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EXTraS – Exploring the X-ray Transient and variable Sky – produced the most sensitive and thorough search for, and caracterization of variability in the soft X-ray sky ever performed. All results were released in a public archive. The project extracted all temporal domain information for about 400,000 serendipitous X-ray sources detected by the ESA XMM-Newton observatory since its launch, on time scales ranging from ~0.1 s to ~10 years, and on fluxes spanning from 10-9 to 10-15 erg cm-2 s-1 in the 0.2-10 keV energy range. The analysis included different lines:

An XMM-Newton observation, featuring sources that display variability on time scales ranging from hours (left column) to weeks (right column)

1) Aperiodic, short-term variability, on time scales ranging to the instrumental resolution to the duration of an observation. Background-corrected light curves were produced with both uniform and adaptive time binning (using the Bayesian block approach), in multiple energy ranges; power density spectra were also generated. Such products were characterized by computing a large set of synthetic parameters, describing different aspects of each source variability.

2) Search for pulsations. Using each photon’s time tag, a periodicity search were performed by running an FFT-based approach taking into account the properties of “red” noise related to each source aperiodic variability; a more refined study of all candidate signals was carried out, based on the epoch folding technique.

Discovery of ~1 s pulsations in the extreme ULX NGC5907 ULX-1 (Israel et al. 2017)

3) Search for new transients, focusing on sources that rose above detection threshold just in a fraction of the exposure time (and were therefore missed by the standard, time-integrated source detection). A time-resolved source detection approach was used, based on the selection of promising time intervals and detector regions based on the Bayesian block algorithm.

4) Long-term variability, among different observations. A large number of sources have been observed more than once, due to the overlap of different observations at different epochs. A photometric catalogue combining all flux measurements and upper limits to the flux of all sources in all pointed and slew data was produced. Tests for variability and a basic caracterization were performed on the resulting, long-term light curves.

All results and products were released to the community in a public archive, including a database of all parameters describing variability for all sources, and about 17 million files (light curves, hardness ratios, folded light curves, power spectra, …). This is a very rich resource for the study of almost all classes of astrophysical objects, for investigations ranging from the search for rare events to population studies.

Our team published a series of early results, including the ground-breaking discovery of ~1 s pulsations from the extreme Ultraluminous X-ray Source NGC5907 ULX-1, proving the source to be powered by an accreting neutron star, shining about 500 times above its Eddington limit. Such a luminosity, coupled to an extremely high observed spin up rate, challenge current accretion models (see Israel et al., 2017, on the Science web site or on arXiv).

EXTraS was a collaborative effort of six European partners: Istituto Nazionale di Astrofisica (INAF, Italy, coordinator); Scuola Universitaria Superiore IUSS Pavia (Italy), Consiglio Nazionale delle Ricerche – Istituto di Matematica Applicata e Tecnologie Informatiche “E. Magenes” (CNR-IMATI, Italy); University of Leicester (UK); Max Planck Gesellschaft zur Foerderung der Wissenschaften – Max Planck Institut fuer extraterrestrische Physik (MPG-MPE, Germany); Friedrich-Alexander Universitat Erlangen-Nuremberg – Erlangen Center for Astroparticle Physics (ECAP, Germany). EXTraS was funded (2014-2016) by the European Union within the Seventh Framework Programme (FP7-Space).

For more details, please see the project paper.

Page maintained by Andrea De Luca Last modified: 7-6-2022