Is 4U 0114+65 an eclipsing HMXB?

Abstract

We present the pulsation and spectral characteristics of the 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 upto 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 timeresolved spectral analysis reveal moderate absorption column density (N H -2-20 × 10 22 atoms cm −2 ) and a relatively low equivalent width (∼ 30 eV & 12 eV of the iron K α and K β lines respectively) as opposed to the typical X-ray spectra of HMXBs during eclipse where the equivalent width is ∼ 1 keV. Both XIS and 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.

XMM-Newtonspectroscopy of the accreting magnetar candidate 4U0114+65

“…It is lower in the beginning of the observation, when the absorption is higher. These values are consistent with those found by (Pradhan et al 2015) using Suzaku. The spectral index α remains constant and <1, indicating an efficient Comptonising process (see Table A.1).…”

“…The observations were carried out in the time interval between 21-05-2015 20:43:19 UT MJD = 57 163.8634 and 22-05-2015 10:01:53 UT, corresponding to an average orbital phase φ ≈ 0.7 (using a P orb = 11.5983 d and the ephemeris of Pradhan et al 2015), which covers approximately 5% of the orbit. Our XMM-Newton observation was performed well outside the minimum in the long-term light curve (see Pradhan et al 2015 for Suzaku observations performed during the minimun). The data can be found in the XMM archive under the ID 0764650101.…”

“…We Notes. To calculate the orbital phase, we used as the initial epoch T 0 = 55 763.0; the value was taken from Pradhan et al (2015).…”

“…The source 4U 0114+65 is a non-eclipsing system (Pradhan et al 2015) and hosts an X-ray pulsar with a ∼2.6 h spin period, which makes it one of the slowest known X-ray pulsars. In order to explain such a slow spin period, (Li & van den Heuvel 1999) suggested that this object could have been born as a magnetar.…”

Radiography in high mass X-ray binaries

“…Also, Hainich et al (2020) showed that there was no systematic difference between the stellar properties in classic SgXBs and in SFXTs, hinting at similar wind micro-structures (Driessen et al 2019). Finally, Pradhan et al (2018) concluded that the higher variability of the emission in SFXTs compared to SgXBs was likely due to mechanisms inhibiting accretion near the accretor, rather than direct clump capture, although flares in SFXTs have recently been found by Ferrigno et al (2020) to be associated with massive structures approaching the accretor. To summarize, X-ray variability is, at best, an indirect tracer of the clumpiness of the wind because of intermediate regions where the clumps are mixed and where other instabilities can kick in, in particular those associated to the different accretion regimes.…”