Group training on GPU programming w/ OpenACC

Last Friday we had a group meeting with João Paulo Navarro from NVIDIA. João Paulo gave an OpenACC tutorial to the group, demonstrating how easy it is to accelerate scientific codes on GPUs. With just two lines of code (yes, I said two lines), we made a partial differential equation solver run almost 10x faster on a GPU! It is truly impressive.

We have great plans ahead, with some group projects where we anticipate huge speedups using GPUs. Stay tuned!

IMG_1987 (1)
The Black Hole Group after the OpenACC training with J. P. Navarro (NVIDIA; sitting at the rightmost side).

New grad student in the group obtains FAPESP scholarship

Let’s congratulate Artur Vemado─the newest graduate student in the group─who got a prestigious FAPESP scholarship. Artur just graduated with an astronomy degree at USP. His project will consist of incorporating radiative cooling in the energy equation of hot accretion flows, in order to investigate state transitions in black hole binaries.

New paper: The Multiwavelength Spectrum of NGC 3115

Last week, in collaboration with astrophysicists from USA and China, our group’s paper was accepted for publication in Monthly Notices of the Royal Astronomical Society (MNRAS). The paper is about the properties of the spectral energy density (SED) of the nearby galaxy NGC 3115, that hosts a billion solar mass black hole in a low-luminosity  active galactic nucleus (LLAGN). Behind the spectrum of this galaxy there is a lot of information about the state of the gas flow around the supermassive black hole.

This work is a compilation of observational data of NGC 3115 nucleus followed by modeling of the spectrum considering the electromagnetic processes for the case of a radiatively inefficient accretion flow (RIAF), as the observation suggests. The main part of the work is the analysis of the radio emission that can be well-explained only considering the synchrotron emission from the RIAF, without the need of relativistic jet arising from the LLAGN,

The main result of the paper is a tight constraint on the density profile (ρ) of the accretion flow ρ(r) ∝ r -0.73 +0.01-0.02 which implies an important mass-loss via subrelativistic outflows (i.e. winds) in the RIAF. Our modeling suggests a nonthermal population of electrons in the flow too, similarly to SgrA*—the supermassive black hole in the center of our Galaxy—models.

The main plot of the paper. In the left the thermal SED. In the right is a zoom in radio region of spectrum: the red dash-dotted line is the thermal component of electron population and the blue dashed line is due to the nonthermal electron population, the black solid line is the resulting spectrum.


Postdoctoral position ICTP-SAIFR/USP to join our group

There is a FAPESP postdoctoral position opening for working in our group and collaborating with researchers at ICTP-SAIFR. This position is for a double appointment at USP and ICTP-SAIFR.

Review of applications will begin in January 2018 and will continue until all positions have been filled. Although there is no strict deadline, applications before December 15, 2017 are strongly recommended for positions to begin in 2018.

More information in the website.


Participação de membro do grupo no Jornal Band News

Ontem à noite, participei do Jornal Band News contando a importância da grande descoberta astronômica sobre a colisão das duas estrelas de nêutrons.

Foi um momento histórico, o início da era da astronomia dos multi-mensageiros: observação de ondas gravitacionais e luz vindos de uma mesma fonte astronômica! E a resposta ao mistério da origem dos elementos mais pesados que o ferro da tabela periódica.

Espero que tenha passado os principais aspectos da descoberta, ressaltando a participação de muitos astrônomos no Brasil em vários estados—RN, RJ, SP, SC e SE—e em várias instituições—INPE e UFRN (times do LIGO/VIRGO), e USP, ON, UFRJ, UFSE e UFSC (contrapartida eletromagnética).

Multi-messenger astronomy: detection of gravitational waves and electromagnetic counterparts in neutron star merger

The most recent news in astronomy consists of the report that scientists have managed to record a collision of neutron stars in both gravitational waves and electromagnetic waves. This is a remarkable feat for many reasons. First of all, this is the first time that astronomers detected the gravitational waves that are produced when these neutron stars orbit each other in a deadly inspiral. Equally impressive was the fact that this event was also “seen”: it has been detected and monitored all across the electromagnetic spectrum. There were observations in gamma rays, X-rays, ultraviolet, visible light, infrared and radio. Moreover, the short gamma-ray burst detected shortly after the gravitational waves provided sound evidence that these bursts are associated with the collision of neutron stars. Finally, this merger has given us very sound evidence that such destructive events are responsible for the creation of heavy chemical elements in the periodic table, such as gold and platinum.

