X-ray study of the 'Winking Demon' reveals its flaring secrets
1 September 1999
An X-ray observation of the 'Winking Demon', one of the sky's nearest and X-ray brightest stars, has revealed that its huge X-ray explosions take place in structures similar in relative size to those of our own Sun - contrary to what had been previously suggested. The study by J|rgen Schmitt at Hamburg University and Fabio Favata at the ESA-ESTEC Space Science Department has just been published by the magazine 'Nature' (dated 1 September 1999) and in 'Astronomy and Astrophysics' (currently in press).The Arabs gave the name of Algol - meaning 'demon star' or 'winking star' - to the second brightest object in the Perseus constellation, about a hundred light-years away. The reason is that every 69 hours it dims from second to third magnitude (2.2 to 3.5) over a period of four hours, then taking the same time to regain its brilliance.
The 18th century English astronomer John Goodricke was the first to explain why Algol behaves this way. It is in fact a binary system, composed of two stars of different spectral types: a smaller, very hot and brilliant bluish 'B' star and a cooler, dimmer but more voluminous orange-tinted 'K' star. (* See note). They are some 10 million kilometres from each other and revolve around a central point (barycentre). Orbiting in our line of sight, the two stars periodically pass one in front of the other. The consequence is the regular 'wink' when the dim star eclipses its neighbour, causing the overall brightness to diminish.
Algol is perhaps the best known 'eclipsing binary' but there are many others. Its observation at radio, ultra-violet and X-ray wavelengths has also highlighted the appearance of giant flares, 100,000 times larger than a solar one!
Such differences of magnitude have led to the proposition that giant coronal structures up to a few times larger than the star itself, are the prerequisite for such huge flares. On the Sun, even the largest flares always come from structures small in comparison to the Sun itself.
For their X-ray observation in August 1997, Favata and Schmitt used the detectors aboard BeppoSAX, the Italo-Dutch high-energy astronomy satellite. One of the detectors used was the Low-Energy Concentrator Spectrometer 'LECS' built by the ESA-ESTEC Space Science Department.
They observed Algol for nearly three days and thus covered its entire binary orbit (2.8 days). Approximately 5 hours after the beginning of the observation, a very strong flare appeared, whose evolution dominated the rest of the observation. During the flare, the X-ray dark B star eclipsed the X-ray bright K star (the opposite of what's seen in visible light, in which the dimmer K star eclipses the bright B star), an event never observed previously.
J|rgen Schmitt and Fabio Favata ascertained that the flare was completely occulted during the eclipse. This allowed them to pinpoint its location and size: the flare took place on the K-type star, from a compact region above its South Pole. Using X-ray spectrographic data, they were also able to deduce the physical characteristics of the super-hot plasma and measure the intense energy that was released. This enabled Favata and Schmitt to deduce that the flare did not arise from a very large structure, as postulated previously, but from a structure similar in relative size - less than the stellar radius - to the ones seen on the Sun's surface.
"It is the first time that a stellar flare has really been measured," says Fabio Favata. "Until now, only very indirect methods were available to evaluate a flaring star's corona and they regularly indicated huge structures, often much bigger than the star itself. But we have shown that even for such giant flares their actual size is much smaller, and thus that they are still 'solar-like'."
J|rgun Schmitt adds: "We have also seen that strong magnetic fields are at work to confine the flaring plasma. The temperatures and strength of Algol's flare imply the presence, at distances of about half a stellar radius, of magnetic fields as strong as in the Sun, but only in its sunspots."
There now remains the challenge to reconstruct the solar-like scenario that allows such giant flares. Favata and Schmitt have no doubt that the XMM mission will allow significant advances in this field. The greater resolution, larger collecting area and grating spectrometers of ESA's new X-ray observatory will be able to reveal, for instance, much more about the chemical composition of the flaring plasma. But says Favata: "It may need a fair amount of luck to catch another eclipse like Algol's!"
*Note: Stars are classified in 11 main spectral classes - W, O, B, A, F, G, K, M, R, N and S - a sequence that ranges from very hot (W) to very cool (S). These spectral classes can be related to the star's luminosity or absolute brightness. The result is the 'Hertzsprung-Russel' diagram showing the various kinds of stars. Our Sun is a G-class star, halfway along the main plot, and fortunately for us, its coronal activity is much more subdued than a rapidly-rotating K-type star like Algol!
