The energy-dependent temporal variation of the ROSAT PSPC gain

¹ Max-Plank-Inst. für extraterrestrische Physik, Germany e-mail: alm@rosat.mpe-garching.mpg.de
² Astrophysikalisches Institut Potsdam, Germany e-mail: ghasinger@aip.de
³ USRA/Goddard Space Flight Center e-mail: snowden@lheavx.gsfc.nasa.gov

Send offprint request to: M.A. Prieto

Received: 29 August 1995
Accepted: 24 February 1996

Abstract

The existence of systematic spectral-fit residuals in ROSAT PSPC spectra, and their dependence on time, is by now a well established fact. This paper describes how those residuals may be related to second order variations of the gain of the PSPC as a function of energy and time. As a result, the energy scale used for the interpretation of PSPC spectra can be incorrect producing significant systematic effects, in particular at energies where the effective area of the instrument changes rapidly. A monotonic gain decay of per year is measured at 1 keV. A quantitative description of the PSPC gain-time variation as a function of energy is provided. The functional dependence of this variation is found to be well represented by a second order polynomial.