Session X14: Supermassive Black Holes

The Complete XMM-SERVS Survey: A Sensitive X-ray Survey of the LSST Deep-Drilling Fields

Cosmic X-ray surveys over the past two decades have played a critical role in transforming our understanding of growing supermassive black holes (SMBHs) in the distant universe. I will describe one key survey, now recently completed, advancing this effort: the 13.1 deg² XMM-SERVS survey. XMM-SERVS has successfully mapped three legacy sky fields at 50 ks depth: the SERVS areas of CDF-S, XMM-LSS, and ELAIS-S1. These fields have first-rate multiwavelength coverage already and are LSST/DES Deep-Drilling Fields, MOONS/PFS massive spectroscopy fields, prime TolTEC/ALMA fields, and multi-object reverberation-mapping fields. Multiwavelength fitting of infrared-to-X-ray spectral energy distributions is now being used to characterize the active galactic nuclei (AGNs) and galaxies in these fields, enabling further in-depth investigations. When the follow-up of its 12,000 X-ray sources is complete, XMM-SERVS should dramatically advance studies of links between SMBH accretion and host-galaxy properties, SMBH growth across the full range of cosmic environments, groups/clusters at z = 0.1-2, protoclusters, and other topics.   

The low-mass end of accreting supermassive black holes

The unification theory of black hole accretion suggests that the accretion process is independent of the black hole mass, and we expect to observe similar accretion properties in sources spanning a wide range of black hole mass. The discovery of scaling relations between the black hole mass and X-ray reverberation lag for both accreting supermassive and stellar-mass black holes is a breakthrough in this context. However, the low-mass end of active galactic nuclei (AGN) has never been explored in detail. For this purpose, we construct a sample of the least-massive AGN from the XMM-Newton archive and measure frequency-resolved time delays between the soft and hard X-ray emission as well as test the predictions of the standard alpha-disc model. We utilize a new high-density disc reflection model where the density parameter varies from n_e =10¹⁵ to 10²⁰ cm^-3 and apply it to the broadband X-ray (0.3-10 keV) spectra of the sample. The X-ray spectra reveal soft X-ray excess below around 1.5 keV, which is well modeled by high-density reflection from an ionized accretion disc of density n_e ~10¹⁸ cm^-3 on average. We detected soft reverberation time lags in some of these AGN which provides the strongest supporting evidence for the reflection origin of the soft X-ray excess. The results suggest a radiation pressure-dominated disc with an average of 70% fraction of the disc power transferred to the corona, consistent with that observed in higher mass AGN. We show that the disc density higher than 10¹⁵ cm^-3 can result from the radiation pressure compression when the disc surface does not hold a strong magnetic pressure gradient. We find tentative evidence for a drop in black hole spin at low-mass regimes, which hints that the merger of intermediate-mass black hole pairs could be responsible for the formation of the low-mass end of supermassive black holes, which can be confirmed with the future gravitational wave detector LISA.   

Extracting astrophysics from black hole images

The Event Horizon Telescope (EHT) is providing images of a supermassive black hole. These images are fundamentally related to properties of the accretion disk, since the black holes themselves produce no light. We develop a simple prescription to relate observational features of  the images to phenomenological characteristics of accretion disks such as the intensity profile and disk inclination. Comparing smoothed theoretical results to the EHT image for M87*, we provide constraints on the mass of the black hole and accretion disk profiles for M87*.   

Null Geodesics and Observational Signatures of Kerr Naked Singularities

The Event Horizon Telescope (EHT) recently resolved the supermassive black hole at the center of M87 at the event horizon scale. The reconstructed black hole images show asymmetric rings with interior brightness depressions. This result was used to test modified gravity theories by constraining how the black hole spacetime may deviate from the Kerr solution with black hole spin |a| < 1. This range of the black hole spin was assumed because of the weak cosmic censorship hypothesis, which states that singularities need to be hidden from an observer at infinity by an event horizon. In this work, we propose to use the EHT images to test the weak cosmic censorship hypothesis. We assume the Kerr spacetime is correct for simplicity but relax the spin assumption by surveying a wide range of black holes from spin |a| < 1 to spin |a| > 1. We perform a detailed study on the null geodesics surrounding Kerr naked singularities, including spherical photon orbits and chaotic regions. From the simulations, we predict the structure of the "black hole shadow" and observable signatures of naked singularities. By comparing these predictions with current and future EHT observations, we can test the weak cosmic censorship hypothesis in the Kerr solution.   

