The energetic Universe
X-IFU observations will measure the energy released by accretion onto black holes by winds and outflows, all the way up from the local Universe to z~3, providing solid ground to understand AGN feedback. In nearby galaxies, the amount of gas, energy and metals blown into the circum-galactic medium by both AGN and starbursts will be mapped. By targeting distant GRB afterglows, X-IFU observations will characterise the ambient interstellar medium of high-z galaxies and constrain their prevailing stellar populations. Deep X-IFU observations of obscured distant AGN will be able to unveil their redshift via the Fe K emission line.
X-IFU observations of gamma-ray bursts can play a unique role in the study of metal enrichment as GRBs are the brightest light sources at all redshifts and, for long duration events (LGRBs), occur in star-forming regions. As LGRB progenitors are short-lived massive stars, they provide an ideal probe of the effect of stellar evolution on galaxy chemical enrichment across cosmic time. Beginning with metal free (Population III) stars, the cycle of metal enrichment started when their final explosive stages injected the first elements beyond Hydrogen and Helium into their pristine surroundings, quickly enriching the gas. These ejecta created the seeds for the next generation of stars (population II). Finding and mapping the earliest star formation sites (population III/II stars) is one of the top priorities for future astrophysical observatories. Tracing the first generation of stars is crucial for understanding cosmic re-ionization, the formation of the first seed Super-Massive Black Holes (SMBH), and the dissemination of the first metals in the Universe. Photons from Pop III stars and radiation generated from accretion onto the first SMBH initiate the Cosmic Dawn.
The chemical fingerprint of Pop III star explosions is distinct from that of later generations, opening the possibility to probe the Initial Mass Function (IMF) of the Universe. Stellar evolution studies show that the nucleosynthetic yields of Pop III and Pop II explosions differ significantly. The convolution of these yields with an IMF directly translates to abundance patterns, which can differ up to an order of magnitude depending on the characteristic mass scale of the IMF (Ref. 28). The X-IFU will be able to measure metal abundance patterns for a variety of ions (e.g., S, Si, Fe) for at least 10 medium-bright X-ray afterglows per year with H equivalent column densities as small as 1021 cm-2 and gas metallicities as low as 1% of solar for the denser regions expected in early star-forming zones; in even denser regions the accuracy will be further improved (Figure below). Measuring these patterns using GRBs, combined with Athena studies of AGN sightlines, galaxies and supernovae, will enable us to determine the typical masses of early stars, thereby testing whether or not the primordial IMF is indeed top heavy. For more information see Ref. 29.
Probing the very first stars in the Univers
The measurement of tight correlations between the mass of galaxy bulges, or of the velocity dispersion of their stellar content, and the mass of the SMBH hosted in all galaxies (Refs. 30, 31) strongly indicate that some feedback mechanism must have acted between these components during the galaxy formation and evolution phases. In the last fifteen years, thanks to X-ray observations of quasars and nearby Seyfert galaxies, we have been able to identify AGN driven ultra-fast outflows (UFOs) as one of the plausible mechanisms (Refs. 32-34). UFOs manifest themselves as blushifted resonant absorption lines due to highly ionized iron. They are detectable in the 7-10 keV band (Refs. 34, 35) and are thought to be related to winds that are energetic enough to quench the star formation of the host galaxies. This provides the link between the tiny SMBH and the huge galaxy bulges (Ref. 36, see Ref.37 for a review on this topic). However, we still lack some fundamental pieces of evidence to fully verify this scenario: 1) we do not know what is the launching mechanism of the UFOs, how they deposit their energy into the interstellar medium and how they interact with star forming regions; 2) we have only poor knowledge of the physical conditions of the AGN engine and of the importance of UFOs at high-z, i.e., in epochs where both QSO and starburst activity were at their highest (Refs. 38-40 and references therein).
To investigate the first of the above mentioned questions, we should obtain a detailed characterization of the physical properties of UFOs (column density, ionization state, outflow velocity, location, geometry, covering factor, etc.) and check how they evolve with respect to the distance from the source, maximum outflow velocity, and X-ray or UV luminosity (see Figure below). We must also consider that UFOs are known to be variable (Ref. 36) and so we must be able to investigate the above mentioned properties on time-scales of ~10-100 ks (corresponding to the dynamical time-scale at 10 Rg for a SMBH with M~107-8 Msun). The coupling of the large collecting area and of the exquisite energy resolution of X-IFU provides the capability to perform such kind of studies (Ref. 41) on a sample of at least ~50-80 nearby (z<0.01) and bright (F2-10 keV > 10-11 erg cm-2 s-1) Seyfert galaxies with the accuracy needed to disentangle the right scenario among the radiation-driven (e.g., Refs. 42, 43), momentum-driven (Ref. 44), and magnetically-driven (Ref. 45) accretion disc wind models. Moreover, the ability of the X-IFU to simultaneously obtain high-resolution X-ray spectra and good quality images will allow us to investigate where and how the AGN driven winds interact with the interstellar medium, what is the interplay among the AGN- and the starburst-driven winds, and how these winds enrich the intergalactic matter. In this respect it will be important to study nearby starburst and Luminous Infrared Galaxies (LIRG)/Ultra Luminous Infrared Galaxies (ULIRG) with the main goal to map, from good quality X-ray spectra, regions dominated by collisionally-ionized plasmas (hot gas), non-equilibrium ionization (shocks) and photo-ionized plasmas. Within its nominal life, X-IFU will be able to obtain such information for at least 30 objects thus allowing, for the very first time, to have a clear vision of such phenomena in the nearby Universe.
