Robert A. Austin

Optical imaging chamber for x-ray astronomy

The light emitted by electron avalanches in a parallel plate chamber can be used to image the tracks of photoelectrons liberated by the interaction of an incident x-ray with the gas filling the chamber. The differing morphologies of photoelectron tracks and high-energy charged particle tracks can be used for background rejection. The initial direction (before scattering) of the liberated photoelectron also contains information about the polarization of the incident radiation. We have built a small test chamber with which we have imaged photoelectron tracks using an intensified CCD camera. Our results show that optical imaging could be used in a hard x-ray imaging polarimeter useful for astronomy.

Instrumentation for X-ray Astronomy

Less than five decades ago, the first X-ray observations of the sky were made using simple devices such as film and geiger counters with crude collimators. These instruments were carried aloft by sounding rockets and made observations lasting only a few minutes at most. Today, orbiting observatories, utilizing high-resolution CCDs at the focus of arc sec optics, have lifetimes measured in years. To maintain the pace of discovery in X-ray astronomy, detectors must continue to evolve into devices of ever increasing sensitivity and sophistication. Further progress depends upon a host of technologies: grazing incidence optics, proportional counters, semiconductors, calorimeters, etc. In this article we present a brief qualitative overview of these technologies and of the principles behind them, as well as some examples of how they are employed in scientific missions for X-ray observations at energies up to 100 keV.

High-pressure gas-scintillation-proportional counter for the focus of a hard-x-ray telescope

We are developing a high-pressure Gas Scintillation Proportional Counter for the focus of a balloon-borne hard - x-ray telescope. The device has a total active diameter of 50 mm, of which the central 20 mm only is used, and is filled with xenon + 4 percent helium at a total pressure of 106 Pa giving a quantum efficiency of greater than 85 percent up to 60 keV. The detector entrance is sealed with a beryllium window, 3-mm thick, which provides useful transmission down to 6 keV, well below the atmospheric cut- off at balloon float altitudes. Scintillation light exits the detector via a UV transmitting window in its base and is registered by a Hamamatsu position-sensitive crossed-grid- readout photomultiplier tube. Initial testing is underway and preliminary measurements of light yield, energy resolution and spatial resolution will be reported. Simulations show that a spatial resolution of 0.5 mm FWHM or better should be achievable up to 60 keV, and this is well matched to the angular resolution and plate scale of the mirror system. The energy resolution has been measured to be around 5 percent at 22 keV. Full details of the instrument design and its performance will be presented. A first flight is scheduled for the Fall of 99, on a stratospheric balloon to be launched form Fort Sumner, New Mexico.

X-ray emission from accretion disks in active galactic nuclei

We constructed a grid of relativistic models for standard high-relative-luminosity accretion α-disks around supermassive Kerr black holes (BHs) and computed X-ray spectra for their hot, effectively optically thin inner parts by taking into account general-relativity effects. They are known to be heated to high (∼106–109 K) temperatures and to cool down through the Comptonization of intrinsic thermal radiation. Their spectra are power laws with an exponential cutoff at high energies; i.e., they have the same shape as those observed in active galactic nuclei (AGNs). Fitting the observed X-ray spectra of AGNs with computed spectra allowed us to estimate the fundamental parameters of BHs (their mass and Kerr parameter) and accretion disks (luminosity and inclination to the line of sight) in 28 AGNs. We show that the Kerr parameter for BHs in AGNs is close to unity and that the disk inclination correlates with the Seyfert type of AGN, in accordance with the unification model of activity. The estimated BH masses Mx are compared with the masses Mrev determined by the reverberation mapping technique. For AGNs with luminosities close to the Eddington limit, these masses agree and the model under consideration may be valid for them. For low-relative-luminosity AGNs, the differences in masses increase with decreasing relative luminosity and their X-ray emission cannot be explained by this model.

