2. Observations and data analysis
Our RXTE observations of Her X-1 were performed on 1996 July 26. We primarily use data obtained with the PCA, the low energy instrument onboard RXTE. The PCA consists of five nearly identical Xe proportional counters with a total effective area of about (Jahoda et al. 1997). The data extraction has been performed using the RXTE standard data analysis software, ftools 4.0. To avoid contamination of the spectra due to the Earth's X-ray bright limb, only data measured at source elevations more than above the spacecraft horizon were used in the present analysis. Background subtraction was performed with version 1.5 of the background estimator program, using a model where the background is estimated from the rate of Very Large Events in the detector. The diffuse X-ray background and a model for the activation of radioactivity within the detector is added. See Jahoda 1996 for a description of the PCA.
For the spectral analysis, version 2.2.1 of the PCA response matrix was used (Jahoda, 1997, priv. comm.). The spectral modeling of data from an observation of the Crab pulsar made available to us by the RXTE PCA and HEXTE teams with this matrix suggests that the matrix is well understood on the 1% level (see Dove et al. 1998 for a discussion of these data). Using this version of the PCA response matrix, the overall description of the Crab continuum is good, with the remaining deviations mainly being below 3 keV and around the Xe K edge at 35 keV. To avoid these uncertainties we used data from 3 to 18 keV only, which is sufficient for an analysis of the dips because of the power law nature of the Her X-1 X-ray spectrum and due to the dependency of the photoionization cross section (see discussion in Sect. 3.2). To obtain meaningful values and to take into account the remaining uncertainties of the matrix, we added a 1% systematic error to all PCA channels. The spectral analysis was performed with XSPEC, version 10.00p (Arnaud 1996).
© European Southern Observatory (ESO) 1999
Online publication: February 23, 1999