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

¹ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica – Palermo, via U. La Malfa 153, 90146 Palermo, Italy
e-mail: romano@ifc.inaf.it
² ISDC Data Center for Astrophysics, Université de Genève, 16 chemin d’Écogia, 1290 Versoix, Switzerland
³ Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802, USA
⁴ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica – Milano, via E. Bassini 15, 20133 Milano, Italy
⁵ Harvard–Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁶ INAF, Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone, Italy
⁷ Istituto Universitario di Studi Superiori, Piazza della Vittoria 15, 27100 Pavia, Italy
⁸ INFN, Sezione di Pavia, via A. Bassi 6, 27100 Pavia, Italy
⁹ INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
¹⁰ Institut für Astronomie und Astrophysik, Eberhard Karls Universität, Sand 1, 72076 Tübingen, Germany

Received: 26 January 2015
Accepted: 12 February 2015

Abstract

Supergiant fast X-ray transients (SFXTs) are high mass X-ray binaries associated with OB supergiant companions and characterized by an X-ray flaring behaviour whose dynamical range reaches 5 orders of magnitude on time scales of a few hundred to thousands of seconds. Current investigations concentrate on finding possible mechanisms to inhibit accretion in SFXTs and to 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 × 10³⁸ erg s^-1. This extends the total source dynamic range to ≳10⁶, 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.