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A nonthermal “bomb” producing a strong flare in Sgr A*

Our paper titled “A nonthermal bomb explains the near-infrared superflare of Sgr A*” has been accepted for publication in The Astrophysical Journal Letters, and it is freely available in arXiv. The work was developed by PhD student Eduardo Gutiérrez, from the GARRA group at the Argentine Institute of Radio Astronomy, and Prof. Rodrigo Nemmen and PhD student Fabio Cafardo from the Black Hole Group at USP. In the Letter, we propose a physical mechanism to explain an unprecedented powerful flare that took place in Sgr A* last year.

On 13 May 2019, the supermassive black hole at our Galactic center experienced the strongest near-infrared flare detected so far. The observations made with the Keck Telescope led Prof. Tuan Do, from the University of California, Los Ángeles, and collaborators to publish a Letter reporting the results. The following animation is a superpositions of the images taken by the Keck telescope, and shows the strong variable emission detected that night:

Interestingly, the flare was catched when the brightness was already diminishing, which suggests that its peak luminosity might have been even higher!

Sgr A* is known to experience regular flares in several bands of the spectrum, extending from radio up to X-rays; however, the NIR flare on May 2019 was much brighter than any other ever before. In our work, we propose a possible physical mechanism which might have been the responsible for the flaring emission. We frame the event under the term nonthermal bomb. But what do we mean by this term?

Screenshot from 2020-01-27 15-10-32

The accretion flow onto our galactic center is extremely thin; very little amount of matter feeds the black hole. These low-density flows are usually called Radiatively Inefficient Accretion Flows (RIAFs) because most of the gravitational energy released by the matter is swallowed by the hole and not radiated. Given the very low densities, the plasma in these flows is extremely collisionless and particles have difficulties to exchange energy between themselves efficiently. As a consequence, they may not reach thermal equilibrium, and a fraction of them can be nonthermal. In fact, it is thought that a small population of nonthermal electrons steadily present in the accretion flow is the responsible for the quiescent radio emission of Sgr A*. To represent the ambient conditions in the flow we have followed the standard modelling of a RIAF around Sgr A* in the steady state (see Yuan et al. 2003). The following image shows the Spectral Energy  Distribution (SED) predicted by this model, which is in very good agreement with the multiwavelength data. The different colors in the plot represent the different emission processes that take place in the flow.

Over the background ambient responsible for the quiescent emission, an additional transient process must occur in order to produce a flare like the one we are dealing with. Do et al. (2019) suggested that a large increase in the accretion rate (for example, the accretion of a denser blob of matter) might be what caused the enhanced emission. On the contrary, we propose a different mechanism: that a huge amount of magnetic energy was released in the accretion flow in a bursting event and was able to accelerate an additional amount of electrons to relativistic energies. Those we do not state which particular acceleration mechanism is working, the most plausible culprits  are magnetic reconnection and turbulence acceleration. Since we dealt with a time-dependent process we took into account the evolution of this population of accelerated relativistic particles as they cool by synchrotron emission and are advected towards the hole. Interestingly, given the length-scales of the accretion flow in Sgr A*, both of these processes, namely cooling by synchrotron and advection, are of the same order that the detected flare. In the following figure we show the flare data and the fitting we obtain with our model.

Screenshot from 2020-02-26 12-46-27

Despite some degeneration in the parameters, our model is able to explain the flare emission and fit the data very accurately. We also make predictions in X-rays and mm wavelengths (Event Horizon Telescope band) that might help to test our model against others. We expect that this works motivate further investigation on particle acceleration and bursting events in RIAFs, and in particular in Sgr A*.

Link to the article in arXiv

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

Paper: Gamma-ray observations of low-luminosity active galactic nuclei

The majority of the activity around nearby (z ~ 0) supermassive black holes (SMBHs) is found in low-luminosity active galactic nuclei (LLAGN), the most of them being classified as low ionization nuclear emission regions (LINERs). Although these sources are well studied from radio up to X-rays, they are poorly understood in gamma-rays. In this work we take advantage of the all sky-surveying capabilities of the Large Area Telescope on board Fermi Gamma ray Space Telescope to study the whole Palomar sample of LLAGN in gamma-rays.

