Periodic Reporting for period 1 - BlackHoleMergers (Formation of Black Hole Mergers in Dense Stellar Systems)
Reporting period: 2019-12-01 to 2021-11-30
Gravitational waves (GWs) from the merger of binary black holes (BBHs) have only recently been possible to observe using the pioneering LIGO and Virgo detectors. To date around 50 BBH mergers have been observed and confirmed, which not only have resulted in an enormous amount of new science and open questions, but also marked the beginning of a new era in physics: Gravitational Wave Astrophysics.
However, the fundamental question of how these BBHs form in our Universe is still open. In this regard the astrophysical community has proposed several possible formation channels that currently are being tested using -- for the first time -- real GW data. In general, the proposed channels can be divided into the following two categories: i) Isolated BBH formation. In this case BBHs form as a result of isolated stellar evolution. ii) Dynamical BBH formation. In this case two BBHs form as a result of close encounters involving two or more BHs in dense stellar regions. One of the main questions is how to disentangle these two categories using GWs alone, as BHs, as the name indicates, do not emit any light!
To use data to probe the astrophysical origin of BBH mergers, one naturally needs accurate models to compare to data. However, as the field of GW astrophysics is still relatively young, we do not have at the moment a complete (theoretical) understanding of how BBHs (might) pair up in the scenarios that are most relevant to test with data at the moment. Therefore, to learn about our Universe using GWs we must first, or at least in parallel to the observations, have a good prediction and understanding of what astrophysical environments we want to look for and probe. That has been my main focus as a Marie Curie Fellow, with a special emphasis on the dynamical formation channel.
Astronomy has inspired humans for thousands of years, but our view on the Universe has always been limited to what we can infer from the light emitted from distant stars. With the possibility of detecting GWs we have now opened up a new window into the Universe from which we can see parts we never have been able to see before. So far, we have for the first time seen the formation and interaction of BHs, but as with all other experiments, we also expect to see something new. But what could that be? Time, data and accurate models will soon tell!
However, the fundamental question of how these BBHs form in our Universe is still open. In this regard the astrophysical community has proposed several possible formation channels that currently are being tested using -- for the first time -- real GW data. In general, the proposed channels can be divided into the following two categories: i) Isolated BBH formation. In this case BBHs form as a result of isolated stellar evolution. ii) Dynamical BBH formation. In this case two BBHs form as a result of close encounters involving two or more BHs in dense stellar regions. One of the main questions is how to disentangle these two categories using GWs alone, as BHs, as the name indicates, do not emit any light!
To use data to probe the astrophysical origin of BBH mergers, one naturally needs accurate models to compare to data. However, as the field of GW astrophysics is still relatively young, we do not have at the moment a complete (theoretical) understanding of how BBHs (might) pair up in the scenarios that are most relevant to test with data at the moment. Therefore, to learn about our Universe using GWs we must first, or at least in parallel to the observations, have a good prediction and understanding of what astrophysical environments we want to look for and probe. That has been my main focus as a Marie Curie Fellow, with a special emphasis on the dynamical formation channel.
Astronomy has inspired humans for thousands of years, but our view on the Universe has always been limited to what we can infer from the light emitted from distant stars. With the possibility of detecting GWs we have now opened up a new window into the Universe from which we can see parts we never have been able to see before. So far, we have for the first time seen the formation and interaction of BHs, but as with all other experiments, we also expect to see something new. But what could that be? Time, data and accurate models will soon tell!
I have over the past two years studied several aspects of black hole scatterings in dense environments to establish new understanding of what observational imprints such formation channels leave in the GW signals. Part of my work has been centered on how to observationally probe the formation origin of BBH mergers that are eccentric, i.e. non-circular, when they become visible in the LIGO/Virgo detector bands. In this regard I have established two new research areas during my MC Fellowship, each of which are introduced in the following:
1) Formation of black hole mergers in gas-disks (AGN) around super massive BHs.
