Letter to the Editor

BeppoSAX observation of 4U 1705-44: detection of hard X-ray emission in the soft state

¹ INAF – IASF di Palermo, via Ugo La Malfa 153, 90146 Palermo, Italy e-mail: Santina.Piraino@ifc.inaf.it
² IAAT, University of Tübingen, Sand 1, 72076 Tübingen, Germany
³ Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, via Archirafi 36, 90123 Palermo, Italy
⁴ Department of Physics and Astronomy, University of Iowa, Iowa City, IA 52246, USA
⁵ Universitá degli Studi di Cagliari, Dipartimento di Fisica, SP Monserrato-Sestu, KM 0.7, 09042 Monserrato, Italy

Received: 10 May 2007
Accepted: 19 June 2007

Abstract

Context.4U 1705-44 is one of the best-studied type I X-ray burster and atoll sources.

Aims.Since it covers a wide range in luminosity (from a few to 50 10³⁶ erg s^-1) and shows clear spectral state transitions, it represents a good laboratory for testing the accretion models proposed for atoll sources.

Methods.We analyzed the energy spectrum accumulated with BeppoSAX observations (43.5 ks) in August 2000 when the source was in a soft spectral state.

Results.The continuum of the wide-band energy spectrum is well-described by the sum of a blackbody (0.56 keV) and a Comptonized component (seed-photon temperature ∼1 keV, electron temperature 2.7 keV, and optical depth 11). A hard tail was detected at energies above ~25 keV. The latter can be modeled by a power law having a photon index ~2.9, which contributes ~11% of the total flux in the range 0.1–200 keV. A broad emission line, possibly from a relativistic accretion disk, models the feature in the Fe K line region of the spectrum.

Conclusions.This is the first time that a high-energy tail has been observed during a soft state of the source.