A COMPARISON BETWEEN THE HERCULES-X-1 PRE-ECLIPSE AND ANOMALOUS DIPS

We report the properties of three pre-eclipse and two anomalous dips f rom Hercules X-l using data from the EXOSAT Observatory. The anomalous dips are shorter, lasting similar to 1 hr compared to lower limits of similar to 3 hr for the pre-eclipse dips, and shallower, never result ing in more than 91 +/- 4% of the 2 - 17 keV flux being obscured, comp ared to the pre-eclipse dips where up to 97 +/- 1% of the flux can be obscured. Fits to the 2 - 17 keV anomalous and pre-eclipse dip spectra require the presence of at least two spectral components, confirming the result of Choi et al. (1994). One component results from absorptio n by cool material with an abundance of 0.42 +/- 0.13 that of cosmic m aterial while the second component shows no evidence for extra absorpt ion and is mos probably X-rays scattered in an extended accretion disk corona (ADC). The average intensity of the ADC component is 4.0 +/- 2 .0% of that outside the pre-eclipse and anomalous dips. During the pre -eclipse dips absorption as high as 3.3 10(24) atoms cm(-2) is observe d, a factor 2.5 higher than seen in the anomalous dips. At times, duri ng the pre-eclipse dips only, the absorbed component is not detected a nd an energy independent reduction is observed. This is probably due t o extremely high (> 7 10(24) atoms cm(-2)) absorption removing the com ponent from the instrument bandpass. This may also be an explanation f or the energy independent reduction seem during an entire pre-eclipse dip by Ushimaru et al. (1989).