A primeira fotografia de um buraco negro

Ontem foi um dia muito especial para o nosso grupo de pesquisa, com a divulgação da primeira imagem do horizonte de eventos de um buraco negro da história, pela colaboração Event Horizon Telescope (EHT) (press releases da NSF e ESO).

A primeira imagem de um buraco negro: o buraco negro supermassivo na galáxia M87. Crédito: Event Horizon Telescope Collaboration.

Este resultado científico é um grande divisor de águas na astronomia. A partir de agora, na astrofísica de buracos negros, vamos nos referir à era “antes do EHT” e “depois do EHT”. Buracos negros, que eram antes entidades abstratas—comumente ilustradas em filmes e desenhos animados, mas das quais tínhamos somente observações indiretas—agora tem uma imagem concreta.

A ilustração publicada no site xkcd revela bem as gigantescas escalas do astro. O horizonte de eventos em M87 tem 38 bilhões de km de diâmetro, que é um pouco maior que o nosso Sistema Solar! Apesar disso, lá dentro há uma quantidade de matéria seis bilhões de vezes maior que a massa do Sistema Solar. São números quase inimagináveis para quem não é da área.

Crédito: xkcd.

É interessante mencionar que, ao olharmos para a mancha escura no centro da imagem acima, estamos nos defrontando com um imenso vazio cósmico. A massa de seis bilhões de Sóis que existe dentro do buraco negro de M87 está totalmente concentrada num ponto central chamado de singularidade. E entre a singularidade e o horizonte de eventos—nome que damos para superfície absolutamente negra do buraco negro—não há nada. É literalmente um coração das trevas, como o título do romance de Joseph Conrad.

A auréola dourada mostra a radiação eletromagnética com comprimento de onda de 1.3 mm, emitida pelo gás nas partes internas do turbilhão espiralando na direção do horizonte de eventos—chamado de disco de acreção—pouco antes de cair dentro do buraco negro e se perder para sempre do nosso universo.

A partir de agora, nosso grupo vai se debruçar sobre os seis artigos científicos publicados pela colaboração EHT, e disseca-los nas nossas reuniões semanais de Journal Club. Será muito divertido!

New paper: precessing winds from a low-luminosity AGN

We have a new paper out in collaboration with Rogemar Riffel and others accepted in MNRAS: Precessing winds from the nucleus of the prototype Red Geyser.

Riffel, Nemmen, et al., MNRAS, in press

We studied the Akira galaxy, which was named after the Akira manga by Edmond Cheung. Its companion galaxy is called Tetsuo. Akira is an interesting galaxy because it hosts a supermassive black hole fed at quite low rates—we call it a low-luminosity active galactic nucleus.

The black hole seems to be ejecting gas quite vigorously. In fact so vigorously that the BH outflow is capable of quenching star formation in the galaxy. Cheung et al. called this galaxy a “red geyser”.

We observed the nucleus of the galaxy (where the black hole is located) with Gemini integral field spectroscopy (IFU) in order to characterise the black hole outflow. This is a powerful technique because it gives us high-spatial resolution information on several emission and absorption lines.

Equivalent widths of hydrogen alpha in Akira, measured with Gemini. The outflow is marked in orange. Notice that the EWs are telling us that this outflow is consistent with being produced from a black hole (AGN), not from stars.

Below is the money plot of the paper. It tells us that the outflow coming from the black hole is changing its orientation as it propagates away from the galactic nucleus! How to interpret this?

First of all, we do not think we are seeing a jet because this galaxy does not show any extended radio structures. We think this is a subrelativistic, uncollimated wind as shown in the illustration below. We interpreted this as a precessing wind, with the likely cause of the precession being a misalignment between the accretion disk and the BH spin aka Lense-Thirring precession.

Cartoon for the precessing wind launched by the supermassive black hole in the Akira galaxy. The only difference is that Akira is not a radio galaxy, so we do not see extended relativistic jets as shown in the cartoon. Credit: ESA/AOES Medialab.

For more information, please read the paper.

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

Ivan defends his MsC dissertation

Let’s congratulate Ivan on his brilliant masters dissertation defense. His dissertation’s title is “Winds and feedback from supermassive black holes accreting at low rates”. In this work, Ivan performed a suite of hydrodynamical simulations of hot accretion flows with a large dynamical range and long durations (comparable to the viscous timescale), aiming at better understanding black hole wind production and feedback in low-luminosity AGNs hosted by quiescent galaxies.

We have a paper coming out soon, where we will report the results of this work. Stay tuned!

From left to right: Diego Falceta Gonçalvez, Rodrigo Nemmen, Ivan de Almeida, Thaisa Storchi Bergmann and Roderik Overzier.

The evaluation committee was composed of Thaisa Storchi Bergmann, Roderik Overzier and Diego Falceta Gonçalvez.

This work was funded by a FAPESP scholarship, grant number 2016/24857-6. It has made use of the following computing facilities:

  • Laboratory of Astroinformatics (IAG/USP, NAT/Unicsul; FAPESP grant 2009/54006-4)
  • Aguia cluster, HPC resources of Universidade de São Paulo

Raniere visiting Torino to continue exploration of the unknown gamma-ray sky

Raniere will be visiting the University of Torino over the next year, working with Prof. Francesco Massaro, in order to continue our group’s research to understand the unidentified gamma-ray sources observed with Fermi Large Area Telescope. In particular, he will use a suite of optical observations to try to pinpoint the nature of such sources.

