How the Most Energetic Explosions in the Universe Are Formed

Simone Dichiara

We welcome Simone Dichiara as a guest blogger. Simone is the first author of a paper that is the subject of our latest press release. He is an Assistant Research Professor at Penn State University and currently works at the Mission Operations Center of the Neil Gehrels Swift Observatory, where he serves as an Observational Duty Scientist and as Instrument Scientist for the X-Ray Telescope (XRT) onboard. Since the early years of his master’s degree, Simone has focused his research on a particular class of astrophysical transient sources known as Gamma-Ray Bursts (GRBs), which are among the most energetic explosions in the Universe. His research continued during his Ph.D. at the University of Ferrara, under the supervision of Cristiano Guidorzi and Lorenzo Amati, and throughout his postdoctoral appointments at the National Autonomous University of Mexico and at NASA Goddard Space Flight Center. He has extensive experience in observational time-domain astronomy and has been directly involved in observational campaigns using several ground- and space-based observatories operating across a broad range of wavelengths, from the optical band to very high-energy gamma rays. He has served as Principal Investigator (PI) or Co-Investigator (Co-I) on multiple approved observing programs with the Chandra X-ray Observatory.

Neutron stars are among the most extreme objects known in astrophysics. These compact stars are formed in the explosive deaths of massive stars and compress down to become between 1.4 and 2 times the mass of the Sun into a sphere only tens of kilometers across (roughly the size of a city). Their density is so high that a single teaspoon of neutron star material would weigh billions of tons.

When two of these cosmic monsters orbit each other and eventually collide, they produce extremely powerful bursts of radiation called "gamma-ray bursts," (GRBs) which are among the most luminous electromagnetic events in the Universe. These brief flashes of extremely energetic gamma-ray radiation can momentarily outshine all other sources in the sky detected by gamma ray space-based observatories. Although they last from only a fraction of a second to a few minutes, during that time they can release as much energy as the Sun will emit over its entire lifetime.

Localization of a neutron star collision

Several instruments onboard different satellites were used to pinpoint the sky position of this powerful explosion first seen on September 6, 2023. (After this, it was given the name GRB 230906A). Determining the direction from which such energetic signals originate could be a complex task, especially when the initial detection constrains the source location to a relatively large region of the sky. By combining and triangulating the information received from multiple detectors, scientists were able to significantly reduce the uncertainty in the source position. This progressive refinement of the localization allowed ground- and space-based telescopes to rapidly repoint and begin detailed follow-up observations.

A crucial role was played by the Neil Gehrels Swift Observatory, which enabled the identification of the X-ray counterpart of the event. The most precise localization of the source, however, was achieved by the Chandra X-ray Observatory, which, thanks to its exceptional imaging capabilities, made it possible to study the cosmic environment in which the collision occurred and to securely identify the host galaxy associated with the event.

An unusual host galaxy

The precise Chandra localization achieved through our program, enabled follow-up observation with other facilities, including NASA’s Hubble Space Telescope (HST) and the ESO's Very Large Telescope (VLT) that played a crucial role for the identification of the galaxy in which this explosion occurred.

GRB 230906A: a collision between two neutron stars.
Credit: X-ray: NASA/CXC/Penn State Univ./S. Dichiara; IR: NASA/ESA/STScI; Illustration: ERC BHianca 2026 / Fortuna and Dichiara, CC BY-NC-SA 4.0;
Image Processing: NASA/CXC/SAO/P. Edmonds

Optical and near infrared observations revealed that this cosmic encounter occurred within an extremely peculiar galaxy. Unlike most galaxies known to host short GRBs, this one appears exceptionally faint. Moreover, HST and VLT observations revealed that the GRB lies within a complex system of interacting galaxies. The surrounding galaxies show clear signs of past tidal encounters, and the GRB position coincides with a "tidal tail" (an extended stream of stars and gas pulled out during these interactions).

Go with the tide: a cosmic collision forms within the tidal streams of material produced by galactic interaction

All the observational evidence points to a remarkable scenario: this binary neutron star collision likely occurred within a tiny dwarf galaxy (thousands of times less massive and smaller than the Milky Way) that itself formed out of tidal debris produced by galaxy interactions. These interactions stretch and tear galaxies apart, pulling out vast streams of gas and stars known as tidal tails. Within these turbulent environments, pockets of gas can collapse and give rise to newly formed, self-gravitating dwarf systems called "tidal dwarf" galaxies. Our data suggest that the neutron star binary originated in one of these small cosmic structures. This would represent an entirely new and previously unobserved formation channel for binary neutron star systems. In such systems, tidal interactions increase the rate of star formation producing a new population of neutron stars that ended up being bound together gravitationally and eventually merge producing these big explosions.

In other words, the same gravitational forces that tear galaxies apart may also create the conditions for some of the Universe’s most spectacular collisions.

We found a gold factory in a hopeless place

Neutron star collisions are thought to be among the primary cosmic factories of elements such as gold and platinum. When two neutron stars collide, they eject neutron-rich matter that rapidly forms atoms heavier than iron (so-called “heavy elements”). Indeed, these collisions are one of the Universe’s main sites of heavy-element production.

If this event occurred within tidal debris in a galaxy group, it carries important implications. Galaxy interactions may not only enhance star formation and produce compact binaries, but they also disperse newly forged heavy elements directly into the gas surrounding a galaxy. (known as the circumgalactic medium). Such events could help explain the high metal abundances observed in some stars in the halo of our Galaxy, showing that even the most chaotic environments can become efficient cosmic forges.

Sometimes, even in the most unlikely places, the cosmos builds its gold.