Periodic Reporting for period 2 - B Massive (Binary massive black hole astrophysics)
Período documentado: 2021-03-01 hasta 2022-08-31
The Action B Massive is framed in the context of theoretical and observational astrophysics, the branch of science that has the goal of understanding the evolution of the Universe and of the astrophysical objects (from galaxies to planets) within it. Among those, massive black hole binaries are perhaps the most extreme, fascinating yet elusive ones. In fact, as the mutual gravitational pull causes galaxies to merge with each other frequently along cosmic history, the massive black holes hosted at their center are expected to sink into the nucleus of the merger remnant and form a binary. Still, the evidence of the existence of these binaries is sparse and their role in galaxy evolution is poorly understood. Establishing observationally their existence will be a milestone for contemporary astronomy, providing a fundamental missing piece in the puzzle of galaxy formation, piercing through the (hydro)dynamical physical processes shaping dense galactic nuclei from parsec scales down to the event horizon, and probing gravity in extreme conditions.
We can both see and listen to massive black hole binaries. Remarkably, besides arguably being among the brightest objects shining in the Cosmos, they are also the loudest gravitational wave sources in the Universe. As such, we shall take advantage of both the type of messengers – photons and gravitons – they are sending to us, which can now be probed by all-sky time-domain surveys and radio pulsar timing arrays respectively. The main goal of B Massive is to combine theoretical astrophysics, radio an gravitational-wave astronomy and time-domain surveys, with state of the art data analysis techniques to: i) observationally prove the existence of massive black hole binaries, ii) understand and constrain their astrophysics and dynamics, iii) enable and bring closer in time the direct detection of gravitational waves with pulsar timing arrays.
Although fairly abstract theoretical objects, the impact of massive black hole binaries on our society cannot be underestimated. Black holes are among the most iconic physical objects, teasing the imagination and inspiring people of all ages and socio-economic backgrounds. Moreover, massive black hole binaries are the main driver of future space borne observatories like the Laser Interferometer Space Antenna (LISA), an ESA-led endeavour to observe gravitational waves in space which will have a huge economic and technological impact for the European Union.
We can both see and listen to massive black hole binaries. Remarkably, besides arguably being among the brightest objects shining in the Cosmos, they are also the loudest gravitational wave sources in the Universe. As such, we shall take advantage of both the type of messengers – photons and gravitons – they are sending to us, which can now be probed by all-sky time-domain surveys and radio pulsar timing arrays respectively. The main goal of B Massive is to combine theoretical astrophysics, radio an gravitational-wave astronomy and time-domain surveys, with state of the art data analysis techniques to: i) observationally prove the existence of massive black hole binaries, ii) understand and constrain their astrophysics and dynamics, iii) enable and bring closer in time the direct detection of gravitational waves with pulsar timing arrays.
Although fairly abstract theoretical objects, the impact of massive black hole binaries on our society cannot be underestimated. Black holes are among the most iconic physical objects, teasing the imagination and inspiring people of all ages and socio-economic backgrounds. Moreover, massive black hole binaries are the main driver of future space borne observatories like the Laser Interferometer Space Antenna (LISA), an ESA-led endeavour to observe gravitational waves in space which will have a huge economic and technological impact for the European Union.
The B Massive action made significant progress both on the theoretical and the observational side.
We set-up and run a large number of N-body and hydrodynamical simulations of massive black hole binaries evolving in stellar environments and gaseous disks respectively. These simulations aim at understanding the dynamical evolution of the binary system, especially of its orbital elements: the semimajor axis and the eccentricity. We also implemented a sophisticated treatment of massive black hole binaries and triplets into the state of the art semi-analytic galaxy evolution model L-Galaxies. From the gas and star properties of the galaxy merger, the algorithm integrates the evolution of the orbital elements accounting for gas and star driven dynamics, tracks the accretion rate onto the two holes and the occurrence of triple and multiple interactions. The code has already been used to compute the expected gravitational wave signal produced by the cosmic population of massive black hole binaries in the pulsar timing array (PTA) band, and to produce mock realization of the full sky which will serve as playground to understand the properties of a realistic population of massive binaries and their host.
Parallel to the theoretical work, the observational side of the Action has focused mostly on gravitational wave detection with PTAs. PTAs are in fact expected to observe a stochastic gravitational wave background coming from the incoherent superposition of the signals emitted by hundreds of thousands of adiabatically inspiralling massive black hole binaries throughout the Universe. Right after the start of the action, a sospicious common red signal has been observed by the NANOGrav PTA in their ensemble of pulsars. The Action then took a central role in preparing and in analyzing an updated version of the European PTA (EPTA) data, including up to seven years of new observations compared to the dataset published in 2016. A preliminary version of the dataset, including the best six pulsars in the array, has been already analyzed and shows a clear common red signal of unknown nature. We are now working on the full dataset and we are sharpening our analysis tools in order to be able to detect any specific spatial correlation of the signal among pairs of pulsars, which would be the smoking gun of its gravitational wave nature. To this purpose, we have worked on a new software package for assessing the robustness of the Bayesian samplers used in the analysis.
