SIMBAD references

We report the properties of three pre-eclipse and two anomalous dips from Hercules X-1 using data from the EXOSAT Observatory. The anomalous dips are shorter, lasting ∼1hr compared to lower limits of ∼3hr for the pre-eclipse dips, and shallower, never resulting in more than 91±4% of the 2-17keV flux being obscured, compared to the pre-eclipse dips where up to 97±1% of the flux can be obscured. Fits to the 2-17keV anomalous and pre-eclipse dip spectra require the presence of at least two spectral components, confirming the result of Choi et al. (1994ApJ...422..799C). One component results from absorption by cool material with an abundance of 0.42±0.13 that of cosmic material while the second component shows no evidence for extra absorption and is most 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 3.3x10²⁴atoms/cm² is observed, a factor 2.5 higher than seen in the anomalous dips. At times, during the pre-eclipse dips only, the absorbed component is not detected and an energy independent reduction is observed. This is probably due to extremely high (>7x10²⁴atoms/cm²) absorption removing the component from the instrument bandpass. This may also be an explanation for the energy independent reduction seem during an entire pre-eclipse dip by Ushimaru et al. (1989PASJ...41..441U).