A ten-year-long peak of the X-ray flux from the burster 4U 1724-307 in the globular cluster Terzan 2: Evolution of the donor star or the influence of a third star?

We analyze observations of the burster 4U 1724-307 (1E 1724-3045) in the globular cluster Terzan 2 from 1971 until 2001. Uhuru, OSO-8, Einstein, and EXOSAT observations are used. In addition, we analyze data from the TTM telescope onboard the Mir/Kvant observatory and from the ASM and PCA instruments onboard the RXTE observatory. Based on PCA/RXTE scanning observations, we have mapped the sky in the Galactic-center region. The derived light curve of the burster 4U 1724-307 shows the source to be variable on a time scale of 30 years: over the first 20 years of observations (1970–1990), the source flux was approximately constant, while in the 1990s, it rose over ∼5 years and declined over approximately the same period, approaching its original value. We discuss several scenarios to explain the behavior of the light curve, including the evolution of the mass outflow rate from the donor-star surface, the episodic appearance of a second source in the globular cluster, and gravitational microlensing. As one of the scenarios, we also consider the possibility that the passage of a third star affects the accretion rate in the binary 4U 1724-307. We show that if this event is responsible for the 10-year-long rise in the luminosity of the burster 4U 1724-307, then the size and period of this binary can be estimated.