# Supergiant Fast X-ray Transients uncovered by the EXTraS project: flares reveal the development of magnetospheric instability in accreting neutron stars

## TL;DR

This study analyzes X-ray flares from Supergiant Fast X-ray Transients using XMM-Newton data, revealing that their flaring activity aligns with magnetospheric instability models, and provides detailed flare characteristics.

## Contribution

It offers a comprehensive analysis of SFXT flares using Bayesian methods, linking observed properties to magnetospheric instability theories and accretion processes.

## Key findings

- 144 flares identified across a luminosity range of 10^32 to 10^36 erg/s.
- Flares' temporal properties match predictions from Rayleigh-Taylor instability models.
- Rapidly rotating neutron stars may experience sporadic accretion from temporary disks.

## Abstract

The low luminosity, X-ray flaring activity, of the sub-class of high mass X-ray binaries called Supergiant Fast X-ray Transients, has been investigated using XMM-Newton public observations, taking advantage of the products made publicly available by the EXTraS project. One of the goals of EXTraS was to extract from the XMM-Newton public archive information on the aperiodic variability of all sources observed in the soft X-ray range with EPIC (0.2-12 keV). Adopting a Bayesian block decomposition of the X-ray light curves of a sample of SFXTs, we picked out 144 X-ray flares, covering a large range of soft X-ray luminosities (1e32-1e36 erg/s). We measured temporal quantities, like the rise time to and the decay time from the peak of the flares, their duration and the time interval between adjacent flares. We also estimated the peak luminosity, average accretion rate and energy release in the flares. The observed soft X-ray properties of low-luminosity flaring activity from SFXTs is in qualitative agreement with what is expected by the application of the Rayleigh-Taylor instability model in accreting plasma near the neutron star magnetosphere. In the case of rapidly rotating neutron stars, sporadic accretion from temporary discs cannot be excluded.