Alfonso Collura

AXAF High-Resolution Camera (HRC): calibration and recalibration at XRCF and beyond

The high resolution camera (HRC) is a microchannel plate based imaging detector for the Advanced X-Ray Astrophysics Facility (AXAF) that will be placed in a high earth orbit scheduled for launch in August, 1998. An end-to-end calibration of the HRC and the AXAF high resolution mirror assembly (HRMA) was carried out at the Marshall Space Flight Centers X-Ray Calibration Facility (XRCF). This activity was followed by several modifications to the HRC to improve its performance, and a series of flat field calibrations. In this paper, and the following companion papers, we discuss the calibration plans, sequences, and results of these tests. At the time of this conference, the HRC has been fully flight qualified and is being integrated into the science instrument module (SIM) in preparation for integration into the AXAF spacecraft.

Calibration of the UV/ion shields for the AXAF High-Resolution Camera

The high resolution camera (HRC) is one of AXAFs two focal plane instruments. It consists of two detectors: the HRC-I which is optimized for direct imaging of x-ray sources; and the HRC-S which is optimized as the spectroscopic read-out of the low energy transmission grating (LETG). Both detectors are comprised of a chevron pair of micro-channel plates (MCPs) with a crossed grid charge detector (CGCD) and a UV/ion shield (UVIS). The role of the UVIS is to minimize the detectors sensitivity to low energy electrons, ions and UV light, while providing sufficient x-ray transmission in the 0.1 to 10 keV x-ray band. In this paper, we report on the results of the flight UVIS calibration measurements. Specifically, x-ray and UV transmission measurements obtained at the HRC X-ray Test Facility of the Smithsonian Astropysical Observatory, and x- ray transmission measurements of UVIS witness samples obtained at a synchrotron light source facility.

Two very luminous variable X-ray sources in M82

We report on a variability study of X-ray sources in the starburst galaxy M82, based on ROSAT and Einstein High-Resolution Imager observations. In particular, we concentrate our analysis on two bright sources which exhibit significant variability. The brightest source in M82 is located in the central core of the galaxy, is variable within individual ROSAT observations, and indication are that it might have varied between the Einstein and the ROSAT observations. It is the most luminous X-ray binary candidate yet known. The other source is located outside the crowded central region and was very bright in the Einstein observation, but was not detected by ROSAT, despite the larger effective area of the instrument and the much longer exposure. The detection of variability poses strong constraints on the X-ray luminosity of individual components of the X-ray sources. Even if both sources are radiating at the Eddington limit, their mass would be at least several solar masses, making them candidates for extragalactic black holes.

Performance and calibration of the AXAF High-Resolution Camera II: the spectroscopic detector

The high resolution camera (HRC) is one of two focal plane detector systems that will be flown on the Advanced X-ray Astrophysics Facility (AXAF). The HRC consists of two microchannel plate (MCP) detectors: one to provide large area, high position resolution imaging and timing (HRC-I), and a second (HRC-S) to provide a readout for the AXAF low energy transmission gratings. Each detector is composed of a chevron pair of CsI coated MCPs with a crossed grid charge detector and an Al/polyimide UV/ion shield. In this paper, we describe the operation, performance and calibration of the spectroscopic detector. In particular, we discuss the absolute quantum efficiency calibration, the point spread function of the instrument combined with the AXAF telescope, the count rate linearity, the spatial linearity, and the internal background of the instrument. Data taken in the laboratory and at the x-ray Calibration Facility at Marshall Space Flight Center are presented.

An Explanation of the ROSAT High-Resolution Imager Ultraviolet Sensitivity

The UV sensitivity to bright stars observed during operations of the high-resolution imager (HRI) on the X-ray-XUV Roentgen Satellite (ROSAT) has been commonly interpreted as a result of enhanced transmission solely in the wavelength range 1500-1700 A of the HRIs UV/ion shield. We report on recent laboratory UV transmission measurements of HRI flight spare filters that show transmissions much higher than previously assumed for wavelengths longward of approximately 2000 A. We present a model for the UV response of the HRI based on these measurements. Using this model and the known UV spectra of the A stars Vega (α Lyrae, V = +0.03) and β Carinae (V = +1.68), we successfully predict the observed HRI count rates for these two stars.

Measurement of optical constants n and k of lexan and polyimide

We present preliminary results on a program aimed at characterizing the optical properties of materials of potential usage in filters for soft x-ray detectors. In particular, we discuss a method that we have used to derive and model the refractive index n and the extinction coefficient k of thin plastic film materials. The method is based on best fit estimates of the parameters of a quanto- mechanical model describing k. The value of n is then evaluated using the Kramers-Kronig relationship. This method has provided accurate values of previously unknown optical constants of polyimide and lexan allowing to model the transmission of multilayer filters such as the aluminized polyimide filters of the HRC on board Chandra X-ray Observatory.

X-ray optics made from thin plastic foils

New design concepts and materials can be used to produce very lightweight, thin foil approximations, to Wolter I and other x-ray optics. Structures are designed around a central hub and spacers that connect one spoked wheels. Figure defining, thin pins span the distance between the wheels. Thin, metal coated or multilayered, plastic foils can be formed into cones, cylinders or spirals for x-ray telescopes or lenses. Imaging and spectroscopic data obtained with x- ray lenses are presented and they indicate that a 60 cm diameter, 4.65 m focal length x-ray telescope can have a half power diameter of < 2 arcmin.

X-ray Astronomy Calibration and Testing Facility (XACT) at Osservatorio Astronomico di Palermo G.S. Vaiana

We describe the X-ray Astronomy Calibration and Testing Facility of the Osservatorio Astronomico di Palermo G.S. Vaiana. The facility, including a 16 meter vacuum beam line, a 1 m diameter test chamber, a X-ray source system, X-ray detectors, an advanced data acquisition and control system and other minor tools and accessories, features characteristics of vacuum cleanliness and versatility almost unique among facilities of this size. It started operating in June 1993 and will soon be completed with further equipments to improve energy resolution and to extend its capabilities to the UV band up to wavelengths of the order of 3100 A. Possible applications are test and calibration of filters, development and calibration of detectors, reflectivity measurements, testing of small X-ray telescopes. Presently it is employed for the calibration of UV-Ion shields of the High Resolution Camera of AXAF-I.

Simple gas scintillation proportional counter soft x-ray detector for laboratory usage

We describe a simple Gas Scintillation Proportional Counter for laboratory usage built at the G.S. Vaiana Observatory in Palermo. Its spectral resolution (about a factor 2 better than traditional Gas Proportional Counters), the easy realization and its relatively moderate cost make it a valid alternative to Gas Flow Proportional Counters for the detection of soft X-rays when an improved energy resolution is required.

In-Flight Calibration of the ROSAT HRI Ultraviolet Sensitivity

Comparing measured and estimated count rates of a few selected sample stars, we con—rm the validity and provide the in-—ight calibration of the ROSAT HRI UV/visible eUective area model in Zombeck et al. The count rate estimates for Betelgeuse derived with this model are in agreement with the measured HRI upper limit. This result is also con—rmed in an erratum by