We report the detection of a sudden spin-up of the 11 s anomalous X-ray pulsar 1RXS J170849.0 - 4000910 in regular timing observations made with the Rossi X-ray Timing Explorer . The event , which occurred between MJD 51446 ( 1999 September 25 ) and 51472 ( 1999 October 21 ) , is well characterized by an increase in the rotational frequency of magnitude | \Delta \nu / \nu| = ( 6.2 \pm 0.3 ) \times 10 ^ { -7 } and an increase in the rate of spin down | \Delta \dot { \nu } / \dot { \nu } | = ( 1.38 \pm 0.25 ) \times 10 ^ { -2 } .
These values are very similar to those of glitches observed in the Vela radio pulsar and other young radio pulsars .
The event therefore suggests that the internal structure of this anomalous X-ray pulsar is similar to those of the radio pulsars .
In particular , it implies that the fractional moment of inertia in neutron superfluid that is not corotating with the crust is \geq 1 % .
The detection of a glitch in this anomalous X-ray pulsar constrains models for the origin of glitches in neutron stars .
Most notably , it challenges models that preclude glitches in long-period pulsars , and , under the magnetar hypothesis , suggests that large glitches can occur in hot neutron stars .
The glitch is consistent with the predictions of the magnetar model for anomalous X-ray pulsars , but accretion-powered scenarios can not be excluded using our observations alone .