Searching for Orbital Periods of Supergiant Fast X-ray Transients
Gordon E. Sarty
Received September 2, 2010; revised October 30, 2010; accepted November 15, 2010
Building on the currently active program of observing High Mass X-ray Binaries (HMXBs), we describe here a challenging extension of the project for experienced AAVSO observers who want to push their observing to the limit. A new subclass of HMXBs, the Supergiant Fast X-ray Transients (SFXTs), have been recently discovered in INTEGRAL data and are still poorly understood. In these systems, a neutron star is accreting matter from the strong wind of its supergiant companion (typically of spectral type O–B), resulting in a conspicuous emission of x-ray radiation. At odds with the other previously known HMXBs, the SFXT sources display an extreme variability in x-ray output. The origin of this variability is presently unknown, and has led in the past three years to the suggestion that the SFXT sources might host neutron stars with ultra-strong magnetic fields or supergiant stars with peculiar ultra-dense clumpy winds. Our new program aims at determining the orbital parameters of all the known systems in this class (fifteen objects) in order to distinguish between the different theoretical models proposed to interpret the behavior of the SFXTs.