INTEGRAL spectral variability study of the atoll 4U 1820-30: first detection of hard X-ray emission

Abstract

We study the 4-200 keV spectral and temporal behaviour of the low mass X-ray binary 4U 1820-30 with INTEGRAL during 2003-2005. This source as been observed in both the soft (banana) and hard (island) spectral states. A high energy tail, above 50 keV, in the hard state has been observed for the first time. This places the source in the category of X-ray bursters showing high-energy emission. The tail can be modeled as a soft power law component, with the photon index of ~2.4, on top of thermal Comptonization emission from a plasma with the electron temperature of kT_e~6 keV and optical depth of τ~4. Alternatively, but at a lower goodness of the fit, the hard-state broad band spectrum can be accounted for by emission from a hybrid, thermal-nonthermal, plasma. During this monitoring the source spent most of the time in the soft state, usual for this source, and the >~4 keV spectra are represented by thermal Comptonization with kT_e~3 keV and τ~6-7.

14 pages, 4 figures, accepted for publication by ApJ

References in corpus (1)

• Spitzer Reveals Infrared Optically-Thin Synchrotron Emission from the Compact Jet of the Neutron Star X-Ray Binary 4U 0614+091

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• Study of the reflection spectrum of the accreting neutron star GX 3+1 using XMM-Newton and INTEGRAL
• ALMA observations of 4U 1728-34 and 4U 1820-30: first detection of neutron star X-ray binaries at 300 GHz
• The Galactic LMXB Population and the Galactic Centre Region
• New insights on the puzzling LMXB 1RXS J180408.9-342058: the intermediate state, the clocked type-I X-ray bursts and much more
• The first detection of Compton Reflection in the Low-Mass X-ray Binary 4U1705-44 with INTEGRAL and BeppoSAX
• The evolving radio jet from the neutron star X-ray binary 4U 1820$-$30
• The accretion/ejection link in the neutron star X-ray binary 4U 1820-30 I: A boundary layer-jet coupling?
• INTEGRAL and BeppoSAX observations of the transient atoll source 4U 1608-522: from quiescent to hard spectral state
• AstroSat view of `Clocked` burster GS 1826-238:broad-band spectral nature of persistent and burst emission during soft state
• Insight-HXMT Detections of Hard X-ray Tails in Scorpius X-1