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4U 1957+11: a persistent low-mass X-ray binary and black-hole candidate in the high state?

Rudy Wijnands·Jon M. Miller·Michiel van der Klis
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Abstract

We report on several pointed Rossi X-ray Timing Explorer observations of the enigmatic low-mass X-ray binary (LMXB) 4U 1957+11 at different X-ray luminosities. The luminosity of the source varied by more than a factor of four on time scales of months to years. The spectrum of the source tends to get harder when its luminosity increases. Only very weak (1%-2% rms amplitude; 0.001-10 Hz; 2-60 keV) rapid X-ray variability was observed during the observations. A comparison of the spectral and temporal behaviour of 4U1957+11 with other X-ray binary systems, in particular LMC X-3, indicates that 4U 1957+11 is likely to be a persistent LMXB harboring a black hole and which is persistently in the black-hole high state. If confirmed, it would be the only such system known.

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Rudy Wijnands, Jon M. Miller, Michiel van der Klis. 2001-11-09. 4U 1957+11: a persistent low-mass X-ray binary and black-hole candidate in the high state?. https://doi.org/10.1046/j.1365-8711.2002.05137.x

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