B-ReX: Reverse Transition in Be/X-ray Binaries

• B-ReX is a reverse state transition in Be/X-ray binaries, characterized by a shift from strong, regular type I outbursts and spin-up to weak, irregular outbursts and spin-down in systems like EXO 2030+375.
• Multiwavelength observations with Swift/XRT, NuSTAR, and optical spectroscopy reveal that a decreased Hα equivalent width signals a restructuring of the Be star’s disk, impacting accretion torque.
• The study identifies a quasi-periodic (~21-year) pattern in torque reversals and disk changes, providing actionable insights into the coupling between optical diagnostics and X-ray accretion processes.

“B-ReX” (Editor’s term) may be used to denote the reverse-transition episode observed around 2015–2016 in the transient Be X-ray binary pulsar EXO 2030+375, as reported in “EXO 2030+375 Restarts in Reverse” (Kretschmar et al., 2017). In that episode, a system that had shown regular type I X-ray outbursts near periastron and a long monotonic spin-up evolved into progressively fainter and less regular outbursts, flattening of the spin-up to almost zero, and a return to spin-down. The episode is of interest in Be/X-ray-binary research because it ties torque evolution and outburst phenomenology to changes in the Be star’s circumstellar disk, traced optically through the H$\alpha$ line equivalent width, while also constraining whether the source had entered centrifugal inhibition of accretion.

1. Astrophysical setting

EXO 2030+375 is described as a well-known transient Be X-ray binary discovered during a giant outburst in 1985. The system consists of a neutron star accreting matter from the decretion disk of a Be companion, and its orbital period is given as 46 days (Kretschmar et al., 2017). In this class of systems, type I X-ray outbursts near periastron are expected when the neutron star interacts with the Be-star disk at closest approach.

Since 1991, normal outbursts had occurred at nearly every periastron passage. These outbursts were initially bright, and the neutron star was spinning up. In the standard Be/XRB picture summarized by the source, this combination indicates efficient feeding of the neutron star by the Be disk, producing both periodic X-ray brightening and positive torque. The system therefore serves as a long-baseline case study of how decretion-disk evolution modulates accretion geometry and neutron-star spin behavior.

2. Long-term torque and outburst history before the 2015 transition

The source presents a structured long-term history in which the outburst amplitude, outburst phase, and spin evolution changed coherently over multi-year intervals (Kretschmar et al., 2017).

Epoch	Reported behavior	Interpretation in the source
1992–1994	Bright outbursts; spin-up	Efficient disk-fed accretion
Afterward	Sudden flux drop; global spin-down trend	Reduced angular-momentum transfer
1995	Type I outbursts shifted 8–9 days earlier in orbital phase	Changed disk-interaction geometry
From 2002 onward	Outbursts brightened again; spin reversed back to spin-up	Re-established stronger feeding
Summer 2006	Giant outburst significantly accelerated the pulsar	Strong torque episode
More than eight years after 2006	Regular outbursts and monotonic spin-up, with gradually decreasing amplitude	Long-lived but slowly weakening accretion state

This chronology is significant because it shows that neither torque nor the timing of type I outbursts is fixed in EXO 2030+375. Instead, both depend on the state of the Be disk. A plausible implication is that EXO 2030+375 provides an unusually clear example of decadal disk-driven state changes in a Be/XRB, with the 2015–2016 reverse transition echoing an earlier low state.

3. The 2015 decline and emergence of a low-flux state

A central result is that in early 2015 the source behavior changed again. The outbursts became progressively fainter and less regular; first every second outburst was much fainter than usual, and eventually there was hardly any outburst activity (Kretschmar et al., 2017). At the same time, the long-term spin-up flattened to almost zero.

The optical record changed in parallel. A decrease in the H$\alpha$ equivalent width was reported, indicating that the disk surrounding the mass donor had changed. The source interprets the H$\alpha$ equivalent width as evidence that the Be-star disk had likely shrunk or otherwise become less effective at feeding the neutron star. Because H$\alpha$ equivalent width is commonly used as a proxy for the size or strength of the circumstellar disk, this provides a direct link between the optical state of the donor and the X-ray torque state.

