GX 13+1

R. Corbet, Goddard Space Flight Center (GSFC) and Universities Space Research Association, reports on behalf of the RXTE ASM team at GSFC and the Massachusetts Institute of Technology: "Analysis of the RXTE ASM lightcurve of the low-mass x-ray binary GX 13+1 (= 4U 1811-17) obtained between 1996 Feb. 23 and Nov. 14 suggests the presence of a modulation with a period of 24.7 +/- 1 days. The folded lightcurve on this period shows a mean peak-to-peak modulation of approximately 15 percent. This periodicity is apparently confirmed by the Ariel 5 ASM lightcurve of GX 13+1, as a Fourier Transform of this shows that the largest peak, for periods < 125 days, occurs at 25.2 +/- 0.15 days, consistent with the RXTE result. In the RXTE data, the modulation is most pronounced at low energies (1.3-4.8 keV) and is reduced at higher energies (4.8-12.2 keV). In addition, there is an anticorrelation between hardness ratio and intensity in the RXTE data, which may suggest a similarity with Cyg X-2 (Wijnands et al. 1996, Ap.J. 473, L45). If this periodicity is a persistent property of this source, it may also be present in the variability of the infrared counterpart."