Observing GW190521-like binary black holes and their environment with LISA

Abstract

Binaries of relatively massive black holes like GW190521 have been proposed to form in dense gas environments, such as the disks of Active Galactic Nuclei (AGNs), and they might be associated with transient electromagnetic counterparts. The interactions of this putative environment with the binary could leave a significant imprint at the low gravitational wave frequencies observable with the Laser Interferometer Space Antenna (LISA). We show that LISA will be able to detect up to ten GW190521-like black hole binaries, with sky position errors 1 deg 2 . Moreover, it will measure directly various effects due to the orbital motion around the supermassive black hole at the center of the AGN, especially the Doppler modulation and the Shapiro time delay. Thanks to a careful treatment of their frequency domain signal, we were able to perform the full parameter estimation of Doppler and Shapiro-modulated binaries as seen by LISA. We find that the Doppler and Shapiro effects will allow for measuring the AGN parameters (radius and inclination of the orbit around the AGN, central black hole mass) with up to percent-level precision. Properly modeling these low-frequency environmental effects is crucial to determine the binary formation history, as well as to avoid biases in the reconstruction of the source parameters and in tests of general relativity with gravitational waves.

Quiescent and Active Galactic Nuclei as Factories of Merging Compact Objects in the Era of Gravitational Wave Astronomy

“…The variation in the distance between the emitter and the observer causes a Doppler shift in the arrival time of GW pulses, the gravitational time dilation causes relativistic Doppler shift and gravitational redshift, whilst the SMBH gravitational field causes a delay in the time arrival of the GW signal. The aforementioned three effects, called Roemer, Einstein, and Shapiro delay [533], causes a shift in the measured GW signal that might be measurable in both low- [498,[534][535][536][537][538][539] and high-frequency detectors [165,540,541].…”

“…It is worth noting that a BBH with mass m bin = 50M at a distance r = 1 mpc from an SMBH with mass M SMBH = 10 6 (10 8 )M revolves around the SMBH in a time P 2 3(0.3) yr, thus a detector with a mission lifetime (τ life ) longer than the BBH merging time and the orbital period could measure the Doppler modulation in the source signal [534][535][536]539] and related-effects [498,538,542,543].…”

“…A sufficiently long monitoring of the source could also lead to detection of lensing signatures caused by the SMBH [538,542,544]. The probability to observe such an effect depends on the properties of BBHs forming around an SMBH and could be O(1 − 3%) with both LISA and TianGO [538,542], potentially enabling to measure the SMBH mass at a level of 0.01 − 1% [538,539]. Similarly to electromagnetic wavelets, GWs from BBHs around an SMBH could be affected by aberration, an effect that can cause a shift in the GW signal as significant as the "standard" Doppler shift and potentially detectable by a LISA-like mission, provided that the BBH is orbiting within a few 10 3 Schwarzschild radii [543].…”

A supplementary radiation-reaction force between two binaries

“…This last possibility in particular can considerably modify the dynamics of a two-body system, e.g. by inducing Kozai-Lidov oscillations [23][24][25], resonances [26][27][28] or Doppler shifts in the waveform [29][30][31][32][33][34][35][36][37][38]. From astronomical observations, many-body systems are very common in the universe [39][40][41], so that they could constitute a non-negligible fraction of the mergers observed in GW observatories [42,43].…”