Raniere’s visit will be funded by a FAPESP BEPE scholarship, grant number 2018/24801-6.

GNU Scientific Library (GSL) partially ported to NVIDIA GPUs: cuSL

Recently, we embarked in the adventure of porting a “Monte Carlo radiative transfer in curved spacetimes” code from multi-threaded CPUs to GPUs. This work is important for three reasons:

  1. It is fun!
  2. It is important because most general relativistic radiative transfer procedures currently available neglect inverse Compton scattering, which is particularly important to the group since we want to understand the nonthermal emission of active galactic nuclei
  3. Radiative transfer can be slow; on the other hand, it can usually be made massively parallel with a couple of algorithmic improvements

These are some of the reasons why we decided to port a radiative transfer code to GPUs. We have reasons to believe that we can achieve speed-ups of a factor of ~100 times compared to a parallel, OpenMP (CPU) version of the code when using a modern GPU such as a GTX 1080 Ti. This can be a game-changer to allow faster modelling of the radiation from accreting black holes.

We are collaborating with colleagues from the computer science department at the university (Alfredo Goldmann and Matheus Tavares Bernardino). This is work in progress and we have some exciting preliminary results, which we unfortunately cannot publicly share yet. We hope to soon have a paper reporting these results and share the GPU-accelerated black hole radiative transfer code on Github.

In the meantime, we have one software deliverable from this project which may be useful for other researchers: we partially ported some functions of the GNU Scientific Library (GSL) to CUDA. More specifically, we ported the following functions:

  • gsl_ran_dir_3d: generate random vectors in the 3D space
  • gsl_sf_bessel_K0_scaled_e, gsl_sf_bessel_K1_scaled_e, gsl_sf_bessel_Kn: different sorts of “exotic” Bessel functions
  • gsl_ran_chisq: generate random numbers drawn from the chi-square distribution
  • cheb_eval_e: Chebyshev polynomial
  • gsl_sf_lnfact_e, gsl_sf_fact_e: factorial
  • gsl_ran_gamma_double: Gamma distribution
  • gsl_sf_psi_int_e: Digamma (Psi) function
  • gsl_poly_eval: polynomial evaluation

We called this GSL CUDA-port cuSL. It is publicly available on Github. Hopefully, this may be useful to other researchers that are doing mathematical physics computations using CUDA and NVIDIA GPUs.

Roberta joins the group

I’m Roberta Duarte Pereira and I’m a physicist graduated at IFSC-USP. I just started my master’s degree in the Black Hole Group at IAG-USP under the supervision of Prof. Rodrigo Nemmen.
Since my childhood, one of my main passions in life is Astronomy, mainly Black Holes because they are so interesting and they always fascinated me. During my time in undergrad course, Computational Physics also called my attention. So I decided to connect both of my passions.
Recently, I started a project – under the supervision of Professor Rodrigo—in astrophysical simulations with deep learning. It’s a project that may bring some new insights about how we see numerical simulations nowadays and we could gain so much in this area!
I’m very happy and greatly motivated with this project and also for being in the Black Hole Group.

Roberta will work on applying machine learning and deep learning to numerical simulations of black holes, in collaboration with João Paulo Navarro from NVIDIA.

Welcome aboard, Roberta! We are glad you chose to come work in our group, and excited for the discoveries in the computational universe that will come from your project.

Roberta Duarte Pereira and Rodrigo Nemmen

Group funded by FAPESP

We are very happy to announce that our research group is receiving a large, competitive grant from FAPESP (Jovem Pesquisador, R$415k, grant 2017/01461-2). Besides including funding to support the group’s computational needs, this grant includes 1 PhD, 1 MsC and 2 undergraduate scholarships.

415k reais may not look like much when converted to dollars, but this amount of funding is quite hard to get lately in Brasil, especially for junior faculty.

Fermi Symposium + Collaboration meeting + NASA GSFC visit

The PhD students of the group working on gamma-ray observations—Fabio and Raniere—spent the last two weeks in Washington DC and surroundings. They went to the Fermi LAT Collaboration meeting at George Washington University, where they interacted with gamma-ray astronomers in the Fermi Collaboration. Raniere presented his ongoing analysis of the gamma-ray emission of a population of nearby AGNs.

Raniere de Menezes presenting his work on low-luminosity AGNs at the Fermi LAT Collaboration Meeting, Washington DC.

Following the Collaboration meeting, the students presented their research at the Fermi Symposium in Baltimore. Raniere presented a poster about his work on the pulsar populations in Milky Way globular clusters—which is about to be submitted for publication—while Fabio gave a talk describing his analysis of the gamma-ray emission from the Galactic Center on constraints on Sgr A* physics.

Fabio Cafardo presenting his work on the gamma-ray emission of Sgr A* at the Fermi Symposium, Baltimore.

After the symposium, Fabio and Raniere spent a couple of days visiting NASA Goddard Space Flight Center to discuss their research with GSFC scientists.

Their visit was possible thanks to NASA funds, grant xxxxxx.

Undergrad working on GPU research advances to international phase of research symposium

Matheus Tavares Bernardino, an undergraduate student working in our group, presented his work on the acceleration of black hole radiative transfer with OpenACC in GPUS at the undergraduate science symposium at USP and now will compete in the international phase of the symposium which will happen in November.

Congratulations, Matheus!

Matheus (right) and collaborator Alfredo Goldmann (left) happily showcasing the poster