The gravitational waves, which are ripples in spacetime that occur when these massive objects orbit each other, were picked up by the LIGO twin detectors in the United States, as well as by Virgo, a gravitational wave detector in Italy. These are the same detectors that have been picking up gravitational waves from the mergers of black holes – the first detection gave 3 LIGO scientists the 2017 Nobel prize in Physics. However, this was the first time that scientists were able to detect gravitational waves from a merger of neutron stars, rather than black holes.

Just like black holes, neutron stars are one of the possible outcomes for a star when it runs out of fuel and collapses due to its own gravity. In order to become a neutron star, a star has to be very massive – much more than the Sun – but not extremely massive, otherwise it will collapse into a black hole. Mergers of black holes are thought to produce very little – if any – light, so they are difficult to “see” with traditional (electromagnetic) astronomy, although we are now able to “hear” them due to the gravitational waves produced in these collisions. Mergers of neutron stars, however, are a different story.

Less than two seconds after the gravitational waves were detected by LIGO and Virgo, the space telescope Fermi detected gamma rays coming from the same region where the gravitational waves came from. Alarms were sounded and telescopes all over the world, as well as in space, started to sweep the sky in order to pinpoint the location of this event. The Swope telescope, in Chile, was the first to report the location as the galaxy NGC 4993, in the constellation of Hydra, about 130 million light-years distant from the Earth.


Above, we see the detections of gravitational waves as well as observations across the entire electromagnetic spectrum. Source: LIGO.

The gamma rays detected by Fermi are what we call a short gamma-ray burst (sGRB). Before this detection, there were strong suggestions that these sGRB were produced when two neutron stars collide, but this was the first conclusive evidence that this is indeed what happens.

Once the telescopes were pointed to the sky, astronomers noticed a blob at the very place where this collision occurred. This blob is what we call a kilonova: a violent explosion that occurs when two neutron stars merge. Upon measuring the electromagnetic spectrum of this kilonova, astronomers detected the presence of heavy chemical elements, such as gold and platinum. This confirmed theoretical predictions that stated that these heavy elements are produced in such collisions, which are so strong that neutrons are literally forced into the nuclei of atoms, making them heavier and therefore creating heavier elements. This implies that the idea that we are all made of stardust also applies to our jewelry.

This detection of gravitational waves and the subsequent observation of this collision across all types of electromagnetic radiation marks the beginning of multi-messenger astronomy: we are now able to study the same event occurring in the universe using two very different types of information. It was a massive effort done by thousands of scientists scattered across the globe and is a small sample of the true power of scientific collaborations.

Speeding up black hole radiative transfer with GPUs

Today we’ve had the first meeting of the group with the goal of accelerating radiative transfer calculations around black holes with graphical processing units (GPUs). The idea is that by porting the code to exploit the massive parallelism of GPUs, we will see a dramatic speedup (hopefully ~10-100x). We have an enthusiastic group of students!

We decided to proceed with NVIDIA CUDA rather than OpenCL due to the better documentation and more astrophysical examples available in CUDA to guide us.

We will try to have a beta version of the GPU-accelerated code by December, and a release candidate on May 2018. The main challenges–as usual with GPGPU–is writing the appropriate kernels and GPU memory management in order to reduce host-device data transfers as much as possible.

Stay tuned for exciting news on astrophysical high-performance computing by our group!


XLI SAB 2017

During the week of September 4th to 8th, 2017, the XLI SAB (Brazilian Astronomical Society) meeting took place in São Paulo. It was a week  when astronomers from all corners of Brazil got together to discuss about astronomy and astrophysics.

The SAB was created in 1974 and every year the association promotes this meeting. The main objective of the reunion is to integrate Brazilian researchers and to plan the future of the astronomy in Brazil.

The XLI SAB was a great event, it was close to 300 participants presenting their current work. It is a very prolific environment for any astronomer or astrophysicist to discuss and learn with other researchers.

The members of the IAG Black Hole Group were present and exhibited their interesting works about black holes and high energy astrophysics. Gustavo Soares, Phd student, presented about “Spectral simulations of accreting black holes”, Raniere Menezes, Phd student, presented other poster with the title “Optical observations of blazar candidates and unknown gamma-ray sources” and Ivan Almeida, MSc student, presented his work about “The multiwavelength spectrum of NGC 3115: Hot accretion flow properties“.

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