To fully understand if and how these winds acted along cosmic time to shape the SMBH-galaxy co-evolution, we must finally couple the knowledge on the launch and SMBH-host galaxy interaction mechanisms obtained at low redshift with a reliable census of the UFOs and of their properties at high-z, i.e., we must answer the second of the above mentioned questions. Up to now, UFOs have been measured only in a handful of mostly lensed (i.e., brightness enhanced) QSO at high redshift (Refs. 32, 46-49). This clearly demonstrates that the capability to collect photons is fundamental to obtain the information we need. The Athena collecting area will enable investigations of the properties of ionized and outflowing absorbers in large samples of QSO (LX 1044erg/s) up to redshift z~4 (Ref. 50). Moreover, the wide angle surveys performed using WFI should produce rich samples of hundreds of QSO suitable for detailed spectroscopic studies with the X-IFU. These studies are expected to allow reliable estimates of the physical parameters of UFOs at the knee (L*) of the X-ray luminosity function, which dominate the growth of SMBH at z ~ 1-4. We expect to be able to fully test the scenarios that assume the AGN driven winds as the mechanism to self-regulate the star-formation and SMBH growth along cosmic times.
Outflows from supermassive black holes
Thanks to its high energy resolution and sensitivity, the X-IFU will allow the measurement of SMBH spins with unprecedented accuracy in a large number of AGN even beyond the local Universe.
The angular momentum is, in addition to mass, the other fundamental parameter that characterizes astrophysical black holes and therefore a proper census of SMBH in the Universe requires the measurement of their spins. This is also of fundamental importance in order to understand the black hole growth history (Ref.51), particularly the relative roles of mergers and chaotic accretion, that tend to reduce the spin, versus prolonged accretion, that generally increases it. Moreover a systematic study of SMBH spins would shed light on the relation between SMBH rotation and its outflow power in the form of relativistic jets. Since the influence of the spin is felt only up to a few gravitational radii, X-ray observations, probing the innermost regions of SMBH systems, are the main tool for such measurements.
The simplest and most widely applicable way of measuring SMBH spins is via time-averaged spectral fitting of relativistically broadened Fe Kalpha lines (Ref. 52). The larger the spin, the closer to the horizon will be the innermost stable circular orbit (ISCO) of a prograde accretion disk resulting in a larger broadening of the reflected lines. Broad lines are present in at least ~ 30-40% of bright nearby type 1 AGN (Refs.53, 54) and reliable estimates of SMBH spins have already been made in about 20 AGN with this technique (e.g., 55-58). The Fe line is always accompanied by a reflection continuum in hard X-rays and, if the reflecting matter is at least partly ionized, also in the soft X-rays. With its large effective area over a broad energy range X-IFU will permit the simultaneous use of the iron line and of the soft X-ray reflection continuum to measure black hole spins also at intermediate redshifts. As an example, the spin of a maximally rotating black hole spin in PB5062, a luminous (LX ~ 3 x 1046 erg s-1) QSO at z=1.77 can be recovered with a precision of 20% in a 100 ks observation.
One of the difficulties in these measurements is to disentangle the different AGN spectral components that come from regions at different distances from the SMBH. With its excellent energy resolution the X-IFU will easily separate the broad lines from the narrow features, which are ubiquitous in AGN and originate from more distant matter (Ref. 59) and this will again allow to increase drastically the sample of measured SMBH spins.
X-IFU will also be able to map the inner regions of the accretion disks in the time-energy plane. Any deviation from axial symmetry in the disk emissivity (e.g., associated with hot spots) will lead to a characteristic variability of the iron line (Ref. 60), with arcs being traced out on the time-energy plane (Ref. 61). Evidence of hot spots were found in XMM-Newton data (Refs. 62-63) and they are of great diagnostic power for tracing the inner turbulent flow of the disk in the strong gravity environment. General Relativity makes specific predictions for the arc forms and from a fit of these features one can derive SMBH mass and spin as well as the disk view inclination.
