COMMISSION 27 OF THE I. A. U. 
      INFORMATION BULLETIN ON VARIABLE STARS
                   NUMBER 380

                            Konkoly Observatory
                            Budapest
                            1969 September 14


         ON THE PERIOD CHANGES OF PULSARS

      For the pulsar NP 0527 it has been suggested that
it was ejected from the Crab nebula in the supernova 
explosion in A.D. 1054, since its 1.2deg angular separation from
the Crab pulsar NP 0532 is small in comparison with the
average density of pulsars, and the dispersion measures of
these two pulsars differ by only 10% (E. C. Reifenstein,
III, W. D. Brundage, D. H. Staelin, Phys, Rev. Let. 22, 311,
1969). This hypothesis implies that the tangential velocity
of NP 0527 is about 0.15c = 45000 km/sec, corresponding to
an annual proper motion of 4.8". In a blink search for a high
velocity star around the position of NP 0527 A. J. Perry
(IBVS 347) found no such object down to about 18.5 magn,
      But the high velocity of NP 0527 can be tested in
another way. Even, when an object has a constant space
velocity, its radial velocity is changing at a yearly rate
of v^2t/r, where v denotes the tangential velocity of the
object in km/sec, t the number of seconds in a year, and r
the distance of the object in km. This change in radial
velocity gives rise to an apparent period change of
DeltaP = v^2tP/rc per year. For an object with v = 0.15c at the
distance of the Crab nebula (2000 ps) and having the period
of NP 0527 (3.745s), the period change would be 12700 nsec/
year. This is a figure 2-3 orders of magnitude higher than
those obtained for the period changes of other pulsars
(s. Table, below), with the exception of NP 0532 and
PSR 0833-45 for which the observed period changes amount
roughly to the same value. Therefore, if NP 0527 was ejected
from the Crab nebula in the supernova explosion in A. D. 1054,
it should show a comparatively rapid period increase.
      The above mentioned effect always works in the same
direction: it causes an increase of the period. Therefore,
the observation that pulsar periods are (with only one
anomalous temporary exception of PSR 0833-45, s. IAU Circ
2140) continuously increasing, does not necessarily indicate
in all cases a real physical change, so much the more as
there are some indications that pulsars were thrown out at
high speeds in supernova explosions (G. Burbidge, F. Hoyle,
Nature 221, 847). I have calculated the tangential velocities
and the corresponding annual proper motions which are 
necessary to give the observed period changes for B pulsars
as consequences of the changes in radial velocity. In the
Table P is the observed period, DeltaP the observed period
change, r the distance in parsecs estimated from the
dispersion measure (Nature, 222, June 7. 1969) and v the
calculated tangential velocity. The calculated velocities
are one order of magnitude lower than the value attributed
to NP 0527.

                                            Annual
Pulsar    P        P         r      v       proper
          sec    nsec.y^-1   ps   km sec^-1 motion
CP 0328   0,715     60      430    3500      1,5"
CP 0834   1,274    124      390    3300      1,8
CP 0950   0,253     37       60    1600      5,6
CP 1133   1,188    109      130    1900      3,0
HP 1506   0,740    167     >600   >6300       -
CP 1919   1,337     26      230    1150      1,0


      Though the period changes of NP  0532 and PSR 0833-45 
might be intrinsic (if not caused by a variation of 
integrated electron content along the line of eight, particularly
in the vicinity of the source), it is premature to
draw inferences for the evolution of pulsars from the observed
period changes.




                          L. DETRE
                      Konkoly Observatory
                          Budapest