MNRAS

And the second paper on the arxiv today is Daria’s masterpiece! pAGN (which Daria says you should read “pagan”) is a brand new, super cool code that implements the hydrodynamics of AGN disks, at least in their most popular one-dimensional fashion. Those solutions have been around for a long time but their details were, well, let’s say unclear. Daria went through everything from beginning to end, coming up with the “one-stop solution for your AGN disc needs” (that was actually the working title of the paper…). So pip install pAGN and have fun.

D. Gangardt, A. A. Trani, C. Bonnerot, D. Gerosa.
Monthly Notices of the Royal Astronomical Society 530 (2024) 3986–3997. arXiv:2403.00060 [astro-ph.HE].
Open source code.

We do population analysis in gravitational waves all the time now. That is: we compare many observations from GW experiments against many simulated datapoints from simulations. But what if you only have one observation? That could be a LIGO guy that is kind of an outlier (think GW190521) or maybe a datapoint from a future detector (think LISA) that feels lonely in his parameter space. Don’t look further, this is stats for you (and Matt’s last paper as a grad student…)

M. Mould, D. Gerosa, M. Dall’Amico, M. Mapelli.
Monthly Notices of the Royal Astronomical Society 525 (2023) 3986–3997. arXiv:2305.18539 [astro-ph.HE].

We want more! With gravitational-wave data, some quantities like the masses of the black holes are much easier to see than others. But those others are very interesting, notably spins that process and orbits that are eccentric, because they would tell us how black hole binaries came to be in the first place. So while it would be great to see those, it’s also being very hard. Some tentative claims have been made with current data, but nothing unambiguous so far. In this paper led by Isobel from Cambridge, we show that (surprise surprise…) the signals needs to be long enough before one can tell eccentricity and spin precession apart.

I. Romero-Shaw, D. Gerosa, N. Loutrel.
Monthly Notices of the Royal Astronomical Society 519 (2023) 5352–5357. arXiv:2211.07528 [astro-ph.HE].

Together with my postdoc Nate, we’re proceeding our investigations on supermassive, spinning binary black holes surrounded by accretion disks (that is: a ton of gas around big monsters at the center of galaxies!). In today’s paper, we dig a bit deeper into what happens when the disk breaks. That presumably stops the interactions between the gas and the black-hole spins which could make all this funky astrophysics (spins that moves, disks that breaks, etc) actually observable with future gravitational-wave detectors. More needs to be done of course, but here we are.

N. Steinle, D. Gerosa.
Monthly Notices of the Royal Astronomical Society 519 (2023) 5031–5042. arXiv:2211.00044 [astro-ph.HE].

Big stars burn everything they have, die fast, and produce big black holes. So when you see two black holes together, it’s likely that the big black hole comes from the big star. Or maybe not? Before dying, the big star can drop some mass onto the other guy, making it bigger! So now, the initially big star still produces the first black hole, but, at the end of the day, that might not be the more massive black hole anymore! This scenario is called “mass-ratio reversal” and our astrophysics friends have put together many models out there showing this is indeed possible for a good fraction of the black holes that produce gravitational-wave events. So here we ask the data: given the events LIGO and Virgo have seen so far, what’s the evidence for mass-ratio reversal in binary stars? Read Matt’s paper to find out.

M. Mould, D. Gerosa, F. S. Broekgaarden, N. Steinle.
Monthly Notices of the Royal Astronomical Society 517 (2022) 2738–2745. arXiv:2205.12329 [astro-ph.HE].

Breaking things is fun! In the previous paper of this series, we looked at accretion disks around massive black-hole binaries and found things were going awry. We kept on finding configurations that our implementation could not handle… And now we know this is real! Finding disk solutions when the spin of the black hole has a large misalignment is just not possible! And that’s because the disk really breaks into different sections. We’ve now checked it with state-of-the-art hydrodynamical numerical simulations that not only confirm what we suspected but also show some funny things (like breaking being prevented by disk spirals, etc). I was serious, breaking things is real fun!

Check out Rebecca’s beautiful movies!

R. Nealon, E. Ragusa, D. Gerosa, G. Rosotti, R. Barbieri.
Monthly Notices of the Royal Astronomical Society 509 (2022) 5608–5621. arXiv:2111.08065 [astro-ph.HE].

Supermassive black hole inspiral and spin evolution are deeply connected. In the early stages when black holes are brought together by star scattering and accretion, spin orientations can change because of interactions with the environment. Later on, when gravitational waves are driving the mergers, spins change because of relativistic couplings. In this paper we try to follow this complicated evolution in a full cosmological framework, using products of the Illustris simulation suite, a new sub-resolution model, and post-Newtonian integrations.

M. Sayeb, L. Blecha, L. Z. Kelley, D. Gerosa, M. Kesden, J. Thomas.
Monthly Notices of the Royal Astronomical Society 501 (2021) 2531-2546. arXiv:2006.06647 [astro-ph.GA].

New paper today! We’ve been working on this for a very long time but three weeks of lockdown forced us to finish it. It’s about distorted (aka warped) accretion discs surrounding black holes. If the black hole is spinning and part of a binary system, the disc behaves in a funny way. First, it’s not planar but warped to accomodate these external disturbances. Second, disc and black hole interacts and tend to reach some mutual agreement where the disc is flat and the black-hole spin is aligned. We find it’s not that easy and things are actually much more complicated: read the paper to know more about non-linear fluid viscosities, critical obliquity, mass depletion, etc.

ps. Here is a Twitter thread by P. Armitage.

D. Gerosa, G. Rosotti, R. Barbieri.
Monthly Notices of the Royal Astronomical Society 496 (2020) 3060-3075. arXiv:2004.02894 [astro-ph.GA].

Spoiler alert: this paper is a bit sad.

Stellar-mass black-hole binaries are now detected by LIGO on a weekly basis. It would be really cool if LISA (a future space mission targeting low-frequencies gravitational waves) could see them as well. We could do a lot of cool stuff, in both the astro and the theory side of things. In today’s paper, we try to figure out how easy or hard it will be to extract these signals from the LISA noise. Well, it’s hard. In terms of the minimum signal-to-noise ratio required, we find that this is as high as 15. The number of expected detection becomes discouragingly low unless the detector behaves a bit better at high frequencies or black holes with 100 solar masses start floating around.

C. J. Moore, D. Gerosa, A. Klein.
Monthly Notices of the Royal Astronomical Society 488 (2019) L94-L98. arXiv:1905.11998 [astro-ph.HE].

Supermassive black holes in binaries and their accretion discs… Spins align on some timescale, but migration also takes place. Do gas discs have enough time to align the spins? Well, the secret is the mass ratio: light secondaries might prevent primaries from aligning. A great collaboration between gravitational-wave and planet researchers!

D. Gerosa, B. Veronesi, G. Lodato, G. Rosotti.
Monthly Notices of the Royal Astronomical Society 451 (2015) 3941-3954. arXiv:1503.06807 [astro-ph.GA].

Black-hole kicks are powerful. I mean, really powerful. They can even eject supermassive black holes from the heavier galaxies in our Universe. And then these galaxies are left “empty”.

D. Gerosa, A. Sesana.
Monthly Notices of the Royal Astronomical Society 446 (2015) 38-55. arXiv:1405.2072 [astro-ph.GA].

My very first paper was on alignment between supermassive black holes and their accretion disks. Maybe it takes a bit longer…

G. Lodato, D. Gerosa.
Monthly Notices of the Royal Astronomical Society 429 (2013) L30-L34. arXiv:1211.0284 [astro-ph.CO].