ROSAT Status Report #22
March 31, 1992
ROSAT HRI Spatial Response
The MPE/GSFC/SAO ROSAT calibration team has reached the conclusion that the HRI point-spread function (PSF) has an unexpected component that redistributes roughly 10 percent of point-source photons beyond a central core (radius ~ 10") and into a region that extends out to about 5 arcmin, an effect which is relatively independent of energy. In addition, the X-ray mirror is expected to scatter photons beyond the 10" core, but this energy-dependent effect amounts to less than 6% at energies below 1 keV. The functional form given below provides a reasonable description of the combined mirror/HRI PSF out to about 100" and has been used in successful deconvolution-modeling applications with flight data:
PSF = A1*EXP(-0.5*(R/S1)**2) + A2*EXP(-0.5*(R/S2)**2) + A3*EXP(-R/S3),
where the radial distance R is in arc seconds, and
A1 = 0.9638, S1 = 2.1858",
A2 = 0.1798, S2 = 4.0419",
A3 = 0.001168, S3 = 31.69"
This PSF was derived by combining observations of HZ43, AR Lac, and
LMC X-1 and, in this sense, represents an energy-averaged response
function. Analysis of these same flight data indicates the presence
of an azimuthal asymmetry in the core (R < 12") of the PSF;
investigation into the causes and extent of this effect are
continuing.
Given the existence of this extended PSF, some proposed scientific objectives (e.g., studies of dust halos, jets of AGN, etc.) will perhaps require longer exposure times than originally planned. It is important that ROSAT observers not be misled by this unexpected behavior of the HRI. Use of the above function will help ensure proper interpretation of possible source extent seen with the HRI.
This finding that broader-than-expected components are needed (in addition to the expected Gaussian-like core) to fit the HRI response was developed through analysis of flight data from Capella, Sirius, HZ43, Cyg X-2, LMC X-1 and LMC X-3, as well as ground-test data from the HRI Telescope Test and Flat-Field Test in the Panter X-ray test facility in Neuried, Germany. The resulting halo appears in all data taken with the flight model HRI, both ground and flight measurements.
A working hypothesis, developed to interpret these findings, attributes the presence or absence of the halo, in different HRI detectors, to the presence of the so-called "electrostatic shield", which was added to the flight model prior to final delivery. The halo is absent in data taken during the Neuried Telescope Test with an HRI detector that was configured without an "electrostatic shield". The shield was introduced following the Telescope Test as a remedy for background problems that were observed during that test. It is 0.3 micron polypropylene, coated on the side not facing the microchannel plate (MCP) with 300 Angstroms of aluminum and is positioned about 1 mm from the front surface of the MCP, held at the same electrical potential as the front MCP surface.
The mechanism suggested to explain the halo involves X rays that interact in the CsI-coated "webbing" area between the MCP pores (which comprise about 65% of the frontal area of the forward MCP). Some of the photoelectrons produced in this process are able to travel a considerable distance from the interaction site in the field-free region between the MCP and the electrostatic shield. When these electrons are finally "captured" in an MCP pore and initiate an electron avalanche, the event position no longer corresponds to the x-ray position.
Laboratory tests of this hypothesis are being set up, and until the effect is studied further, the halo can not be considered understood. Nevertheless, its existence is not in question, and ROSAT observations must be interpreted accordingly (i.e., with the above PSF function or subsequent, improved representations).
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