Dale Edward Graessle

Chandra Observations of the Young Pulsar PSR B0540?69

The young pulsar PSR B0540-69 was one of the first targets observed with the Chandra X-Ray Observatory. The high angular resolution of Chandra allows us to resolve the compact nebula surrounding the pulsar. We have determined a position for PSR B0540-69 of R.A. = 05h40m11221,decl. = -69°195498 (J2000) with a 1 σ radial uncertainty of 07. Combining our measurements of the pulsar period with previous measurements covering a span of 12 years, we derive a braking index of 2.082 ± 0.005 (95% confidence). The spectrum of the pulsed emission is consistent with a power law with a photon index of 1.83 ± 0.13. The compact nebula has a softer spectrum with a photon index of 1.85-2.26.

EXAMINING DUST GRAIN MODELS USING THE REDDENING AND X-RAY DUST HALO OF TERZAN 2

We use the X-ray dust halo of the low-mass X-ray binary 4U 1724–307, located in the globular cluster Terzan 2, to probe the interstellar medium along this line of sight (LOS). The X-ray dust halo arises from X-rays scattering off of interstellar dust grains. Using a low optical depth sight line to determine the Chandra ACIS point-spread function, we extracted the radial profile as a function of energy and used it to determine the H column density (N H) and cloud location along the LOS for several dust grain models, including the commonly-used models of MRN and WD. The resulting N H values were used to determine the reddening E(B – V), which was then compared with the average E(B – V) for this sight line found by other workers. We found that for this LOS, only the ZDA BARE-AC-S, BARE-GR-FG, and BARE-GR-S models yield reddenings within 1σ of the literature average.

Iridium optical constants from synchrotron reflectance measurements over 0.05- to 12-keV x-ray energies

We present optical constants derived from synchrotron reflectance measurements of iridium-coated X-ray witness mirrors over 0.05-12 keV, relevant to the Chandra X-ray Observatory effective area calibration. In particular we present for the first time analysis of measurements taken at the Advanced Light Source Beamline 6.3.2 over 50-1000 eV, Chandras lower-energy range. Refinements to the currently tabulated iridium optical constants (B. L. Henke et al., At. Data Nucl. Data Tables 54, 181-343, 1993 and on the Web at http://www-cxro.lbl.gov/optical_constants/) will become important as the low-energy calibration of Chandras X-ray detectors and gratings are further improved, and as possible contaminants on the Chandra mirror assembly are considered in the refinement of the in-flight Ir absorption edge depths. The goal of this work has been to provide an improved tabulation of the Ir optical constants over the full range of Chandra using a self-consistent mirror model, including metallic layers, interface roughness, contaminating overlayer, and substrate. The low-energy data present us with a considerable challenge in the modeling of the overlayer composition, as the K-absorption features of C, O, and N are likely to be present in the ~10A overlayer. The haphazard contamination and chemical shifts may significantly affect optical constants attributed to this overlayer, which will distort the iridium optical constants derived. Furthermore, the witness mirror contamination may be considerably different from that deposited on the flight optics. The more complex modeling required to deal with low-energy effects must reduce to the simpler model applied at the higher energies, which has successfully derived optical constants for iridium in the higher energy range, including the iridium M-edges, already used in the Chandra calibration. We present our current results, and the state of our modeling and analysis, and our approach to a self-consistent tabulation.

Feasibility study of the use of synchrotron radiation in the calibration of AXAF - Initial reflectivity results

The 1-percent calibration accuracy goal of the Advanced X-ray Astrophysics Facility is being approached by way of an experiment at the National Synchrotron Light Source that will demonstrate the accuracy achievable in reflectance measurements conducted on coated flat mirrors in the 50 eV-12 keV energy range. The coatings will be of commercially produced Au, Ni, and Ir, deposited either by sputtering or by e-beam deposition. Optical constants will be estimated via the reflectance vs. angle-of-incidence method.

Iridium optical constants from x-ray transmission measurements over 2 to 12 keV

Precise transmission measurements of free standing iridium foils and of iridium coatings supported on thin polyimide film have been made at the X8 beamline of the National Synchrotron Light Source, at energies from 2 to 12 keV. These measurements were conducted to provide iridium optical constants in support of NASAs Advanced X-ray Astrophysics Facility (AXAF). Transmission data were collected at small energy increments across iridium M and L absorption edges to study detailed edge structures. From these data, the imaginary components of the index of refraction were computed. The data also allows computation of the real parts, using the Kramers-Kronig dispersion integral. Preliminary results indicate a measurement accuracy of better than one percent for transmission. Absorption coefficients deviate by varying amounts from values predicted from Henke data table, which is consistent with the accuracy claimed for those table.s Nonuniform thickness in our iridium foils may be a source of errors in our analysis and improved foils are being sought. Additional measurements will also be needed with foils of different thickness to account and correct for possible effects of sample thickness on the determination of absorption edge fine structure. To our knowledge, transmission measurements of this degree of accuracy and precision have not been previously reported in the literature for iridium in the 2- 12 keV energy range.