Anomalous temporal behaviour of Her X-1

Abstract

DURING January 1975 the Leicester Sky Survey experiment aboard Ariel V observed anomalous X-ray emission the during off state of Her X-1. Figure 1a shows the positive observations preceded by an extended period when no radiation was detected, a 3σ limit being shown by horizontal bars. The counting rates are averages over an entire satellite orbit and arise from photons in the range 2.4 to 19.8 keV. The counting rate scale may be converted to Uhuru units for a typical Her X-1 spectrum by multiplying by 1.27. Figure 1b shows that about half of the following on state was observed but not the actual turn on. If the turn on had occurred at phase 0.2 in the first observed cycle, the midpoint of the anomalous emission would have preceded the turn on by 16.5 d. A turn on extrapolated from Copernicus data (A. C. Fabian, private communication), taken two 35-d cycles later, reduces this to 14 d.