TY - JOUR

T1 - Warped disks during type II outbursts in Be/X-ray binaries

T2 - Evidence from optical polarimetry

AU - Reig, P.

AU - Blinov, D.

PY - 2018/11/1

Y1 - 2018/11/1

N2 - 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.

AB - 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.

KW - Polarization

KW - Stars: emission-line Be

KW - Stars: individual: 4U0115+63

KW - X-rays: binaries

UR - http://www.scopus.com/inward/record.url?scp=85056114204&partnerID=8YFLogxK

U2 - 10.1051/0004-6361/201833649

DO - 10.1051/0004-6361/201833649

M3 - Article

AN - SCOPUS:85056114204

VL - 619

JO - ASTRONOMY & ASTROPHYSICS

JF - ASTRONOMY & ASTROPHYSICS

SN - 0004-6361

M1 - 33649

ER -