Letter to the Editor

Hard X-ray flares in IGR J08408–4503 unveil clumpy stellar winds

¹ Institut d'Astrophysique et de Géophysique, Université de Liège, Allée du 6-Août 17, Bâtiment B5c, 4000 Liège, Belgium e-mail: leyder@astro.ulg.ac.be
² Science Data Centre, Université de Genève, Chemin d'Écogia 16, 1290 Versoix, Switzerland
³ Observatoire de Genève, Université de Genève, Chemin des Maillettes 51, 1290 Sauverny, Switzerland
⁴ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁵ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna, via Gobetti 101, 40129 Bologna, Italy (formerly IASF/CNR, Bologna)

Received: 29 August 2006
Accepted: 14 February 2007

Abstract

Context.A 1000-s flare from a new hard X-ray transient, IGR J08408–4503, was observed by INTEGRAL on May 15, 2006 during the real-time routine monitoring of IBIS/ISGRI images performed at the INTEGRAL Science Data Centre. The flare, detected during a single one-hour long pointing, peaked at 250 mCrab in the 20–40 keV energy range.

Aims.Multi-wavelength observations, combining high-energy and optical data, were used to unveil the nature of IGR J08408–4503.

Methods.A search in all INTEGRAL public data for other bursts from IGR J08408–4503 was performed, and the detailed analysis of another major flare is presented. The results of two Swift Target of Opportunity observations are also described. Finally, a study of the likely optical counterpart, HD 74194, is provided.

Results.IGR J08408–4503 is very likely a supergiant fast X-ray transient (SFXT) system. The system parameters indicate that the X-ray flares are probably related to the accretion of wind clumps on a compact object orbiting about 10¹³ cm from the supergiant HD 74194. The clump mass loss rate is of the order of 10^-6 yr^-1.

Conclusions.Hard X-ray flares from SFXTs allow to probe the stellar winds of massive stars, and could possibly be associated with wind perturbations due to line-driven instabilities.