Journal Article

<i>Swift</i> observations of two supergiant fast X-ray transient prototypes in outburst

R. Farinelli, P. Romano, V. Mangano, C. Ceccobello, L. Ducci, S. Vercellone, P. Esposito, J. A. Kennea and D. N. Burrows

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 424, issue 4, pages 2854-2863
Published in print August 2012 | ISSN: 0035-8711
Published online August 2012 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2012.21422.x
Swift observations of two supergiant fast X-ray transient prototypes in outburst

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ABSTRACT

We report on the results from observations of the most recent outbursts of XTE J1739−302 and IGR J17544−2619, which are considered to be the prototypes of the supergiant fast X-ray transient class. They triggered the Swift/Burst Alert Telescope on 2011 February 22 and March 24, respectively, and each time a prompt Swift slew allowed us to obtain the rich broad-band data we present. The X-ray Telescope light curves show the descending portion of very bright flares that reached luminosities of ∼2 × 1036 and ∼5 × 1036 erg s−1. The broad-band spectra, when fitted with the usual phenomenological models adopted for accreting neutron stars, yield values of both high-energy cut-off and e-folding energy consistent with those obtained from previously reported outbursts from these sources. In the context of more physical models, the spectra of both sources can be well fitted either with a two-blackbody model or with a single unsaturated Comptonization model. In the latter case, the model can be either a classical static Comptonization model, such as COMPTT, or the recently developed COMPMAG model, which includes thermal and bulk Comptonization for cylindrical accretion on to a magnetized neutron star. We discuss the possible accretion scenarios derived by the different models, and we also emphasize the fact that the electron density derived from the Comptonization models, in the regions where the X-ray spectrum presumably forms, is lower than that estimated using the continuity equation at the magnetospheric radius and the source X-ray luminosity, and we give some possible explanations.

Keywords: X-rays: binaries; X-rays: individual: XTE J1739−302; X-rays: individual: IGR J17544−2619

Journal Article.  6627 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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