Big news for our beloved black hole journal club meetings! Ivan will be now the new organizer of this important event in our department.

Fabio Cafardo was in charge of organizing the JC between 2016 and February 2021. Since he is graduating very soon, we now welcome Ivan Almeida to the important role of being the JC organizer. Thanks Fabio for being such a fantastic organizer, paying attention to keeping it on time, asking a lot of questions and making it overall fun! We will miss you.

Está disponível uma bolsa de doutorado direto FAPESP, para trabalhar no Grupo de Buracos Negros do Prof. Rodrigo Nemmen no IAG-USP, dentro do Projeto Jovem Pesquisador FAPESP “O Universo Extremo: Buracos Negros e o Telescópio Fermi“.

O projeto a ser desenvolvido é na área de astrofísica de altas energias, envolvendo observações em raios gama de buracos negros supermassivos com o Telescópio Espacial Fermi. O trabalho envolverá o estudante em colaborações internacionais do Prof. Nemmen.

A bolsa é livre de impostos e a FAPESP oferece apoio para os custos de mudança. O salário inicia em R$ 2043/mês, chegando a R$ 3726/mês no último ano de doutorado. A Reserva Técnica para participação em eventos, compra de material etc é de 30% do valor anual da bolsa (R$ 13414/ano).

Os candidatos interessados deverão entrar em contato por email com o Prof. Rodrigo Nemmen para entrevista, onde serão discutidos: • experiência em computação e pesquisa do candidato • motivação para fazer pós-graduação • redação • conhecimentos básicos em (astro)física. Os candidatos devem incluir no e-mail:

Histórico escolar de graduação (e de pós-graduação, se houver)

Link para o CV Lattes

Um ou dois contatos de referências

O candidato deverá passar primeiro o processo seletivo para o programa de Doutorado Direto em Astronomia do IAG-USP, com inscrições até 25 de Junho de 2020. Potenciais interessados também podem entrar em contato com o Prof. Nemmen para tirarem dúvidas, antes de se candidatarem ao programa de pós-graduação em Astronomia do IAG-USP.

One FAPESP PhD scholarship is available in the Black Hole Group of Prof. Rodrigo Nemmen at IAG-USP, within the Projeto Jovem Pesquisador FAPESP “The Extreme Universe: Black Holes And The Fermi Telescope”. The PhD project is in the field of high-energy astrophysics, involving the analysis of Fermi Large Area Telescope gamma-ray observations and associated physics. The work will involve the student in the scientific collaborations of Prof. Nemmen.

The salary is tax-free, and the funding agency provides relocation funding. The salary begins at R$ 2043/month, reaching R$ 3726/month in the last year of the graduate program. The scholarship includes funds for attending events in the amount of R$ 13414/year.

Candidates should contact Prof. Rodrigo Nemmen. If short-listed, they will be interviewed by the PI where they will be asked to discuss their: • research and computational experience • motivation for pursuing graduate school • writing skills • (astro)physics knowledge. The candidates should include in their e-mail:

Undergraduate transcripts (and graduate, if available)

Curriculum vitae

One or two reference contacts

The candidate must be accepted in the selection process for the programa de Doutorado Direto em Astronomia do IAG-USP (deadline: June 25, 2020). Candidates who are interested and have any questions should contact Prof. Nemmen.

Está disponível uma bolsa de mestrado FAPESP, para trabalhar no Grupo de Buracos Negros do Prof. Rodrigo Nemmen no IAG-USP, dentro do Projeto Jovem Pesquisador FAPESP “O Universo Extremo: Buracos Negros e o Telescópio Fermi“.

O projeto a ser desenvolvido envolve análise de observações em raios gama de buracos negros supermassivos com o Telescópio Espacial Fermi. O trabalho envolverá o estudante em colaborações internacionais do Prof. Nemmen.

A bolsa é livre de impostos e a FAPESP oferece apoio para os custos de mudança. O salário inicia em R$2043/mês. A Reserva Técnica para participação em eventos, compra de material etc é de 10% do valor anual da bolsa (R$2452/ano).

Os candidatos interessados deverão entrar em contato por email com o Prof. Rodrigo Nemmen para entrevista, onde serão discutidos: • experiência em computação e pesquisa do candidato • motivação para fazer pós-graduação • redação • conhecimentos básicos em (astro)física. Os candidatos devem incluir no e-mail:

Histórico escolar de graduação

Link para o CV Lattes

Um ou dois contatos de referências

O candidato deverá passar primeiro o processo seletivo para o programa de Mestrado em Astronomia do IAG-USP, com inscrições até 25 de Junho de 2020. Potenciais interessados podem entrar em contato com o Prof. Nemmen para tirarem dúvidas, antes de se candidatarem ao programa de pós-graduação em Astronomia do IAG-USP.

