Star of the Month
X Persei AKA: HR 1209; HD 24534; 4U 0352+309
Position: 03 hrs 55 min 23.1 sec +31 degrees 02 min 45 sec
Due south at 22.17 (GMT) on 15 December 2021
X Persei (shown with a cross)
Image: Simbad (http://simbad.u-strasbg.fr/simbad/)
If you look below Zeta Persei, a 2.9 magnitude star at the foot of the constellation of Perseus, you can spot a 6.8 magnitude bluish-white star of no apparent interest which is 2,643 light years away. Even if I remark that it is a variable star called X Persei, you might only be moderately interested. However if I then told you that it is a high mass X-ray binary system, you may be more excited. How can a seemingly ordinary star be an X-ray source? The answer lies with the hidden member of the binary system, a neutron star which is also a pulsar with a particularly long pulse of 837 seconds; just under 14 minutes. The visible companion is a rapidly rotating Be star (see the Double Star for December 2021 for two more Be stars) which throws off matter as it spins, and this disc of expelled matter is gathered up by the neutron star. This accretion process produces X-ray flares – similar to a black hole, but on a much smaller scale – and between 1973 and 1979, it produced particularly strong X-ray flares and was duly registered as 4U 0352+309 in the final Uhuru catalogue of X-ray sources in 1978. Since then, the X-ray flares have quietened down. This may or may not be associated with the visible member’s loss of its circumstellar excretion shell in 1989-90, when it briefly became an ordinary B star. It has now reverted to being a Be star and it is possible that the strength of the X-ray may increase in the future.
However X Persei was known to be a variable star long before pulsars and X-ray sources were discovered; that X Persei is an X-ray source is just a coincidence. First noted back in 1888, its magnitude varied between 6 and 7 and in the first half of the 1960s it was varying between 6.0 and 6.5. This irregular variability is a result of its circumstellar shell and it is hence a member of the family of Gamma Cassiopeiae variables; the archetype Gamma Cassiopeiae being the central star in the famous W of Cassiopeia. Then it suddenly brightened to 6.0 in 1988 before dimming to 6.8 where it remained more or less unchanged up to 1992. This is obviously related to the loss of its circumstellar shell. There may have been an earlier less marked “naked” period in the mid-1970s. To find X Persei, look for the first bright star above the Pleiades, which is Zeta Persei. Just below Zeta Persei are two sixth magnitude stars which thus form a triangle with the brighter star. X Persei is the left-hand or eastern star of the pair. When you look at X Persei through binoculars or a telescope, be grateful that your eye and indeed all life is protected from X Persei’s X-ray flares by our planet’s thick atmosphere.