Orbital Modulation and Long-Term Variability in MAXI J0903-531 and XTE J1829-098 from Swift BAT, Fermi GBM, MAXI and RXTE PCA Observations

MAXI J0903-531 (J0903) and XTE J1829-098 (J1829) are transient X-ray pulsars with periods of ~14s and ~7.8 s respectively. J0903 has an optical counterpart that is classified as a Be star, and J1829 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 curves of both sources using a variety of all-sky survey X-ray instruments. For J0903 we use data from the Swift BAT and the Fermi GBM, while for J1829 we use the RXTE PCA, Swift BAT, and MAXI. For J0903, we find that since the source was reported as being in outburst in April 2021, it has regularly shown outbursts on a period of ~59 days detected in both the BAT and the GBM. In addition, the GBM shows that, apart from orbital modulation of the pulse frequency, there has been a rather steady decrease in pulse frequency. The length of the period derived from the outburst recurrence intervals is suggestive of a small difference from an orbital period previously derived from pulse-timing measurements with NICER. For J1829 we find evidence that the BAT and MAXI light curves are modulated on the ~244 day period previously reported from RXTE PCA monitoring observations, with source brightening confined to a relatively narrow phase range. This is unlike J0903 where the orbital modulation occurs over a larger 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. Both sources therefore show periodic outbursts, and intervals during which strong outbursts did not occur. 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. The location of J0903 in the pulse period/orbital period diagram is also typical for a Be star HMXB. However, given the apparently rather long orbital period of J1829, its pulse period is relatively short. We present our analyses of the orbital modulation in both sources, and their long-term variability, and discuss the factors that may cause the similarities and differences between the two sources.