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Astron. Astrophys. 349, 819-824 (1999)

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An anomalous low-state from the globular cluster X-ray source X 1732-304 (Terzan 1)

M. Guainazzi, A.N. Parmar and T. Oosterbroek

Astrophysics Division, Space Science Department of ESA, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands

Received 11 June 1999 / Accepted 5 July 1999


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 [FORMULA] erg s-1, 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 [FORMULA]0.34 keV blackbody together with a power-law with a photon index of [FORMULA]. 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.

Key words: accretion, accretion disks – stars: individual: X 1732-304 – stars: neutron – Galaxy: globular clusters: individual: Terzan 1 – X-rays: general

Send offprint requests to: M. Guainazzi

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© European Southern Observatory (ESO) 1999

Online publication: September 13, 1999