistical errors, whose importance has been well established in astronomical applications, uncertainties
in instrument calibration are generally ignored. Despite wide recognition that uncertainties in calibration can
cause large systematic errors, robust and principled methods to account for them have not been developed, and
consequently there is no mechanism by which they can be incorporated into standard astronomical data analysis.
Here we present a framework where they can be encoded such that they can be brought within the scope of
analysis. We describe this framework, which is based on a modified MCMC algorithm, and propose a format
standard derived from experience with effective area measurements of the ACIS-S detector on Chandra that can
be applied to any instrument or method of codifying systematic errors. Calibration uncertainties can then be
propagated into model parameter estimates to produce error bars that include systematic error information.
We study the gain variations in the HRC-I over the duration of the
Chandra mission. We analyze calibration observations of AR Lac obtained yearly at the nominal aimpoint and at 20 offset locations on the detector. We show that the gain is declining, and that the
time dependence of the gain can be modeled generally as a linear
decrease in PHAs. We describe the spatial and temporal characteristics
of the gain decline and discuss the creation of time-dependent gain
correction maps. These maps are used to convert PHAs to PI channels, thereby removing spatial and temporal dependence, and allowing source pulse-height distributions to be compared directly regardless of
observation date or location on the detector.
Instrument response uncertainties are almost universally ignored in current astrophysical X-ray data analyses. Yet modern X-ray observatories, such as Chandra and XMM-Newton, frequently acquire data for which photon counting statistics are not the dominant source of error. Including allowance for performance uncertainties is, however, technically challenging in terms of both understanding and specifying the uncertainties themselves, and in employing them in data analysis. Here we describe Monte Carlo methods developed to include instrument performance uncertainties in typical model parameter estimation studies. These methods are used to estimate the limiting accuracy of Chandra for understanding typical X-ray source model parameters. The present study indicates that, for ACIS-S3 observations, the limiting accuracy is reached for ~ 104 counts.
The Chandra Low Energy Transmission Grating Spectrometer (LETGS) is
comprised of 3 micro-channel plate (MCP) segments and is primarily
used with the High Resolution Camera spectroscopic array (HRC-S).
In-flight calibration data observed with the LETG+HRC-S show that
there are non-linear deviations in the positions of some lines by as
much as 0.1 Å. These deviations are thought to be caused by spatial non-linearities in the imaging characteristics of the HRC-S detector. Here, we present the methods we used to characterize the non-linearities of the dispersion relation across the central plate of the HRC-S, and empirical corrections which greatly reduce the observed non-linearities by a factor of 2 or more on the central MCP.
The HRC-S is a microchannel plate detector on board Chandra and is primarily used for spectroscopic observations with the Low Energy Transmission Grating Spectrometer (LETGS) in place. Photons are detected via signals read out from evenly spaced wires underneath the plates and positions are computed by centroiding around the strongest amplifier signals. This process leads to gaps in between the taps where no events are placed. A deterministic correction is then made during ground processing to these event locations to remove the gaps. We have now developed a new, empirical degap corrections from flight data. We describe the procedure we use, present comparisons between the new degap and lab-data based degap, and investigate the temporal stability of the degap corrections.
The dispersion relation for the Chandra Low Energy Transmission
Grating Spectrometer (LETGS) is known to better than 1 part in 1000
over the wavelength range 5-150 Å. A recent resolution of a data processing software bug that lead to a systematic error in the
computation of photon wavelengths has allowed us to trace further
discrepancies in the dispersion relation to the boundaries between
different microchannel plate segments of the HRC-S imaging detector.
However, data acquired during in-flight calibration with the HRC-S
detector have always shown the presence of additional non-linear
deviations in the positions of some spectral lines by as much as
0.05 Å, which is of the order of a full width half maximum
(FWHM) of a line profile. These latter effects are thought to be caused by spatial non-linearities in the imaging characteristics of the HRC-S detector. Here, we present an improved dispersion relation for the LETG+HRC-S and new methods to help characterize the spatial non-linearities. We also describe an empirical approach that might be used to help improve the position determination of photon events.
Accurate calibration of the Chandra Low Energy Transmission Grating (LETG) higher-order (|m|>1) diffraction efficiencies is vital for proper analysis of spectra obtained with the LETG's primary detector, the HRC-S, which lacks the energy resolution to distinguish different orders. Pre-flight ground calibration of the LETG was necessarily limited to sampling a relatively small subset of spectral orders and wavelengths, and virtually no higher-order data are available in the critical region between 6 and 10 Å. In this paper, we describe an analysis of diffraction efficiencies based on in-flight data obtained using the LETG's secondary detector, the ACIS-S. Using ACIS, the relative efficiency of each order can be studied out to
|mλ| ~ 80 Å, which is nearly one-half of the LETG/HRC-S wavelength coverage. We find that the current models match our results well but can be improved, particularly for the even orders just longward of the Au-M edge at 6 Å.
We present the in-flight effective area calibration of the Low Energy Transmission Grating Spectrometer (LETGS), which comprises the High Resolution Camera Spectroscopic readout (HRC-S) and the Low Energy Transmission Grating (LETG) aboard the Chandra X-ray Observatory. Previous studies of the LETGS effective area calibration have focused on specific energy regimes: 1) the low-energy calibration for which we compared observations of Sirius B and HZ 43 with pure hydrogen non-LTE white dwarf emission models; and 2) the mid-energy calibration for which we compared observations of the active galactic nuclei PKS 2155-304 and 3C 273 with simple power-law models of their seemingly featureless continua. The residuals of the model comparisons were taken to be true residuals in the HRC-S quantum efficiency (QE) model. Additional in-flight observations of celestial sources with well-understood X-ray spectra have served to verify and fine-tune the calibration. Thus, from these studies we have derived corrections to the HRC-S QE to match the predicted and observed spectra over the full practical energy range of the LETGS. Furthermore, from pre-flight laboratory flatfield data we have constructed an HRC-S quantum efficiency uniformity (QEU) model. Application of the QEU to our semi-empirical in-flight HRC-S QE has resulted in an improved HRC-S on-axis QE. Implementation of the HRC-S QEU with the on-axis QE now allows for the computation of effective area for any reasonable Chandra/LETGS pointing.
The Chandra X-ray Observatory was successfully launched on July 23, 1999, and subsequently began an intensive calibration phase. We present preliminary results from in- flight calibration of the low energy response of the High Resolution Camera Spectroscopic readout (HRC-S) combined with the Low Energy Transmission Grating (LETG) aboard Chandra. These instruments comprise the Low Energy Transmission Grating Spectrometer (LETGS). For this calibration study, we employ a pure hydrogen non-LTE white dwarf emission model (Teff equals 25000 K and log g equals 9.0) for comparison with the Chandra observations of Sirius B. Pre-flight calibration of the LETGS effective area was conducted only at wavelengths shortward of 45 angstroms (E > 0.277 keV). Our Sirius B analysis shows that the HRC-S quantum efficiency (QE) model assumed for longer wavelengths overestimates the effective area on average by a factor of 1.6. We derive a correction to the low energy HRC-S QE model to match the predicted and observed Sirius B spectra over the wavelength range of 45 - 185 angstroms. We make an independent test of our results by comparing a Chandra LETGS observation of HZ 43 with pure hydrogen model atmosphere predictions and find good agreement.