ejaavso137

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Maurizio Falanga
International Space Science Institute (ISSI), Hallerstrasse 6, CH-3012 Bern, Switzerland

Enrico Bozzo
Roland Walter
ISDC, Geneva Observatory, University of Geneva, Chemin d'Ecogia 16, CH-1290 Versoix, Switzerland

Gordon E. Sarty
University of Saskatchewan, Departments of Psychology and Physics and Engineering Physics, 9 Campus Drive, Saskatoon, Saskatchewan S7N 5A5, Canada

Luigi Stella
INAF-Obsservatorio Astronomicao di Roma, via Frascati 33, 00040 Monteporzio Catone (Rome), Italy

Received September 2, 2010; revised October  30, 2010; accepted November 15, 2010

Abstract

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.