Issue |
A&A
Volume 471, Number 1, August III 2007
|
|
---|---|---|
Page(s) | L17 - L20 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361:20077841 | |
Published online | 26 June 2007 |
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
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.
Key words: accretion, accretion disks / stars: individual: 4U 1705-44 / stars: neutron / X-rays: stars / X-rays: binaries / X-rays: general
© ESO, 2007
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