How is EXTraS Going?

It is a very busy time for our team. Less than five months are left, before the EXTraS project will officially end. The final version of all results and products will be released to the community in late December, this year.

A huge load of work has been already successfully completed, searching for, and characterising all temporal-domain information in XMM/EPIC data.

At INAF/IASF in Milan, we focused on aperiodic variability. Light curves were extracted for almost all (>95%) sources listed in the XMM serendipitous source catalog, with both uniform time bins and with the Bayesian block approach. FFT spectra have also been produced. A set of more than 50 parameters have been computed, to characterise different aspects of each source variability, on time scales ranging from a fraction of a second to ~1 day.

At INAF/OA Roma, we have been systematically searching for pulsations on the largest possible sample of sources (about 80% of the objects in the XMM serendipitous source catalog). An FFT-based search technique was used, taking into account the effects of any aperiodic variability possibly superimposed to the periodic signal. Upper limits to the pulsed fraction have been computed for all sources which do not show any significant periodicity.

At IUSS Pavia, our team analysed all EPIC data with a brand new approach, based on the Bayesian block algorithm, to search for new, faint transients - sources that rose above the detection threshold for a short time. More than 2,000 candidate transients have been detected on time scales ranging from tens of seconds to ~1 hr. Such sources are typically missed by the source detection run as a part of the standard XMM data processing, and thus are not included in the XMM serendipitous source catalog.

At the University of Leicester, long-term variability has been studied. For all sources located in sky regions observed more than once, we collated flux measurements from the XMM serendipitous source catalogue, as well as from a improved and enlarged version of the XMM slew survey catalogue. This was completed, in case of non-detection at a specific epoch, by upper limits computed from the related EPIC data. Test and characterisation of variability were performed on the resulting long-term light curves, displaying the behaviour of sources on time scales ranging from ~days to ~a decade.

To ensure the highest possible quality of EXTraS products for the community, a careful screening, both using automatic algorithms and by direct visual inspection, is being carried out at MPE in Garching.

All time-domain results, coupled to X-ray spectral information as well as to multi-wavelength archival data, are being used by our team at ECAP (Bamberg) to perform phenomenological classification of the sources, the first ever tried on a sample of hundreds of thousands of soft X-ray sources.

All EXTraS products are being ingested in the LEDAS high-energy data archive at Leicester, where user-friendly visualisation tools for users are also being developed.

For further details on our recent work, you can have a look at a series of posters presented at the last XMM Workshop in ESAC.

The scientific information included in our output is indeed huge. We are publishing a small sample of selected results. See e.g. our detection of a flare from a very early protostar (Pizzocaro et al.2016), and the discovery of the first pulsating, accreting neutron star in the Andromeda galaxy (Esposito et al. 2016) — more to come! We trust that the EXTraS catalogue, describing variability over a range of more than nine orders of magnitude in time scale, for hundreds of thousands of X-ray sources spanning six orders of magnitude in flux, will be a very useful resource in almost all fields of astrophysics.

In this perspective, we are glad to invite you in Italy to participate in a scientific workshop devoted to variability in the X-ray sky. We will meet in Pavia, from November 21 to 23, to discuss about X-ray time-domain studies of all kind of astrophysical sources, from nearby stars to extragalactic, rare transient events. EXTraS products will also be presented with detail in a dedicated hands-on session. See you in Pavia!