D. J. Yentis

The ROSAT All-Sky Survey: a Catalog of Clusters of Galaxies in a Region of 1 steradian around the South Galactic Pole

A field of 1.013 sr in the ROSAT All-Sky Survey (RASS), centered on the south Galactic pole (SGP), has been searched in a systematic, objective manner for clusters of galaxies. The procedure relied on a correlation of the X-ray positions and properties of ROSAT sources in the field with the distribution of galaxies in the COSMOS digitized database, which was obtained from scanning the plates of the UK Schmidt IIIa-J optical survey of the southern sky. The study used the second ROSAT survey database (RASS-2) and included several optical observing campaigns to measure cluster redshifts. The search, which is a precursor to the larger REFLEX survey encompassing the whole southern sky, reached the detection limits of both the RASS and the COSMOS data and yielded a catalog of 186 clusters in which the lowest flux is 1:5 � 10 � 12 ergs cm � 2 s � 1 in the 0.1–2.4 keV band. Of these 157 have measured redshifts. Using a flux limit of 3:0 � 10 � 12 ergs cm � 2 s � 1 a complete subset of 112 clusters was obtained, of which 110 have measured redshifts. The spatial distribution of the X-ray clusters out to a redshift of 0.15 shows an extension of the Local Supercluster to the Pisces-Cetus supercluster ðz <� 0:07Þ, and an orthogonal structure at higher redshift ð0:07 < z < 0:15Þ. This result is consistent with large-scale structure suggested by optical surveys. Subject headings: catalogs — galaxies: clusters: general — X-rays: galaxies: clusters On-line material: machine-readable table

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.

The COSMOS/UKST Catalog of the Southern Sky

We have prepared an ‘object’ catalog of the Southern sky south of + 2.5 degrees Declination from COSMOS scans of the IIIa-J and Short Red surveys taken with the UK Schmidt Telescope. The catalog consists of upwards of 500 million objects down to the limit of the plates. A version of the catalog in compact form is available for distribution, and a database management system has been produced which allows rapid access to any part of the catalog. We also describe the creation of a catalog of clusters of galaxies derived from the galaxies appearing in the object catalog. The cluster catalog contains some 70 000 candidate clusters in the Southern hemisphere and forms the basis for further major programs of follow-up study.

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.

High-Resolution Spectroscopy of G191-B2B in the Extreme-Ultraviolet

We report a high-resolution (R = 3000-4000) spectroscopic observation of the DA white dwarf G191-B2B in the extreme-ultraviolet band 220-245 A. A low-density, ionized He component is clearly present along the line of sight, which if completely interstellar implies a He ionization fraction considerably higher than is typical of the local interstellar medium. However, some of this material may be associated with circumstellar gas, which has been detected by analysis of the C IV absorption-line doublet in a Hubble Space Telescope/Space Telescope Imaging Spectrograph spectrum. A stellar atmosphere model assuming a uniform element distribution yields a best fit to the data that includes a significant abundance of photospheric He. The 99% confidence contour for the fit parameters excludes solutions in which photospheric He is absent, but this result needs to be tested using models allowing abundance gradients.

Proposed multilayer-grating designs for the Astrophysical Plasmadynamic Explorer (APEX): an EUV high-resolution spectroscopic SMEX

APEX is a proposed mission for a Small Explorer (SMEX) satellite. The instrument is a suite of 8 near-normal incidence EUV spectrometers and is the outgrowth of 17 years of research at NRL on multilayer coatings and holographic ion-etched diffraction gratings. A prototype spectrometer has been flown successfully on a sounding rocket. We have examined different multilayer and gratings designs and produced a configuration optimized for the proposed science. APEX will achieve a peak effective area of at least 30-50 cm2 in the range 90-275 Å with resolution ~10,000, significant improvements on Chandra and EUVE.

