The INTEGRAL long monitoring of persistent Ultra Compact X-ray Bursters

TL;DR

This study systematically analyzes INTEGRAL data of Ultra Compact X-ray Binaries, revealing their average spectral properties across a broad energy range and their typical low/hard accretion state.

Contribution

It provides the first comprehensive spectral analysis of multiple Ultra Compact X-ray Binaries using combined INTEGRAL, BeppoSAX, and SWIFT data, highlighting their common low/hard state characteristics.

Findings

Most sources are in a low/hard state with dominant Comptonised emission.

Spectra are well fitted by a disk-blackbody plus Comptonised component.

Accretion rates are generally below 10^{-9} solar masses per year.

Abstract

The combination of compact objects, short period variability and peculiar chemical composition of the Ultra Compact X-ray Binaries make up a very interesting laboratory to study accretion processes and thermonuclear burning on the neutron star surface. The improved large optical telescopes and more sensitive X-ray satellites have increased the number of known Ultra Compact X-ray Binaries allowing their study with unprecedented detail. We analyze the average properties common to all ultra compact Bursters observed by INTEGRAL from ~0.2keV to ~150keV. We have performed a systematic analysis of the INTEGRAL public data and Key-Program proprietary observations of a sample of the Ultra Compact X-ray Binaries. In order to study their average properties in a very broad energy band, we combined INTEGRAL with BeppoSAX and SWIFT data whenever possible. For sources not showing any significant flux variations along the INTEGRAL monitoring, we build the average spectrum by combining all available data; in the case of variable fluxes, we use simultaneous INTEGRAL and SWIFT observations when available. Otherwise we compared IBIS and PDS data to check the variability and combine BeppoSAX with INTEGRAL/IBIS data. All spectra are well represented by a two component model consisting of a disk-blackbody and Comptonised emission. The majority of these compact sources spend most of the time in a canonical low/hard state, with dominating Comptonised component and accretion rate lower than ~10^{-9}Msolar/yr, not depending on the used model to fit the data.