COMMISSION 27 OF THE I. A. U.
           INFORMATION BULLETIN ON VARIABLE STARS
                        Number 3500

                                           Konkoly Observatory
                                           Budapest
                                           7 August 1990

                                           HU ISSN 0374 - 0676


     Optical monitoring of the 60 ms X-ray pulsar candidate
                       Wackerling 2134

  Observations with the Einstein Observatory by Caraveo et al. (1989) have led to
the identification of the relatively bright variable X-ray point source IE1024.0-5732
with the emission line early-type star Wack 2134 (Wackerling,1969). Their timing
analysis has shown that the X-ray source has a periodicity P ~ 60 ms, and optical
spectra indicates the star in Wack 2134 is probably of O5 type. Hence Caraveo et al.
infer indirectly that the Wack 2134 system is a binary, with the hot massive star
losing matter to a spinning neutron star. The observed period then would make this
system the fastest X-ray pulsar to date.
  With this short period in mind, we have examined Wack 2134 for the possible
presence of optical pulsations. The system has been observed on three separate
occasions in white light, using a photometer sampling at a rate of 1 kHz on the
University of Tasmania's 1 m telescope. In each case, the data train was examined
using several 2048K point Fast Fourier Transforms. Both single FFT's of the data
reduced to 3 ms time resolution, and the average FFT of several overlapped data
segments at 1ms were generated. The low frequency end of the single FFT spectrum
from the best data run, 105 min of data obtained on 19 December 1989, is shown in
Figure 1. The plot in Figure 1 was generated by grouping the power spectrum into
blocks of 128 points, and determining the maximum and mean power in each block.
Here the maxima are shown in the upper plot while the relatively smoother means are
shown underneath. The minima from each of the 128 point blocks lie along the
frequency axis. As can be seen in Figure 1, no significant spikes are evident..
  To place a 90 percent confidence level upper limit on the amplitude of any
modulation, the method described by Lewin et al. (1988) has been used. In the
frequency range of 10 Hz to 20 Hz, the 19 December 1989 data set an upper limit,
for any pulsed optical component, of ~ 7.1 mag fainter than the white light flux from
Wack 2134. Since the apparent visual magnitude of Wack 2134 is ~ 12.6, this
implies an upper limit around magnitude 19.7 for any optical pulsations.


                                [FIGURE 1]

 Figure 1: The average FFT power spectrum of Wack 2134 obtained on
           19 December 1989, as discussed in the text.



              S. W. Dieters, K. M. Hill and R. D. Watson
              University of Tasmania
              GPO Box 252C
              Hobart, Tasmania
              Australia, 7001.


References:
Caraveo, P.A., Bignami, G.F. and Goldwurm, A., 1989. Astrophys. J., 338, 338 [BIBCODE 1989ApJ...338..338C ]
Lewin, W.H.G., van Paradijs, J. and van der Klis, M., 1988. Space Sci. Rev.
  46, 273, [BIBCODE 1988SSRv...46..273L ]
Wackerling, L.R., 1969. Mem. R.A.S., 73, 153. [BIBCODE 1970MmRAS..73..153W ]