A unique star

3 June 2014. Using ESA’s X-ray telescope XMM-Newton a team of Potsdam astronomers together with collegues from Belgium and the USA have found X-ray pulsations of a unique star. It is a celestial wonder with a body of a normal star but with the magnetic field much stronger than normal. The race is now on to understand why it behaves in this way.
A unique star

The optical + X-ray image uses DSS. Credit: L. Oskinova (University Potsdam), ESA and STScI Digitized Sky Survey. DSS

The star is known as Xi1 Canis Majoris, it lies some 1500 light years away yet it can be seen with the naked eye. This is because the star is very bright and hot with a surface temperature of about 27500 K, and its mass approximately 15 times that of the Sun. This massive star draws particular attention to itself because it has an immensely strong magnetic field that was discovered by Swetlana Hubrig (AIP, Potsdam). The field is 5000 times stronger that the magnetic field of the Sun!

The star's magnetic field is carried into space by a river of particles flowing outwards from the star. This is known as a stellar wind and is extensively studied by Helge Todt and Wolf-Rainer Hamann, both from UP, Potsdam.

The star also shines brightly in X-rays. Lidia Oskinova (UP, Potsdam) was leading the study of this star using the European Space Agency (ESA) telescope XMM-Newton. Despite the star's fearsome temperature, it is not hot enough to emit appreciable X-rays. Instead, the X-rays are thought to come from shock waves in the star's magnetic field.

The team of Potsdam astronomers from AIP and Potsdam University, together with colleagues from three different institutes on both sides of Atlantic, discovered something very unexpected when analyzing the X-ray observations. The star's X-ray emission pulsated, rising and falling every 5 hours. Such pulsations have never before seen from normal stars. The results of this study is published in the journal Nature Communications.

The only stars previously known to give off pulsating X-rays were stellar remnants known as degenerate stars. These stars are very small, a white dwarf is about the size of the Earth, while a neutron star is only 10 km across. The mechanisms which produce X-ray pulsations in these degenerate object cannot work for Xi1 CMa that consists of normal matter.

A clue why Xi1 CMa is pulsating in X-rays may come from the fact that it pulsates in optical light as well. This optical variability has been known for a century. The pulses are extremely stable, changing by less than a second per century. The Potsdam team used the data from another ESA space mission Hipparcos that observed Xi1 CMa among millions of other stars in 1989 and 1993. The team compared optical and X-ray observations and found that they are in unison.

X-ray pulsations of Xi1 CMa are telling that stellar winds and stellar interiors are linked much more close than it was thought previously. The on-going and future collaboration between the AIP and theUP in obtaining new observations and developing sophisticated cutting edge models of stellar winds and stellar magnetospheres shall reveal the mystery of newly discovered stellar X-ray pulsars.

Link to publication in Nature Communications

ESA press release


++ A joint press release by the University of Potsdam (UP) and the AIP ++