This note documents the Epoch 4 background model files. Both the faint (L7_240) and bright (Sky_VLE) source background model files are available,

The names of the model files are

faint sources:
pca_bkgd_faintl7_e4v19991214.mdl
pca_bkgd_faint240_e4v19991214.mdl

bright sources:
pca_bkgd_skyvle_e4v20000131.mdl
pca_bkgd_allskyactiv_e4v20000131.mdl

Models for PCUs 0 and 2 are made from Epoch 4 background data. For the other PCUs, which are occasionally switched off, the Epoch 4 model is based on the equivalent Epoch 3 model. Two adjustments are made: (a) the Epoch 3 PHA channel boundaries are adjusted to match the Epoch 4 energy scale, and (b) the Epoch 3 L7/VLE count rate is adjusted to match the expected Epoch 4 L7/VLE count rate. Estimates of the systematic error are given for both the faint source and the bright source models.