INTEGRAL study of temporal properties of bright flares in Supergiant Fast X-ray Transients

TL;DR

This study analyzes the timing and duration of bright X-ray flares in Supergiant Fast X-ray Transients using INTEGRAL data, revealing characteristic timescales, heavy-tailed distributions, and correlations that support a model involving fractal stellar winds.

Contribution

It provides the first detailed characterization of flare timing and outburst durations in SFXTs, identifying typical timescales and correlations, and supports a wind model with fractal structure.

Findings

Identified characteristic timescales separating different outbursts.

Found heavy-tailed distributions for outburst durations.

Discovered a correlation between total emitted energy and outburst duration.

Abstract

We have characterized the typical temporal behaviour of the bright X-ray flares detected from the three Supergiant Fast X-ray Transients showing the most extreme transient behaviour (XTEJ1739-302, IGRJ17544-2619, SAXJ1818.6-1703). We focus here on the cumulative distributions of the waiting-time (time interval between two consecutive X-ray flares), and the duration of the hard X-ray activity (duration of the brightest phase of an SFXT outburst), as observed by INTEGRAL/IBIS in the energy band 17-50 keV. Adopting the cumulative distribution of waiting-times, it is possible to identify the typical timescale that clearly separates different outbursts, each composed by several single flares at ks timescale. This allowed us to measure the duration of the brightest phase of the outbursts from these three targets, finding that they show heavy-tailed cumulative distributions. We observe a correlation between the total energy emitted during SFXT outbursts and the time interval covered by the outbursts (defined as the elapsed time between the first and the last flare belonging to the same outburst as observed by INTEGRAL). We show that temporal properties of flares and outbursts of the sources, which share common properties regardless different orbital parameters, can be interpreted in the model of magnetized stellar winds with fractal structure from the OB-supergiant stars.