The black hole group has a new master: Artur Vemado defended his MsC dissertation, entitled “radiative cooling and state transitions in stellar mass black holes”. The defense was very successful.

Here, Artur reported his numerical simulations of black hole accretion flows where he incorporated radiative cooling (with some approximations otherwise the problem is essentially intractable!). We observe the self-consistent emergence of a hot corona enveloping a cold thin accretion disk. Artur quantified the inner radius of the thin disk, the size of the corona, and how these properties respond to varying the mass accretion rate onto the black hole. The resulting simulated black holes are similar to observations of stellar mass black holes in binary systems.

We are looking forward to reporting these exciting results on the emergence of the corona (not the covid-19!) and truncated disk in an upcoming publication.

Many thanks to FAPESP funding through grant 2017/25710-1.

Congratulations to the now Dr. Gustavo R. R. Soares, for a successful PhD thesis defense! 🎉🍾

The thesis is entitled “Accretion discs, jets, and black hole spins: a study of blazars” and was done under my supervision. The whole defense was entirely online, following the social distancing recommendations of the World Health Organization and the São Paulo State government, in order to ensure the safety of all involved with respect to COVID-19.

The defense lasted for almost five hours (!!), with the thesis committee members in two countries—Brasil and US—and in three states in Brasil: Pernambuco, Rio de Janeiro and São Paulo.

Gustavo presenting his thesis work

Thanks to Dr. Soares’s work, we now we know a bit more about the role of black holes in the universe, and how the supermassive ones power relativistic jets.

Gustavo will begin a postdoc at Oregon State University in the Fall. We are all wishing Gustavo a huge success for his future career!

Our thanks to the Brazilian science funding agencies CAPES, CNPq and FAPESP. Without them, this work would not have been possible.

On the week of March 2nd to 6th, the Black Hole Group attended the IAU Symposium 359: Galaxy Evolution and Feedback Across Different Environments, or simply the GALFEED symposium. The event occurred at the beautiful setting of Bento Gonçalves in the south of Brasil. This is the wine region of the country, which specializes in the merlot variety.

This was an incredible event which led to many fruitful interactions between the group members and the galaxy evolution community. Beginning on March 2nd, Fabio and Raniere gave poster flash talks where they had the daunting challenge of summarizing their posters in only one minute. Not so easy, but they did a great job!

Queue of students waiting to present their work

Fabio Cafardo presenting his PhD work on Sgr A*

Raniere de Menezes presenting his work on low-luminosity AGNs

On Tuesday, it was Ivan and Gustavo’s turn to face the one-minute-present-all-your-research challenge. And again, it went fantastic!

Gustavo Soares talking about blazars and their spins

Ivan Almeida presenting his numerical simulations of black hole winds at low Eddington ratios

On Wednesday, we surprised Fabio with a surprise birthday party: it was his 40th birthday! There was cake and presents!

Fabio’s birthday. From left to right: Raniere, Gustavo, Fabio, Ivan, Rodrigo and Roderik

On Thursday, it was Rodrigo’s turn. He surprised the audience by beginning hist talk about spin estimates for M87*…

Rodrigo begins his talk on the spin of M87*. Certainly not the best of pictures…

… and then switching gears to talk about the first AI simulation of a black hole—the work of graduate student Roberta Pereira.

Rodrigo Nemmen starts with a cyberpunk intro screen, high resolution astrometry of student poster locations, and the fundamental black hole astrophysics equations (simulation time<<PhD thesis timescale) @nemmen#galfeed2020pic.twitter.com/j0EiPkCmd7

After Marta Volonteri’s review talk on the cosmic evolution of massive black holes, Fabio gave a presentation about the current status of his analysis of Fermi LAT observations of the Galactic Center—we are finishing the first paper of the series which should be submitted very soon.

Fabio Cafardo’s talk at the 43th SAB meeting.

After Fabio’s talk, we’ve had Ivan’s talk on his numerical hydrodynamical simulations of radiatively inefficient accretion flows and their winds.

Ivan Almeida’s talk at the 43th SAB meeting.

Roberta Pereira presented her poster on applying deep learning to predict the future of accreting black holes, which are an extreme example of spatiotemporally chaotic systems.