Outbursts from Galactic stellar-mass black holes and neutron stars span orders of magnitude in mass accretion rate, and evolve over days, weeks, and months (Ref. 64-66). In contrast, the same dynamic range in AGN is not accessible on human timescales. The high flux observed from Galactic sources ensures very high sensitivity in the crucial Fe K band, wherein the most highly ionized gas –likely tied to the region closest to the black hole– is observed. Discoveries made in the Fe K band in stellar-mass black holes help to direct and sharpen subsequent observations of AGN. In short, Galactic compact objects represent rapidly evolving, high-flux proxies that are vital to understanding the much larger classes of Seyfert galaxies and quasars.
Theoretical studies demand that black holes must be fuelled by accretion disks wherein magnetic processes mediate the transfer of mass and angular momentum (Ref. 68-70). In addition, the angular momentum can be transported throughout the accretion flow via the non-axisymmetric waves and shocks, related to the self-gravitating parts of disks in active galactic nuclei, or to the regions tidally excited by the companion star in black hole binaries (Ref. 71).
For decades, one were only able to observe the effects of the fundamental disk physics (thermal spectra, jet ejection episodes), but unable to probe the underlying process that fuels black holes in Seyfert and quasar accretion modes. Very new observations may now indicate that disk winds can be used to constrain the emergent magnetic field of accretion disks (Ref. 67). This opens a long-awaited window on the fundamental physics of disk accretion and enables connections to numerical simulations. This is also a window on mechanical feedback from black holes, since feedback modes may depend on the disk magnetic field strength and configuration (Refs. 72,73).
Equipped with the X-IFU, Athena will have the power to reveal the fundamental physics that drives accretion onto black holes. The keys are spectral resolution, and sensitivity. Whereas current telescopes can only offer initial constraints on disk physics in 100 ksec, the X-IFU will be able to constrain magnetic fields via winds in just hundreds of seconds – the dynamical time scale of such winds. The evolution of the outflow properties (mass outflow rate, kinetic power) and disk fields will finally be accessible on their natural time scale (see Figure below).
Jets may be the main agents of feedback from black holes into environments as large as galaxy clusters. In both stellar-mass black holes and AGN, momentary dips in the X-ray flux have been associated with relativistic jet ejection events (Ref. 74-76). These dips last 100s of seconds in stellar-mass black holes. Whereas prior missions have only been able to observe continuum variations that do not reveal velocities or accelerations, X-IFU will be able to obtain sensitive line spectra as mass is transferred from the disk into the jet, providing a new and unparalleled view of the jet launching process.
For black holes that accrete at rates more than a few per cent of the Eddington limit, the inner region of the flow will be dominated by radiation rather than gas pressure (Ref. 77). Observationally, two microquasars have shown spectacular X-ray variability on timescales of order of 50-100 s, that confirm the limit-cycle nature of the underlying process (Refs. 78,79). Moreover, the observed interplay between the wind outflow launched in some states of the sources and such ‘heartbeat’ variations has shed some light on the plausible disk stabilizing force (Ref. 78). The magnetic fields, or, in general, the intrinsically stochastic nature of the turbulent dissipation must be an important agent here (Ref. 81). X-IFU observations of these and other targets will reveal to exquisite detail the structure of these winds and allow a detailed study of disk wind launching mechanisms and open a window into the study of fundamental disk physics.
A remarkable case that links the stellar BH to the SMBH of AGN is the massive BH at the center of our own galaxy, Sgr A*. With its 4× 106 Msun mass and consequently a time scale of ~30 min at the ISCO, its clean close environment due to the low accretion rate and the fact that it is relatively nearby, Sgr A* offers the opportunity to study the details of its steady accretion flow and the non-thermal flares that take place within a few Schwarzschild radii from the horizon. Even if challenging because of the confusion in the inner 5″ of the galaxy, the X-IFU will allow to study the spectral lines of the Sgr A* accreting plasma (Ref. 82) providing insights in the physical conditions and dynamics of the flow. X-IFU will also be able to explore in detail the variable fluorescent spectral lines from the molecular clouds of the central zone which are reflecting radiation from ancient outbursts of the BH (Refs. 83-84). With its excellent spectral-imaging performances the X-IFU will obtain Molecular Cloud line diagnostics unreachable to present instruments and crucial for the reconstruction of Sgr A* light curve in the past 1000 yr, probing the link between dormant SMBH and their past active phases.
Black hole winds
X-IFU simulated observation lasting only 100 seconds of the Black Hole binary GRS1915+105. A disk wind, as reported in Ref. 67 has been simulated. Strong spectral features can be clearly seen in the spectrum, including Fe Kalpha (6.65–6.75 keV), the resolved doublet Fe Kalpha (6.95–7.00 keV), Fe Kalpha shifted by 0.01c (7.00–7.05 keV), Fe XXVI Kalpha shifted by 0.03c, Fe XXV Kbeta (7.65–7.90 keV) and Fe XXVI Kbeta (8.20–8.35 keV). This rich set of features will enable unprecedented studies of the structure of the disk winds.
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