Giant outburst from the supergiant fast X-ray transient IGR J17544-2619: accretion from a transient disc?

TL;DR

This paper reports on a giant outburst from the SFXT IGR J17544-2619, with unprecedented luminosity and dynamic range, suggesting transient disc formation as the accretion mechanism.

Contribution

It presents the first evidence challenging existing models by showing a giant outburst likely caused by a transient accretion disc in an SFXT.

Findings

Peak luminosity of 3×10^38 erg/s during outburst

Dynamic range exceeding 10^6, the largest recorded for an SFXT

Tentative detection of 11.6 s pulsations

Abstract

Supergiant fast X-ray transients (SFXTs) are high mass X-ray binaries associated with OB supergiant companions and characterised by an X-ray flaring behaviour whose dynamical range reaches 5 orders of magnitude on timescales of a few hundred to thousands of seconds. Current investigations concentrate on finding possible mechanisms to inhibit accretion in SFXTs and explain their unusually low average X-ray luminosity. We present the Swift observations of an exceptionally bright outburst displayed by the SFXT IGR J17544-2619 on 2014 October 10 when the source achieved a peak luminosity of $3\times10^{38}$ erg s$^{-1}$. This extends the total source dynamic range to $\gtrsim$10$^6$, the largest (by a factor of 10) recorded so far from an SFXT. Tentative evidence for pulsations at a period of 11.6 s is also reported. We show that these observations challenge, for the first time, the maximum theoretical luminosity achievable by an SFXT and propose that this giant outburst was due to the formation of a transient accretion disc around the compact object.