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Preprint 13a/99

Disappearing Pulses in Vela X­1

P. Kretschmar (1;2), I. Kreykenbohm (2), J. Wilms (2), R. Staubert (2), W. A. Heindl (3), D. E. Gruber (3), R. E. Rothschild (3)

(1) INTEGRAL Science Data Centre, Ch. d'Ecogia 16, 1290 Versoix, Switzerland
(2) Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany
(3) CASS, University of California at San Diego, La Jolla, CA 92093, U.S.A.

To appear in: Proc. of the 5th Compton Symposium, ed. M. McConnell

Abstract. We present results from a 20 h RXTE observation of Vela X­1, including a peculiar low state of a few hours duration, during which the pulsation of the X­ray emission ceased, while significant non­pulsed emission remained. This "quiescent state" was preceded by a "normal state" without any unusual signs and followed by a "high state" of several hours of increased activity with strong, flaring pulsations. While there is clear spectral evolution from the normal state to the low state, the spectra of the following high state are surprisingly similar to those of the low state.

Key words: -

Paper (201k gzip'ed Postscript including figures)

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