Flaring source in NGC 6540
Copyright: ESA/XMM-Newton; A. De Carlo (INAF)
Copyright: ESA/XMM-Newton; A. De Carlo (INAF)
An enigmatic X-ray source revealed as part of a data-mining project for
high-school students shows unexplored avenues hidden in the vast archive
of ESA’s XMM-Newton X-ray Observatory.
When XMM-Newton was launched in 1999, most students who are finishing
high school today were not even born. Yet ESA’s almost two-decade old
X-ray observatory has many surprises to be explored by the next
generation of scientists.
A taste of new discoveries was unveiled in a recent collaboration
between scientists at the National Institute of Astrophysics (INAF) in
Milan, Italy, and a group of twelfth-grade students from a secondary
school in nearby Saronno.
The fruitful 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.
“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,” says Andrea De Luca, one of the scientists who
coordinated the student project.
“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.”
Copyright
ESA/XMM-Newton; A. De Carlo (INAF)
High-school students
Copyright INAF
Copyright INAF
Scientists at INAF in Milan have been cooperating with local schools for a few years, hosting several groups of students at the institute for a couple of weeks and embedding them in the activities of the various research groups.
“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,” explains Ruben
Salvaterra, another scientist involved in the programme.
“Once they were ready to explore the data archive, they proved very effective and resourceful.”
The six students analysed about 200 X-ray sources, looking at their
light curve – a graph showing the object’s variability over time – and
checking the scientific literature to verify whether they had been
studied already.
Eventually, they identified a handful of sources exhibiting interesting
properties – a powerful flare, for example – that had not been
previously reported by other studies.
“One of the sources stood out as especially intriguing,” says Andrea.
Featuring the shortest flare of all analysed objects, this source
appears to be located in the globular cluster NGC 6540 – a dense
grouping of stars – and had not been studied before.
After presenting their findings to the scientists in a seminar, the
students went back to school. But the work for Andrea, Ruben and
collaborators had only just begun.
“The source identified by the students displays brightness changes like
no other known objects, so we started looking more in detail,” says
Ruben.
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.
Stars like our Sun shine moderately in X-rays, and occasionally undergo
flares that boost their brightness like the one observed in this source.
However, such events normally last much longer – up to a few hours or
even days.
On the other hand, short outbursts are observed in binary star systems
hosting a dense stellar remnant such as neutron star, but these
outpourings of X-rays are characterised by a much higher luminosity.
“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,” explains collaborator Sandro Mereghetti, lead author of
the paper presenting the results.
Another 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 lurking 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 systematic study of variability that led to the compilation of the
EXTraS catalogue, together with this first attempt at data mining,
suggests that we have opened a new, unexplored window on the X-ray
Universe,” adds Sandro.
The team plans to study the newly identified source in greater detail to
better understand its nature, while searching for more similar objects
in the archive.
“It is exciting to find hidden jewels like this source in the XMM-Newton
archive, and that young students are helping us find them while
learning and having fun,” concludes Norbert Schartel, XMM-Newton project
scientist at ESA.
Notes for Editors
For further information, please contact:
Andrea De Luca
INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica
Milano, Italy
INFN, Pavia, Italy
Email: andrea.deluca@inaf.it
Ruben Salvaterra
INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica
Milano, Italy
Email: ruben.salvaterra@inaf.it
Sandro Mereghetti
INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica
Milano, Italy
Email: sandro.mereghetti@inaf.it
Norbert Schartel
XMM-Newton Project Scientist
European Space Agency
Email: norbert.schartel@esa.it
Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
Email: markus.bauer@esa.int
“EXTraS discovery of a peculiar flaring
X-ray source in the Galactic globular cluster NGC 6540” by S. Mereghetti
et al. 2018 is published in Astronomy & Astrophysics, DOI: 10.1051/0004-6361/201833086.
The students involved in this project are Razvan Patrolea, Lorenzo
Apollonio, Elena Pecchini, Cinzia Torrente, Bartolomeo Bottazzi-Baldi
and Martino Giobbio from Liceo scientifico G.B. Grassi in Saronno,
Italy. They discovered the peculiar source during a two-week internship
at INAF, Milan, in September 2017, as part of an initiative supported by
the Italian Ministry of Education, University and Research.
The discovery was made as a result of the Exploring the X-ray Transient
and variable Sky (EXTraS) project, a EU/FP7 project devoted to a
systematic variability study of the X-ray sources in the XMM-Newton
public archive.
For further information, please contact:
Andrea De Luca
INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica
Milano, Italy
INFN, Pavia, Italy
Email: andrea.deluca@inaf.it
Ruben Salvaterra
INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica
Milano, Italy
Email: ruben.salvaterra@inaf.it
Sandro Mereghetti
INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica
Milano, Italy
Email: sandro.mereghetti@inaf.it
Norbert Schartel
XMM-Newton Project Scientist
European Space Agency
Email: norbert.schartel@esa.it
Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
Email: markus.bauer@esa.int
Source: ESA/Space Science
