- Be stars and Be/X-ray binaries -
 

Be/X-ray binaries are X-ray sources composed of a Be star and a compact object -- in all cases in which there is any certainty, the compact object is a neutron star. The high-energy radiation is believed to arise due to accretion of material associated with the Be star by the compact object.

The name "Be star" is used as a general term describing an early-type non-supergiant star, which at some time has shown emission in the Balmer series lines. Nowadays, we know that a Be star is an otherwise normal main-sequence to giant early-type star which is surrounded by a disc of material lost from the star. The causes that give rise to the disc are not well understood. Different mechanisms (fast rotation, non-radial pulsation, magnetic loops) have been proposed, but it is not yet clear if any of them seems can explain the observed phenomenology on its own. Currently the mainstream mood among people in the know seems to favour again models in which magnetic fields play the main role (perhaps with the help of non-radial pulsations), but still there are no measurements of magnetic fields in Be stars or any idea of how you generate a magnetic field in a radiative envelope.

This disc contributes line (the defining characteristic is HI emission, but generally also FeII and - for early spectral types - HeI) and continuum (due to free-free and free-bound interactions) emission to the spectral energy distribution of the Be star. The disc contribution is small in the blue region, except shortwards of the Balmer discontinuity, but increases steadily at longer wavelengths, resulting in what is known as "infrared excess". When the overall (star+disc) energy distribution is observed, this results in a redder spectrum than expected for the spectral type. This effect is known as "circumstellar reddening".

My paper "On the nature of Be/X-ray binaries", 1998, A&A 338, 505 (astro-ph 9807158) is a bit outdated now, but the phenomenology described is still the same.
A recent review by Janus Ziólkowski can be found at astro-ph 0208455

Be/X-ray binaries are generally distant objects placed on the Galactic plane. They are therefore at least moderately reddened. The observed spectral energy distribution will be a combination of the stellar spectrum + circumstellar emission reddened by the interstellar absorption. As a consequence, it is very difficult indeed to separate the circumstellar reddening and the interstellar reddening and hence to determine (even with approximation) things like the distance to the sources - the distance to the source is actually a very important observational parameter, since it is needed to determine the luminosities of X-ray sources, that then theoriticians will use to understand the physics of neutron stars.

An important part of my research is therefore devoted to obtaining good measurements of those parameters. A first step is obtaining accurate spectral types for the sources. For this purpose, we need spectroscopy with moderate resolution. Since the sources are reddened, this normally means using big telescopes, like the 4.2-m William Herschel Telescope, which we recently used to determine the spectral class of the B = 17.3 counterpart to the Be/X-ray transient V0332+53 in Negueruela et al., 1999, MNRAS 307, 695 (also at astro-ph 9903228).

 

Projects

Since I did my PhD on the optical properties of Be/X-ray binaries, I have always been involved in the study of these objects. At the moment, my main lines of research in this field are:

 

 
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Written and maintained by Ignacio N.. Last update on 18th January 2006.