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Preprint 04/00

Temporal Evolution of X­ray Lags in Cygnus X­1

K. Pottschmidt (1), J. Wilms (1), M.A. Nowak (2), W.A. Heindl (3), D.M. Smith (4), and R. Staubert (1)

(1) Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany
(2) JILA, University of Colorado, Boulder, CO 80309­440, U.S.A.
(3) Center for Astronomy and Space Sciences, Code 0424, University of California at San Diego, La Jolla, CA 92093, U.S.A.
(4) Space Sciences Laboratory, University of California at Berkeley, Berkeley, CA 94720, U.S.A.

To appear in: Astronomy & Astrophysics Letters

Abstract. We present the long term evolution of the frequency­dependent X­ray time lags of the black hole candidate Cygnus X­1 as measured in 1996 and 1998 with the Rossi X­ray Timing Explorer (RXTE). Lag spectra measured during the 1996 June soft state are very similar to those seen during 1996 December and most of 1998 while Cyg X­1 was in its hard state. During state transitions, however, the shape and magnitude of the X­ray lag is highly variable and tends to be much larger than outside of the state transitions. This behavior is most obvious in the 1-10 Hz band. The increase of the X­ray lag during the state transitions might be related to the formation and destruction of the synchrotron radiation emitting outflows present during the hard state.

Key words: stars: individual (Cyg X­1) - binaries: close - X­rays: stars

Paper (56k gzip'ed Postscript including figures)

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