Fifteen Years of Restlessness in the Sky X (Astronomy)

The catalog of the Extras project – a collection accessible to anyone, coordinated by Andrea De Luca of INAF of Milan, of all the X photons collected from 1999 to 2015 by the Epic camera of the Esa space telescope for high energy Xmm-Newton – is from today described in every aspect, from the algorithms developed for data analysis to the products of the same analyzes, in an article published in Astronomy & Astrophysics

The night sky as we are used to looking at it – planes and artificial satellites aside – offers a rather static panorama. There is the apparent motion due to the Earth’s rotation, and the even slower movements of the Moon and the planets. Sure, a meteor passes by every now and then, but stars, nebulae and galaxies seem to have been standing there for millennia. It was not for nothing that the ancients called them fixed stars and considered them immutable. However, it is a false impression, due to our limited sensory abilities. In reality the universe is teeming with events and changes, some periodic and others unpredictable: radio wave beams emitted by pulsars, jets of matter ejected by quasars, neutron stars that merge producing gamma-ray bursts, black holes regurgitating energy for having swallowed too much matter,

Time-varying sources, object of astronomy in the temporal domain, as specialists call it. And it is precisely to look for this variability that from 2014 to 2017 the astrophysicists of the Extras project – coordinated by Andrea De Luca of  INAF of Milan and financed by the European Union with 2.5 million euros – examined the list of all X photons collected over about 15 years by the Epic camera of the European space telescope for high energy Xmm-Newton . The catalog they extracted was put online, available to anyone . And to make it easier for all interested astronomers to use, it is now out onAstronomy & Astrophysics an article that describes it in every aspect, from the algorithms developed for data analysis to the products of the same analyzes, complete with statistical properties, potentials and limits.

Andrea De Luca, astrophysicist at the INAF IASF in Milan, coordinator of the Extras project © INAF

“With the Extras project (acronym for Exploring the X-ray variable and Transient Sky) we have systematically characterized the temporal variability of over 400 thousand X-ray sources revealed by Xmm-Newton from the year of its launch – 1999 – until 2015: phenomena aperiodic, periodic and transient, on a time scale from about one second to several years “, explains De Luca to Media Inaf. “We have produced an enormous amount of results, all available to the astronomical community since the beginning of 2017. By exploring the public archive, with little work, it is possible to find rare phenomena and very peculiar sources, but also to derive the properties of entire already known source classes. Anyone can search for their favorite source. We hope to offer useful information for the study of very different astrophysical problems, but also for the design and implementation of new experiments dedicated to temporal variability studies ».

In addition to sharing all the products (about 20 million) and algorithms developed during the project with the community, Extras has also obtained numerous scientific results. Some examples chosen from those we have also reported on Media Inaf : the detection of pulsating signals from Andromeda , the identification of the most distant and brightest X pulsar known to date, the recent observation of an X flare from a brown dwarf and the discovery of a variable source of X-rays  made together with six high school students during a school-work alternation activity.

Ruben Salvaterra, astrophysicist at the INAF IASF in Milan and member of the Extras team © INAF

“Our search for transient phenomena allowed us to identify even an extremely difficult event to observe: we saw live the explosion of a supernova in another galaxy, more than a billion light-years away from us,” adds another. team astronomer, Ruben Salvaterra, also of the INAF of Milan. “Supernovae are easily observable in the optical band of the electromagnetic spectrum, but it is a deferred vision: several days after the explosion, the radioactive decay of the elements produced in the collapse of the star produces a lot of light in the optical band, which astronomers manage to observe for several weeks. The moment of the explosion, on the other hand, is marked by a very intense flash of radiation in the X-ray band. It is a phenomenon that can give us very valuable information on the mechanism of the explosion. It only lasts a few minutes: to see it, you have to look in the right direction. In fact, a telescope should be aimed before the explosion happens … but we cannot predict supernovae, so the only possibility is that a star explodes right in front of the telescope,target . We need a lot of luck, so far only one has been seen, with the Swift satellite . But when you collect a large amount of data like that of Xmm-Newton, the possibility of finding this phenomenon becomes concrete… and in fact, looking at all the observations, we found it ».

Featured image: Xmm-Newton. Credits: Esa (Image by C. Carreau)

To know more:

  • Read on Astronomy & Astrophysics the article “ The EXTraS Project: Exploring the X-ray transient and variable sky ”, by A. De Luca, R. Salvaterra, A. Belfiore, S. Carpano, D. D’Agostino, F. Haberl, GL Israel, D. Law-Green, G. Lisini, M. Marelli, G. Novara, AM Read, G. Rodriguez-Castillo, SR Rosen, D. Salvetti, A. Tiengo, G. Vianello, MG Watson, C. Delvaux, T. Dickens, P. Esposito, J. Greiner, H. Haemmerle, A. Kreikenbohm, S. Kreykenbohm, M. Oertel, D. Pizzocaro, JP Pye, S. Sandrelli, B. Stelzer, J. Wilms and F. Zagaria

Provided by INAF

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