M. P. Kowalski

USA Experiment and RXTE Observations of a Variable Low-Frequency Quasi-periodic Oscillation in XTE J1118+480

The USA experiment on ARGOS and RXTE have exensively observed the X-ray transient XTE J1118+480 during its recent outburst in 2000 April--June. We present detailed monitoring of the evolution of a low frequency QPO which drifts from 0.07 Hz to 0.15 Hz during the outburst. We examine possible correlations of the QPO frequency with the flux and spectral characteristics of the source, and compare this QPO to low frequency QPOs observed in other black hole candidates.Using the USA Experiment on the Advanced Research and Global Observation Satellite and the Rossi X-Ray Timing Explorer, we have extensively observed the X-ray transient XTE J1118+480 during its recent outburst in 2000 April-June. We present detailed monitoring of the evolution of a low-frequency quasi-periodic oscillation (QPO) that drifts from 0.07 to 0.15 Hz during the outburst. We examine possible correlations of the QPO frequency with the flux and spectral characteristics of the source, and we compare this QPO with low-frequency QPOs observed in other black hole candidates.

On-blaze operation of a Mo/Si multilayer-coated, concave diffraction grating in the 136–142-Å wavelength region and near normal incidence

The efficiency and resolving power of a concave, 2400-groove/mm, blazed diffraction grating that had a Mo/Si multilayer coating were determined. The multilayer coating had a peak reflectance of 55% at 140-A near normal incidence. The efficiency of the multilayer grating for wavelengths in the 136-139-A range was 2% near normal incidence. This efficiency was a factor of 150 greater than the efficiency of a sister replica Au-coated grating in the same wavelength region. The resolving power of the multilayercoated grating in the third order of a V viii transition with a first-order wavelength of 140.451 A was 9100. Comparisons with the Au-coated grating indicated that the application of the multilayer coating did not affect the resolving power or the blaze angle.

Multilayer-coated laminar grating with 16% normal-incidence efficiency in the 150-Å wavelength region

We characterized a laminar grating with a Mo/Si multilayer coating by using synchrotron radiation and atomic force microscopy. The grating substrate had 2400 grooves/mm, 40-A groove depth, and 2080-A groove width. The microroughness of the grating substrate was 5 A rms. The multilayer coating was optimized to have peak normal-incidence reflectance at a wavelength near 150 A. For an angle of incidence of 10 degrees the peak grating efficiency was 16.3% in the +1 order and 15.0% in the -1 order. The efficiency in the zero order was lower by a factor of 40 owing to the excellent matching of the groove depth and groove width to the wavelength of the incident radiation. By dividing the grating efficiencies by the measured reflectance of the multilayer coating, we obtained inferred groove efficiencies of 34% and 32% in the +1 and -1 orders, respectively.

OBSERVATION OF X-RAY VARIABILITY IN THE BL LACERTAE OBJECT 1ES 1959+65

This paper reports X-ray spectral observations of a relatively nearby (z = 0.048) BL Lac object 1ES 1959+65, which is a potential TeV emitter. The observations include 31 short pointings made by the Unconventional Stellar Aspect (USA) experiment on board the Advanced Research and Global Observation Satellite (ARGOS), and 17 pointings by the Proportional Counter Array (PCA) on board the Rossi X-Ray Timing Explorer (RXTE). Most of these observations were spaced by less than 1 day. 1ES 1959+65 was detected by the ARGOS USA detector in the range 1-16 keV, and by the PCA in the 2-16 keV range but at different times. During the closely spaced RXTE observations beginning on 2000 July 28, the ending of one flare and the start of another are visible, associated with spectral changes, where the photon index Γ ranges between ~1.4 and ~1.7, and the spectrum is harder when the source is brighter. This implies that 1ES 1959+65 is an X-ray-selected BL Lac type (XBL) blazar, with the X-ray emission likely to originate via the synchrotron process. The USA observations reveal another flare that peaked on 2000 November 14 and doubled the flux within a few days, again associated with spectral changes of the same form. The spectral variability correlated with the flux and timing characteristics of this object that are similar to those of other nearby BL Lac objects and suggest relativistic beaming with a Doppler factor δ ≥ 1.6 and magnetic fields on the order of a few milligauss. We also suggest that the steady component of the X-ray emission—present in this object as well as in other XBLs—may be due to the large-scale relativistic jet (such as measured by Chandra in many radio-loud active galactic nuclei) but pointing very closely to our line of sight.

