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The halo of the Milky Way provides a laboratory to study the properties of the shocked hot gas that is predicted by models of galaxy formation. There is observational evidence of energy injection into the halo from past activity in the nucleus of the Milky Way 1 – 4 ; however, the origin of this energy (star formation or supermassive-black-hole activity) is uncertain, and the causal connection between nuclear structures and large-scale features has not been established unequivocally. Here we report soft-X-ray-emitting bubbles that extend approximately 14 kiloparsecs above and below the Galactic centre and include a structure in the southern sky analogous to the North Polar Spur. The sharp boundaries of these bubbles trace collisionless and non-radiative shocks, and corroborate the idea that the bubbles are not a remnant of a local supernova 5 but part of a vast Galaxy-scale structure closely related to features seen in γ-rays 6 . Large energy injections from the Galactic centre 7 are the most likely cause of both the γ-ray and X-ray bubbles. The latter have an estimated energy of around 10 56 erg, which is sufficient to perturb the structure, energy content and chemical enrichment of the circumgalactic medium of the Milky Way. Observations from the eROSITA telescope reveal soft-X-ray-emitting bubbles extending above and below the Galactic plane, which arose from energy injected into the Galactic halo from past activity in the Galactic centre.

Quasi-Periodic Eruptions (QPEs) are extreme high-amplitude bursts of X-ray radiation recurring every few hours and originating near the central supermassive black holes in galactic nuclei. It is currently unknown what triggers these events, how long they last and how they are connected to the physical properties of the inner accretion flows. Previously, only two such sources were known, found either serendipitously or in archival data, with emission lines in their optical spectra classifying their nuclei as hosting an actively accreting supermassive black hole. Here we present the detection of QPEs in two further galaxies, obtained with a blind and systematic search over half of the X-ray sky. The optical spectra of these galaxies show no signature of black hole activity, indicating that a pre-existing accretion flow typical of active nuclei is not required to trigger these events. Indeed, the periods, amplitudes and profiles of the newly discovered QPEs are inconsistent with current models that invoke radiation-pressure driven accretion disk instabilities. Instead, QPEs might be driven by an orbiting compact object. Furthermore, their observed properties require the mass of the secondary object to be much smaller than the main body and future X-ray observations may constrain possible changes in the period due to orbital evolution. This scenario could make QPEs a viable candidate for the electromagnetic counterparts of the so-called extreme mass ratio inspirals, with considerable implications for multi-messenger astrophysics and cosmology.

We present the results of our studies of the aperiodic optical flux variability for SS Cyg, an accreting binary systemwith a white dwarf. The main set of observational data presented here was obtained with the ANDOR/iXon DU-888 photometer mounted on the RTT-150 telescope, which allowed a record (for CCD photometers) time resolution up to 8 ms to be achieved. The power spectra of the source’s flux variability have revealed that the aperiodic variability contains information about the inner boundary of the optically thick flow in the binary system. We show that the inner boundary of the optically thick accretion disk comes close to the white dwarf surface at the maximum of the source’s bolometric light curve, i.e., at the peak of the instantaneous accretion rate onto the white dwarf, while the optically thick accretion disk is truncated at distances 8.5 × 10 9 cm ∼10 R WD in the low state. We suggest that the location of the inner boundary of the accretion disk in the binary can be traced by studying the parameters of the power spectra for accreting white dwarfs. In particular, this allows the mass of the accreting object to be estimated.

We show how the simultaneous presence of a strong quasi periodic oscillation (QPO) of period 10 seconds in the optical and X-ray lightcurves of theX-ray transient XTE J1118+480 can be used to obtain information about the nature of the accretion flow around the source.The unusually high optical-to-X-ray flux ratio and the QPO observed simultaneously in both energy bands suggest that a significant fraction ofthe optical flux might originate close to the central source,where most of the X-rays are produced and be indicative of a magnetically dominated corona.We also show how the temporal evolution of the QPO can provide us with information on both the inner radius and the viscous properties of the optically thick accretion disc.[PUBLICATION ABSTRACT]

