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X-ray pulsar takes the plunge

posted Jul 27, 2016, 6:41 AM by Jeremy Drake
Smithsonian astrophysicist Vallia Antoniou and University of Crete astronomer Andreas Zezas are leading an international team studying the Small Magellanic Cloud using an extensive series of observations made by the Chandra X-ray Observatory. Lying "just" 200,000 light years away, the SMC is our closest galaxy.  Much smaller than the Milky Way and termed a "dwarf irregular galaxy", the SMC harbours an interesting population of stars.  Having undergone extensive episodes of star formation over the last few million to tens of millions of years - very recently in astronomical terms when the age of the Universe is measured in billions of years - it is a good place to go hunting for interesting younger and more massive stars. One such object retrieved from the net of Chandra observations is the X-ray pulsar SXP214.

X-ray pulsars are magnetized neutron stars in a close binary system with a more normal stellar companion. The neutron star scavenges
gas from the companion though gravitational attraction, and the ionized gas is channeled onto the magnetic poles of the star to form X-ray emitting hotspots at temperatures of millions of degrees.  Like a lighthouse, the hotspots rotate in and out of sight giving rise to X-ray pulsations - hence the X-ray pulsar monicker.

SXP214 was discovered by Southampton University astronomer Malcolm Coe from X-ray observations made back in 2009. It is a neutron star spinning with a period of about 200s in an eccentric orbit around a "Be star".  Be-type stars are young stars several times more massive than the Sun and rotate so rapidly that they fling material off themselves to form a circumstellar disk.  This disk proved key to understanding the Chandra observations of SXP214.  In a study lead by Smithsonian astrophysicist JaeSub Hong, the X-ray emission from the neutron star during the 14 hours of observations was found to both brighten and become "softer" - become relatively more bright at lower X-ray energies.  The neutron star was caught in the act of plunging through the disk.  Material in the disk fed the accretion onto the star, making its X-ray emission brighter. As it emerged, the lower energy X-rays that were absorbed by the murky disk began to shine through.  The study was published in the 2016 July 20 edition of the Astrophysical Journal, and also features in an American Astronomical Society NOVA research highlight.