Gamma-rays from the accretion flow

The observational properties of LLAGN favor a scenario for their central engines which is quite different from that of more luminous AGN: since the SMBHs are accreting at low rates they are in the radiatively inefficient accretion flow (RIAF) mode (Narayan & Yi 1994) rather than radiatively efficient geometrically thin accretion disks (Shakura & Sunyaev 1973).  RIAFs store most of the viscous energy and advect it into the SMBH. The viscous heating affects mainly the ions, while the radiation is produced primarily by the electrons.

The presence of both RIAFs and jets in LLAGN produces a rich multiwavelength spectral energy distribution (SED) in which a gamma-ray component should be expected. There are multiple possible origins for the gamma-ray emission. In the vicinity of the event horizon, the ion temperature of a RIAF can reach 10¹² K (Yuan & Narayan 2014), thus enabling proton-proton collisions and the production of neutral pions, which subsequently decay into pairs of GeV photons (Mahadevan et al. 1997). Furthermore, synchrotron self-Compton (SSC) emission is expected from the jet. Quantifying and modeling the gamma-ray emission from LLAGN is the main goal of this work.

Results

Precisely, the four radio-brightest LLAGN in the sample are identified as significant gamma-ray emitters, all of which are recognized as powerful Fanaroff-Riley I galaxies. These results suggest that the presence of powerful radio jets is of substantial importance for observing a significant gamma-ray counterpart even if these jets are misaligned with respect to the line of sight (see figure below, left panel). We also find that most of the X-ray-brightest LLAGN do not have a significant gamma-ray and strong radio emission, suggesting that the X-rays come mainly from the accretion flow in these cases (see figure below, right panel).

Lgamma_Lradio_noerrorbar  Gamma_Xrays_noerrorbar

When comparing different emission models to the SEDs of these gamma-ray-bright LLAGN, we find that they are well described by a jet-dominated model in the form of a one-zone synchrotron-SSC jet scenario, similar to what is expected when observing blazars (see figure below, left panel). We also find that models invoking the origin of the electromagnetic radiation in a RIAF fail to explain the observed gamma-ray emission but are able to reproduce the radio-to-X-ray emission (see figure below, right panel). Since the gamma-ray-bright LLAGN consist of powerful radio galaxies, it is not entirely unexpected that their emission can be dominated by a jet even if misaligned to the observer’s line of sight.

NGC315_SED_SSCNGC315_SED_RIAF

The original discussion of this work can be found in de Menezes et al. (2020).

This work was supported by FAPESP (Fundação de Ampara à Pesquisa do Estado de São Paulo) under grants 2016/25484-9, 2018/24801-6 (R. de Menezes), 2017/01461-2 (R. Nemmen) and 2019/10054-7 (I. Almeida); and many other institutions.

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Attending the biggest event of AI and Machine Learning in Latin America: Khipu

From November 10th to November 15th, Khipu happened in Montevideo, Uruguay. Khipu was the best biggest event of artificial intelligence and machine learning in Latin America. The speakers of this event are all AI leaders in their fields and big companies such as Google, Deepmind, Apple and Facebook were sponsoring the event. The competition to be in one of the 200 participants of this event was highly competitive. It was one in a lifetime experience that I will try to sum up here. 

On the very first day, they welcomed us with a small lecture about motivation in AI fields and with a toast event. In this toast event, I met Nando de Freitas (Principal Scientist at Deepmind and of the main speakers there) and Cho Kyunghyun (Research at Facebook AI and NYU Professor), we talked about my project, about the event and about Brazil! They were really nice and friendly to me and my friend.

