Insight-HXMT Detections of Hard X-ray Tails in Scorpius X-1

TL;DR

This study uses Insight-HXMT observations to detect and analyze hard X-ray tails in Sco X-1, revealing their variation along the Z-track and exploring potential emission mechanisms, contrasting with Cyg X-1's spectrum.

Contribution

First detection and detailed analysis of hard X-ray tails in Sco X-1 across different spectral states using Insight-HXMT data.

Findings

Hard X-ray tails are present throughout the Z-track.

The tails harden and fade from HB to FB.

Different mechanisms may explain the tails on HB/NB and FB.

Abstract

Using the observations of the high-energy (HE) detector of the Hard X-ray Modulation Telescope (Insight-HXMT) for Scorpius X-1 in 2018, we search for hard X-ray tails in the hard X-ray spectra in ~30-200 keV. The hard X-ray tails are found throughout the Z-track on the hardness-intensity diagram and they harden and fade away from the horizontal branch (HB), through the normal branch (NB), to the flaring branch (FB). Comparing the hard X-ray spectra between Cyg X-1 and Sco X-1, it is concluded that the hard X-ray spectrum of Cyg X-1 shows high-energy cutoff, implying a hot corona in it, but the high-energy cutoff does not reveal in the hard X-ray spectrum of Sco X-1. Jointly fitting the HE spectrum with the medium-energy and low-energy spectra of Sco X-1 in ~2-200 keV, it is suggested that the upscattering Comptonization of the neutron star (NS) emission photons by the energetic free-falling electrons onto the NS or by the hybrid electrons in the boundary layer between the NS and the accretion disk could be responsible for the hard X-ray tails of Sco X-1 on the HB and NB, but neither of the two mechanisms can be responsible for the hard X-ray tail on the FB. Some possible origins for the peculiar hard X-ray tail of FB are argued.