Orbital Modulation and Long-Term Variability in XTE J1829-098 from Swift BAT, MAXI, and RXTE PCA Observations

XTE J1829-098 is a transient X-ray pulsar with a period of ~7.8 s. It has a probable IR counterpart that may be a highly reddened OB star, although a red giant could not be excluded. We have investigated the long-term X-ray light curve of J1829 using the RXTE PCA, Swift BAT, and MAXI. We find that the BAT and MAXI light curves are clearly modulated on the ~244 day period previously reported from PCA monitoring observations, with outbursts confined to a relatively narrow phase range. We examined the PCA light curve of J1829 using a longer duration dataset than previously reported on, and between approximately Dec 2008 to the end of observations in Nov 2011, strong outbursts did not occur. The BAT and MAXI light curves also show that the source entered an inactive state at this time. The source returned to an active state again around the end of 2014. Such behavior is typical of Be X-ray binary systems with the absence of outbursts likely related to the dissipation of the Be star’s decretion disk. Be star X-ray binaries show an overall correlation between pulse period and orbital period, although with some scatter in the relationship. The pulse period of J1829 is rather shorter than predicted by the correlation, and we speculate that this might cause the short phase duration of the outbursts due to magneto-centrifugal inhibition of accretion.