Long-term stability of a Mo/Si multilayer structure

The normal-incidence reflectance of a Mo/Si multilayer mirror, with peak reflectance near 130 A, was measured over a period of 20 months by using synchrotron radiation. The measured reflectances were unchanged over this period of time, and this indicates that the material layers and interfaces were stable.

Normal-incidence reflectance of W/B 4 C multilayer mirrors in the 34–50-Å wavelength region

The measured normal-incidence reflectances of W/B(4)C multilayers mirrors are 2% in the 34-50-A wavelength region and are consistent with interfacial roughness in the range of 3-4 A.

Efficiency of a multilayer-coated, ion-etched laminar holographic grating in the 14.5–16.0-nm wavelength region

The efficiency of an ion-etched laminar holographic grating was measured at near-normal incidence in the 14.5-16.0-nm wavelength range. The grating had an electron-beam-evaporated Mo/Si multilayer coating matched to the grating groove depth. The efficiency peaked at 16.3% in the first inside order at 15.12 nm and 15.0% in the first outside order at 14.94 nm. These are believed to be the highest efficiencies obtained to date from a multilayer-coated laminar grating at near-normal incidence in the EUV (lambda<30.0nm) . Zero and even orders were almost completely suppressed. The grating groove efficiency in the first order approached the theoretical limit of 40.5%.

Dual-waveband operation of a multilayer-coated diffraction grating in the soft x-ray range at near-normal incidence

A concave diffraction grating (2400 grooves/mm) coated with a Si/Mo multilayer has an efficiency of 2.5% at 290 A and a resolving power of 14,000 in third order of 156 A.

Polarization and efficiency of a concave multilayer grating in the 135–250-Å region and in normal-incidence and Seya–Namioka mounts

A molybdenum/silicon multilayer coating was applied to a holographic ion-etched blazed grating substrate that had 2400 grooves/mm and a radius of curvature of 2.2 m. Scanning probe microscopy yielded the same surface microroughness (5 Å rms) before and after deposition of the multilayer. The efficiency and polarization performance of the grating was measured by synchrotron radiation in the 135-250-Å wavelength region. In the second grating order and the second Bragg order of the multilayer coating, the peak normal-incidence efficiency was 7.5% at a wavelength of 147 Å, representing a groove efficiency of 27%. At an angle of incidence of 35°, the polarization performance of the grating was 95%-100% in the 210-250-Å wavelength region. In a Seya-Namioka spectrometer mount at an angle of incidence of 30°-40°, the grating is a nearly perfect polarizing optical element in the wavelength bands between 125 and 300 Å, which are covered by the multilayer coating.

Quantum efficiency of cesium iodide photocathodes at soft x-ray and extreme ultraviolet wavelengths

We have measured the quantum efficiency for normal incidence radiation of microchannel plate detectors which use opaque or semitransparent photocathodes made of fluffy CsI, solid CsI, or both. At wavelengths below ~44 A, detectors with fluffy CsI semitransparent photocathodes are more efficient than those with opaque photocathodes of solid CsI, but the opposite is true at longer wavelengths. Fluffy CsI semitransparent photocathodes with surface densities between 150 and 400 μg/cm2 are optimum at soft x-ray wavelengths, and we have obtained efficiencies of 35 and 41% at 8 and 44 A, respectively, for a single-layer photocathode. The measured peak efficiency for an opaque layer of solid CsI, deposited at 0° coating angle and 5000 A (226 μg/cm2) in thickness, is 56% at 110 A.