The wide-area XMM-XXL X-ray survey is used to explore the fraction of obscured AGN at high accretion luminosities, \\(L_X (\\rm 2-10 \\, keV) > 10^{44} \\, erg \\,s ^{-1}\\), and out to redshift \\(z\\approx1.5\\). The sample covers an area of about \\(\\rm14\\,deg^2\\) and provides constraints on the space density of powerful AGN over a wide range of neutral hydrogen column densities extending beyond the Compton-thick limit, \\(\\rm N_H\\approx10^{24}\\,cm^{-2}\\). The fraction of obscured Compton-thin (\\(\\rm N_H=10^{22}-10^{24}\\,cm^{-2}\\)) AGN is estimated to be \\(\\approx0.35\\) for luminosities \\(L_X(\\rm 2-10\\,keV)>10^{44}\\,erg\\,s^{-1}\\) independent of redshift. For less luminous sources the fraction of obscured Compton-thin AGN increases from \\(0.45\\pm0.10\\) at \\(z=0.25\\) to \\(0.75\\pm0.05\\) at \\(z=1.25\\). Studies that select AGN in the infrared via template fits to the observed Spectral Energy Distribution of extragalactic sources estimate space densities at high accretion luminosities consistent with the XMM-XXL constraints. There is no evidence for a large population of AGN (e.g. heavily obscured) identified in the infrared and missed at X-ray wavelengths. We further explore the mid-infrared colours of XMM-XXL AGN as a function of accretion luminosity, column density and redshift. The fraction of XMM-XXL sources that lie within the mid-infrared colour wedges defined in the literature to select AGN is primarily a function of redshift. This fraction increases from about 20-30% at z=0.25 to about 50-70% at \\(z=1.5\\).

In the last decade, a combination of high sensitivity, high spatial resolution observations and of coordinated multi-wavelength surveys has revolutionized our view of extra-galactic black hole (BH) astrophysics. We now know that supermassive black holes reside in the nuclei of almost every galaxy, grow over cosmological times by accreting matter, interact and merge with each other, and in the process liberate enormous amounts of energy that influence dramatically the evolution of the surrounding gas and stars, providing a powerful self-regulatory mechanism for galaxy formation. The different energetic phenomena associated to growing black holes and Active Galactic Nuclei (AGN), their cosmological evolution and the observational techniques used to unveil them, are the subject of this chapter. In particular, I will focus my attention on the connection between the theory of high-energy astrophysical processes giving rise to the observed emission in AGN, the observable imprints they leave at different wavelengths, and the methods used to uncover them in a statistically robust way. I will show how such a combined effort of theorists and observers have led us to unveil most of the SMBH growth over a large fraction of the age of the Universe, but that nagging uncertainties remain, preventing us from fully understating the exact role of black holes in the complex process of galaxy and large-scale structure formation, assembly and evolution.

Almost 50% of galaxies in the local Universe are in clusters or groups coexisting with both hot and cold gas components. In the present study, we observationally probed the cold-gas content of X-ray-selected massive galaxy clusters with spectroscopic redshift measured from the SDSS/SPIDERS survey. This paper focuses on the most massive structures: galaxy clusters with a mean mass of M\\(_{500c}\\) = 2.7\\(\\times 10^{14}\\) M\\(_{\\odot}\\). We used a large number of background quasar optical spectra from SDSS DR16 to probe the diffuse T\\(=\\)10\\(^4\\)K gas in their intracluster medium. We first analysed a sample of spectra with known MgII absorbers, and then blindly stacked about 16,000 archival spectra at the redshifts of the foreground galaxy clusters. We tentatively (\\(3.7 \\sigma\\) significance) detect MgII in the clusters with an equivalent width EW(MgII \\(\\lambda\\)2796) of 0.056\\(\\pm\\)0.015 Å, corresponding to a column density of log [N(MgII)/cm\\(^{-2}\\)]=12.12\\(\\pm0.1\\). We tested our methodology by generating 22,000 mock SDSS spectra with MgII absorbers from TNG50 cosmological magnetohydrodynamical simulations, combining photo-ionisation modelling and ray tracing. We also performed bootstrapping stacking at different cluster redshifts and stacked quasar spectra with no intervening clusters in the line of sight to measure the significance of our detection. These results are in line with the findings of recent, similar observational studies but challenge predictions from TNG simulations. Together, our findings indicate that large amounts of cold gas may be found in the most massive structures of the Universe.

