Is 4U 0114+65 an eclipsing HMXB?

Abstract

We present the pulsation and spectral characteristics of the High-Mass X-ray Binary (HMXB) 4U 0114+65 during a Suzaku observation covering the part of the orbit that included the previously known low-intensity emission of the source (dip) and the egress from this state. This dip has been interpreted in previous works as an X-ray eclipse. Notably, in this Suzaku observation, the count rate during and outside the dip vary by a factor of only 2-4 at odds with the eclipses of other HMXBs, where the intensity drops up to two orders of magnitude. The orbital intensity profile of 4U 0114+65 is characterized by a narrow dip in the RXTE-ASM (2-12 keV) light curve and a shallower one in the Swift-BAT (15-50 keV), which is different from eclipse ingress/egress behaviour of other HMXBs. The time-resolved spectral analysis reveal moderate absorption column density (N_H - 2-20 × 10²² atoms cm^-2) and a relatively low equivalent width (∼30 and 12 eV of the iron Kα and Kβ lines, respectively) as opposed to the typical X-ray spectra of HMXBs during eclipse where the equivalent width is ∼1 keV. Both X-ray Imaging Spectrometer and HXD-PIN data show clear pulsations during the dip, which we have further confirmed using the entire archival data of the IBIS/ISGRI and JEM-X instruments onboard INTEGRAL. The results we presented question the previous interpretation of the dip in the light curve of 4U 0114+65 as an X-ray eclipse. We thus discuss alternative interpretations of the periodic dip in the light curve of 4U 0114+65.