Evolution of Spin, Orbital, and Superorbital Modulations of 4U 0114+650

Abstract

We report a systematic analysis of the spin, orbital, and superorbital modulations of 4U 0114+650, a high-mass X-ray binary that consists of one of the slowest spinning neutron stars. Using the dynamic power spectrum, we found that the spin period varied dramatically and is anticorrelated with the long-term X-ray flux variation that can be observed using the Rossi X-ray Timing Explorer ASM, Swift BAT, and the Monitor of All-sky X-ray Image. The spin-up rate over the entire dataset is consistent with previously reported values; however, the local spin-up rate is considerably higher. The corresponding local spin-up timescale is comparable to the local spin-up rate of OAO 1657−415, indicating that 4U 0114+650 could also have a transient disk. Moreover, the spin period evolution shows two ∼1000-day spin-down/random-walk epochs that appeared together with depressions of the superorbital modulation amplitude. This implies that the superorbital modulation was closely related to the presence of the accretion disk, which is not favored in the spin-down/random-walk epochs because the accretion is dominated by the direct wind accretion. The orbital period is stable during the entire time span; however, the orbital profile significantly changes with time. We found that the depth of the dip near the inferior conjunction of the companion is highly variable, which disfavors the eclipsing scenario. Moreover, the dip was less obvious during the spin-down/random-walk epochs, indicating its correlation with the accretion disk. Further monitoring in both X-ray and optical bands could reveal the establishment of the accretion disk in this system.

Possible Periodic Dips in the Pulsating Ultraluminous X-ray Source M51 ULX-7

“…These Roche-lobe overflow high-mass X-ray binaries (HMXBs) could show orbitalphase-dependent dips that can be interpreted as the obscuring effect by the bulge similar to low-mass X-ray binaries (Naik et al 2011;Hu et al 2013). In addition, the large size and the extended atmosphere of the companion star in a wind-fed HMXB make it possible to have an atmospheric eclipse that behaves like a dip and the X-rays from the NS are not fully obscured (Grundstrom et al 2007;Hu et al 2017). A high inclination angle is needed for both scenarios.…”

Understanding the coexistence of spin-up and spin-down behaviors in long period X-ray pulsars

“…Sanjurjo-Ferrrin et al (2017) further confirmed the long term spin-up rate for 2S 0114+65, and discussed the long pulsation period in terms of the quasi-spherical settling accretion (Sakura et al 2012), but the spin-up behavior was not discussed. Hu et al (2017) proposed possible transient accretion disk may account for the spin-up of 2S 0114+65. No quantitative calculation is done for the the possible transient disk in Hu et al (2017).…”

“…The neutron star in 4U 2206+54 shows a long-term spin-down trend with a mean rate of ∼ 5 × 10 −7 s s −1 in last 15 years (Wang 2013;Torrejon et al 2018). While the neutron star in 2S 0114+65 undergoes a long-term spin-up trend with a mean rate of ∼ −10 −6 s s −1 in last 30 years (Hall et al 2000;Wang 2011;Sanjurjo-Ferrrin et al 2017), though sometimes it shows a short term variation of spin-down and spin-up behaviours (Hu et al 2017). The period derivatives of these long period X-ray pulsars are not explored in full details yet.…”

The super-orbital modulation of supergiant high-mass X-ray binaries

“…Two groups are typically distinguished; the classical systems, which show a virtually persistent high X-ray luminosity, and the supergiant fast X-ray transients (SFXTs), which feature a still highly debated and peculiarly prominent variability in X-rays (Martínez-Núñez et al 2017). The bulk of the X-ray radiation from the SgXBs can be reasonably well explained as being due to the accretion of the stellar wind from the OB supergiant onto a highly magnetized neutron star (NS), with no evidence supporting the presence of longlived accretion disks (Bozzo et al 2008;Shakura et al 2012;Romano et al 2015;Hu et al 2017). No clear indication has yet been reported of systematic differences between the properties of the supergiant stellar winds in classical systems and SFXTs (Hainich et al, in prep.…”