A Quasar-based Supermassive Black Hole Binary Population Model: Implications for the Gravitational Wave Background

The nanohertz gravitational wave background (GWB) is believed to be dominated by GW emission from supermassive black hole binaries (SMBHBs). Observations of several dual-active galactic nuclei (AGN) strongly suggest a link between AGN and SMBHBs, given that these dual-AGN systems will eventually form bound binary pairs. Here we develop an exploratory SMBHB population model based on empirically constrained quasar populations, allowing us to decompose the GWB amplitude into an underlying distribution of SMBH masses, SMBHB number density, and volume enclosing the GWB. Our approach also allows us to self-consistently predict the number of local SMBHB systems from the GWB amplitude. Interestingly, we find the local number density of SMBHBs implied by the common-process signal in the NANOGrav 12.5-yr data set to be roughly five times larger than previously predicted by other models. We also find that at most ∼25% of SMBHBs can be associated with quasars. Furthermore, our quasar-based approach predicts ≳95% of the GWB signal comes from z ≲ 2.5, and that SMBHBs contributing to the GWB have masses ≳108 M ⊙. We also explore how different empirical galaxy–black hole scaling relations affect the local number density of GW sources, and find that relations predicting more massive black holes decrease the local number density of SMBHBs. Overall, our results point to the important role that a measurement of the GWB will play in directly constraining the cosmic population of SMBHBs, as well as their connections to quasars and galaxy mergers.

Brightness Asymmetry of Black Hole Images as a Probe of Observer Inclination

The Event Horizon Telescope recently captured images of the supermassive black hole in the center of the M87 galaxy, which shows a ring-like emission structure with the south side only slightly brighter than the north side. This relatively weak asymmetry in the brightness profile along the ring has been interpreted as a consequence of the low inclination of the observer (around 17° for M87), which suppresses the Doppler beaming and boosting effects that might otherwise be expected due to the nearly relativistic velocities of the orbiting plasma. In this work, we use a large suite of general relativistic magnetohydrodynamic simulations to reassess the validity of this argument. By constructing explicit counterexamples, we show that low inclination is a sufficient but not necessary condition for images to have low brightness asymmetry. Accretion flow models with high accumulated magnetic flux close to the black hole horizon (the so-called magnetically arrested disks) and low black hole spins have angular velocities that are substantially smaller than the orbital velocities of test particles at the same location. As a result, such models can produce images with low brightness asymmetry even when viewed edge on.

The Eccentric Nature of Eccentric Tidal Disruption Events

Upon entering the tidal sphere of a supermassive black hole, a star is ripped apart by tides and transformed into a stream of debris. The ultimate fate of that debris, and the properties of the bright flare that is produced and observed, depends on a number of parameters, including the energy of the center of mass of the original star. Here we present the results of a set of smoothed particle hydrodynamics simulations in which a 1M ⊙, γ = 5/3 polytrope is disrupted by a 106 M ⊙ supermassive black hole. Each simulation has a pericenter distance of r p = r t (i.e., β ≡ r t/r p = 1 with r t the tidal radius), and we vary the eccentricity e of the stellar orbit from e = 0.8 up to e = 1.20 and study the nature of the fallback of debris onto the black hole and the long-term fate of the unbound material. For simulations with eccentricities e ≲ 0.98, the fallback curve has a distinct, three-peak structure that is induced by self-gravity. For simulations with eccentricities e ≳ 1.06, the core of the disrupted star reforms following its initial disruption. Our results have implications for, e.g., tidal disruption events produced by supermassive black hole binaries.

Review of Three Lectures on Complexity and Black Holes by Leonard Susskind

The future prospects for anyone falling into a black hole are bleak. For one thing, there is no chance (according to our present state of knowledge) of ever getting out again. Worse, one is facing certain destruction when one meets the "singularity" (or its inconceivably dense physical manifestation, whatever that may be) inside. However, there is an "event horizon," the point of no return, separating the overly curious infalling astronaut from the doom he or she faces at the singularity. Suppose Alice the Astronaut wants to see what's behind the horizon (never mind the consequences). How much time would Alice have to look around and see what's happening, before reaching the end of her worldline? Conventional wisdom, until relatively recently, was that she would have some amount of time, perhaps hours. Passing the event horizon of a supermassive black hole would not seem like any kind of a milestone to the infalling individual; it is only an outside observer who would notice something out of the ordinary.

