Preface to Excerpts From the 2025 ESAC Conference on X‐Ray Quasi‐Periodic Eruptions and Repeating Nuclear Transients

Super-massive binary black holes and emission lines in active galactic nuclei

In the last , following pioneering papers by Ya B Zeldovich and E E Salpeter, in which a powerful energy release from nonspherical accretion of matter onto a black hole (BH) was predicted, many observational studies of black holes in the Universe have been carried out. To date, the masses of several dozen stellar-mass black holes in X-ray binary systems and of several hundred supermassive black holes in galactic nuclei have been measured. The estimated radii of these massive and compact objects do not exceed several gravitational radii. For about ten stellar-mass black holes and several dozen supermassive black holes, the values of the dimensionless angular momentum have been estimated, which, in agreement with theoretical predictions, do not exceed the limiting value of . A new field of astrophysics, so-called black hole demography, which studies the birth and growth of black holes and their evolutionary connection to other objects in the Universe, namely stars, galaxies, etc., is rapidly developing. In addition to supermassive black holes, massive stellar clusters are observed in galactic nuclei, and their evolution is distinct from that of supermassive black holes. The evolutionary relations between supermassive black holes in galactic centers and spheroidal stellar components (bulges) of galaxies, as well as dark-matter galactic haloes are brought out. The launch into Earth's orbit of the space radio interferometer RadioAstron opened up the real possibility of finally proving that numerous discovered massive and highly compact objects with properties very similar to those of black holes make up real black holes in the sense of Albert Einstein's General Relativity. Similar proofs of the existence of black holes in the Universe can be obtained by intercontinental radio interferometry at short wavelengths (the international program, Event Horizon Telescope).

Black Holes and the Supermassive Compact Object at the Galactic Center: Multi-arts of Thought and Nature

view Abstract Citations (47) References (38) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Binary Pairs of Supermassive Black Holes: Formation in Merging Galaxies Valtaoja, L. ; Valtonen, M. J. ; Byrd, G. G. Abstract In a previous paper by Sillanpaa et al. we find evidence for a binary pair of supermassive black holes in OJ 287. Here we study a process in which supermassive binary black holes are formed in nuclei of supergiant galaxies due to galaxy mergers. There is growing evidence that mergers of galaxies are common and that supermassive black holes in centers of galaxies are also common. Consequently, it is expected that binary black holes should arise in connection with galaxy mergers. We first consider the merger process in a galaxy which is modeled after M87. We include Chandrasekhar's dynamical friction and the tidal break-up of the companion galaxy in calculating the orbits of the companion galaxies. After the disruption of the companion we follow the orbits of the central black holes. We derive the capture probability of a companion as a function of its mass. Assuming a correlation between the galaxy mass and the black holes mass, we calculate the expected mass ratio in binary black holes. The binary black holes formed in this process are long lived, surviving longer than the Hubble time unless they are perturbed by black holes from successive mergers. The properties of these binaries agree with Gaskell's observational work on quasars and its interpretation in terms of binary black holes. Publication: The Astrophysical Journal Pub Date: August 1989 DOI: 10.1086/167683 Bibcode: 1989ApJ...343...47V Keywords: Black Holes (Astronomy); Galactic Nuclei; Interacting Galaxies; Chandrasekhar Equation; Mass Distribution; Quasars; Astrophysics; BLACK HOLES; GALAXIES: INTERACTIONS; GALAXIES: NUCLEI; QUASARS full text sources ADS | data products SIMBAD (2) NED (2)

We review the results of multi‐scale, hydrodynamical simulations of major mergers between galaxies with or without central supermassive black holes (SMBHs) to investigate the orbital decay of SMBH pairs in galactic nuclei and the formation of massive SMBH seeds via direct gas collapse. Both SPH simulations and AMR simulations are carried out. The complex balance between heating and cooling is modeled via an effective EOS with varying adiabatic index γ apparopriate for the conditions of an intense nuclear starburst such as that expected during and after the merger. Prominent gas inflows due to tidal torques produce nuclear disks at the centers of merger remnants whose properties depend sensitively on the details of gas thermodynamics. In parsec‐scale resolution simulations starting with two SMBHs originally at the centers of the two galaxies, a SMBH binary forms very rapidly, less than a million years after the merger of the two galaxies, owing to the drag exerted by the surrounding gaseous nuclear disk. Binary formation is significantly suppressed if heating, by e.g. radiative feedback from the accreting SMBHs, renders cooling negligible.The nuclear disk rearranges its mass distribution in response to a second, internal gas inflow occurring while the binary sinks. The inflow is driven by spiral instabilities imprinted by the final collision between the two galactic cores. In simulations with 0.1 pc resolution, the gas inflow continues all the way down to the center and peaks at >104 M⊙/yr, producing a Jeans‐unstable supermassive central cloud (with mass a few times 108 M⊙) only 105 yr after the merger. If the collapse continues, the cloud could form a massive black hole seed (Mseed>105 M⊙) after prior formation of a supermassive star or quasi‐star. The massive SMBH seed can grow up to a billion solar masses in less than a billion years by accreting the surrounding nuclear gas. If the gas‐rich merger occurs at z>8, this is then a new, attractive way to explain the rapid emergence of the bright QSOs discovered by the Sloan Digital Sky survey at z>6, which does not require the assumption of primordial gas composition in order to suppress cooling below 104 K and star formationas in models starting from unstable, isolated protogalactic disks. If there is a pre‐existing pair of SMBHs their orbital decay stalls at parsec scales because, as a result of the formation of the supermassive cloud, the nuclear disk density decreases outside the cloud, yielding much weaker dynamical friction. We envision a new scenario in which direct formation of massive black hole seeds and SMBH binary formation are mutually exclusive; if a SMBH is already present in the nuclear disk it can stabilize it and weaken the secondary inflow via its energetic feedback, maintaining a high enough density where the SMBHs are located and assisting their sinking.