All of the lectures were interesting but I will give special attention to some of them: Ian Goodfellow talking about GANs, Yoshua Bengio talking about Perspectives in AI, Nando de Freitas talking about Reinforcement Learning, Cho Kyunghyun talking about Recurrent Neural Networks and Jeff Dean closing the event with an amazing talk about the challenges in AI. All the lectures are available here: http://tv.vera.com.uy/buscar?search=Khipu

Another important lecture was “How To Write a Good Paper”. So important and very helpful ideas!

Another highlight in this event was Nando de Freitas talking about my project with me when he visited my poster and commenting about my project in his lecture. It was a really special moment for sure. He even posted in Twitter and retweet my tweet talking about the project. I received a supportive comment from Cho Kyunghyun as well.

It is impossible to talk about Khipu without talking about Jeff Dean (GoogleAI leader and one of Tensorflow creators). I talk with him for a few minutes in a coffee break, he is also very friendly and we talked about my project and about GoogleAI opportunities. In his awesome lecture about challenges in AI, he also comments about my project and I found that one of the biggest highlights in the week. I took a picture with him and posted on Twitter, he commented.

I even met some amazing people from DeepMind and GoogleAI there, they gave me a lot of advice and talked about the life in DeepMind and in GoogleAI. I even met Feryal Behbahani from DeepMind and Women in ML and Oriol Vinyals from DeepMind (main AlphaStar researcher)

This event was so important with so many big names in AI. I am very thankful for Khipu because this event because I have so many opportunities now and also big thank you because they paid all the expenses with travel and accommodation. I really look forward to Khipu 2020! It is a really life changing experience.  

Credits of pictures: @Khipu_AI

Paper: Black hole growth in high redshift radio-loud quasars studied with the ESO Very Large Telescope

Our paper, VLT/SINFONI study of black hole growth in high redshift radio-loud quasars from the CARLA survey”has been accepted for publication in the Monthly Notices of the Royal Astronomical Society, and the pre-print appeared on astro-ph today. The paper was led by former graduate student Murilo Marinello, and this work formed a major part of his PhD dissertation published in April this year.

The new study focused on 35 distant, radio-loud quasars, the majority of which were selected from the Clusters Around Radio-Loud AGN (CARLA) survey. These quasars were known to have large black hole masses, emit luminous radio emission, and tend to be found in dense regions of the early universe. Therefore, they are believed to be good candidates for the distant progenitors of massive (elliptical) galaxies that dominate the universe today. The masses of their supermassive black holes had previously been estimated using the virial black hole mass method applied to their SDSS spectra. Due to their high redshifts, however, the only emission line available to make these measurements in these optical SDSS spectra was the CIV line. This line is known to be affected by non-gravitational effects (winds or outflows) and is thus not optimal for the virial black hole mass estimate. In this project, we therefore re-observed the quasars in the near-infrared using the SINFONI spectrograph on the Very Large Telescope in Chile. This allowed us to access the redshifted Ha broad emission line, and thus determine the black hole masses more accurately. This makes a big difference, as can be seen in the figure below showing the nice symmetric Ha line on the top and the distorted CIV line at the bottom for one of our quasars:

LineFit_example

Together with a determination of the accretion rates of the quasars, which can be estimated from their luminosities, the new black hole masses were used to also derive the growth histories of these supermassive black holes. One major finding was that if these quasars had always been accreting at the same rates as measured at the current time, it would not have been possible for them to obtain their high observed masses within the cosmic time available since the Big Bang. The logical conclusion is thus that these quasars must have experienced a phase of much faster growth in the past. This can be nicely illustrated in the following figure:

GROWTH_TRACK_new

The red points are the CARLA quasars from this study. The black solid lines show the growth tracks we found to be the ones describing their most likely histories. These tracks consist of two phases: a rapid growth phase starting from a one thousand solar mass black hole seed at z ~ 20, growing at the Eddington limit to a hundred million or more solar masses at z ~ 6, followed by a second, slower phase at the observed lower Eddington ratios until z ~ 2-3. As such, it is possible that the CARLA quasars are direct descendants of the luminous quasars found at z ~ 6-7.

