EDP Sciences
Free access
Volume 423, Number 3, September I 2004
Page(s) 969 - 973
Section Galactic structure, stellar clusters and populations
DOI http://dx.doi.org/10.1051/0004-6361:20040335

A&A 423, 969-973 (2004)
DOI: 10.1051/0004-6361:20040335

A transient high-coherence oscillation in 4U 1820-30

T. Belloni1, I. Parolin1 and P. Casella2, 3

1  INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
    e-mail: belloni@merate.mi.astro.it
2  INAF - Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
3  Dipartimento di Fisica, Università degli Studi "Roma Tre", via della Vasca Navale 84, 00146 Roma, Italy

(Received 25 February 2004 / Accepted 11 May 2004)

We re-analyzed two Rossi X-Ray Timing Explorer archival observations of the atoll source 4U 1820-30 in order to investigate the detailed time-frequency properties of the source during the intervals when a ~7 Hz QPO was detected by Wijnands et al. (1999, ApJ, 512, L39). We find that in both observations, in addition to a QPO signal lasting a couple of minutes as previously reported, there is a much narrower transient oscillation with a life time of only a few seconds. Within this time, the oscillation is consistent with being coherent. Its integrated fractional rms is around 10% and its frequency 7.3 Hz and 5.7 Hz in the two observations. We discuss the possible association of this QPO with other oscillations known both in Neutron-Star and Black-Hole systems, concentrating on the similarities with the narrow 5-7 Hz oscillations observed at high flux in Black-Hole Candidates.

Key words: accretion: accretion disks -- stars: neutron -- X-rays: binaries

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