COMMISSION 27 OF THE I. A. U.
        INFORMATION BULLETIN ON VARIABLE STARS
                    NUMBER 720

                                   Konkoly Observatory
                                   Budapest
                                   1972 September 29


 ON THE NATURE OF THE OPTICAL VARIATIONS OF HZ Her=Her X1



   The variable star HZ Her was recently identified with a
close binary system, containing the X-ray pulsar Her X1 (1-3).
Using 249 old plates (1907-1972) we investigated the optical
periodical variability(4)(see Fig. 1). The moments of optical
minima coincide with the epochs of X-ray eclipses derived in
(5). The period p=1.70017d is stable during 65 years. The light
variations are symmetrical to phase 180d; the light intensity
monophonically increases from 0d to 180d and decreases from 180d
to 360d. The hemisphere of the optical component turned to the
                      X-ray source is three times brighter
                      than the opposite one. Therefore
                      we suggest that the reflection 
                      effect is the main cause of the light
                      variations. The X-ray radiation,
                      falling on the surface of the visible
   [FIGURE 1]
                      component is absorbed in the
                      photosphere and reradiated in the optical
                      and ultraviolet spectral bands.
                      The importance of X-ray heating of
                      the normal component in X-binary
                      system was pointed out in (6,7). The
spectral class lies in the range B2-A7 V (8-10), the earlier
spectrum is at the brightness maximum. In this picture the 
latest observed spectrum reflects the properties of the visual
star. Its mass must be of the order of 2M_Sun, its distance is
about 1.5 kpc and its z coordinate is about 750 pc. At this
distance the X-ray luminosity of the system will be about 3.10^35-
10^36 erg/sec and exceeds the optical luminosity of the A7V star
(~10^34 erg/sec). This is in rather good accordance with the 
estimation of X-ray energy needed for increasing the brightness of
the optical component due to reflection effect. The dispersion
of points near the maximum of the light curve is real and probably 
correlates with the cycle of the X-ray radiation, which
is observed only 9 days during every 36 day cycle (5).If there
is a real correlation, the maximal optical luminosity 
corresponds to the phase 0.5 of the 36 day cycle, when the X-ray
flux in the direction to the Earth is absent.


Sternberg Astronomical Institute,
            Moscow


                    A.M. CHEREPASHCHUK, YU.N. EFREMOV,
                    N.E. KUROCHKIN, N.I. SHAKURA,
                    R.A. SUNYAEV.





References:

1) W. Liller, 1972, IAU Circ. No. 2415. [BIBCODE 1972IAUC.2415....1L ]
2) J.N. Bachcall, N.A. Bachcall, 1972, IAU Circ. No. 2427. [BIBCODE 1972IAUC.2427....1B ]
3) N.E. Kurochkin, 1972, IAU Circ. No. 2436. [BIBCODE 1972IAUC.2436....1F ]
4) N.E. Kurochkin, 1972, Astron.Circ. No. 717. [BIBCODE 1972ATsir.717....1K ]
5) H. Tananbaum et al., 1972, Ap.J., 174, L 143. [BIBCODE 1972ApJ...174L.143T ]
6) I.S. Shklowsky, 1967, Astron.Zh., 44, 930. [BIBCODE 1967AZh....44..930S ]
7) N.I. Shakura, R.A. Sunyaev, 1972, Astron.Aph. (in press). [BIBCODE 1973A&A....24..337S ]
8) B.W. Bopp, G. Grupsmith, P. Vanden Bout, 1972, IAU Circ.
  No. 2434. [BIBCODE 1972IAUC.2434....1B ]
9) S.D. Crampton, C.L. Morbey, 1972, IAU Circ. No. 2428. [BIBCODE 1972IAUC.2428....1B ]
10) A. Davidsen, J.P. Henry, J. Middleditch, H.E. Smith, 1972,
  preprint.