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A 24 day period for the low-mass X-ray binary GX 13+1 was previously proposedon the basis of 7 years of RXTE ASM observations (Corbet 2003) and it wassuggested that this was the orbital period of the system. This would make itthe one of the longest known orbital periods for a Galactic low-mass X-raybinary powered by Roche lobe overflow. We present here the results of: (i)K-band photometry obtained with the SMARTS Consortium CTIO 1.3 telescope on 68nights over a 10 month interval; (ii) Continued monitoring with the RXTE ASM,analyzed using a semi-weighted power spectrum instead of the data filteringtechnique previously used; and (iii) Swift BAT hard X-ray observations.Modulation near 24 days is seen in both the K-band and additional statisticallyindependent ASM X-ray observations. However, the modulation in the ASM is notstrictly periodic. The periodicity is also not detected in the Swift BATobservations, but modulation at the same relative level as seen with the ASMcannot be ruled out. If the 24 day period is the orbital period of system, thisimplies that the X-ray modulation is caused by structure that is not fixed inlocation. A possible mechanism for the X-ray modulation is the dipping behaviorrecently reported from XMM-Newton observations.

Properties of the 24 Day Modulation in GX 13+1 from Near-Infrared and X-ray Observations