A low/hard state outburst of XTE J1550-564

¹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy e-mail: colombo@merate.mi.astro.it; homan@merate.mi.astro.it; campana@merate.mi.astro.it
² Dipartimento di Scienze, Università dell'Insubria, Via Valleggio 11, 22100 Como, Italy
³ Astronomical Institute “A. Pannekoek” and Center for High-Energy Astrophysics, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands e-mail: michiel@astro.uva.nl

Corresponding author: T. Belloni, belloni@merate.mi.astro.it

Received: 4 March 2002
Accepted: 6 May 2002

Abstract

We present the results of the analysis of 11 RXTE/PCA observations of the January 2002 outburst of the black-hole transient XTE J1550-564. The outburst was rather short, with an e-folding time of ~11 days in the PCA band 3–25 keV. The source was seen in an extreme low/hard state. The energy spectra could be fitted with a power law slightly steepening ( to ~1.5) in time, with a high-energy cutoff decreasing from ~300 keV to ~100 keV and some additional features between 6 and 7 keV, while the 3–150 keV flux decreased by a factor of 17. The power spectra were fitted with two broad Lorentzian components, whose characteristic frequencies decreased with the source flux, and whose fractional rms contribution was rather constant. The timing parameters derived can be identified with those observed in other systems. These results show that, although during previous, more luminous, outbursts, a very complex behavior was observed, at low luminosities XTE J1550-564 behaves in a way comparable to most Black-Hole Candidates.