Probing supermassive black hole binaries with pulsar timing

Abstract

The observation of gravitational-waves from merging supermassive black holes will be transformative: the detection of a low-frequency gravitational-wave background can tell us if and how supermassive black holes merge, inform our knowledge of galaxy merger rates and supermassive black hole masses, and enable the possibility of detecting new physics at nanohertz frequencies. All we have to do is time pulsars.Supermassive black hole (SMBH) mergers are the strongest sources of gravitational-waves (GWs) in the Universe. SMBH mergers are expected to follow galaxy mergers, since likely all massive galaxies host central SMBH, e.g. [1]. Briefly, the black holes fall to the center of the newly-formed galaxy through dynamical friction and form a binary. This binary should harden by ejecting stars crossing their orbit -a process called stellar hardening. When the binary is separated by a centiparsec to a milliparsec, gravitational waves (GWs) drive the binary to merge. These steps are described in more detail in [2].However, there is currently sparse observational evidence for sub-parsec separated SMBHB systems. In fact, under the assumption of a static spherical galaxy model, SMBHBs may stall at their final parsec of separation, and never merge -this is known as the final parsec problem [3]. This stalling happens when, for example, the black holes run out of stars to eject -called loss cone depletion. Even though the system is emitting GWs, it would take longer than a Hubble time for the binary to merge via GW emission only. However, if one assumes a triaxial galactic model, loss cone depletion is no longer an issue [4].There are other ways to overcome the final parsec problem: a circumbinary accretion disk can torque the binary, carrying away energy and angular momentum, helping it to overcome its final parsec of separation and merge. Environmental interactions with gas and stars also induce eccentricity in the binary [5]. An eccentric binary will emit GWs at higher harmonics, rapidly decreasing the time to coalescence of the binary, and enabling it to merge within a Hubble time. Pulsar Timing ArraysA pulsar timing array (PTA) is a galactic-scale GW detector. With an array of millisecond pulsars, one can search for nanohertz frequency GW signals originating from the most massive SMBHB systems in their slow inspiral phase, Refs [6,7]. GWs transiting the galaxy change the proper distance between the pulsars and the Earth by tens of meters per light year, inducing delays or advances in the pulsar's pulse arrival times. These time delays are tens to hundreds of nanoseconds over a decade, highlighting the importance of millisecond pulsars as the basis of our experiment: no other natural object has this kind of timing precision.The nanohertz GWs in the PTA band likely originate from SMBHBs in the 10 8 − 10 9 ⊙ range, with periods of years to decades, hence, they are millions of years from merging. A nanohertz GW background should be generated from the cosmic merger history of SMBHs and will likely be

The NANOGrav 12.5 yr Data Set: Observations and Narrowband Timing of 47 Millisecond Pulsars

“…The absolute number of TOAs, as well as the number of timing model parameters (including parameterizations of the noise), play a determining role in the time it takes to perform GW analyses of PTA data (van Haasteren & Vallisneri 2015, 2014Ellis et al 2013;Lentati et al 2013b). Advanced data analysis techniques to handle this deluge of TOAs will need to be significantly improved if we want to avoid delays on the numerous science deliverables offered by PTAs (Cordes et al 2019;Fonseca et al 2019;Goulding et al 2019;Kelley et al 2019;Lynch et al 2019;Mingarelli 2019;Siemens et al 2019;Stinebring et al 2019;Taylor et al 2019).…”

A Second Look at 12 Candidate Dual AGNs Using BAYMAX

“…Understanding which environmental factors are most important for dual SMBH activity allows for a better understanding about black hole growth and its relation (or lack thereof) to galaxy-galaxy interactions. Additionally, as progenitors to SMBH mergers, the rate of dual AGNs has implications for the rate of expected gravitational wave events that will be detected by pulsar timing arrays (e.g., Mingarelli 2019) and future space-based interferometry.…”

Review of pulsar timing array for gravitational wave research

“…[46] that 5 years after the detection of background waves, individual sources are expected to be detected. Supermassive black holes of approximately 10 8 M currently inspiralling in the PTA band are supposed to be the source for LISA, proposed to launch on early 2030 [46]. So, the prevailing uncertainties in the detection of gravitational waves from PTA's could have big implication on LISA.…”