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

TL;DR

This study investigates the 24-day modulation in GX 13+1 using near-infrared and X-ray data, suggesting the modulation may be due to variable structures rather than a strict orbital period.

Contribution

It provides multi-wavelength observational evidence for the 24-day modulation and discusses its possible non-orbital origin, challenging previous assumptions about the system's orbital period.

Findings

Modulation near 24 days observed in K-band and ASM X-ray data.

The 24-day modulation is not strictly periodic in ASM data.

No clear detection of the 24-day period in Swift BAT data.

Abstract

A 24 day period for the low-mass X-ray binary GX 13+1 was previously proposed on the basis of 7 years of RXTE ASM observations (Corbet 2003) and it was suggested that this was the orbital period of the system. This would make it the one of the longest known orbital periods for a Galactic low-mass X-ray binary 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 68 nights over a 10 month interval; (ii) Continued monitoring with the RXTE ASM, analyzed using a semi-weighted power spectrum instead of the data filtering technique previously used; and (iii) Swift BAT hard X-ray observations. Modulation near 24 days is seen in both the K-band and additional statistically independent ASM X-ray observations. However, the modulation in the ASM is not strictly periodic. The periodicity is also not detected in the Swift BAT observations, but modulation at the same relative level as seen with the ASM cannot be ruled out. If the 24 day period is the orbital period of system, this implies that the X-ray modulation is caused by structure that is not fixed in location. A possible mechanism for the X-ray modulation is the dipping behavior recently reported from XMM-Newton observations.