Multi-wavelength extragalactic nuclear transients, particularly those detectable as multi-messengers, are among the primary drivers for the next-generation observatories. X-ray quasi-periodic eruptions (QPEs) are the most recent and perhaps most peculiar addition to this group. Here, we report a first estimate of the volumetric rate of QPEs based on the first four discoveries with the eROSITA X-ray telescope onboard the Spectrum Roentgen Gamma observatory. Under the assumption, supported by a suite of simulated light curves, that these four sources sample the intrinsic population somewhat homogeneously, we correct for their detection efficiency and compute a QPE abundance of \\(\\mathscr{R}_{{\\rm vol}} = 0.60_{-0.43}^{+4.73} \\times 10^{-6}\\,\\)Mpc\\(^{-3}\\) above an intrinsic average \\(\\log L_{\\rm 0.5-2.0\\,keV}^{\\rm peak} > 41.7\\). Since the exact lifetime of QPEs (\\(\\tau_{\\rm life}\\)) is currently not better defined than between a few years or few decades, we convert this to a formation rate of \\(\\mathscr{R}_{\\rm vol}/\\tau_{\\rm life}\\approx 0.6 \\times 10^{-7} (\\tau_{\\rm life}/10\\,\\mathrm{y})^{-1}\\,\\)Mpc\\(^{-3}\\,\\)year\\(^{-1}\\). As a comparison, this value is a factor \\(\\sim10\\,\\tau_{\\rm life}\\) times smaller than the formation rate of tidal disruption events. The origin of QPEs is still debated, although lately most models suggest that they are the electromagnetic counterpart of extreme mass ratio inspirals (EMRIs). In this scenario, the QPE rate would thus be the first-ever constraint (i.e. a lower limit) to the EMRI rate from observations alone. Future discoveries of QPEs and advances in their theoretical modeling will consolidate or rule out their use for constraining the number of EMRIs detectable by the LISA mission.

We present near-infrared spectra, obtained with SINFONI and XShooter observations at ESO VLT, of nine dusty, red QSOs at 1.210^{21} - 10^{22} cm^{-2}) and show a broad Ha component in the NIR spectra. We complement this sample with twelve additional sources taken from the literature with similar properties resulting in a total sample of 21 X-ray obscured, intermediate type (1.8-1.9), dusty reddened QSOs. From the broad Ha line we have computed the BH masses through the virial formula and derived Eddington ratios. Moreover, from optical/IR multi-component SED fitting we have derived the stellar mass of their host galaxies and their SFRs. We find that most of the sources in our sample are hosted in starburst and main sequence star-forming galaxies with Eddington ratios lambda>0.1. We find a strong trend with the BH mass i.e. less massive objects are scattered below and above the local relation while the most massive ones are mainly located above it. We also studied the evolution of these sources on the M_BH-M_star plane compared to a sample of optically blue type--1 QSOs and we find that obscured red QSOs show a ratio of M_BH to M_star that increases with redshift which is consistent with or slightly lower than what has been found for blue QSOs. These sources may represent the blow-out phase at the end of the rapid BH growth and immediately preceding the classical blue QSOs typically sampled in optical surveys. They in fact show evidence of outflows in the ionized gas component, but their BH has already fully formed.

It has been argued that recycled gas from stellar mass loss in galaxies might serve as an important fuelling source for black holes (BHs) in their centers. Utilizing spectroscopic samples of galaxies from the Sloan Digital Sky Survey (SDSS) at \\(z = 0-0.35\\) and the Large Early Galaxy Astrophysics Census (LEGA-C) survey at \\(z = 0.6-1\\) that have X-ray coverage from XMM-Newton or Chandra, we test this stellar mass loss fuelling scenario by investigating how AGN activity and BH growth vary with the break strength at 4000 \\(Å\\), \\(\\rm D_{n}4000\\) (which is closely related to the age of stellar populations), as younger galaxies are considered to have higher stellar mass loss rates. We found that when controlling for host-galaxy properties, the fraction of log \\(L_{\\rm X}\\)/\\(M_\\star\\) > 32 (which roughly corresponds to Eddington ratios \\(\\gtrsim 1\\)%) AGN and sample-averaged black hole accretion rate (\\(\\rm \\overline{BHAR}\\)) decrease with \\(\\rm D_{n}4000\\) among \\(\\rm D_{n}4000\\) \\(\\lesssim\\) 1.9 galaxies, suggesting a higher level of AGN activity among younger galaxies, which supports the stellar mass loss fuelling scenario. For the oldest and most massive galaxies at \\(z = 0-0.35\\), this decreasing trend is not present anymore. We found that, among these most massive galaxies at low redshift, the fraction of low specific-accretion-rate (31 \\(<\\) log \\(L_{\\rm X}\\)/\\(M_\\star\\) \\(<\\) 32) AGNs increases with \\(\\rm D_{n}4000\\), which may be associated with additional fuelling from hot halo gas and/or enhanced accretion capability.