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.
Over the last decade more than five thousand gamma-ray sources were detected by the Large Area Telescope (LAT) on board Fermi Gamma-ray Space Telescope. Given the large positional uncertainty of the telescope, nearly 30% of these sources remain without an obvious counterpart in lower energies; these are called unassociated gamma-ray sources (UGSs). This motivated the release of several catalogs of gamma-ray counterpart candidates and several follow up campaigns in the last decade.
Majority is dominated by blazars
Between the associated sources, the large majority is composed by blazars, divided into BL Lacs, with a characteristic lineless spectrum (see figure below), and flat spectrum radio quasars (FSRQs), with broad emission lines and radio spectral index α < 0.5 (defined by the flux density S_ν ∝ ν^−α). In this sense, some of the most successful catalogs of gamma-ray candidate counterparts are the WISE Blazar-Like Radio-Loud Sources (WIBRaLS) catalog and the Kernel Density Estimation selected candidate BL Lacs (KDEBLLACS) catalog, both selecting blazar-like sources based on their infrared colors from the Wide-field Infrared Survey Explorer (WISE).
In this work we characterized these two catalogs, clarifying the true nature of their sources based on their optical spectra from Sloan Digital Sky Survey (SDSS) data release (DR) 15, thus testing how efficient they are in selecting true blazars. If a WIBRaLS2 or KDEBLLAC source is a true blazar, its spectrum may look like the following:
Typical FSRQ spectrum.
Typical BL Lac spectrum.
Based on the optical SDSS spectra, we found that at least ~30% of each catalog is composed by confirmed blazars, with quasars (QSOs) being the major contaminants in the case of WIBRaLS2 (~58%) and normal galaxies in the case of KDEBLLACS (~38.2%). We found that specially in the case of KDEBLLACS, the contaminants are mainly concentrated in the edges of the WISE color-color diagram (see figure below) and can be easily separated from the spectroscopically confirmed BL Lacs.
At least 30% of KDEBLLACS is composed by blazars.
The major contaminants in KDEBLLACS are QSOs and normal galaxies and they are concentrated mainly in the edges of the WISE color-color diagram.
Some sources in the Fermi-LAT catalogs are considered blazar candidates of uncertain type (BCUs) because the adopted association methods select a counterpart that satisfies at least one of the following conditions: i) An object classified as blazar of uncertain or transitional type in Roma-BZCAT. ii) A source with multiwavelength data indicating a typical two-humped blazar-like spectral energy distribution (SED) and/or a flat radio spectrum. BCUs are divided into three sub-types:
– BCU I: the counterpart has a published optical spectrum which is not sensitive enough for classifying it as FSRQ or BL Lac.
– BCU II: there is no available optical spectrum but an evaluation of the SED synchrotron peak position is possible.
– BCU III: the counterpart shows typical blazar broadband emission and a flat radio spectrum, but lacks a optical spectrum and reliable measurement of the synchrotron peak position.
In 4FGL, 1155 sources are considered as BCUs. Our analysis based on the optical spectra available in SDSS DR15 allowed us to give a conclusive classification for 11 of them: 2 BL Lacs, 4 BL Lacs with spectra dominated by the host galaxy, and 5 FSRQs. The SDSS spectral analysis also allowed us to find 25 new BL Lac objects which will be included in future releases of Roma-BZCAT.
This work contributes to a better understanding of the γ-ray sky in the Fermi-LAT era. In particular, the community will benefit from the characterization of WIBRaLS2 and KDEBLLACS in population studies of blazars and in subsequent programs of spectroscopic follow-up needed to confirm the nature of the UGSs.
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).
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.
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?
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.
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.
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.
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*.
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.
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.
In this work we studied how accreting supermassive black holes generate winds that can potentially interact with the host galaxy. Our target was an active galactic nuclei (AGN) with very low accretion rates, like the famous M87*. Can these underfed systems generate powerful winds that will change the fate of the whole galaxy?
We performed diverse simulations of accretion disc flows around Schwarzschild black holes under an hydrodynamic treatment. Our simulations were some of the longest ones of our knowledge. The results show that some systems can create powerful thermally driven black hole winds that can be related to what we call “AGN feedback”. AGN feedback can be understood as the interaction between the ejected material/energy from the accretion flow and the host galaxy, this effect is crucial to understand galaxy evolution and currently it is a very active topic of research in astronomy. With this work we explored the possibility of thermally driven winds as a mechanism to explain this effect.
In the video below we show one of the simulations. On the top we have the gas density and each horizontal panel is the same disc but with different zoom levels, the scale is in Schwarzschild radius. On the bottom of the video we plot the wind efficiency. In practical terms, higher values here indicate stronger material ejection and production of more powerful winds.
Fabio e Rodrigo começamos o dia fazendo uma apresentação no IAG sobre a ciência do Event Horizon Telescope, e depois transmitimos a coletiva de imprensa da NSF. Nota: não fazemos parte da colaboração Event Horizon Telescope.
A partir daquele momento, houve uma gigantesca procura da imprensa pelo nosso grupo de pesquisa de buracos negros, para comentar sobre a incrível primeira fotografia de um buraco negro.
Nas mídias de vídeo, fomos procurados pela Band News e Globo News (Rodrigo) e Gustavo apareceu no AstroTubers.
Link para matéria na Band News (8 minutos em horário nobre!).
Link para matérias na Globo News (Rodrigo aparece nos 5:00 do vídeo) e aqui.
Ficamos contente que o nosso grupo de pesquisa esteja realizando um papel importante de disseminação e clarificação da ciência para o público—afinal de contas, nossos recursos de pesquisa são custeados pelos impostos pagos pela população.
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).
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.
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.
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.
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.
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!
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 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.
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:
It is fun!
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
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