Spectral state evolution of 4U~1820-30: the stability of the spectral index of Comptonization tail

Abstract

We analyze the X-ray spectra and their timing properties of the compact X-ray binary 4U~1820-30. We establish a number of spectral transitions in this source seen with BeppoSAX and RXTE. During the RXTE observations (1996 -- 2009), the source were approximately 75% of its time in the soft state making the lower banana and upper banana transitions combined with long-term low-high state transitions. We reveal that all of the X-ray spectra of 4U~1820-30 are fit by a composition of a thermal (blackbody) component, a Comptonization component (COMPTB) and a Gaussian-line component. Thus using this spectral analysis we find that the photon power-law index Gamma of the Comptonization component is almost unchangeable (Gamma~2) while the electron temperature kT_e changes from 2.9 to 21 keV during these spectral events. We also establish that for these spectral events the normalization of COMPTB component (which is proportional to mass accretion rate, \dot M increases by factor 8 when kT_e decreases from 21 keV to 2.9 keV. Before this index stability effect was also found analyzing X-ray data for Z-source GX 340+0 and for atolls, 4U~1728-34, GX~3+1. Thus, we can suggest that this spectral stability property is a spectral signature of an accreting neutron star source. On the other hand in a black hole binary Gamma monotonically increases with \dot M and ultimately its value saturates at large \dot M.

25 pages, 13 figures and 5 Tables. The paper is accepted and scheduled for publication in The Astrophysical Journal, for the ApJ April 20, 2013, v 767, 2nd issue

References in corpus (8)

• Evaluating Spectral Models and the X-ray States of Neutron-Star X-ray Transients
• A new Comptonization model for low-magnetized accreting neutron stars in low mass X-ray binaries
• The superorbital variability and triple nature of the X-ray source 4U 1820-303
• X-ray spectral and timing behavior of Scorpius X-1. Spectral hardening during the flaring branch
• GX~3+1: the stability of spectral index as a function of mass accretion rate
• INTEGRAL spectral variability study of the atoll 4U 1820-30: first detection of hard X-ray emission
• Stability of the photon indices in Z-source GX 340+0 for spectral states
• Discovery of kilohertz quasi-periodic oscillations and state transitions in the low-mass X-ray binary 1E~1724-3045 (Terzan 2)

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• ALMA observations of 4U 1728-34 and 4U 1820-30: first detection of neutron star X-ray binaries at 300 GHz
• X-Ray Spectra of The High-Mass X-RAY Binary 4U~1700-37 using BeppoSAX, Suzaku and RXTE Observations
• NGC 4051: Black hole mass and photon index-mass accretion rate correlation
• Iron-line and continuum variations in the XMM-Newton and Suzaku spectra of the neutron-star low-mass X-ray binary 4U 1636-53
• A Comprehensive Study of Thermonuclear X-ray Bursts from 4U 1820-30 with NICER: Accretion Disk Interactions and a Candidate Burst Oscillation
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• BEPPOSAX and RXTE spectral study of the low-mass X-ray binary 4u~1705-44. Spectral hardening during the banana branch
• Broadband X-ray emission and the reality of the broad iron line from the Neutron Star - White Dwarf X-ray binary 4U 1820-30
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• Monte Carlo Simulations of Thermal Comptonization Process in a Two Component Advective Flow around a Neutron Star
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