In the local universe, there is a strong correlation between the masses of the supermassive black holes and the masses of their host galaxies. Since the more massive galaxies are also found, on average, in more massive dark matter halos, there is an indirect connection between the mass of the black hole and the mass of its halo. We therefore also tested whether the black holes in the CARLA quasars already “know” that they are located in dense galaxy environments:

SD_MBH_reli1.png

We found a weak, low significance correlation between the black hole masses and the surface density of galaxies that surround them (the latter is a measure of the environment or halo mass of the CARLA quasars), and therefore do not find strong evidence that the most massive CARLA quasars are also in the most dense environments or massive halos. However, these galaxy surface densities had been previously determined with the Spitzer Space Telescope as part of the CARLA project, and are not very precise. In the future, we will therefore focus on trying to obtain more precise measurements of the environments of the CARLA quasars, and test again for possible correlations between black hole mass and environment.

 

Massive presence of the group at the Brazilian Astronomical Society Meeting

The Black Hole Group had a massive presence at the 43th meeting of the Brazilian Astronomical Society—Sociedade Astronômica Brasileira, SAB.

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.

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

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? 🙂

Gustavo Soares’s talk at the 43th SAB meeting.

Group taught course at ICTP-SAIFR high-energy astrophysics school

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. 

Here is the material that we covered:

  1. Active galactic nuclei and blazars (Nemmen). Two lectures, 3 hours. Lecture 1, lecture 2
  2. Fermi LAT (Cafardo). Lecture, 1 hour
  3. Fermi LAT hands-on session (Cafardo). 1.5 hours

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:

  1. 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. 
  2. Relativistic jets in active galactic nuclei, arXiv:1812.06025. Up-to-date review about AGN jets
  3. Foundations of black hole accretion disk theory. Focuses on black hole accretion, with a general relativistic treatment

Here are some pictures from the event.

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!

Paper: Optical characterization of WISE selected blazar candidates

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:

 

Results

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.

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 detailed discussion can be found here: https://arxiv.org/abs/1908.05229

This work was supported by FAPESP (Fundação de Ampara à Pesquisa do Estado de São Paulo) under grants 2016/25484-9, 2018/24801-6 and 2017/01461-2; and many other institutions.

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Paper: The spin and magnetic flux of M87*

My paper estimating the spin and magnetic flux of the central supermassive black hole of the galaxy M87—the so-called M87* object—was just accepted for publication in the Astrophysical Journal Letters (ApJL). The black hole in M87* is the first in history to have its event horizon imaged with the very large baseline interferometric apparatus of the Event Horizon Telescope (EHT).

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:

  1. 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.
  2. 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:

  1. the Faraday rotation is caused by the hot accretion flow in front of a source of synchrotron emission
  2. the accretion flow is roughly spherical and characterized by a power-law radial density profile
  3. 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.

New paper: Winds and feedback from supermassive black holes fed at low rates

We have a new paper out now on arXiv and submitted to the MNRAS: Winds and feedback from supermassive black holes accreting at low rates: Hydrodynamical treatment. This paper is the outcome of my MSc dissertation.

winds-arxiv

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.

For more details, please read our paper.

This work was supported in part by FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo) under grant 2017/01461-2 and grant 2016/24857-6.fapesp_logo-lowres

Imagem do buraco negro: repercussão na imprensa brasileira

Esta Quarta-feira, 10 de Abril de 2019, foi um dia que ficará para a história, como o dia em que conseguimos o impossível: a humanidade conseguiu fazer a primeira fotografia de um buraco negro.

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!).

Gustavo no AstroTubers
Rodrigo na Globo News 1/2 (ao vivo, 19:45)

Link para matérias na Globo News (Rodrigo aparece nos 5:00 do vídeo) e aqui.

Link para vídeo no Jornal da Band

E tem mais: fomos procurados também pela Veja, Isto É, Revista Pesquisa FAPESP, Estadão, Jornal da USP e Fantástico.

Matéria de capa no Estadão com entrevista de Rodrigo.

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.