Bright Earth data has been obtained throughout the lifetime of the ROSAT mission. Bright Earth observations spanning approximately two week segments were coadded to produce N ( ) maps. Each map is binned to produce 32 arcsec pixels. In each spatial bin, the PHA spectrum was extracted and fitted with a Pearson function with an exponential tail. The Bright Earth Gain (BE gain) is assigned the value of the mean from fitted pulse height distibution. This value typically ranges from pulse height channel 3 to 8. The end result of this Brigh Earth ``build'' is therefore a data cube, with BE gain as a function of time during the mission and position on the detector. The data cube is stored as a single FITS file. This file is updated periodically and is included in this distribution. The FITS file is used by the program 'xgain' to determine the gain state histogram.
To acquire a gain state from the bright earth build, an interpolation occurs according to the two submaps with time signatures immediately before and after the observation time. A gain value is derived from the functional form for the spatial variation of the gain for each of the submaps and then averaged to find the gain at the time of the observation. Discontinuities in the gain curve exist for changes in the high voltage level (HV) for the instrument. These gain shifts are evident in Figure 1. Observation times for sources near these HV changes will be extrapolated to produce an accurate gain level since the gain near these transitions will reflect an average of the two states. Table 2 lists the times of the HV changes during the ROSAT lifetime.
Table 2: HRI Gain History