While focusing on the Hot and Energetic Universe, the breakthrough capabilities (e.g., excellent spectral resolution combined with high throughput and fast timing capabilities) of the X-IFU will enable a wealth of new science investigations to be performed for a wide range of astrophysical sources of great interest to the broader astronomical community. This so-called observatory science covers very different topics and we highlight here only a few of them. We also highlight the potential of X-IFU observations in unforeseen discoveries thanks to the Athena fast Target of Opportunity observation capability, in particular in science triggered by new Gravitational Wave detectors.
Athena investigations of the solar system will answer questions still open following the pioneering work carried out with Chandra and XMM–Newton, and will add enormously to our understanding of the interactions of the solar wind with planetary bodies, and between space plasmas and magnetic fields. The X-IFU will determine the species, and thus the origin (solar wind or Io’s volcanoes), of the ions responsible for Jupiter soft X-ray aurora, and will test theories of ion acceleration in the planet magnetosphere through line broadening velocity measurements (see Fig below). High sensitivity observations of X-ray fluorescence from the Galilean moons will allow surface composition measurements, and studies of the Io plasma torus will shed light on the yet unknown mechanisms energizing its X-ray emission. A search for X-ray aurorae on Saturn with the X-IFU will reach much greater depth than possible so far. The X-IFU will spectroscopically map Mars extended exosphere through differing solar wind conditions and seasons, as well as the very extended comae of comets transiting in the Sun neighbourhood.
The X-IFU will drastically improve our knowledge of the consequences of X-ray incidence on exoplanets, a crucial element in order to understand the effects of atmospheric mass loss and, more generally, of the chemical and physical evolution of planet atmospheres especially in the early evolutionary stages. In a few selected nearby known planetary systems hosting hot Jupiters, X-IFU will search for ingress/eclipse/egress effects during planetary orbits. In a wider sample of planetary systems X-IFU can confirm/improve the statistical evidence of Star-Planet Interactions and search for those variability features that are imprints of such interactions. Athena may also discover unexpected spectral signatures (and their orbital modulation) of planetary atmospheres due to the host stars high energy radiation and particle emission.
The strong stellar winds of massive stars (mass loss rate of 10-7 M⊙ yr-1 or more, velocity of 1000 km s-1 or more) make them key players in feedback processes within galaxies, whatever their redshift. However, large discrepancies remain in the evaluation of the wind properties, impeding a proper understanding of massive star evolution and feedback. The X-ray emission forms inside the stellar winds, so they are a sensitive probe of their physical properties. Time resolved high-resolution spectroscopy, collected by X-IFU, will thus provide major breakthroughs in this field. For example, the instability of the line driving mechanism causing the wind outflow produces small-scale structures as well as shocks leading to X-ray emission. Taking advantage of this common origin, the degree of wind inhomogeneity in single stars is probed by examining short-term variations as well as line profile shapes in a large sample of objects. The sensitivity of current X-ray observatories, however, limited such studies to statistical analyses of the global X-ray fluxes for a few cases only, while X-IFU will be able to perform in-depth analyses of tens of stars (see figure below).
In addition to this short-term, stochastic variability, stellar winds may also display changes recurrent with the stellar rotation period, notably due to the presence of magnetic confinement, pulsational activity, or co-rotating features. The presence of such variations could just be hinted at in the best datasets currently at hand, but X-IFU will permit detailed Doppler mapping, allowing to derive the precise properties of these features. This will help solving the riddle of their physical origin – e.g. before the discovery of the above-mentioned hints, it was generally assumed that what produces Discrete Absorption Components in the UV domain would be unable to generate high-energy signatures, or that surface pulsations would not be sufficiently strong to significantly modify the winds.
Finally, in massive binary systems, where the two stellar winds collide, precise line profile studies (in particular of the Fe-K line complex at 6.7 keV which specifically arises in such hot shocked plasma) will for the first time be performed. Following the stars as they rotate around each other, the X-ray emission will change due to a changing view of the collision zone and (when eccentric) the changing strength of the collision. The study of these variations yields the immediate post-shock conditions in the wind interaction zone while Doppler tomography will directly map it. This will allow to test precisely the physical processes at work, e.g. presence of collisionless shocks, of non-equilibrium plasma, of fluorescence on nearby cooler material, or of charge exchange, as well as to solve remaining mysteries (e.g. the scarce number of colliding winds able to accelerate particles to relativistic speeds). X-IFU will thus dramatically increase our understanding of the plasma physics in the most important stellar feedback drivers.
