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Transient Relativistic eXplosions

Periodic Reporting for period 4 - TReX (Transient Relativistic eXplosions)

Reporting period: 2021-04-01 to 2022-09-30

Relativistic transients are among the most interesting phenomena observed in our Universe. These transients include events like tidal disruption of a star by a super massive black hole that resides in a galactic center, a long gamma-ray burst that arises when a massive star collapses and forms a black hole, a merger of two black holes that emits copious gravitational and a merger of two neutron stars. The last two phenomena were at the focus of attention in recent years as those are the sources of gravitational waves that have been detected for the first time by the LIGO and Virgo gravitational waves observatories. The issues addressed under this project involve a wide range of seemingly unrelated relativistic transients. They arise in different situations - as clear from the brief description above, however they have a lot in common. Our goal is to understand what happens in the event, interpret observations and predict new ones. A basic theme of this project is to apply methods and ideas developed to one object to another.

The events we explore here take place is distances of millions or even billions of light years away from Earth. This is lucky, had they been nearer their radiation could have damaged life on Earth. However addressing these question is important for society as they involve new regimes of physics that cannot be explored in any other way. They involve gravitational fields that are hundred of billions time stronger than the gravitational field of Earth, they involve velocities that approach 0.999999 of the speed of light and particles of incomprehensible energies, both much larger than what we can imagine ever reaching on Earth. They involve densities that are trillion times larger than densities that we can explore and magnetic field that are billion time stronger than what can be achieved. It is only by studying these phenomena that we can learn how the laws of physics behave in the extreme. Two important examples that have been studies are gravitational waves and the origin of Gold. As wildly broadcasted to the world gravitational waves were discovered in the fall of 2015, some 100 years after Einstein's prediction of their existence. As we anticipated gravitational waves were measured from these phenomena that we explore here (black hole mergers and neutron star mergers). A second example involves the origin of heavy elements like Gold. Light elements like Oxygen, Carbon Silicon and others, up to Iron are produces in stars like our Sun. However until recently it was wildly debated how heavier elements like Gold Uranium and Plutonium are produced. We have suggested long time ago and have shown within this ERC project that those elements are produced in binary neutron star mergers that we explore here.

These study explore new regime in the Universe whose understanding may pave the way to a new way we look on our world.

The overall research objectives is to understand physics and astrophysics under the most extreme conditions and to learn from then about the basic laws of physics in regimes that cannot be explored otherwise.
The work performed is divided to four work packages, each one dealing with a different phenomena. However there are strong links between the different parts.

With the discovery of gravitational waves from a binary neutron star merger in August 2017 we clearly focused on the first packages that deals with the electromagnetic counterpart that will accompany this phenomena. In fact already before August 2017 we have predicted some of the features that were observed. Once the event was detected we participated in some of the most important observations (in particular optical and radio observation) and in their interpretations and we have outline the basic picture of how the event looked. We have also worked on the interpretation of the observations to the question of whether or not neutron star mergers are the sites of production of heavy elements, the so called r-process elements, such as Gold, Uranium Plutonium and so on.

Tidal disruption events (TDEs), in which a star that wanders into the vicinity of a super massive black hole is torn apart by the black hole pose an interesting enigma. The standard model, accepted for thirty years or so predicts a luminosity that is much larger than what is observed. We consider this as an "inverse energy crisis'' and we have proposed an innovative solution to this puzzle, called elliptical or eccentric accretion (in which we have an elliptical accretion disk instead of the regular circular accretion models). After suggesting this model we have begun exploring its properties, first using linearized stability analysis and then using detailed numerical simulations. Additionally, at times super massive black hole are at the core of active galactic nuclei (AGN). These black hole operate as powerhouses that are powered by accretion. If a star is disrupted star within such a system its debris will encounter the disk producing a very different observational phenomena. Somewhat surprisingly it was never explored how a TDE will look like in such a case. We have explored for the first time a tidal disruption event that takes place around an AGN.

In a third part of the project we explored gamma-ray bursts (GRBs) these are the brightest cosmic explosions involving ultra relativistic jets that produce very high energy radiation. We have explored the physical conditions within the emission regions of GRBs and in particular we have shown that recent observations of sub-TeV radiation from GRB 190114C reveal the detailed conditions within the emission regions of GRBs and support our "pair balance" accretion model that we proposed in our earlier ERC grant. A significant part of this part involved exploration of the cocoon that arises when the jet propagates. The observations of the electromagnetic counterpart of the neutron star merger, mentioned earlier, revealed that cocoon emission was important also there, demonstrating nicely the interconnection between the seemingly unrelated different parts of this project. This part of the research culminated in early 2021 when a new window was opened towards GRBs with the discovery of TeV emission from GRB 202119c. We were the first to interpret the implications of this emission. Later on our detailed analysis demonstrated that these observations are fully consistent with the predictions of the predictions of our "detailed balance" model for particle acceleration in collisionless shocks.
The results are clearly beyond the state of the art. We have opened new venues concerning all aspects of our research - new aspects of the electromagnetic cocoon emission, new models for tidal disruption events and new phenomena that were never explored before, like a TDE in an AGN.

In all cases the answers found have revealed that our approaches are promising but at the same time we these have opened new questions that we plan to explore until the end of this project.