It is widely accepted that active galactic nuclei (AGN) are hosting a supermassive black hole in their center. The supermassive black hole is actively fueled by surrounding gas through an accretion disk, which produces a broad band continuum (from X-ray to radio emission). The hard photons from the accretion disk create the photoionized plasma around the central black hole, which emits a number of broad emission lines. Therefore, one of the signatures of the strong activity in galaxies is the emission of the broad spectral lines (line widths of several 1000 km/s), which are seen only in a fraction of AGN, so called Type 1 AGN. These broad emission lines often show very complex line profiles, usually strongly variable in time. Here we will describe the basic properties of the broad emission lines and how can we use them to derive the properties of the central supermassive black hole, i.e., the mass and spin, or see signatures of supermassive binary black holes.

Galaxy mergers are a crucial pathway for the growth and evolution of galaxies at the cosmic center. Supermassive black holes (SMBHs) are commonly present at the centers of galaxies. During galaxy mergers, the central SMBHs may form binary black hole systems and, in some cases, merge. Detecting and studying supermassive binary black hole systems within galaxies is a significant aspect of galaxy evolution research. Active black holes produce prominent broad emission lines, which serve as effective probes of such activity. The presence of two distinct sets of broad emission lines with velocity differences has traditionally been considered a key indicator for identifying binary black holes. However, the probability of both black holes in a binary system being active is extremely low. More commonly, binary systems consist of one active and one quiescent black hole. The spectral signature of such systems is characterized by a significant velocity offset between the broad emission lines and the system's narrow emission lines. This velocity difference has also led to the discovery of "recoiling" black holes, driven by gravitational wave radiation from closely bound binary systems. Recoiling black holes are crucial observational targets for studying black hole mergers, binary orbital evolution, and galaxy mergers.We have identified a class of recoiling black holes with partially obscured nuclear regions. Dust in the nuclear region attenuates the intense radiation from the active black hole, allowing the host galaxy's emission to become visible alongside the nuclear radiation and broad-line features. This discovery provides a unique perspective for exploring the physical connection between the evolution of binary black hole systems and galaxy evolution. Further studies using multi-wavelength photometry, high-resolution spectroscopic analysis, and long-term spectral monitoring of partially obscured supermassive binary black hole systems are expected to reveal the physical processes underlying black hole mergers and galaxy mergers, as well as their significant role in galaxy evolution.

Stars passing too close to a super massive black hole (SMBH) can produce tidal disruption events (TDEs). Since the resulting stellar debris can produce an electromagnetic flare, TDEs are believed to probe the presence of single SMBHs in galactic nuclei, which otherwise remain dark. In this paper, we show how stars orbiting an IMBH secondary are perturbed by an SMBH primary. We find that the evolution of the stellar orbits are severely affected by the primary SMBH due to secular effects and stars orbiting with high inclinations with respect to the SMBH-IMBH orbital plane end their lives as TDEs due to Kozai-Lidov oscillations, hence illuminating the secondary SMBH/IMBH. Above a critical SMBH mass of $\approx 1.15 \times 10^8$ M$_{\odot}$, no TDE can occur for typical stars in an old stellar population since the Schwarzschild radius exceeds the tidal disruption radius. Consequently, any TDEs due to such massive SMBHs will remain dark. It follows that no TDEs should be observed in galaxies with bulges more massive than $\approx 4.15\times 10^{10}$ M$_{\odot}$, unless a lower-mass secondary SMBH or IMBH is also present. The secular mechanism for producing TDEs considered here therefore offers a useful probe of SMBH-SMBH/IMBH binarity in the most massive galaxies. We further show that the TDE rate can be $\approx 10^{-4}-10^{-3}$ yr$^{-1}$, and that most TDEs occur on $\approx 0.5$ Myr. Finally, we show that stars may be ejected with velocities up to thousands of km s$^{-1}$, which could contribute to the observed population of Galactic hypervelocity stars.

