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anomalous x - ray pulsars ( axps ) are an unusual class of astrophysical objects .
there are currently only five confirmed axps : 4u  0142 + 61 , 1e  1048.15937 , 1e  1841045 , rxs  j170849.0400910 , and 1e  2259.1 + 586 . all five are found in the plane of the galaxy ; and two of the five certain members of the class appear to be located at the geometric centers of apparent supernova remnants .
axp characteristics can be summarized as follows ( see israel , mereghetti , & stella 2002 , for a review ) : they exhibit x - ray pulsations in the range @xmath1512  s ; they have pulsed x - ray luminosities in the range @xmath2  erg  s@xmath3 ; they spin down regularly ; their x - ray luminosities are much greater than the rate of loss of rotational kinetic energy inferred from the observed spin - down ; they have spectra that are characterized by thermal emission of @xmath4  kev with evidence for a hard tail in some sources .
soft gamma repeaters also exhibit axp - like pulsations in quiescence ( e.g. kouveliotou et al . 1998 ) ; the connection between axps and sgrs is intriguing but not yet clear .
the results here summarize those reported by gavriil & kaspi ( 2001 ) .
the results presented here were obtained using the proportional counter array ( pca ) on board the rossi x - ray timing explorer ( _ rxte _ ) .
our observations consist primarily of short snapshots taken on a monthly basis .
in addition , we used a handful of archival observations ; the exposures in these observations vary .
phase - coherent timing of axps has shown that the rotational stability of some axps is comparable to those of some radio pulsars .
the rotational stability of 1e  2259.1 + 586 , first reported by kaspi , chakrabarty , & steinberger ( 1999 ) , has now persisted over 4.5  yr , although the inclusion of @xmath5 has recently been necessary .
4u  0142 + 61  has been an extremely stable rotator over 4.4  yr of _ rxte _ monitoring . for rxs
j170849.0400910 , phase coherent timing has been accomplished in the 1.4  yr since the glitch reported by kaspi , lackey , & chakrabarty ( 2000 ) . in these data ,
we find a significant positive @xmath5 .
this indicates a decay of the negative @xmath0 , expected for long - term glitch recovery , as seen in glitching radio pulsars ( e.g. shemar & lyne 1996 ) .
1e  1841045  shows rotational stability but with considerably higher red noise ( see gotthelf et al .
, this volume ) .
1e  1048.15937  has exhibited far noisier behavior , making phase - coherent timing using monthly and weekly observations virtually impossible .
interestingly there is a correlation between timing stability and @xmath0 : the sources with the smallest @xmath0 are the most stable .
arrival time residuals for 1e  2259.1 + 586 , 4u  0142 + 61  and rxs  j170849.0400910  are shown in figure  [ fig : res ] .
axp spectra are generally best fit by a two - component model consisting of a photoelectrically absorbed blackbody with a hard power - law tail ( israel et al .
whether these two components are physically distinct is an open question ( see zel , psaltis , & kaspi 2001 ) . to investigate this
, we compared the pulse profile morphology of the axps in two energy bands . figure  [ fig : energy ] displays the average pulse profiles of rxs  j170849.0400910 , 1e  2259.1 + 586   and 1e  1048.15937  in the energy bands 24  kev and 68  kev . from figure
[ fig : energy ] it is clear that axp pulse profiles exhibit different degrees of energy dependence ; rxs  j170849.0400910  shows high energy dependence , while 1e  1048.15937  shows no energy dependence .
we have also used our _ rxte _ data to monitor the pulsed flux of the axps as a function of time .
rapid ( @xmath6 months ) flux variability would challenge the magnetar model and accreting sources generally show flux variability correlated with @xmath0 .
given the large field - of - view of the pca and the low count rates for the sources relative to the background , total flux measurements are difficult with our _ rxte _ data .
instead , we have determined the pulsed component of the flux , by using the off - pulse emission as a background estimator .
the flux time series for 1e  2259.1 + 586 , 4u  0142 + 61  and rxs  j170849.0400910 are displayed in figure  [ fig : flux ] .
we do not find evidence for any large variability in the pulsed flux for any source , and have set @xmath7 upper limits on variations @xmath12030% ( depending on the source ) .
this is surprising given previous reports of large ( factor of 510 ) total flux variations in 1e  2259.1 + 586  and 4u  0142 + 61  ( baykal & swank 1996 ; oosterbroek et al . 1998 ) .
assuming a constant pulsed fraction , this suggests that more than one of the axps happen to be much more quiescent during the _ rxte _ monitoring than in the past .
iwasawa , koyama , & halpern ( 1992 ) reported a significant change in the pulse morphology of 1e  2259.1 + 586  in 1.214  kev _ ginga _ observations obtained in 1990 , such that the leading pulse had amplitude roughly half that of the trailing pulse .
if correct , this has important implications for the magnetar model , which predicts such pulse morphology changes in the event of a restructuring of the magnetic field , as might occur following a major sgr - like outburst .
motivated by this finding , we searched for pulse profile changes in our _ rxte _ observations of all the axps .
we have not detected any large pulse profile variations .
this justifies our other analysis procedures which assume a fixed profile .
we rule out variations in features having amplitude @xmath820% of the peak amplitude at the @xmath7 level , although the limit depends on source and integration time .
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we report on long - term monitoring of anomalous x - ray pulsars ( axps ) using the _ rossi x - ray timing explorer_. using phase - coherent timing , we find a wide variety of behaviors among the sources , ranging from high stability ( in 1e  2259.1 + 586  and 4u  0142 + 61 ) , to instabilities so severe that phase - coherent timing is not possible ( in 1e  1048.15937 ) . 
 we note a correlation in which timing stability in axps decreases with increasing @xmath0 . 
 the timing stability of soft gamma repeaters in quiescence is consistent with this trend , which is similar to one seen in radio pulsars . 
 we consider high signal - to - noise ratio average pulse profiles as a function of energy for each axp , and find a variety of behaviors . 
 we find no large variability in pulse morphology nor in pulsed flux as a function of time .    
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