Warped disks during type II outbursts in Be/X-ray binaries: evidence from optical polarimetry

Abstract

Context. Current models that explain giant (type II) X-ray outbursts in Be/X-ray binaries (BeXB), are based on the idea of highly distorted disks. They are believed to occur when a misaligned and warped disk becomes eccentric, allowing the neutron star to capture a large amount of material. The BeXB 4U 0115+63 underwent two major outbursts in 2015 and 2017. Aims. Our aim is to investigate whether the structural changes in the disk expected during type II outbursts can be detected through optical polarimetry. Methods. We present the first optical polarimetric observations and new optical spectra of the BeXB 4U 0115+63 covering the period 2013-2017. We study in detail the shape of the Hα line profile and the polarization parameters before, during, and after the occurrence of a type II X-ray outburst. Results. We find significant changes in polarization degree and polarization angle and highly distorted line profiles during the 2017 X-ray outburst. The degree of polarization decreased by ∼ 1%, while the polarization angle, which is supposed to be related with the disk orientation, first increased by ∼ 10 • in about two months and then decreased by a similar amount and on a similar timescale once the X-ray activity ceased. Conclusions. We interpret the polarimetric and spectroscopic variability as evidence for the presence of a warped disk.

Study of recent outburst in the Be/X-ray binary RX J0209.6−7427 with AstroSat: a new ultraluminous X-ray pulsar in the Magellanic Bridge?

“…The Type II outbursts are more luminous (typical luminosity > 10 37 ergs s −1 ), less common and these outbursts are probably caused by episodes of sudden mass eruption from the Be star or due to accretion from the warped disc ⋆ E-mail: amar.deo.chandra@gmail.com of the Be star which does not lie in the orbital plane of the binary system. However the exact mechanism of the eruptions is still poorly understood (Reig 2011;Cheng et al 2014;Reig & Blinov 2018).…”

Optical and Near-Infrared Monitoring of Gamma-ray Binaries Hosting Be Stars

“…Spectropolarimetric observations of Be stars over decades have shown variation in V-band polarization as the Be disk dissipates and a new disk shows up [40]. Polarimetric observation of the Be/X-ray binary 4U 0115+63, covering a giant outburst, has shown that the polarization degree and angle changed significantly while the Be disk warped [41].…”

Eccentric neutron star disk driven type II outburst pairs in Be/X-ray binaries

“…Thanks to its very well constrained orbital parameters, many models for Be/X-ray binaries are based on this source . Giant outbursts in this system have been associated with the presence of a tilted disk around the Be star (Reig et al 2007;Martin et al 2011;Kato 2014) and the timescale between subsequent giant outbursts has been estimated to be 3 yrs, even though sometimes these outbursts can occur 1-1.5 yrs apart (Reig et al 2007;Reig & Blinov 2018). Note that, for this system, the ZLK oscillations of the Be star disk might operate on a timescale that matches the observed 3 yr frequency of type II outbursts, provided that the disk is not completely destroyed during the outburst .…”

“…In general, type II outbursts last several binary orbital periods (Kretschmar et al 2012). Reig & Blinov (2018) found that type II outbursts often occur in pairs with the second one being less luminous than the first. Type II outbursts may be explained by the presence of a highly misaligned Be star disk that becomes eccentric due to the Von Zeipel-Lidov-Kozai (ZLK) mechanism (von Zeipel 1910;Kozai 1962;Lidov 1962) and transfers mass through Roche lobe overflow onto the neutron star (Martin et al 2014b,a;Fu et al 2015a;.…”