The supergiant fast X-ray transients XTE J1739-302 and IGR J08408-4503 in quiescence with XMM-Newton 

¹ ISDC – Science Data Centre for Astrophysics, University of Geneva, Chemin d'Écogia 16, 1290 Versoix, Switzerland e-mail: enrico.bozzo@unige.ch
² INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00044 Rome, Italy
³ Dipartimento di Fisica – Università di Roma Tor Vergata, via della Ricerca Scientifica 1, 00133 Rome, Italy
⁴ International Space Science Institute (ISSI) Hallerstrasse 6, 3012 Bern, Switzerland
⁵ INAF – Osservatorio Astronomico di Brera, via Emilio Bianchi 46, 23807 Merate (LC), Italy
⁶ Institut d'Astrophysique et de Géophysique de l'Université de Liège, 17 allée du 6 août, 4000 Liège, Belgium

Received: 18 January 2010
Accepted: 9 April 2010

Abstract

Context. Supergiant fast X-ray transients are a subclass of high mass X-ray binaries that host a neutron star accreting mass from the wind of its OB supergiant companion. They are characterized by an extremely pronounced and rapid variability in X-rays, which still lacks an unambiguous interpretation. A number of deep pointed observations with XMM-Newton  have been carried out to study the quiescent emission of these sources and gain insight into the mechanism that causes their X-ray variability.

Aims. We continued this study by using three XMM-Newton observations of the two supergiant fast X-ray transient prototypes XTE J1739-302 and IGR J08408-4503 in quiescence.

Methods. An in-depth timing and spectral analysis of these data have been carried out.

Results. We found that the quiescent emission of these sources is characterized by both complex timing and spectral variability, with multiple small flares occurring sporadically after periods of lower X-ray emission. Some evidence is found in the XMM-Newton spectra of a soft component below ~2 keV, similar to that observed in the two supergiant fast X-ray transients AX J1845.0-0433 and IGR J16207-5129 and in many other high mass X-ray binaries.

Conclusions. We suggest some possible interpretations of the timing and spectral properties of the quiescent emission of XTE J1739-302 and IGR J08408-4503 in the context of the different theoretical models proposed to interpret the behavior of the supergiant fast X-ray transients.