Finally, Gustavo won one of the best poster prizes at the meeting, and was awarded a talk at the meeting. Wait, is that an award? 🙂

From August 8 to 13, our group taught two courses at the high-energy astrophysics school at ICTP-SAIFR. The first was taught by myself and gave a broad overview of the active galactic nuclei phenomenon, including blazars. The second course was about Fermi LAT observations and taught by Fabio Cafardo, a PhD student in the group who is working of gamma-ray astronomy.

Fabios’s lecture also included a fun hands-on tutorial, teaching the students to analyze gamma-ray observations of the blazar TXS 0506+056. This is the famous blazar which was observed to emit gamma-rays flares and produce a high-energy neutrino at the same time: the second source ever observed in multimessenger astronomy. The first was the neutron star collision observed in gravitational waves with LIGO/Virgo and electromagnetic radiation.

Here is the official abstract for the AGNs and blazars course:

I will give a broad overview of the phenomenology and theory behind the active galactic nuclei (AGN) phenomenon. I will give a particular emphasis on systems which produce relativistic jets such as blazars, given their importance in multimessenger astronomy. I will cover the basic physics of gas accretion and jet production from Kerr black holes. I will also give an overview of the electromagnetic signature from AGNs and blazars, focusing on their gamma-ray emission commonly observed with the Fermi, HESS, MAGIC telescopes, and in the future CTA.

And here is the official description of the Fermi LAT tutorial:

The Fermi Gamma-ray Observatory has revolutionized our understanding of the high-energy universe. Over the last 10 years, the Fermi Large Area Telescope has been observing the entire sky from space every three hours in the 100 MeV to 500 GeV energy range. In this lab activity, We will give a short presentation highlighting the main results and importance of the Fermi Telescope—particularly for blazar and dark matter indirect searches. The talk will be followed by a hands-on tutorial where the students will get familiar with the analysis of space-based gamma-ray observations.

If you want to run tutorial #3 at home, you can download all the material from the third lecture and perform the analysis without needing to install a lot of additional software. We prepared the tutorial such that you only need to install one software and run an install script.

I suggested some reading for the students interested in diving deeper into AGN physics:

Physical processes in active galactic nuclei, Blandford (cf. from p171 onwards in the PDF). Even though this is a quite dated treatment—from 20 years ago!—and a lot has changed since then, this paper does a great job in summarising the basic physics of the AGN phenomenon.

Rodrigo Nemmen and Pasquale Blasi tackling questions from the audience. Credit: Ivan Almeida.Fabio Cafardo teaching the Fermi LAT hands-on tutorial. Credit: Rodrigo.Another shot of Fabio during his course. Credit: Ivan.Some of the diverse audience at the school. Credit: Fabio.

Thanks Fabio Iocco for the invitation. This was fun!

The fact that some image of an event horizon would be announced on April 10th was not a surprise to our group. What caught us by surprise—since we are not members of the Event Horizon Telescope Collaboration (EHTC)—was the fact that the black hole imaged was in M87 and not in our Galactic Center (for more information, check out the six papers outlining this incredible, game-changing discovery).

Here is a fantastic trip, based on real astronomical observations, to the center of the galaxy M87 where M87* is located.

Anyhow, a couple of days after the results were announced, while I was reading the first paper of the series, I was struck by a particular paragraph at the end of Section 6:

I immediately realized that I could produce a more precise estimate of the black hole spin of M87* based on the power of the relativistic jet. Kerr black holes can be completely described by only two numbers: the mass and the spin. There have been several measurements of the mass of M87* with a better than 10% uncertainty on the mass, for example using stellar or gas dynamics or the size of black hole shadow. Getting the spin however is a completely different story and much more difficult. Measuring the spin from the shadow is currently out of question because the images are not sharp enough to the degree that would allow us to get confident estimates. And many other methods in the literature suffer from issues such as large uncertainties in the data or model parameters.

I thought that perhaps I could contribute an interesting estimate of the spin of M87*. Wouldn’t that be a nice—and hopefully quick—paper? Over the next couple of days, I devoted myself entirely to getting this estimate right and assessing whether it would be worth of publishing. I was nervous because if I got that the spin a* is very low or consistent with zero then the result would not be very interesting and not worth writing a paper about it. What I found surprised me. And led me to write my quickest paper up to date: it took me two weeks from the beginning of the analysis up to having a manuscript submitted to ApJL.

