Long-term hard X-ray monitoring of 2S 0114+65 with INTEGRAL/IBIS

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 with X-ray luminosities of 10³⁵-4 × 10³⁶ erg s^-1 from 20 to 100 keV due to accretion rate changes in different orbital phases. In several observations when 2S 0114+65 was bright, we found a pulse period evolution of ∼2.67 h to 2.63 h from 2003 to 2008, with a spin-up rate of the neutron star ∼1.09 × 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 18-100 keV band which changed with the orbital phases generally could be described with a power-law model with a high-energy exponential cut-off. The variation of the power-law photon index over orbital phase anticorrelates with hard X-ray flux, and the variation of E_cut has a positive correlation with the hard X-ray flux, implying a 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 cut-off. We discover that the hard X-ray tails are only detected when the column density is very low. Thus, a high column density leads to the disappearance of the hard X-ray tails in this wind-fed neutron star accretion binary. Our results would help to understand the origin, evolution and properties of this peculiar class of superslow pulsation neutron stars in high-mass X-ray binaries.