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A close binary corona emitting with the power of 10,000 X-ray Suns

posted Feb 6, 2014, 8:08 AM by Jeremy Drake   [ updated Feb 6, 2014, 8:08 AM ]
In a post from 2013 May, I described how the magnetic field of a star like the Sun depends on its rotation rate - the faster the rotation, the stronger the magnetic fields that are generated.  Magnetic fields on the Sun can be seen indirectly by their effect on the the solar atmosphere.  The gas is cooler within strong magnetic fields and is manifest as the dark sunspots on the solar disk.  These fields sustain million degree gas just above the visible surface of the Sun - the solar corona - and are responsible for flares and coronal mass ejections that hurl magnetised globs of hot plasma throughout the solar system at speeds of several hundred kilometers a second.  But the Sun is a slow rotator, with a weak magnetic field compared to some stars.  

AR Lacertae is one of the class of RS Canun Venaticorum type of variable stars. These systems comprise a pair of solar-like stars in close orbit with periods as short as a day or two.  Their tidal interaction makes them synchronous rotators - the rotation periods of the stars are the same and synchronised to their orbital period, much like that of the Moon (but not the Earth!).  AR Lac contains G-type and K-type subgiant stars that rotate once every two days - more than ten times faster than the Sun.  This rotation generates very strong magnetic fields and huge "starspots" on the stellar surfaces that cause visible light variations as the spots rotate into and out of view.  The X-ray intensity from the coronae of AR Lac is 10,000 times that of the Sun.  AR Lac is also special because it is an eclipsing system, and we can use the X-ray eclipses to work out how far the hot coronal gases extend above the stellar surface.  The Chandra X-ray observatory has monitored AR Lac for the past 13 years and we used the data to work out that the corona is brightest on the G star (see the illustration) and extends about one stellar radius above its surface.  This is several times larger the solar X-ray corona.   So why is it as much as 10,000 times brighter than the Sun in X-rays?  The X-ray emitting power of a plasma varies with the square of the plasma density and the hot plasma in the AR Lac coronae is also more dense than in the solar corona.  We could also compare the data to observations from earlier X-ray satellites, and found the X-ray intensity has remained fairly constant for at least the last 30 years.  This suggests the AR Lac stars do not undergo strong magnetic cycles like the Sun, whose X-ray intensity changes by a factor of 10 or more with an 11 year period.  This work was published in the 2014 February 5 edition of the Astrophysical Journal.