Year: 2019
Type: article
Abstract: Massive black hole (MBH) binaries, which are expected to form following the merger of their parent galaxies, produce gravitational waves that will be detectable by pulsar timing arrays at nanohertz frequencies (year periods). While no confirmed, compact MBH binary systems have been seen in electromagnetic observations, a large number of candidates have recently been identified in optical surveys of active galactic nucleus (AGN) variability. Using a combination of cosmological, hydrodynamic simulations; comprehensive, semi-analytic binary merger models; and analytic AGN spectra and variability prescriptions; we calculate the expected electromagnetic detection rates of MBH binaries as periodically variable AGN. In particular, we consider two independent variability models: (i) Doppler boosting due to large orbital velocities and (ii) hydrodynamic variability in which the fuelling of MBH accretion discs is periodically modulated by the companion. Our models predict that numerous MBH binaries should be present and distinguishable in the existing data. In particular, our fiducial models produce an expectation value of 0.2 (Doppler) and 5 (hydrodynamic) binaries to be identifiable in the Catalina survey (CRTS), while 20 and 100 are expected after 5 yr of LSST observations. The brightness variations in most systems are too small to be distinguishable, but almost |$1{{\ \rm per\ cent}}$| of AGN at redshifts |$z$| ≲ 0.6 could be in massive binaries. We analyse the predicted binary parameters of observable systems and their selection biases, and include an extensive discussion of our model parameters and uncertainties.
DOI: 10.1093/mnras/stz150
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Awards: Collaborative Research: Mergers of Massive Black Holes at the Centers of Galaxies, G5282558976, THE PURPOSE OF THIS PROPOSAL IS TO IDENTIFY A POPULATION OF COMPACT SUPERMASSIVE BLACK HOLE BINARIES (SMBHBS) IN GALACTIC NUCLEI. SMBHBS ARE BELIEVED TO FORM FREQUENTLY IN GALAXY MERGERS AND ARE A FUNDAMENTAL INGREDIENT IN THE HIERARCHICAL ASSEMBLY OF GALAXIES. THEY ARE ALSO PRIME TARGETS FOR ON-GOING AND PLANNED GRAVITATIONAL WAVE EXPERIMENTS DESIGNED TO TEST OUR FUNDAMENTAL UNDERSTANDING OF GRAVITY. WE WILL ANALYZE ARCHIVAL UV X-RAY AND IR OBSERVATIONS TO TEST THE BINARY HYPOTHESIS FOR A SAMPLE OF APPROXIMATELY 150 QUASARS NEWLY IDENTIFIED AS BINARY CANDIDATES BASED ON PERIODICITIES IN THEIR OPTICAL LIGHT-CURVES. WE WILL TEST THE RELATIONS PREDICTED BY BINARY MODELS BETWEEN THE OPTICAL UV AND X-RAY PERIODICITES (ALL DIRECTLY PRODUCED IN THE ACCRETION DISKS NEAR THE BHS) AND THEIR IR LIGHT ECHOES (PRODUCED FARTHER OUT IN A DUSTY CIRCUMBINARY TORUS). WE WILL CAREFULLY COMPARE OUR FINDINGS WITH A SUITABLY DESIGNED CONTROL SAMPLE OF NON-PERIODIC QUASARS WITH SIMILAR PROPERTIES. WE WILL INTEPRET THE INFERRED ABUNDANCE OF BINARIES BY COMPARING THEM TO THEORETICAL MODELS FOR THE UNDERLYING SMBHB POPULATION. FINALLY WE WILL PERFORM FOLLOW-UP OPTICAL OBSERVATIONS OF THE MOST PROMISING CANDIDATES. THIS PROPOSAL WILL SUBSTANTIALLY LEVERAGE RECENT THEORETICAL WORK INCLUDING PREDICTIONS BY OUR OWN GROUP FOR PERIODIC VISIBLE UV AND X-RAY SIGNALS FROM BINARIES THROUGH HYDRODYNAMICAL ACCRETION RATE FLUCTUATIONS AND THROUGH RELATIVISTIC DOPPLER BOOSTING. THE RESULTS WILL BE APPLIED TO SMBHB CANDIDATES IDENTIFIED OVER THE PAST YEAR. OUR PROPOSAL WILL DIRECTLY SUPPORT NASA'S STRATEGIC PLAN SUB- GOAL 3D: "DISCOVER THE ORIGIN STRUCTURE EVOLUTION AND DESTINY OF THE UNIVERSE AND SEARCH FOR EARTH-LIKE PLANETS".
OpenAlex ID: W2890572905