The Beat of the EXTraS Sky
The detection and investigation of periodic phenomena have always played a major role in astronomy. This is particularly true when looking at the X-ray sky. In fact, in the 60's, the detection of X-ray pulsations from Centaurus X-3 and of their properties as a function of time allowed scientists to identify the accretion of matter onto rotating compact objects (such as white dwarfs, neutron stars and black holes) as a new and extremely efficient emission mechanism.
Since then, hundreds of X-ray pulsators have been discovered and today we know that many of the known X-ray periodic signals arise from the rotation of a compact star or the orbital motion in a binary system. Among these sources there are also different classes of isolated (not accreting) neutron stars. For the greatest part of the known X-ray pulsators the detection and accurate measurement of their X-ray coherent signals is the only way to infer information of paramount importance in order to characterize the compact object itself, such as the intensity of their magnetic fields, the object age, the system geometry, the matter torque, etc. Additionally, every time we looked for new coherent signals in the archival data of a new mission we found new classes of pulsating X-ray compact objects or we extended the flux/luminosity interval over which the physics of the accretion emission mechanism can be investigated.
Fig. 1 – Chandra ACIS time series of the shortest orbital period (5.4 minutes) binary system discovered by ROSAT satellite in 1999 (Israel et al. 1999, A&A 349, L1; 2002, A&A 386, L13; Ramsay et al. 2002, MNRAS 332, L7): the system hosts two white dwarfs likely emitting owing to a new emission mechanism. Artistic view of the binary which is expected to be a strong gravitational wave emitter (right; credit NASA).
In the last two decades the improvements of the instrument capabilities translated into an almost exponential increase of the number of detected sources. Systematic timing analysis carried out on the light curves of these sources enabled to discover and to study both new X-ray pulsators in very rare evolutionary phases and others which have became prototypes of new classes. As an example, the signal search in about 5,000 objects from the Einstein and EXOSAT data clearly showed that there is a number of soft X-ray pulsars with different properties from those of both accretion-driven and rotation-powered neutron stars: these objects are today known as Magnetars. The timing analysis of about 60,000 sources detected in the ROSAT archive led to the detection of X-ray modulations arising from the orbital motion of double degenerate ultrashort period binaries, hosting two white dwarfs (one of these systems possess the shortest orbital period known; see Figure 1). X-ray pulsations at a period of about 13s were also discovered in the ROSAT light curve of HD497985 in the post-common envelope evolutionary phase. The system likely hosts the most massive and fastest spinning white dwarf likely the first discovered system thought to evolve into a supernova Ia (Israel et al 1997 ApJ, 474, L53; Mereghetti et al 2009 Science, 325, 1222).
Fig. 2 – The X-ray sky in terms of detected sources by current X-ray missions (colors mark the numbers of source detections). Within the EXTraS project there is a working package aimed at searching for new coherent signals among about 400,000 time series. More than 100 new X-ray pulsators are expected to be detected as consequence of this effort. Insets show examples of power spectra and folded light curves obtained by means of the EPIC instruments.
Our capability of detecting X-ray signals is a strong function of the number of source photons, the observation length for uninterrupted pointings and the time resolution. In this respect the XMM EPIC data in imaging mode are the best we can rely upon (with respect to those of present and past missions) in order to maximize our signal search sensitivity owing to the unprecedented throughput, time resolution (in imaging mode) and to the long XMM orbit. With more than 400,000 time series in the archival data and more than 100 new X-ray pulsators expected to be detected in the EPIC frames, possibly also from object belonging to still unknown classes, EXTraS would represent the most powerful coherent signal discovery project ever carried out so far in high energy astrophysics (see Figure 2).