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Identifier

000465801

Title

Optical polarimetry in Be/X-ray binaries : warped and precessed disks during type-II outbursts

Alternative Title

Οπτική πολωσιμετρία σε δυαδικά συστήματα "Be/X-ray binaries": παραμορφωμένοι δίσκοι κατά τη διάρκεια εκρήξεων τύπου-ΙΙ

Author

Τζουβάνου, Αναστασία Χ.

Thesis advisor

Reig, Pablo

Reviewer

Ζέζας, Ανδρέας
Αντωνιάδης, Ιωάννης

Abstract

The mechanisms by which matter from the circumstellar disk of the Be star is transferred to the compact object, which in most cases is a neutron star, to trigger the X-ray outbursts observed in Be/X-ray binaries remain an open question. Current theoretical models suggest that Type-II outbursts occur when the compact object captures an enormous amount of gas from a warped, highly misaligned, and eccentric Be disk. The aim of this work is to test the current theories of warped, highly misaligned, and eccentric Be disk thought optical polarimetry. We used optical polarimetric observations of three Be/X-ray binaries for ten years, from 2013 until 2023, to explore the characteristics of their disk and compare them with those of classical isolated Be stars without companion. Our results, after accounting for instrumental polarization and interstellar medium effects, show that two out of the three systems exhibit intrinsic polarization similar to isolated Be stars, with polarization degrees below 2%. An exception to this was the third Be/X-ray binary system - EXO 2030+375 - which exhibits intrinsic polarization up to 10%. This high degree of polarization has been associated with the alignment of ferromagnetic grains from the Be disk with the neutron star’s magnetic field. Significant changes over time were observed both in polarization degree as well as in polarization angle in all three binary systems. This variability indicates the presence of a warped and precessed disk, in agreement with theoretical models. The hypothesis that the disk around the Be star grows freely and gets more and more warped, precessed, and eccentric and eventually transfers mass to the neutron star is in line with our observational work. These findings enhance our understanding of the interaction between neutron stars and Be star decretion disks in Be/X-ray binaries.

Language

English

Subject

Circumstellar matter

Emission-line

Neutron – stars

Polarization

Αστέρες νετρονίων

Γραμμές εκπομπής Βe δυαδικά συστήματα ακτίνων-Χ

Περιαστρική ύλη

Πόλωση

Issue date