Our project will allow for a significant step forward in our characterization and understanding of variable X-ray phenomenaon time scales ranging from ~0.1 s to ~10 years, and on flux ranges spanning from 10-9 to 10-15 erg cm-2 s-1 in the 0.2-10 keV energy range.
The extremely large range of variability timescales and luminosities explored by EXTraS ensures that the scientific discovery space is enormous. Based on our estimates of the sensitivity of our search algorithms and of the equivalent sky coverage of the XMM-Newton/EPIC observations, we can make predictions about the possible harvest of variable sources in our catalogue. These go from Galactic objects to the most distant cosmological ones suggesting that the EXTraS results will have a great impact in many areas of astrophysics and cosmology and be of interest for a very broad community. The blind nature of our search will allow us to measure (or give strong limits) on the intrinsic occurrence rate of these sources and to perform population studies. Unexpected, serendipitous discoveries can be also foreseen as the parameter space that is still completely unexplored is extremely large.
Without demanding completeness, in the following we will characterize some of the most interesting transients that are expected to be detected within the EXTraS project. Indicative numbers of detections are given on the basis of previous observational results and theoretical models. We adopted here a very conservative 6.5σ detection threshold (corresponding to a factor 2-5 larger flux with respect to the Figure), which implies an essentially negligible contamination from spurious detections.
1. Flare stars
Stellar flares are the result of the energy release in magnetic reconnection in late-type stars, both main sequence FGKM stars and binaries composed of giant stars (RS CVn variables). Hundreds of stellar flares will be detected. It will be possible to study, on a statistical basis, the duration, duty cycle and amount of energy released in flares of different stellar groups. This is also important to better define the habitable zone of stars.
2. Low mass X-ray binaries (LMXBs)
Thermonuclear X-ray bursts on the surface of accreting neutron stars (NS) yield a sudden X-ray flux increase with rise times <2s, decay times ~10-100s, and peak luminosities >0.1 LEdd (>10-9 erg cm-2 s-1 at 8kpc). We expect 3-300 and 0.3-30 bursts from persistent and transient LMXBs, respectively, in the Galactic Center region, assuming a burst recurrence time of 1-100 thousand s and a 10% duty cycle. Particularly interesting are the theory-challenging "burst only sources", for which a single burst detection would put stringent constraints on the still unknown persistent flux at the burst on-set. Bursts from M31, the closest spiral galaxy, can also be detected.
3. Supergiant fast X-ray transients (SFXTs)
High mass X-ray binaries accreting from the wind of OB supergiants, show 100-1000s X-ray flares. They spend 3-5% of time in bright flares (LX~1037 erg s-1) and 40-75% in intermediate flaring state (LX~1034 erg s-1). We expect 3-5 SFXTs active in bright flaring and 40-75 sources in an intermediate state of flaring, detectable throughout the whole Galaxy. The SFXT census is essential to unveil the evolutionary path and formation rate of massive stars.
Extremely magnetized neutron stars, sporadically emit short (<1 sec) powerful bursts (>1040 erg s-1). Only 20 magnetars are known, but their total number in the Galaxy is much higher (>500) and they can constitute a substantial fraction of the total neutron star population. We expect a couple of new magnetars in our final catalogue. Any new addition will be a crucial piece of information to constrain the magnetar population, its relation to the overall population of neutron stars, and will also impact on our comprehension of the short GRBs.
5. Tidal disruption events
The disruption of a star by a supermassive black hole produces a bright flare in the UV and X-rays. Such an event has been recently observed in a galaxy at z=0.35 (Swift J1644+57). A few sources up to the highest redshift (z=5-6) can be expected but even a single detection would represent a fundamental test of the extreme astrophysical regimes and strong gravity conditions. Their non-detection will be equally important, providing more stringent limits on the statistic of these sources.
6. Low luminosity GRBs
GRBs such as GRB060218 with lower luminosity and lower peak energy with respect to the classical GRB, possibly belonging to a distinct class of event. We expect up to a dozen such events in the EXTraS catalogue up to z~2. Their detection would constrain their rate up to high redshift and provide clues on their progenitor(s). In the case of non-detections, our limits would represent the strongest available constraint on the LL-GRB population.
The break-out of the supernova shock wave is believed to produce a burst of X-ray emission as shown in SN 2008D, which was serendipitously detected in X-rays during a pointed observation of the galaxy NGC 2770 at 27 Mpc. We expect to detect ten events like SN2008D up to 500 Mpc (or z=0.1) allowing a measure of the SN rate which is independent from optical surveys, that may be biased (target survey) and suffer from uncertainties due to dust extinction. If no 2008D-like event is identified, this will imply either that the X-ray emission seen in SN2008D was related to the launch of a jet or that the conditions for the shock breakout are met only in very peculiar cases.
A sub-class of radio-loud Active Galactic Nuclei with emission dominated by a relativistic jet aligned with the line of sight, that shows extreme variability on all timescales and at all wavelengths. We expect to identify a few tens of blazar flares including both new and known sources. This will allow us to characterize their variability duty-cycle on short timescales and derive constraints on various model parameters (particle injection and acceleration, magnetic field, BH mass, etc.).
9. Other transient sources
Already detected at other wavelengths or expected from theory but still missing detection, ranging from galactic events (e.g. tidal disruption events of minor bodies falling onto NSs or BHs) to cosmic ones (e.g. orphan afterglows of GRB seen off-axis, GRBs from the first very massive stars).