Students spot mysterious X-ray source

Students spot mysterious X-ray source
A peculiar X-ray source spotted in the globular cluster NGC 6540 © ESA/XMM-Newton; A. De Carlo (INAF)

A mysterious X-ray source revealed as part of a data-mining project for high-school students shows unexplored avenues hidden in the archive of the European Space Agency’s (ESA) XMM-Newton X-ray Observatory.

ESA’s XMM-Newton Observatory, which is nearly two decades old, still has areas to be explored by the next generation of scientists. An idea of the new discoveries was unveiled in a recent collaboration between scientists at the National Institute of Astrophysics (INAF) in Milan, Italy, and a group of students from a secondary school as they discovered a mysterious X-ray source.

The interaction was part of the Exploring the X-ray Transient and variable Sky project, EXTraS, an international research study of variable sources from the first 15 years of XMM-Newton observations.

Andrea De Luca, one of the scientists who coordinated the student project said: “We recently published the EXTraS catalogue, which includes all the X-ray sources – about half a million – whose brightness changes over time as observed by XMM-Newton and lists several observed parameters for each source.

“The next step was to delve into this vast dataset and find potentially interesting sources, and we thought this would be an exciting challenge for a student internship.”

Working with the students to discover the X-ray source

Ruben Salvaterra, another scientist involved in the programme said: “For this particular project, the students received an introduction about astronomy and the exotic sources we study with X-ray telescopes, as well as a tutorial on the database and how to use it,

“Once they were ready to explore the data archive, they proved very effective and resourceful.”

The students involved in the study analysed about 200 X-ray sources, looking at their light curve and checking the scientific literature to verify whether they had been studied already. Eventually, they identified a handful of sources exhibiting interesting properties that had not previously been reported by other studies.

One source stood out and featured the shortest flare of all analysed objects, this source appears to be located in the globular cluster NGC 6540 and had not been studied before.

Salvaterra said: “The source identified by the students displays brightness changes like no other known objects, so we started looking more in detail”.

According to ESA, an otherwise low-luminosity source of X-rays, XMM-Newton saw it brighten by up to 50 times its normal level in 2005, and quickly fall again after about five minutes.

What was discovered?

Collaborator Sandro Mereghetti, lead author of the paper presenting the results, said: “This event is challenging our understanding of X-ray outbursts: too short to be an ordinary stellar flare, but too faint to be linked to a compact object”

One possibility is that the source is a so-called chromospherically active binary, a dual system of stars with intense X-ray activity caused by processes in their chromosphere, an intermediate layer in a star’s atmosphere. But even in this case, it does not closely match the properties of any known object of this class.

The scientists suspect that this peculiar source is not unique, and that other objects with similar properties are in the XMM-Newton archive but have not yet been identified because of the combination of low luminosity and short duration of the flare.

The team plans to study the newly identified source in greater detail to better understand its nature, while searching for similar objects in the archive.

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