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.