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Astron. Astrophys. 357, L17-L20 (2000)

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Letter to the Editor

Temporal evolution of X-ray lags in Cygnus X-1

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

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

Received 28 February 2000 / Accepted 4 April 2000


We present the long term evolution of the frequency-dependent X-ray time lags of the black hole candidate Cygnus X-1 as measured in 1996 and 1998 with the Rossi X-ray 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 X-1 was in its hard state. During state transitions, however, the shape and magnitude of the X-ray 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 X-ray 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 X-1 – stars: binaries: close – X-rays: stars

Send offprint requests to: K. Pottschmidt

Correspondence to: K. Pottschmidt (katja@astro.uni-tuebingen.de)

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© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000