Tracing a Black Hole: Probing Cosmic Darkness in Anthropocenic Times

In April 2019, the Event Horizon Telescope (EHT) project released an unprecedented image of a supermassive black hole at the centre of galaxy Messier 87. The image, which shows a dark disc outlined by swirling hot gas circling the black hole’s event horizon, exhibits a 55 million-year-old cosmic event in the Virgo galaxy cluster—a void of stellar mass measuring some 6.5 billion times that of our sun. Situated within today’s (Good) Anthropocene scenario, characterized as it is by both the rise of an inhospitable planet but also a range of good vibes and affirmative mantras, this tracing explores this newly “discovered” black hole in terms of the unthinkable questions and speculative trajectories it raises for education and its futures. Through a series of forays into astrophysics, historical examples of cosmic imaging, and further exploration of the image created by EHT, this tracing outlines the black hole and its apparent horizons in order to propose a strange vantage point from which pedagogical problem-posing might be interrupted, mutated, and relaunched. By turning to that which lies outside of the traditional science classroom—beyond the school, beyond curriculum, indeed, beyond the planet itself—this tracing seeks to probe this black hole event in terms of its weird and weirding pedagogical trajectories so as to speculate on unthought possibilities for resituating (science) education in the age of the Anthropocene.

The X-Ray Outburst of PG 1553+113: A Precession Effect of Two Jets in the Supermassive Black Hole Binary System

Blazar PG 1553+113 is thought to be a host of supermassive black hole binary system. A 2.2 yr quasi-periodicity in the γ-ray light curve was detected, possibly a result of jet precession. Motivated by the previous studies based on the γ-ray data, we analyzed the X-ray light curve and spectra observed during 2012–2020. The 2.2 yr quasi-periodicity might be consistent with the main-flare recurrence in the X-ray light curve. When a weak rebrightening in the γ-ray was observed, a corresponding relatively strong brightening in the X-ray light curve can be identified. The harder-when-brighter tendency in both X-ray main and weak flares was shown, as well as a weak softer-when-brighter behavior for the quiescent state. We explore the possibility that the variability in the X-ray band can be interpreted with two-jet precession scenario. Using the relation between jets and accretion disks, we derive the primary black hole mass ≃3.47 × 108 M ☉ and mass of the secondary one ≃1.40 × 108 M ☉, and their mass ratio ∼0.41.

Toward Determining the Number of Observable Supermassive Black Hole Shadows

We present estimates for the number of shadow-resolved supermassive black hole (SMBH) systems that can be detected using radio interferometers, as a function of angular resolution, flux density sensitivity, and observing frequency. Accounting for the distribution of SMBHs across mass, redshift, and accretion rate, we use a new semianalytic spectral energy distribution model to derive the number of SMBHs with detectable and optically thin horizon-scale emission. We demonstrate that (sub)millimeter interferometric observations with ∼0.1 μas resolution and ∼1 μJy sensitivity could access >106 SMBH shadows. We then further decompose the shadow source counts into the number of black holes for which we could expect to observe the first- and second-order lensed photon rings. Accessing the bulk population of first-order photon rings requires ≲2 μas resolution and ≲0.5 mJy sensitivity, whereas doing the same for second-order photon rings requires ≲0.1 μas resolution and ≲5 μJy sensitivity. Our model predicts that with modest improvements to sensitivity, as many as ∼5 additional horizon-resolved sources should become accessible to the current Event Horizon Telescope (EHT), whereas a next-generation EHT observing at 345 GHz should have access to ∼3 times as many sources. More generally, our results can help guide enhancements of current arrays and specifications for future interferometric experiments that aim to spatially resolve a large population of SMBH shadows or higher-order photon rings.