Following the Colour of the Low Mass X-ray Binary 4U 1820-30 with INTEGRAL

Antonella Tarana; Angela Bazzano; Pietro Ubertini; A. Andrzej Zdziarski; M. Federici

paper

Following the Colour of the Low Mass X-ray Binary 4U 1820-30 with INTEGRAL

arXiv:astro-ph/0610328

Abstract

The 4-200 keV spectral and temporal behaviour of the low mass X-ray binary 4U 1820-30 has been studied 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 revealed 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 $τ 4. A l t er na t i v e l y, b u t a t a l o w er g oo d n esso f t h e f i t, t h e ha r d - s t a t e b r o a d ban d s p ec t r u m c anb e a cco u n t e df or b y e mi ss i o n f r o mah y b r i d, t h er ma l - n o n t h er ma l, pl a s ma . D u r in g t h eo b ser v a t i o n s, t h eso u r ces p e n t m os t o f t h e t im e in t h eso f t s t a t e, a s p r e v i o u s l y r e p or t e d an d t h e$ \ge $4 k e V s p ec t r a c anb er e p r ese n t e d b y t h er ma l C o m pt o ni z a t i o n w i t hk T_{e} 3 k e V an d$ τ~ 6-7.

4 pages, 3 figures, submitted proceeding of the 6th INTEGRAL Workshop "The Obscured Universe" July 2-8, 2006 Space Research Institute Moscow, Russia