Long-term Hard X-ray Monitoring of 2S 0114+65 with INTEGRAL/IBIS

TL;DR

This study presents a comprehensive long-term analysis of the high mass X-ray binary 2S 0114+65 using INTEGRAL/IBIS data, revealing neutron star spin-up, spectral variability, and conditions for hard X-ray tail detection.

Contribution

It provides the first detailed long-term monitoring of 2S 0114+65's spectral and timing properties, including neutron star spin evolution and the conditions affecting hard X-ray tail visibility.

Findings

Neutron star spin-up rate is accelerating.

Spectral properties vary with orbital phase and flux.

Hard X-ray tails are only detected at low column densities.

Abstract

(abridged) We present the results of the long-term hard X-ray monitoring of the high mass X-ray binary 2S 0114+65 with INTEGRAL/IBIS from 2003 to 2008. 2S 0114+65 is a variable hard X-ray source, when 2S 0114+65 was bright, we found a pulse period evolution of $\sim 2.67$ hour to 2.63 hour from 2003 -- 2008, with a spin-up rate of the neutron star $\sim 1.09\times 10^{-6}$ s s$^{-1}$. Compared with the previous reported spin-up rate, the spin-up rate of the neutron star in 2S 0114+65 is accelerating. The spectral properties of 2S 0114+65 in the band of 18 -- 100 keV which changed with the orbital phases. The variation of the power-law photon index over orbital phase anticorrelates with hard X-ray flux, and the variation of $E_{\rm cut}$ has a positive correlation with the hard X-ray flux, implying that the harder spectrum at the maximum of the light curve. The variations of spectral properties over orbital phase suggested 2S 0114+65 as a highly obscured binary system. In some observational revolutions, hard X-ray tails above 70 keV are detected. We study the characteristics of the hard X-ray tails combining JEM-X and IBIS data in the energy range of 3 -- 100 keV. The 3 -- 100 keV spectra of 2S 0114+65 are generally fitted by an absorbed power-law model with high energy cutoff. We discover that the hard X-ray tails are only detected when column density is very low. Thus, high column density leads to disappearance of the hard X-ray tails in this wind-fed neutron star accretion binary.