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Preprint B/00

A Good Long Look at the Black Hole Candidates LMC X-1 and LMC X-3

M.A. Nowak (1), J. Wilms (2), W.A. Heindl (3), K. Pottschmidt (2), J.B. Dove (4;5), M.C. Begelman (1;6)

(1) JILA, University of Colorado, Campus Box 440, Boulder, CO 80309-0440, U.S.A.; mnowak@rocinante.colorado.edu
(2) Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany; wilms@astro.uni-tuebingen.de, katja@astro.uni-tuebingen.de
(3) Center for Astronomy and Space Sciences, University of California at San Diego, La Jolla, CA 92093, U.S.A.; wheindl@mamacass.ucsd.edu
(4) Center for Astronomy and Space Astrophysics, University of Colorado, Boulder, CO 80309-389, U.S.A.; dove@casa.colorado.edu
(5) also, Dept. of Physics, Metropolitan State College of Denver, C.B. 69, P.O. Box 173362, Denver, CO 80217-3362, U.S.A.
(6) also, Dept. of Astrophysics and Planetary Sciences, University of Colorado, Boulder 80309, U.S.A.; mitch@jila.colorado.edu

2001, MNRAS 320, 316 - 326

Abstract. LMC X-1 and LMC X-3 are the only known persistent stellar-mass black hole candidates that have almost always shown spectra that are dominated by a soft, thermal component. We present here results from 170 ksec long Rossi X-ray Timing Explorer (RXTE) observations of these objects, taken in 1996 December, where their spectra can be described by a disc black body plus an additional soft (Gamma ~2.8) high energy power law (detected up to energies of 50keV in LMC X-3). These observations, as well as archival Advanced Satellite for Cosmology and Astrophysics (ASCA) observations, constrain any narrow Fe line present in the spectra to have an equivalent width <~90 eV. Stronger, broad lines ( ~150eV EW, sigma ~1keV) are permitted. We also study the variability of LMC X-1. Its X-ray power spectral density (PSD) is approximately proportional f^-1 between 10^-3 and 0.3 Hz with a root mean square (rms) variability of ~7%. At energies > 5 keV the PSD shows evidence of a break at f > 0.2 Hz, possibly indicating an outer disc radius of <~1000 GM/c² in this likely wind-fed system. Furthermore, the coherence function gamma²(f), a measure of the degree of linear correlation, between variability in the > 5 keV band and variablity in the lower energy bands is extremely low ( < 50%). We discuss the implications of these observations for the mechanisms that might be producing the soft and hard X-rays in these systems.

Key words: accretion - black hole physics - Stars: binaries - X-rays:Stars

Paper (251k gzip'ed Postscript including figures)

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