Update after the SPIE 2016 paper was published by Yaël Nazé.
High-energy irradiation of circumstellar disks during star formation and early stellar evolution are crucial for disk evolution and, eventually, resulting planetary system formation (Ref. 85). The radiation associated with magnetospheric accretion onto CTTS (Classical T Tauri Stars) and young brown Dwarfs is believed to originate in localized structures (accretion streams and hot spots). Therefore, the observed emission changes throughout the stellar rotation cycle and the X-IFU will probe the X-ray line emission from the heated plasma in shocks forming upon impact of the accreting matter on the stellar surface, while accompanying simultaneous optical studies will monitor the line emissions produced in magnetic accretion channels. The viewing geometry crucially determines what we see in X-rays (cf. TW Hya Ref., 86 and V2129 Oph, Ref. 87) and the detailed mapping of the accretion geometry, therefore, requires simultaneous optical and X-ray monitoring for at least one rotation cycle, i.e., typically a few days. Spectro-polarimetric optical monitoring (e.g., with E-ELT/HIRES) simultaneous (e.g for at least one night) with X-IFU observations provides both time-resolved emission line fluxes tracing accretion columns and maps of the large-scale magnetic field structure. With X-IFU for the brightest (FX ~ 3× 10-13 erg cm-2 s-1) CTTSs high resolution time-resolved spectroscopy down to 3 kilo-seconds will allow us i) to explore the variability of the accretion process (on predicted time scales of hours) and/or the modulation due to accretion stream shadowing, ii) to constrain with Doppler line shifts down to 100 − 400 km s−1 the bulk velocity of accreting material, iii) to investigate from simultaneous observations of many density sensitive triplets the controversial issue of density stratification of accreting material, and iv) to address the open issue of the excitation mechanism of the Fe Kα 6.4 line emitted from the circumstellar disk (Ref. 88).
Strong flares (peak LX ~ 1032 erg s-1, peak T ~ 2× 108 K) on young active stars and the likely associated huge Coronal Mass Ejection are likely a major perturbation source either of circumstellar disk evolution and/or of the early planetary system formation (Ref. 89–91). The X-IFU will allow us to investigate the initial phase of the intense flares including mass motions as well as their influence on circumstellar disks and/or early planetary evolution.
Structures on the surface of late-type stars such as dark star spots or bright plages can be expected to produce corresponding signatures in the outermost atmosphere, the corona, which is traced by X-ray emission. Yet, a clear rotational modulation of the X-ray luminosity has been observed only in few cases (eg., the M star AB Dor, Ref. 92). Joint X-IFU and optical spectro-polarimetric monitoring (e.g., with E-ELT/HIRES) will yield unique constraints on the magnetic field structure in active stars, by probing different temperature regimes separately and, consequently, different structures on the stellar surface.
Planetary nebulae are born from material lost in the final stages of low-mass stars’ lives. Many harbour hot bubbles filled with shocked fast winds, whose exact properties (abundances, temperatures, but also impact of heat conduction or charge exchange) can only be derived thanks to high-resolution spectroscopy provided by X-IFU. In addition, X-IFU spectra will allow a detailed study of the unexpected hard X-ray emission observed in many central stars of PNe, revealing its origin.
The X-IFU will provide the spatially-resolved spectral capabilities that have long been wished for in the study of supernova remnants (SNRs) (See Ref. 93 and references therein). Like clusters of galaxies, SNRs are extended sources, ranging in angular scale from less than 0.5 arc minutes (in external galaxies) to, in extreme cases, several degrees in our Galaxy. X-IFU will be able to spatially resolve the spectra of SNRs with unprecedented spectral resolution. This is of great importance to obtain information on the supernova event and explosion mechanism by providing detailed abundance ratios for all elements with Z=6-28 (carbon to nickel) and new insights into the dynamics of the explosion by mapping Doppler shifts/broadening of the ejecta in young SNRs, as well as for understanding the physics of the hot non-equilibrium plasmas in SNRs, its evolution and impact on the interstellar medium.