Near-simultaneous Spectroscopic and Broadband Polarimetric Observations ofBe Stars

Near simultaneous optical spectroscopic (on four nights) and broadband linear continuum (B, V, R,and I bands) polarimetric (on seven nights) observations of 29 Be stars were carried out during 1993November–December. The program Be stars displayed wavelength dependence of intrinsic polarizations with no frequency dependence of polarimetric position angles. Some of the Be stars displayed long-term polarization variability. The Be and Be-shell stars could not be distinguished from one another solely on the basis of their polarization values. Full widths at half-maximum of the Hα profiles and the intrinsic linear continuum polarizations are closely correlated with the projected rotational velocities of the program stars. Photospheric-absorption–corrected equivalent widths of Hα profiles [W(α)] and the radii of Hα-emitting or -absorbing envelopes (Re or Ra) are nonlinearly correlated with the intrinsic continuum polarizations of these stars. However, W(α) and Re are linearly correlated. With large uncertainties, there is a trend of spectral dependence of polarization. Detailed discussion of these results is presented in this paper.

Preliminary results from small-pixel CdZnTe arrays

We have evaluated 2 small-pixel Cadmium-Zinc-Telluride arrays with thickness of 1 and 2 mm that were fabricated for MSFC by Metorex and Baltic Science Institute. Each array was optimized for temperature and collection bias and was then exposed to Cadmium109, Am241 and Fe55 laboratory isotopes to measure the energy resolution for each pixel. The arrays were then scanned with a finely-collimated x-ray beam, of width 100 micron, to examine pixel to pixel and inter-pixel charge collection efficiency. Preliminary results from the array test are presented.

HERO program: high-energy replicated optics for a hard-x-ray balloon payload

We are developing high-energy replicated optics for a balloon-borne hard-x-ray telescope. When completed, the telescope will have around 130 cm2 of effective collecting area at 60 keV, and an angular resolution of <EQ 30 arc seconds, half power diameter. With an array of gas scintillation proportional counters in the focal plane the payload will provide unprecedented sensitivity for pointed observations in the hard-x-ray band. We present an overview of the HERO program, together with test data from the first mirror shell. The overall sensitivity of the full payload is given for planned long- and ultra-long-duration balloon flights.

Detecting X-rays with an optical imaging chamber

The light emitted by electron avalanches in a parallel plate chamber can be used to image the tracks of photoelectrons liberated by the interaction of an incident x-ray with the gas filling the chamber. The different morphologies of photoelectron tracks and minimum ionizing tracks can be used for charged particle rejection. The initial direction (before scattering) of the liberated photoelectron also contains information about the polarization of the incident radiation. We have built a small test chamber with which we have imaged photoelectron tracks using an intensified CCD camera. Our results show that optical imaging could be used in a hard x-ray imaging polarimeter useful for astronomy.

Hybrid gas detector/phoswich for hard x-ray astronomy

A hybrid detector, which combines an optical avalanche chamber with a phoswich, is currently under development. The optical avalanche chamber--a proportional counter designed to give large quantities of light photons during charge multiplication, mounts on the front of the scintillator and gives response at low energies, while the solid scintillator takes over at energies where the gas becomes transparent (> 90 keV). Both sections of the hybrid will be read out by a common set of photomultipliers under the phoswich. The addition of the gas section to the phoswich improves the energy resolution of the instrument by a factor of 2.5 at 25 keV and the spatial resolution by a factor of 10 at the same energy. The net result is an instrument with broad band coverage and high sensitivity which will be used for coded aperture imaging on long duration balloon flights.

A high energy focal-plane gas scintillation proportional counter

We are developing high-pressure gas scintillation proportional counters for the focus of balloon-borne hard-X-ray telescopes as part of our HERO (High-Energy Replicated Optics) program. Each detector has an active diameter of 5 cm, and is filled with a xenon +4% helium mix at a total pressure of 10 ATM. Imaging is via a position-sensitive photomultiplier tube located immediately below a UV transmitting exit window. Two units are currently under construction and evaluation for a night in the spring of 2000. We present details of their design and construction together with data from preliminary X-ray testing.