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

TL;DR

This study analyzes X-ray spectral and timing data of 4U 1820-30, revealing a stable spectral index during spectral state transitions, which may serve as a signature of neutron star accretion, contrasting black hole behavior.

Contribution

It demonstrates the stability of the photon index Gamma around 2 during spectral state changes in 4U 1820-30, highlighting a potential neutron star spectral signature.

Findings

Photon index Gamma remains approximately 2 during spectral transitions.

Electron temperature kT_e varies from 2.9 to 21 keV.

Normalization of COMPTB increases with mass accretion rate.

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.