Relativistic jets are streams of plasma moving at appreciable fractions of the speed of light. They have been observed from stellar-mass black holes (~3 to 20 solar masses, M(⊙)) as well as supermassive black holes (~10(6) to 10(9) M(⊙)) found in the centers of most galaxies. Jets should also be produced by intermediate-mass black holes (~10(2) to 10(5) M(⊙)), although evidence for this third class of black hole has, until recently, been weak. We report the detection of transient radio emission at the location of the intermediate-mass black hole candidate ESO 243-49 HLX-1, which is consistent with a discrete jet ejection event. These observations also allow us to refine the mass estimate of the black hole to be between ~9 × 10(3) M(⊙) and ~9 × 10(4) M(⊙).

In many galactic nuclei, a nuclear stellar cluster (NSC) co-exists with a supermassive black hole (SMBH). In this second one in a series of papers, we further explore the idea that the NSC forms before the SMBH through the merger of several stellar clusters that may contain intermediate-mass black holes (IMBHs). These IMBHs can subsequently grow by mergers and accretion to form an SMBH. To check the observable consequences of this proposed SMBH seeding mechanism, we created an observationally motivated mock population of galaxies, in which NSCs are constructed by aggregating stellar clusters that may or may not contain IMBHs. Based on several assumptions, we model the growth of IMBHs in the NSCs through gravitational wave (GW) mergers with other IMBHs and gas accretion. In the case of GW mergers, the merged BH can either be retained or ejected depending on the GW recoil kick it receives. The likelihood of retaining the merged BH increases if we consider the growth of IMBHs in the NSC through gas accretion. We find that nucleated lower mass galaxies (${\it M}_{\star } \lesssim 10^{9}\, {\rm M_{\odot }}$; e.g. M33) have an SMBH seed occupation fraction of about 0.3–0.5. This occupation fraction increases with galaxy stellar mass and for more massive galaxies ($\rm 10^{9} \ \lesssim {\it M}_{\star } \lesssim 10^{11}\, {\rm M_{\odot }}$), it is between 0.5 and 0.8, depending on how BH growth is modelled. These occupation fractions are consistent with observational constraints. Furthermore, allowing for BH growth also allows us to reproduce the observed diversity in the mass range of SMBHs in the ${\it M}_{\rm NSC}\!-\!{\it M}_{\rm BH}$ plane.

Nowadays we generally accept the idea that the emission from active galactic nuclei (AGNs) is powered by an accreting supermassive black hole (SMBH) at the center of a galaxy. It is also well known that most -probably all- galaxies go through an active phase at some point in their lives, and that several empirical relations connect black hole and host galaxy properties: this suggests that a tight feedback between the evolution of the black hole and the host galaxy exists; hence, in order to go deeper into galaxy evolution, it is crucial for us to learn more about the formation and evolution of the black holes residing in their centers. Variability is a defining property of AGN emission at all wavebands, and concerns both continuum and broad-line emission. It is generally attributed to instabilities in the AGN accretion disk, together with changes in the accretion rate. Since the extent of variations in different wavelength ranges is not the same, variability measurements can help understand the underlying emission mechanism, constraining the size and structure of the emitting region. The present work investigates AGN variability from two different perspectives. The first part of the project tests the efficiency of optical variability as a tool to select AGNs, since optical continuum variability seems to be a universal feature of broad-line AGNs on timescales from months to years, with variations generally ranging from 1% to 10% of the magnitude, but also much larger (50% of the magnitude) in some cases. Testing techniques for AGN identification based on data from ground-based telescopes is of great relevance in the framework of current and future wide-field surveys (e.g., Dark Energy Survey, Large Synoptic Survey Telescope), since we will need reliable methods to detect and classify the wealth of sources they will provide. The second part of the project investigates the variability of broad absorption lines (BALs) in quasi-stellar object (QSO) spectra, in order to deeply understand the physics and structure of AGNs. BALs originate from outflowing winds along our line of sight; winds are thought to originate from the accretion disk, in the very proximity of the central SMBH; we generally think that they are responsible for a triggering of the accretion mechanism onto the SMBH, as they remove angular momentum from the disk and, since they evacuate gas from the host galaxy, they also play a leading role into galaxy evolution. Several works show that BAL equivalent widths can change on typical timescales from months to years. Such variability is generally attributed to changes in the covering factor (due to rotation and/or changes in the wind structure) and/or in the ionization level. We investigate BAL variability, focusing on BAL disappearance, in a sample of more than 1500 QSOs -the largest sample ever used for such an analysis- to gain insight into the structure and co-evolution of the SMBH and the host galaxy.

