The EXTraS project aims at exploring and disclosing the serendipitous content of the EPIC database in the time domain, and to make it available, and easy to use, to the whole community. EXTraS includes different lines of analysis aimed at extending the time domain region already accessible with the most recent release of the XMM-Newton Serendipitous Source Catalog (3XMM).
1. Short-term aperiodic variability
A systematic search and characterization of aperiodic variability on time scales shorter than the observation duration will be performed on the largest possible number of 3XMM sources. EXTraS will take advantage of the information on the (unbinned) time of arrival of each photon stored in EPIC data. The most important objective is to investigate variability on much shorter time scales than allowed by the automatic rebinning of time series implemented in 3XMM (the minimum number of 20 counts per temporal bin implies that a 3XMM source with a count rate of 0.01 cts/s cannot be studied at temporal scales below 2000 s). Different timing analysis approaches will be tested in order to discover and quantify variability down to a time scale of seconds in a very large sample of sources, extending the search to a larger sample than currently explored in 3XMM. As a further step, a detailed characterization of the time series properties will be performed for the first time. Different measures of the variability patterns and properties will be extracted in a systematic and consistent way in order to allow for a phenomenological classification of the sources.Periodicity.
A systematic search for periodicity will be performed on 3XMM sources. We will apply different state-of-the-art timing analysis techniques to unbinned photon times of arrival (3XMM time series, due to their automatic rebinning, are not useful for this aim). We will study about 200,000 sources for periodicities down to about 0.2 s. This will be the first systematic search for periodicity ever performed on the EPIC database and the space for discovery is very large.
A blind search for transient sources will be carried out. We will perform a systematic and consistent analysis on the entire XMM-Newton/EPIC data archive, using automated, dedicated software, developed by our consortium. This will allow us to unveil fast, faint transients that are only above detection threshold for a very short time interval and thus missed by standard XMM pipeline analysis and not listed in the XMM-Newton serendipitous source catalogue. All detected transients will be studied down to the time resolution of the EPIC instrument (73 ms and 2.6 s for the pn and the MOS cameras, respectively, in their most frequently used operating modes).
4. Long-term variability
A systematic search for long-term variability will be performed. We will take advantage of the large number of overlapping pointed observations performed at different epochs. Moreover, although at lower sensitivity, we will extract information on the large fraction of the sky (about 20%) that has been visited by multiple slews. This will require improved software for processing slew data and a systematic re-analysis of the slew database. For each source, count rate measurements / time series at different epochs will be complemented by upper limits that will be computed in the case of non-detections in spatially overlapping observations. A search for variability on such multiple-observation time series will be carried out. Use of existing X-ray catalogues (e.g. ROSAT, Chandra, Swift/XRT, Suzaku) is foreseen. Such a search has never been performed in a systematic way and will allow us to detect and characterize the variability of a huge number of sources on time scales as long as 10 yr.
5. Multiwavelength characterization and classification
A very important objective of the EXTraS project is to perform a phenomenological classification of all detected variable sources. Basically, using a series of pre-defined “features” (temporal, spectral, as well as multi-wavelength properties), the probability that a source belongs to a “group” will be calculated. Such an effort, never performed before in a systematic way on a large sample of X-ray sources, will rely on a consistent automatic characterization of the temporal (and spectral) X-ray properties, but will also use all information available in existing multiwavelength catalogues and databases. This process will build on existing expertise developed within the XMM-Newton Survey Science Center by a team including members of the EXTraS consortium. Results of both characterization and classification will be released to the community and will be crucial for any possible scientific investigation.
6. Compilation of a variable source catalogue
All our results and products will be included in a public Catalogue, which will be the most important product of the project for the community. Particular effort will be devoted to quality control. In order to minimize the number of unusable data (e.g. spurious detections, flawed products, resulting from our automated data analysis pipelines), a very careful and systematic screening of all results will be performed, using both automated and semi-automated (requiring visual inspection) software tools. Extensive expertise in verification and validation software is being contributed by the MPE, which already designed the Data Product Screening Software for two major projects, namely the ROSAT Bright Source Catalogue and the complete XMM-Newton data archive. Our catalogue will be a very rich resource for a broad possible range of investigations and will serve as a reference for the design of future experiments dedicated to X-ray variable phenomena.
A web site will be deployed with an easy-to-use Virtual Observatory-compliant interface to the catalogue. We will generate Virtual Observatory data models for each data product type associated with the EXTraS archive.
7. Release of new software tools
All the software tools developed and/or customized in the project will be made available to the scientific community. To this aim, a particular attention will be put in the design of solutions that exploit efficiently the heterogeneity of the present architectures for High Performance Computing but, at the same time, are portable in the performance upon different platforms and easily reusable in the current distributed computing infrastructures.
The main focus of EXTraS has a natural appeal, being related to rare, exotic phenomena as well as to extreme, poorly known astrophysical sources, offering excellent opportunities to promote exciting science to a general public audience.
As a very specific contribution, our project includes an experimental didactic program that is aimed at directly involving students in our research program, allowing them to participate in the scientific activities. Such a didactic program, which will be interesting per se since it will allow assessment of the potential of a new form of citizen science, could be repeated in the future and its concept could be applied also to other contexts/experiments.