# Type I outbursts in low eccentricity Be/X-ray binaries

## TL;DR

This paper demonstrates through simulations and analysis that nearly circular Be/X-ray binaries can develop eccentric disks via Lindblad resonances, explaining type I outbursts in low-eccentricity systems.

## Contribution

It introduces a novel mechanism where disk eccentricity arises in circular binaries, accounting for observed outbursts previously unexplained by orbital eccentricity.

## Key findings

- Eccentricity growth driven by 3:1 Lindblad resonance in nearly circular binaries.
- Neutron star captures material at disk apastron, causing outbursts.
- Mechanism explains type I outbursts in low-eccentricity Be/X-ray binaries.

## Abstract

Type I outbursts in Be/X-ray binaries are usually associated with the eccentricity of the binary orbit. The neutron star accretes gas from the outer parts of the decretion disk around the Be star at each periastron passage. However, this mechanism cannot explain type I outbursts that have been observed in nearly circular orbit Be/X-ray binaries. With hydrodynamical simulations and analytic estimates we find that in a circular orbit binary, a nearly coplanar disk around the Be star can become eccentric. The extreme mass ratio of the binary leads to the presence of the 3:1 Lindblad resonance inside the Be star disk and this drives eccentricity growth. Therefore the neutron star can capture material each time it approaches the disk apastron, on a timescale up to a few percent longer than the orbital period. We have found a new application of this mechanism that is able to explain the observed type I outbursts in low eccentricity Be/X-ray binaries.