Stellar-mass black holes (BHs) are expected to segregate and form a steep density cusp around supermassive black holes (SMBHs) in galactic nuclei. We follow the evolution of a multimass system of BHs and stars by numerically integrating the Fokker–Planck energy diffusion equations for a variety of BH mass distributions. We find that the BHs ‘self-segregate’, and that the rarest, most massive BHs dominate the scattering rate closest to the SMBH (10 −1 pc). BH–BH binaries form out of gravitational wave emission during BH encounters. We find that the expected rate of BH coalescence events detectable by Advanced LIGO is ∼1–10 2 yr −1 , depending on the initial mass function of stars in galactic nuclei and the mass of the most massive BHs. We find that the actual merger rate is likely ∼10 times larger than this due to the intrinsic scatter of stellar densities in many different galaxies. The BH binaries that form this way in galactic nuclei have significant eccentricities as they enter the LIGO band (90 per cent with e> 0.9), and are therefore distinguishable from other binaries, which circularize before becoming detectable. We also show that eccentric mergers can be detected to larger distances and greater BH masses than circular mergers, up to ∼700 M � . Future ground-based gravitational wave observatories will be able to constrain both the mass function of BHs and stars in galactic nuclei.

We investigated a sample of 15 luminous high-redshift quasars (3.3 z 5.1) to measure the mass of their supermassive black holes (SMBH) and compare, for the first time, results based on C IV, Mg II, and H? emission lines at high redshifts. Assuming gravitationally bound orbits as dominant broad-line region gas motion, we determine black hole masses in the range of Mbh 2 ? 108 up to Mbh 4 ? 1010 M?. While the black hole mass estimates based on C IV and H? agree well, Mg II typically indicates a factor of ~5 times lower SMBH masses. A flatter slope of the H? radius-luminosity relation, a possibly steeper slope of the Mg II radius-luminosity relation, and a slightly larger radius of the Mg II broad-line region than for H? could relax the discrepancy. In spite of these uncertainties, the C IV, Mg II, and H? emission lines consistently indicate supermassive black hole masses of several times 109 M? at redshifts up to z = 5.1. Assuming logarithmic growth by spherical accretion with a mass-to-energy conversion efficiency of = 0.1 and an Eddington ratio Lbol/Ledd calculated for each quasar individually, we estimate black hole growth times of the order of several ~100 Myr which are smaller than the age of the universe at the corresponding redshift. Assuming high-mass seed black holes (M = 103-105 M?) the SMBHs in the z 3.5 quasars began to grow at redshifts z 4, while for the quasars with z 4.5 they started at z 6 to 10. These estimated timescales for forming SMBHs at high redshifts, together with previous studies indicating high quasar metallicities, suggest that the main SMBH growth phase occurs roughly contemporaneously with a period of violent and extensive star formation in protogalactic nuclei.

Destruction of a Star during the Evolution of a Star + Supermassive Black Hole System

Stellar-mass binary black holes (BBHs) may merge in the vicinity of a supermassive black hole (SMBH). It is suggested that the gravitational-wave (GW) emitted by a BBH has a high probability to be lensed by the SMBH if the BBH's orbit around the SMBH (i.e., the outer orbit) has a period of less than a year and is less than the duration of observation of the BBH by a space-borne GW observatory. For such a BBH + SMBH triple system, the de Sitter precession of the BBH's orbital plane is also significant. In this work, we thus study GW waveforms emitted by the BBH and then modulated by the SMBH due to effects including Doppler shift, de Sitter precession, and gravitational lensing. We show specifically that for an outer orbital period of 0.1 yr and an SMBH mass of $10^7 M_\odot$, there is a 3\%-10\% chance for the standard, strong lensing signatures to be detectable by space-borne GW detectors such as LISA and/or TianGO. For more massive lenses ($\gtrsim 10^8 M_\odot$) and more compact outer orbits with periods <0.1 yr, retro-lensing of the SMBH might also have a 1%-level chance of detection. Furthermore, by combining the lensing effects and the dynamics of the outer orbit, we find the mass of the central SMBH can be accurately determined with a fraction error of $\sim 10^{-4}$. This is much better than the case of static lensing because the degeneracy between the lens' mass and the source's angular position is lifted by the outer orbital motion. Including lensing effects also allows the de Sitter precession to be detectable at a precession period 3 times longer than the case without lensing. Lastly, we demonstrate that one can check the consistency between the SMBH's mass determined from the orbital dynamics and the one inferred from gravitational lensing, which serves as a test on theories behind both phenomena. The statistical error on the deviation can be constrained to a 1% level.