Spectral states evolution of 4U 1728-34 observed by INTEGRAL and RXTE: non-thermal component detection

Abstract

We report results of a one-year monitoring of the low mass X-ray binary (LMXB) source (atoll type) 4U 1728-34 with INTEGRAL and RXTE. Three time intervals were covered by INTEGRAL, during which the source showed strong spectral evolution. We studied the broad-band X-ray spectra in detail by fitting several models in the different sections of the hardness-intensity diagram. The soft states are characterised by prominent blackbody emission plus a contribution from a Comptonized emission. The hard states are characterised by the presence of an excess flux with respect to the Comptonization model above 50 keV while the soft component is fainter. To obtain an acceptable fit to the data this excess is modeled either with a power law with photon index Gamma ~ 2 or a Comptonization (CompPS) spectrum implying the presence of hybrid thermal and non-thermal electrons in a corona. This makes 4U 1728-34 one of the few LMXBs of atoll type showing non-thermal emission at high energy. From our analysis, it is also apparent that the presence of the hard tail is more prominent as the overall spectrum becames harder. We discuss also alternative models which can discribe these hard states.

Accepted for publication in MNRAS (accepted 2011 April 20. Received 2011 April 20; in original form 2010 December 07); 9 pages, 7 figures

References in corpus (6)

• Evaluating Spectral Models and the X-ray States of Neutron-Star X-ray Transients
• A new Comptonization model for low-magnetized accreting neutron stars in low mass X-ray binaries
• Linking Jet Emission, X-ray States and Hard X-ray Tails in the Neutron Star X-ray Binary GX 17+2
• The X-ray spectrum of the bursting atoll source 4U~1728-34 observed with INTEGRAL
• INTEGRAL spectral variability study of the atoll 4U 1820-30: first detection of hard X-ray emission
• Disc-Jet coupling in the LMXB 4U1636-53 from INTEGRAL

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