Hard X-ray emission of Sco X-1

TL;DR

This study analyzes the hard X-ray emission of Sco X-1 using INTEGRAL data, revealing a persistent power-law tail up to 300 keV that varies with accretion states and likely originates from non-thermal electron Comptonization.

Contribution

It provides the first detailed analysis of Sco X-1's hard X-ray tail, showing its correlation with accretion disk components and proposing a non-thermal electron origin.

Findings

Hard X-ray tail extends up to 300 keV without cutoff.

Tail amplitude varies by over ten times with accretion state.

Presence of the tail correlates with the inner disk component.

Abstract

We study hard X-ray emission of the brightest accreting neutron star Sco X-1 with INTEGRAL observatory. Up to now INTEGRAL have collected ~4 Msec of deadtime corrected exposure on this source. We show that hard X-ray tail in time average spectrum of Sco X-1 has a power law shape without cutoff up to energies ~200-300 keV. An absence of the high energy cutoff does not agree with the predictions of a model, in which the tail is formed as a result of Comptonization of soft seed photons on bulk motion of matter near the compact object. The amplitude of the tail varies with time with factor more than ten with the faintest tail at the top of the so-called flaring branch of its color-color diagram. We show that the minimal amplitude of the power law tail is recorded when the component, corresponding to the innermost part of optically thick accretion disk, disappears from the emission spectrum. Therefore we show that the presence of the hard X-ray tail may be related with the existence of the inner part of the optically thick disk. We estimate cooling time for these energetic electrons and show that they can not be thermal. We propose that the hard X-ray tail emission originates as a Compton upscattering of soft seed photons on electrons, which might have initial non-thermal distribution.