Astronomy and Astrophysics, volume 349, 819-824 (1999/9-3)
An anomalous low-state from the globular cluster X-ray source X1732-304 (Terzan 1).
GUAINAZZI M., PARMAR A.N. and OOSTERBROEK T.
Abstract (from CDS):
During a BeppoSAX observation of the X-ray burst source X 1732-304 located in the globular cluster Terzan 1 in April 1999, the source was detected in an anomalous low-state with an X-ray luminosity of 1.4x1033erg/s, about 300 times fainter than the lowest previously reported measure of the persistent flux. The spectral shape and intensity of this low-state emission is similar to that of a number of neutron star soft X-ray transients in quiescence, being well fit by a ≃0.34keV blackbody together with a power-law with a photon index of 1.0±1.01.3. These similarities suggest that the same mechanism, such as the inhibition of accretion due to the propeller effect, is also operating in X 1732-304. Alternatively, the X 1732-304 low-state may be due to obscuration of the line of sight to the central neutron star, with only scattered and/or reflected X-rays being detected. The possibility that the source detected by BeppoSAX is not X 1732-304, but a nearby dim source of the cluster field (what would make the observed dimming of X 1732-304 even more remarkable) is also discussed.