EDP Sciences
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Volume 492, Number 1, December II 2008
Page(s) 135 - 143
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20077146
Published online 01 October 2008

A&A 492, 135-143 (2008)
DOI: 10.1051/0004-6361:20077146

A group of outbursts with exponential decays in the Rapid Burster

V. Šimon

Astronomical Institute, Academy of Sciences of the Czech Republic, 251 65 Ondřejov, Czech Republic
    e-mail: simon@asu.cas.cz

Received 22 January 2007 / Accepted 13 August 2008

Aims. We present an analysis focused on a specific group of six intense outbursts (abbreviated as group A) that displays strong persistent emission in the unique low-mass X-ray binary Rapid Burster (MXB 1730–335).
Methods. Observations taken from $AS\!M/RXT\!E$ in the 1.5–12 keV passband were utilized.
Results. We interpret the exponential decay of these outbursts as arising from irradiation of the disk by X-rays strong enough to ionize all of the disk out to its outer edge without self-shadowing, using earlier models. The parameters of irradiation apparently remained stable with a relatively high degree of consistency for all six events of group A, but they underwent a large and rapid change in twelve outbursts of the subsequent set (group B). We argue that this was because of a change in the conditions for the disk irradiation. We interpret the irradiation in terms of the slim accretion disk region. We find that the vertical dimension of the irradiating source, hence its ability to irradiate the disk, decreases with the increasing luminosity of the outbursts with exponential decays in neutron-star, soft X-ray transients of a comparable orbital period length, and is exceptionally small in the Rapid Burster. The group A outburst profiles indicate that the Rapid Burster is an ultra-compact system. The difference between the intensities and profiles of the group A and group B outbursts is not caused by a different absorption and the irradiating source remains the same for both groups. Fluctuations in the outburst profile display striking regularities, not only inside each group, but also between both groups. We interpret them as due to the spiral arms reaching the disk center. The very rapid rise of outbursts in both groups speaks in favor of a small radius for the optically thin, advection-dominated flow prior to an outside-in outburst. We argue in favor of a relatively weak magnetic field of the neutron star in this system and against a significant role for the propeller effect.

Key words: stars: neutron  -- accretion, accretion disks  -- binaries: close  -- circumstellar matter  -- X-rays: binaries  -- stars: individual: Rapid Burster

© ESO 2008