The USA X-ray timing experiment

The USA Experiment is a new X-ray timing experiment with large collecting area and microsecond time resolution capable of conducting a broad program of studies of galactic X-ray binaries. USA is one of nine experiments aboard the Advanced Research and Global Observation Satellite which was launched February 23, 1999. USA is a collimated proportional counter X-ray telescope with about 1000 cm2 of effective area per detector with two detectors sensitive to photons in the energy range 1–15 keV. A unique feature of USA is that photon events are time tagged by reference to an onboard GPS receiver allowing precise absolute time and location determination. We will present an overview of the USA instrument, capabilities, and scientific observing plan as well as the current status of the instrument.

The joint astrophysical plasmadynamic experiment extreme ultraviolet spectrometer: resolving power

The Joint astrophysical Plasmadynamic EXperiment (J-PEX) is a high-resolution extreme ultraviolet (EUV) spectrometer (220-245 Å) used for the study of white dwarf atmospheres. Significant improvements have been achieved in both the normal-incidence gratings and the focal-plane detector since its first successful sounding rocket flight in 2001. The spherical laminar gratings have been replaced by paraboloidal gratings. The substrates of the new gratings have measured slope errors less than 0.35 arcsec. The gratings were recorded holographically and the rulings transferred into the silica substrates by ion etching. This procedure was followed by polymer overcoat to reduce the blaze angle of the groove profile. The detector uses microchannel plates with 6 μm pores and a cross-strip anode, providing 17.9 μm resolution in the dispersion direction. The detector employs a KBr photocathode with a projected efficiency of 0.24 at 256 Å. Using ray tracing simulations, we predict the resolving power expected from the spectrometer during upcoming EUV calibrations with a He II discharge source.

Proposed mission concept for the Astrophysical Plasmadynamic Explorer (APEX): an EUV high-resolution spectroscopic SMEX

APEX is a proposed mission for a Small Explorer (SMEX) satellite. APEX will investigate the density, temperature, composition, magnetic field, structure, and dynamics of hot astrophysical plasmas (log T = ~5-7), which emit the bulk of their radiation at EUV wavelengths and produce critical spectral diagnostics not found at other wavelengths. APEX addresses basic questions of stellar evolution and galactic structure through high-resolution spectroscopy of white dwarf stars, cataclysmic variables, the local interstellar medium, and stellar coronae. Thus APEX complements the Chandra, Newton-XMM, FUSE, and CHIPS missions. The instrument is a suite of 8 near-normal incidence spectrometers (~90-275 Angstroms, resolving power ~10,000, effective area 30-50 cm2) each of which employs a multilayer-coated ion-etched blazed diffraction grating and a microchannel plate detector of high quantum efficiency and high spatial resolution. The instrument is mounted on a 3-axis stabilized commercial spacecraft bus with a precision pointing system. The spacecraft is launched by a Taurus vehicle, and payload size and weight fit comfortably within limits for the 2210 fairing. Of order 100 targets will be observed over the baseline mission of 2 years. These are selected carefully to maximize scientific return, and all were detected in the EUVE and the ROSAT WFC surveys.

USA experiment on the ARGOS satellite: a low-cost instrument for timing x-ray binaries

The Unconventional Stellar Aspect (USA) experiment on the Advanced Research and Global Observations Satellite (ARGOS) of the Space Test Program is a low-cost, quick, yet scientifically ambitious X-ray timing experiment. The research program emphasizes observing neutron stars and black holes for extended periods with sub-millisecond timing resolution. The scientific program is achieved using hardware whose key features are large collecting area (2000 cm2), energy response extending as low as 1 keV, long accumulated observing times (1 month), high time resolution (1 microsecond(s) ), absolute time-tagging (also to 1 microsecond(s) ) achieved using a GPS receiver, sustained high data rates (40 kbps at all times supplemented by 128 kbps as needed), and flexibility in data handling by using a solid state recorder and a radiation- hardened 20 MIPS 32-bit onboard computer to provide onboard processing.