Hard X-ray emission of Sco X-1

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.

9 pages, 7 figures, Accepted for publication in MNRAS

References in corpus (8)

• Modelling the behaviour of accretion flows in X-ray binaries
• INTEGRAL/IBIS all-sky survey in hard X-rays
• When A Standard Candle Flickers
• The synchrotron boiler and the spectral states of black hole binaries
• Spread of Matter over a Neutron-Star Surface During Disk Accretion: Deceleration of Rapid Rotation
• Average hard X-ray emission from NS LMXBs: Observational evidence of different spectral states in NS LMXBs
• Broad-band Spectral Evolution of Scorpius X-1 along its Color-Color Diagram
• INTEGRAL IBIS/ISGRI energy calibration in OSA 10

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• 15 years of Galactic surveys and hard X-ray Background measurements
• Fe K$\alpha$ and Fe K$\beta$ line detection in the NuSTAR spectrum of the ultra-bright Z-source Scorpius X-1
• Upper limits to the number of Oort Cloud Objects based on serendipitous occultation events search in X-rays
• Insight-HXMT Detections of Hard X-ray Tails in Scorpius X-1
• Study on the energy limits of kHz QPOs in Sco X-1 with $RXTE$ and $Insight$-HXMT observations
• The properties of the Galactic Hard X-Ray and soft gamma-ray Background based on 20 years of INTEGRAL/IBIS observations