  In October 1997, radio pulses from the Crab Pulsar underwent abnormal delay.
This was reported by two radio observatories, both of which explained this
frequency dependent and time varying delay as being due to refractive effects
of ionized shells in the Crab Nebula. Both groups also noted that, curiously
and confusingly coincident with the frequency dependent delay, the Crab Pulsar
also underwent an unusual slowing down, which they believed to be unrelated to
the Crab Nebula and instead intrinsic to the Crab Pulsar, resulting in an
additional delay that was frequency independent. However, it now appears that
one of the groups attributes the frequency independent delay also to refractive
effects. This work aims to verify whether at least a part of the frequency
independent delay is indeed due to intrinsic slowing down of the Crab Pulsar.
Timing analysis of the Crab Pulsar's October 1997 event has been done in
X-rays, which are not delayed by the refractive and diffractive effects that
affect radio waves; at X-rays only the intrinsic slowing down should contribute
to any observed delay. Data mainly from the PCA instrument aboard the RXTE
satellite have been used, along with a small amount of data from the PDS
instrument aboard the BeppoSAX satellite. Analysis of the X-ray data, using the
very accurate reference timing mode derived at radio frequencies, strongly
supports the intrinsic slowing down hypothesis. Analysis using the reference
timing model derived self-consistently from the limited X-ray data, which is
less accurate, is not completely unambiguous regarding the above two
hypotheses, but provides reasonable support for the intrinsic slowing down
hypothesis. A significant fraction of the frequency independent delay during
the October 1997 event is indeed due to intrinsic slowing down of the Crab
Pulsar.