Observations

Before digging into the results, what were the observables? The observables I chose were the total power carried by the jet coming from M87* (the jet power) and amount of mass being fed to the black hole—the mass accretion rate which I will also refer to as Mdot. If I have reliable measurements of these two numbers, then I could use current ideas about how black holes produce jets to tie the observations to an estimate of the black hole spin.

Where do these observables come from? The jet power was estimated by Russell et al. (2013) using Chandra X-ray observations of the hot gas around M87*. From the temperature, density and an idea of the volume of such gas, Russell et al. was able to quantify the average power dumped by the jet over a period of about one million years: about 1E43 erg/s. For reference, the black hole in the center of M87 is putting out ten billion times more energy in its environment than the Sun radiates per second.

I should mention that there are several different ways of estimating the jet power in M87* (for reviews, cf. EHTC paper V). I prefer the estimate from X-ray bubbles because it is more robust against time variability.

The mass accretion rate was estimated by Kuo et al. (2014) based on a clever idea originally proposed by Dan Marrone in the context of Sagittarius A* (Sgr A*). Marrone et al. (2006) figured out that if they have a good measurement of the amount of polarization that radiation suffers when it leaves the surroundings of the black hole, then using simple assumptions one can estimate the gas density near the black hole and Mdot. Concretely, one needs to measure the Faraday rotation measure (RM) and assume that one is observing synchrotron radio emission coming from the inner parts of the accretion flow and that his radiation is polarized by the accretion flow itself as it travels outwards and eventually reaches the observer. Kuo et al. (2014) measured the M87* RM and found that Mdot < 9E-4 Msun/year. In other words, Kuo et al. measured an upper limit to Mdot. This means that in one year, M87* eats up one Jupiter worth of mass (actually, less than that).

Importantly, if the observed polarization in M87* is not due to the accretion flow as a “Faraday screen”, then this will affect the estimates of Mdot. I will return to this point further below.

OK, so we have some amount of energy flowing out of the black hole—the jet power—and some upper limit on the amount of energy flowing into it—the Mdot. How can we put this together and estimate the rotation frequency of spacetime?

Theoretical model

It turns out that black holes (BH) are in many ways similar to a car engine. If you wanted to reverse-engineer the energetics of an engine, you would just need to observe its fuel consumption, how much is lost in the exhaust and how much speed it delivers. Then you would have a good idea of how efficient the engine is and start working out how it could achieve such levels of fuel consumption. For a black hole it works exactly in the same way. If you know how much power it produces by accelerating particles in a jet, and if you know how much gas is being fed to the BH, you can work out how “green” it is. Why is this related to the title of this blog post? Because the level to which a BH is economical is related to how fast it rotates. The BH spin is the turbo in the engine: the larger the value of a* is, the larger is the amount of jet power produced by the BH for a given fixed Mdot.

To begin with, I used a model that specifies the efficiency of jet production η as a function of a*. This model is called Blandford-Znajek mechanism named after the researchers that solved Maxwell equations to first order in the curved spacetime of a BH almost forty years ago, and figured out how BHs can power jets (Blandford & Znajek 1977). The Blandford-Znajek model has a couple of free parameters and I needed to anchor these values otherwise I would not learn much about M87* from applying it to my data. I fixed the fudge factors in the model by using a series of advanced numerical simulations of how magnetized plasmas near event horizons behave as time progresses, which have the technical name of general relativistic magnetohydrodynamic (GRMHD) simulations of accretion onto Kerr BHs.

General relativistic MHD simulation carried of a spinning black hole producing jets carried out by A. Tchekhovskoy. This kind of model formed the basis of the model used in this work.

I based my models on the numerical results of my collaborator Sasha Tchekhovskoy, who is an assistant professor ar Northwestern University. The figure below summarizes how efficient BH engines are at producing jets according to the Blandford-Znajek model and GRMHD simulations.

Efficiency of production of relativistic jets by Kerr black holes, as a function of the black hole spin. Credit: Nemmen 2019.

There are two numbers that control the jet efficiency. The first one is the spin, of course. There is a second number as well: the magnetic flux on the event horizon of the BH. Because jets are powered by a helical twisting of magnetic field lines anchored in the event horizon, the power also depends on the magnetic field. Therefore, by modeling M87*’s data we can learn something not only about the spin but also the magnetic field near the BH.

Main results

The two main results of the paper are the following:

I get a robust lower limit on the black hole spin in M87* from the observations: a* > 0.5. This means that the black hole must be rotating at least at half of the maximal possible rotation frequency allowed by general relativity.