Using novel analytical and numerical techniques, I have found that the probability for forming an eccentric merger in a planar disk-like environment can be up to two orders-of-magnitude higher compared to the isotropic case found in most clusters. The probability for a merger to be eccentric in LIGO/Virgo if formed in a planar AGN disk can therefore approach unity, which is highly surprising and extremely relevant for several of the new GW observations. For example, the source GW190521 observed a few months back is the first source with a non-zero eccentricity. On top of this, its masses exceeded the mass gap limit, and a possible electromagnetic counterpart was also seen. However, its non-zero eccentricity was a major mystery before my work in which I combined all the pieces and illustrated this is a natural outcome of BBH mergers forming in AGN disk environments. This finding is now published in Nature with me as first author. We are currently planning many follow up papers on this discovery, many of which have not been submitted yet.
2) Non-zero eccentricity is a clear indicator of a dynamical origin. However, recent progress has also shown that many different kinds of dynamical systems exist that all can give rise to eccentric mergers. For example, few-body scatterings in clusters will result in BBH mergers similar to the one found through single-single GW captures in galactic nuclei. So, how do we tell the difference between these channels using GWs alone? In my recent work, I have shown that modulations of the burst timing signal can be used to tell the difference between these channels. For example, if the eccentric source is formed through a binary-single interaction, which is the dominant mechanism in clusters, then the third object will in some cases be close enough to the inspiraling eccentric BBH that its tidal force generates modulations in the burst arrival times. Another example are eccentric BBHs forming in AGN disks, where the gas drag also can modulate the signal. What I now have shown in a series of papers, that are soon to be published, is that one can probe all these environmental effects through accurate timing of the bursts. This will finally help distinguish different eccentric formation channels apart using GWs alone. All of these ideas are new, and will over the next few years be explored in much more detail.
1) Formation of black hole mergers in gas-disks (AGN) around super massive BHs.
Using novel analytical and numerical techniques, I have found that the probability for forming an eccentric merger in a planar disk-like environment can be up to two orders-of-magnitude higher compared to the isotropic case found in most clusters. The probability for a merger to be eccentric in LIGO/Virgo if formed in a planar AGN disk can therefore approach unity, which is highly surprising and extremely relevant for several of the new GW observations. For example, the source GW190521 observed a few months back is the first source with a non-zero eccentricity. On top of this, its masses exceeded the mass gap limit, and a possible electromagnetic counterpart was also seen. However, its non-zero eccentricity was a major mystery before my work in which I combined all the pieces and illustrated this is a natural outcome of BBH mergers forming in AGN disk environments. This finding is now published in Nature with me as first author. We are currently planning many follow up papers on this discovery, many of which have not been submitted yet.
2) Non-zero eccentricity is a clear indicator of a dynamical origin. However, recent progress has also shown that many different kinds of dynamical systems exist that all can give rise to eccentric mergers. For example, few-body scatterings in clusters will result in BBH mergers similar to the one found through single-single GW captures in galactic nuclei. So, how do we tell the difference between these channels using GWs alone? In my recent work, I have shown that modulations of the burst timing signal can be used to tell the difference between these channels. For example, if the eccentric source is formed through a binary-single interaction, which is the dominant mechanism in clusters, then the third object will in some cases be close enough to the inspiraling eccentric BBH that its tidal force generates modulations in the burst arrival times. Another example are eccentric BBHs forming in AGN disks, where the gas drag also can modulate the signal. What I now have shown in a series of papers, that are soon to be published, is that one can probe all these environmental effects through accurate timing of the bursts. This will finally help distinguish different eccentric formation channels apart using GWs alone. All of these ideas are new, and will over the next few years be explored in much more detail.
The whole field of GW astro has exploded over the past couple of years, not only among scientists but also in the public. My work as a MC Fellow has, despite the COVID-19 lockdowns, led to several social successes one of which was a PhD school on GWs we did at the Niels Bohr Institute 2021 on topics related to BBH mergers. At the moment I'm preparing press releases and outreach material for my Nature paper on AGN disk mergers (see above), which again will have wide impact especially among the public. Many talks on this subject are also lined up, including a public talk at a nightclub in Odense (Denmark). My work was also featured in a 1 hour long podcast in fall 2021. On the scientific site my work on eccentric BBH mergers in AGN disk environments led to the first consistent explanation for the now famous source GW190521, and my new ideas on how eccentric burst sources can reveal about their environment through their modulation opens up a completely new field that will be part of the standard tool-box for many years to come.