This phase is the core of the B-ReX concept in the present usage: a reverse evolution from regular periastron outbursts and positive torque toward irregular, weak activity and renewed negative torque. A common misreading would be to treat the faint state as automatically equivalent to propeller onset; the observations summarized below do not support that conclusion.

4. Multiwavelength observing campaign

To probe the poorly explored low-flux state and the possibility of a transition to centrifugal inhibition of accretion, the authors undertook a campaign with Swift/XRT, NuSTAR, and the Nordic Optical Telescope (NOT), and the conference contribution reports preliminary results from that campaign (Kretschmar et al., 2017).

Swift/XRT provided the backbone of the X-ray monitoring. The program began with a few snapshots and then expanded to extended monitoring, with tighter sampling around expected outburst peaks and wider spacing elsewhere in the orbit. Swift/XRT was used to track flux evolution, pulse detection at low luminosity, spectral absorption, and the timing and torque state.

NuSTAR contributed one 60 ks observation, extending the X-ray coverage to harder energies and complementing Swift/XRT during faint phases and outburst evolution. The NOT contributed four optical spectra of the H$\alpha$ line, which were used to trace the circumstellar disk around the Be star. The campaign design is notable because it combines torque diagnostics, low-flux pulse searches, broadband spectral characterization, and direct disk monitoring within a single observing framework.

5. Accretion regime in the faint state

A key observational result is that low-level pulsed emission was still detected in all targeted X-ray observations. The source states that there was no sign of a magnetospheric cutoff of emission, and therefore no clear propeller effect (Kretschmar et al., 2017). Physically, this means that the system had not yet made a full transition into a state where the neutron-star magnetosphere centrifugally prevents accretion onto the surface.

The paper further argues that the observed faint-state luminosity is still high enough that accretion directly from the stellar wind can account for it. This is an important qualification: the source may have been approaching centrifugal inhibition of accretion, but the data do not indicate that a full propeller regime had already been reached.

The spectral behavior was comparatively stable. Absorption and spectral hardness were rather constant despite large flux changes, except in one observation during the last monitored outburst, where much higher absorption was found. The proposed explanation is geometrical: the neutron star may have passed behind the Be-star disk. This is consistent with the broader Be/XRB picture in which line-of-sight absorption can vary as the compact object traverses or is occulted by dense disk material.

6. Second torque reversal, optical disk restructuring, and quasi-periodicity

Both Fermi/GBM and Swift/XRT showed that EXO 2030+375 had returned to spin-down, constituting a torque reversal. The source emphasizes that this reversal is “remarkably similar in duration and magnitude” to the one seen about 21 years earlier, and that the orbital phase shift of outburst peaks resembles the earlier episode (Kretschmar et al., 2017). Because the same time interval separates the earlier torque reversal and the new one, and also separates the two known giant outbursts, the authors suggest a $\sim 21$ year quasi-period.

The optical spectroscopy provides independent evidence for disk restructuring. The H$\alpha$ profile evolved clearly between the first two NOT observations, on June 15, 2016 (MJD 57554) and August 1, 2016 (MJD 57601). The interpretation offered is a major change in the disk’s structure, density gradient, size, or geometry. After that, the profile remained similar in the remaining observations, suggesting that the disk had reached a new, more stable state. The minimum H$\alpha$ equivalent width measured during the campaign was similar to values seen in earlier low states, including those associated with the 1995 torque reversal and the post-giant-outburst epoch.

The quasi-period claim should be read narrowly. The source suggests a $\sim 21$ year quasi-period; it does not establish a strict periodic law. What is firmly supported is the recurrence of a characteristic pattern: weakening of the Be disk, fading and irregularization of type I outbursts, flattening and reversal of the spin trend, and renewed evidence for altered orbital-phase behavior. In that sense, B-ReX denotes a disk-coupled reverse state transition in a canonical Be/XRB, observed through simultaneous X-ray and optical diagnostics.