Multi-messenger signatures of supermassive black hole binaries

Supermassive black hole binaries (SMBHBs) are the natural end-product of galaxy mergers and should be common in galactic nuclei. At the final stage of their evolution, binaries can be identified from their bright electromagnetic emission, as quasars with periodic variability, and from their strong gravitational radiation (e.g., by Pulsar Timing Arrays-PTAs). I will discuss synergies between electromagnetic and gravitational wave signatures of SMBHBs, connecting both signals to the orbital dynamics of the binary. I will show that currently multi-messenger detections are possible only for the most massive and nearby galaxies, limited by the sensitivity of PTAs, but in the next 5-10yr joint observations will become feasible for a wider variety of binaries.   

Electromagnetic signatures from supermassive binary black holes approaching merger

Theoretical models predict that when two galaxies merge, the supermassive black holes at their nuclei may end up forming close binary systems of sub-parsec scales. The gravitational waves emitted by these systems are the target of current Pulsar Timing Array efforts and future interferometers as LISA (Laser Interferometer Space Antenna). Unlike most stellar-mass binary black holes, supermassive binary black holes (SMBBHs) live and die in gas-rich environments (the core of a merged galaxy) and hence might accrete copious amounts of matter, become active, and emit electromagnetic radiation. Detailed knowledge of specific emission signatures from SMBBHs is crucial to differentiate them from normal AGNs, identifying potential targets for future multi-messenger observations of electromagnetic radiation and gravitational waves. In this talk, I will present novel relativistic predictions of the electromagnetic emission from the surrounding gas of an SMBBH system approaching merger. We ray-traced data from General Relativistic Magnetohydrodynamic (GRMHD) simulations of supermassive binary black holes approaching merger (at a separation ~20M), and generated images, time-dependent spectra, and light curves. We analyze how the spin of the black holes influences the observed emission. We found that prograde black hole spin makes mini-disks brighter because the smaller ISCO angular momentum demands more dissipation before matter plunges to the horizon. However, compared to mini-disks in larger separation binaries with spinning black holes, our minidisks are less luminous: unlike those systems, their mass accretion rate is lower than in the circumbinary disk, and they radiate with lower efficiency because their inflow times are shorter. Finally, we identify specific signatures that might differentiate SMBBHs from normal AGNs.   

Understanding Whether and How Supermassive Black Holes Drive Galaxy Evolution and the Role of Future Eyes

The well-established scaling relationships of the central supermassive black holes of galaxies that persist across across cosmic time can be qualitatively understood if star formation in the host galaxies of these black holes is regulated by feedback from the gas accretion process driven by them. However, whether the reported quantitative  inconsistencies in the scaling relationships between classes of galaxies can be explained in this broad picture, what is the exact nature of the  feedback and  the role of the ubiquitous bipolar jets launched from the accreting system, the connection with galaxy mergers and harassment, and how these phenomena evolve to z~0 are all poorly understood. An overview of the current evidence, including our own results from a multi-frequency investigation of the circum-nuclear environments of a sample of nearby southern active galaxies will be presented. The gaps in our understanding that can be addressed by upcoming facilities will be discussed.   

Super massive black holes from the collapse of smaller black holes: Statistical mechanical and thermodynamic study

 The formation of super massive black holes is considered from the gravitational collapse of N self- gravitating smaller black holes. The statistical mechanics of the system is analyzed by constructing the micro canonical partition function following the method of calculation of the same quantity for a non ideal gas of atoms by replacing the Van der Waal's interaction with gravitational one and the hard  sphere with the black holes. The energy of the system in the low density and high density state is calculated which shows the final object as a super massive black hole. The thermodynamic study of the system by considering the free energy also shows the transition from an interacting gas state to a condensed form by the formation of a bigger and super massive black hole out of large number of    mini black holes.

REFERENCES:

1. S. Mishra and B. K. Lotte , Research gate(DOI: 10.13140/RG.2.2.29134.79687)

 2. T. Padmanabhan, Physics Reports, 188, No. 5, 285 (1990)

3. E. B. Aronson and C. J. Hansen, The Astrophysical Journal, 177, 145 (1972)   

Brown York charges at null boundaries

The Brown-York stress tensor provides a means for defining quasilocal gravitational charges in subregions bounded by timelike hypersurfaces. We consider a generalization of this stress tensor to subregions bounded by null hypersurfaces. This stress tensor can be derived from the on-shell subregion action of general relativity associated with a Dirichlet variational principle and satisfies a covariant conservation equation with respect to any connection induced from a rigging vector at the null boundary. For transformations that act covariantly on the boundary structures, the charges constructed from this Brown-York stress tensor coincide with canonical charges constructed from a version of the Wald-Zoupas procedure while for anomalous transformations, the charges differ by an intrinsic functional of the boundary geometry. Comparing this stress tensor with the stress tensor of the celestial conformal field theory that is the putative holographic dual of quantum gravity in asymptotically flat spacetimes could provide further insights into the nature of flat space holography.