We set-up and run a large number of N-body and hydrodynamical simulations of massive black hole binaries evolving in stellar environments and gaseous disks respectively. These simulations aim at understanding the dynamical evolution of the binary system, especially of its orbital elements: the semimajor axis and the eccentricity. We also implemented a sophisticated treatment of massive black hole binaries and triplets into the state of the art semi-analytic galaxy evolution model L-Galaxies. From the gas and star properties of the galaxy merger, the algorithm integrates the evolution of the orbital elements accounting for gas and star driven dynamics, tracks the accretion rate onto the two holes and the occurrence of triple and multiple interactions. The code has already been used to compute the expected gravitational wave signal produced by the cosmic population of massive black hole binaries in the pulsar timing array (PTA) band, and to produce mock realization of the full sky which will serve as playground to understand the properties of a realistic population of massive binaries and their host.
Parallel to the theoretical work, the observational side of the Action has focused mostly on gravitational wave detection with PTAs. PTAs are in fact expected to observe a stochastic gravitational wave background coming from the incoherent superposition of the signals emitted by hundreds of thousands of adiabatically inspiralling massive black hole binaries throughout the Universe. Right after the start of the action, a sospicious common red signal has been observed by the NANOGrav PTA in their ensemble of pulsars. The Action then took a central role in preparing and in analyzing an updated version of the European PTA (EPTA) data, including up to seven years of new observations compared to the dataset published in 2016. A preliminary version of the dataset, including the best six pulsars in the array, has been already analyzed and shows a clear common red signal of unknown nature. We are now working on the full dataset and we are sharpening our analysis tools in order to be able to detect any specific spatial correlation of the signal among pairs of pulsars, which would be the smoking gun of its gravitational wave nature. To this purpose, we have worked on a new software package for assessing the robustness of the Bayesian samplers used in the analysis.
The Action already helped in making significant progress beyond the state of the art, mainly thanks to its critical contribution to the EPTA endeavour. The detection of a common red signal in PTA data has prompted a flurry of activity within the community. If its gravitational wave nature was to be confirmed, this might well be the most important discovery of the decade. A proper assessment of the nature of the signal needs the development and testing of reliable analysis and interpretation tools. Such development is ongoing in the EPTA collaboration, and B Massive is playing (and will continue to play) a leading role in it. By the end of the Action we will definitely have a solid assessment on the matter.
We are also making significant progress in understanding the dynamical evolution of massive black hole binaries, their global population and the properties of the gravitational wave signal they generate. One of the most heated debates in the binary community is whether disk driven dynamics leads to binary shrinking or expansion. We already demonstrated that self gravitating disks, which are most relevant to massive binaries in the PTA detection range, promote binary shrinking, therefore not posing an issue to the formation of tight binaries emitting gravitational waves. We have also developed the first 3D code able to treat the hydrodynamic interaction of live binaries with circumbinary disks at high resolution, thanks to the implementation of the particle splitting technique in GIZMO. Thanks to this code we will be able to assess the role of gas in binary evolution and draw a comprehensive picture of massive black hole binary dynamics.
Finally, we have performed the first exploration of the gravitational wave signal produced by massive black hole binaries in the PTA band within the framework of a fully self-consistent semianalytic model (SAM), taking into account all observational constraints coming from the observed galaxy and quasar mass and luminosity functions. Using this model we demonstrated that it would be not trivial to produce a signal at the level of the common red noise observed by current PTAs. Therefore, if confirmed, this signal will provide an important constraint to the evolution of the cosmic population of massive binaries. The code will be used to generate full mock Universes and by the end of the Action we will have a clear picture of the full population of massive black hole binaries, constrained by PTA observations.
We are also making significant progress in understanding the dynamical evolution of massive black hole binaries, their global population and the properties of the gravitational wave signal they generate. One of the most heated debates in the binary community is whether disk driven dynamics leads to binary shrinking or expansion. We already demonstrated that self gravitating disks, which are most relevant to massive binaries in the PTA detection range, promote binary shrinking, therefore not posing an issue to the formation of tight binaries emitting gravitational waves. We have also developed the first 3D code able to treat the hydrodynamic interaction of live binaries with circumbinary disks at high resolution, thanks to the implementation of the particle splitting technique in GIZMO. Thanks to this code we will be able to assess the role of gas in binary evolution and draw a comprehensive picture of massive black hole binary dynamics.
Finally, we have performed the first exploration of the gravitational wave signal produced by massive black hole binaries in the PTA band within the framework of a fully self-consistent semianalytic model (SAM), taking into account all observational constraints coming from the observed galaxy and quasar mass and luminosity functions. Using this model we demonstrated that it would be not trivial to produce a signal at the level of the common red noise observed by current PTAs. Therefore, if confirmed, this signal will provide an important constraint to the evolution of the cosmic population of massive binaries. The code will be used to generate full mock Universes and by the end of the Action we will have a clear picture of the full population of massive black hole binaries, constrained by PTA observations.