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Figure 1: ####### ####### 1.5 ##.
Her X­1 is one the first X­ray bi­
nary pulsar discovered by the eminent
UHURU satellite in 1972 (Tananbaum
et al. 1972, Giacconi et al. 1973) and
since then remains one of the most stud­
ied accretion­driven X­ray stellar bina­
ries. The pulsar is an accretion­powered
magnetized rotating (P = 1:24s ) neu­
tron entering an eclipsing X­ray binary
with a low­mass (¸ 2M fi ) optical com­
panion in a practically circular orbit
with a period of 1.7 d. The binary
orbit inclination is i = 85 \Gamma 88 o , the
total duration of the X­ray eclipse is
around 20,000 s. The optical counter­
part, HZ Her, first suggested by Liller
(1972), provides a classical example of
the reflection effect when the strong
orbital modulation in optics is due to
a powerful illumination of the part of
normal star atmosphere facing X­ray
source (Bahcall & Bahcall 1972; Che­
repashchuk et al. 1972). The ampli­
tude of the reflection effect strongly in­
creases in ultraviolet and according to
many optical UBV­photometrical data
is \Deltam V = 1:45; \Deltam B = 1:6; \Deltam U =
2:55.
Already very first UHURU observa­
tions of Her X­1 revealed the presence
of a long­term 34.85­d cycle. Its prop­
erties have been so interesting that prac­
tically all specialized X­ray satellites
studied it (among which Copernicus ,
Ariel­5, Ariel­6 (Davison & Fabian 1974,
1977, Ricketts et al. 1982), HEAO­
Figure 2: ####### # ###### #######.
1 (Gorecki et al. 1982, Soong et al.
1987), Hakucho and TENMA (Nagase
et al. 1984, Ohashi et al. 1984), EX­
OSAT ( ¨
Ogelman & Tr¨umper 1988), Gin­
ga (Deeter et al. 1991), Astron (Shef­
fer et al. 1992), BATSE (Wilson et al.
1994), and RXTE (see RXTE data are
shown in Fig. 1).
The gross shape of the X­ray light
curve consists of a main­on X­ray state
with a mean duration of 7 orbital peri­
ods surrounded by two off­states each
4 orbital cycles in duration, and of a
secondary short­on state of smaller in­
tensity with a typical duration of ¸ 5:5
orbital cycles. The form of the X­ray
light curve is asymmetric: in the main­
on state the X­ray intensity rapidly in­
creases during 1.5­2 orbital cycles, stays
at maximum over 2.5 cycles, and then
decreases to a minimum in the succes­
sive 3 orbital revolutions.
1