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

TL;DR

This study monitors the spectral evolution of the LMXB 4U 1728-34 over a year, revealing the presence of non-thermal high-energy emission during hard states, which is modeled with hybrid electron populations.

Contribution

It is the first detailed analysis showing non-thermal emission in 4U 1728-34, a rare atoll-type LMXB, using broad-band X-ray spectra from INTEGRAL and RXTE.

Findings

Non-thermal high-energy tail detected in hard states.

Harder spectra correlate with more prominent non-thermal emission.

Spectral modeling suggests hybrid thermal and non-thermal electron populations.

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.