COMMISSION 27 OF THE I. A. U.
     INFORMATION BULLETIN ON VARIABLE STARS
                  NUMBER 733

                               Konkoly Observatory
                               Budapest
                               1972 October 30



       OPTICAL VARIABILITY OF Her X-1
                 (HZ Her)




   The star HZ Her which had been identified with the
X-ray source Her X-1 in IAU Circ. 2415 was estimated on
197 Sonneberg plates of 1934 to 1940 and 1959 to 1972. The
optical variability as it is shown by our plates (exposure
times generally between 0.5 hour and 3 hours) is rather 
curious:
   1.) The "occultation" period given first by Tananbaum
et al. (ApJ 174, L 143) for the X-ray intensity cycle can be
verified with evidence for the following intervals:

       JD 242 7666 to 242 8345 and

       JD 243 6607 to 244 1443 (last plate).

The mean light curve assembled by the elements
                       
       t_Min. = 244 1397.584 + 1.70017d.E

is slightly unsymmetric, thus resembling a delta Cephei curve.
Amplitude ~ 1.5 mag.(pg); "occultation centre" of Tananbaum
et al. at phase 0.00.

   2.) In the intervals

       JD 242 7544 to 242 7657 (begin of our series)
    and JD 242 8627 to 242 9789

the star was faint and varied by 0.5 mag. at best. The 1.7-
day period is working also in these intervals and a second
minimum near phase 0.5 is present in the mean light curve.
Note that the short transition time between 1.) and 2.)
centred at JD 242 7661 is an upper limit.

   3.) The 35.7-day cycle of Tananbaum et al. cannot be
found in our material. Especially, between 1972 March 13
and March 20 when the 1.7-day cycle of the X-ray source had
to be "silent" according to the authors quoted, the 1.7-day
period is verified by 22 good plates, with full amplitude.
   Items 1.) and 2.) can simply be explained by physical
variability of the X-emitting component of the pair: during
the intervals 2.) the surface brightnesses of the two 
components were of comparable value whereas in case 1.) the X-
star is enhanced. One wonders whether X-rays perhaps are
sent out in state 1.) only. - Item 3.) supports the fourth
possibility suggested by Tananbaum et al. (l.c., L 149),that
is the precession of the X-pulsar beam with the period of
35.7 days. Clearly, this mechanism does neither strongly 
effect the revolution of the two components nor the other 
optical properties whereas each of Tananbaum's three further
possibilities should have consequences with regard to the
optical light curve. Note that the decrease of the amplitude 
of the 1.7-day cycle in state 2.) obviously has nothing 
to do with the 35.7-day cycle.

For further details see next number of MVS.




Akademie der Wissenschaften der DDR
Zentralinstitut fur Astrophysik
Sternwarte Sonneberg



                        W. WENZEL and H. GESSNER