To start with the abundance patterns in SNRs, it should be noted that SNRs originate from two different types of explosions: core collapse supernovae (spectroscopy classes Type II, Ib/c) and thermonuclear supernovae (Type Ia). There are still major uncertainties regarding these explosions. Core collapse supernovae are caused by the implosion of the cores of massive stars, leading to the formation of a neutron star or black hole. How the energy liberated leads to the ejection of the outer envelope is very uncertain, issues being the roles of neutrinos, instabilities, rotation and magnetic field. Thermonuclear supernovae are caused by CO white dwarf which explodes if its masses approaches the Chandrasekhar limit (1.4 M⊙). Here a main issue is the supernova progenitor system, whether the evolution towards this limit is caused by accretion from a normal stellar companion, or due to merging of two white dwarfs. For both types of supernovae much can be learned from the resulting nucleosynthesis products. With the current instruments the more abundant even elements have been reasonably well observed in young SNRs, but it is only with the X-IFU that their 3D distribution can be mapped through line doppler shift and broadening measurements, and their spectral properties along the line of sight determined. This is crucial information for the models of Supernova explosions. Indeed the level of asymmetry of the ejecta is closely related to the explosion mechanism itself, leading to relatively symmetrical explosion for Type Ia compared to core collapse events, and leading to different nucleosynthesis yields.
Different explosion mechanisms leave also distinct patterns in the abundance of the odd elements, like F, Na, Al, P, Sc, Ti, Cr and Mn. These rare elements reveal in core collapse how much the exposure was to intense neutrino radiation, and for thermonuclear supernovae it will inform us about the initial pre-main sequence abundances of the progenitor, but also deviations from nuclear statistical equilibrium reactions, which can be used to distinguish among different types of explosions. X-IFU will be able to detect these weaker lines, among the wealth of lines from more abundant elements and to map their spatial distribution, expected to be in different regions of the SNR, often grouped according to the layer of the supernova in which they were synthesized.
As for the physics of SNRs, the hot plasmas in SNRs are often out of both ionisation equilibrium and thermal equilibrium, as the plasmas often did not have time enough to relax to equilibrium. Ionisation non-equilibrium in young SNRs results in line emission from lower ionisation species than one would expect given the temperature. Low ionisation species can also be enhanced by dust sputtering, releasing new atoms into the hot plasma from broken up dust particles. Interestingly, some older SNRs have plasmas for which the ionisation degree is higher than expected given the electron temperatures. This could arise by rapid adiabatic cooling of the electrons, whereas the recombination rate is too low to keep up with the cooling. With X-IFU, this can be studied in detail, for individual ions, but also as a function of location and will lead to new insights on the progenitor properties (stellar wind, shell) and interstellar environment.
Temperature non-equilibrium means that the electron temperature can be cooler than the ion temperature. This has important consequences as the electron temperature is easier to measure, but the internal energy is dominated by the ions. With X-IFU the ion temperatures can be measured through the thermal line broadening at the edges of some SNRs, as in the center line broadening will be dominated by different velocities from different parts of the shell. The ion temperatures are also interesting as efficient cosmic-ray acceleration by SNRs shocks may lead to lower ion temperatures (as the internal energy will now be divided between hot plasma and cosmic rays). On the topic of cosmic-ray acceleration, some relatively young SNRs are dominated by X-ray synchrotron radiation. This radiation itself does not need X-IFU to characterise its spectrum, however, X-IFU is important to find signatures of thermal emission, which can be more easily picked up by high resolution spectroscopy. The thermal emission will be used to estimate the plasma densities, which is notably needed to model the gamma-ray emission from these young SNRs
Athena will have a fast Target of Opportunity (ToO) observation capability enabling observations of transient phenomena within hours of the trigger. Transients will be very much at the focus of astrophysics by the late 2020s, thanks to facilities like the LSST in the optical, SKA in radio or the all-sky Gamma-ray monitors like SVOM or others. X-IFU observations of some of these triggers will reveal critical astrophysical information on these sources (e.g., on high-z GRBs), extending into the high-energy domain the observations at longer wavelengths.
Likewise, at the end of the next decade, one may expect that gravitational wave sources will be located to an accuracy that will enable follow-up observations with Athena. Coalescing compact objects in binaries (involving at least a neutron star, with a stellar mass black hole or another neutron star) are expected, during their runaway orbital decay due to gravitational radiation, to produce electromagnetic radiation related to the energetic outflows generated. A relativistic jet may form and produce a short gamma-ray burst, followed by an X-ray to radio afterglow lasting from hours to days. So far no such X-ray counterpart has been found, despite extensive searches (Ref. 94 for a review of the follow-up observations of GW150914). Likewise for high redshift GRBs, X-IFU observations of the X-ray counterparts of compact binary gravitational wave sources would shed light on the nature and properties of their progenitors, their energy output in electromagnetic form, as well as on the properties of their host galaxies and circumstellar environments (see Ref. 95 for a review).
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