I find lower limits on the amount of magnetic flux threading the event horizon, 𝜙 > 5 (𝜙 in dimensionless units typical of GRMHD works). This means that the magnetic fields surrounding the BH are quite strong. This disfavors a whole category of accretion flow models known as “SANE” for M87*.

Lower limits on the spin of M87* as a function of the density power-law index which is a free parameter. These spins were estimated assuming that the BH is in the MAD state, therefore maximizing the jet power for a given spin. Credit: Nemmen 2019

If these bounds were to be violated, then the BH would not be able to pump enough energy into the jet to be consistent with the observed power. Combining these results with the constraints from EHT observations—something that I have not done—should reduce even further the parameter space allowed for M87*.

A few more details about result #1. I actually considered both the cases in which the BH could rotating in the same (prograde) or opposite (retrograde) direction as the accretion flow (however, the angular momentum vectors must be parallel or antiparallel). If M87* is prograde, then the lower limit on the spin is |a∗| ≥ 0.4, otherwise it is |a∗| ≥ 0.5. I was not able to distinguish between the prograde or retrograde scenarios based only on the data available. Hopefully, the upcoming EHT polarimetric observations will shed more light on these issues.

Some comments

Spin

What is the meaning of the spin parameter that I talked about above? The maximal possible rotation frequency allowed by general relativity corresponds to max(a*) = 1. At the maximal spin, the equator of the black hole would be rotating at the speed of light. Above that limit, one interpretation is that the black hole would break-up due to centrifugal forces and a naked singularity would be revealed. Nobody has figured out how to do that—even in theory.

Accretion rate and Faraday rotation measure

I should thank the referee because he/she really helped to improve the quality of the manuscript thanks to the thoughtful comments. One of the interesting points made by the referee was the following:

The upper limit on density relies on the model used by Kuo et al. to relate rotation measure to accretion rate. There are large uncertainties in this estimate! The RM depends not only on density, but also magnetic field strength and geometry, […] along a highly inclined line of sight. […]

This made me think about the underlying assumptions behind the Mdot estimate by Kuo et al. 2014. The idea goes back to Marrone et al. (2006) and requires a model for the density and magnetic field in the accretion flow, relying on the following assumptions:

the Faraday rotation is caused by the hot accretion flow in front of a source of synchrotron emission

the accretion flow is roughly spherical and characterized by a power-law radial density profile

the magnetic field is well ordered, radial, and of equipartition strength

Of course, real accretion flows are messy and turbulent. GRMHD simulations indicate that their magnetic fields are predominantly toroidal rather than radial (e.g. Hirose et al. 2004 and many other works). Marrone et al. argues that the assumption of a radial magnetic field should give only a small error. The outer radius used in the estimate of Mdot should depend on the coherence of the magnetic field. Kuo et al. assume rout=rBondi. If rout<rBondi, then Mdot will be even less than estimated, thereby increasing the lower limits on a* and phi.

Quantifying the impact of the line of sight on Mdot and hence on our estimates of a* and phi is also difficult. Given that the jet in M87 has a low inclination angle to the observer, one possibility that cannot be completely ruled out is that the RM originates from the jet sheath, with the line of sight of the observer not passing through the RIAF. This scenario was explored by Moscibrodzka et al. 2017 using GRMHD simulations. If that is the case, Moscibrodzka et al. concluded that the RM would be consistent with a higher Mdot than we considered (and potentially much higher). This would lower the spin and magnetic flux, as discussed in the letter.

Interestingly, in the models by the EHTC in paper V did not obey the Mdot constraints of Kuo et al. Mdot in their models is a free parameter that is tuned to reproduce the observed compact mm flux. The exact value of Mdot in the general relativistic ray-tracing simulations employed in paper V depends on the electron thermodynamics and spans a wide range.

One feature of current GRMHD simulations tackling the amount of Faraday rotation in RIAFs such as Moscibrodzka et al. (2017) is that they have a very small torus extending up to about 60M. Furthermore, they do not have a high enough spatial resolution to resolve MRI in the outer regions of the disk—which is a crucial ingredient for reliable RM estimates—and do not have long enough durations to establish inflow equilibrium in the outer parts of the disk. While a jet-originated RM is possible, I am afraid that some GRMHD simulations might be underestimating the polarization effects of the RIAF at larger scales and this might impact their conclusions on the amount of Faraday rotation. In conclusion, both models for M87*’s RM—the simple analytical RIAF model a la Marrone and the current round of GRMHD simulations—are incomplete. There is definitely a lot of space for improvements in these calculations.

This work was supported by FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo) under grant 2017/01461-2.