This graphic depicts the likely discovery of a collision between two neutron stars, made by NASA’s Chandra X-ray Observatory and other telescopes, in a tiny galaxy buried in a huge stream of gas, as described in our latest press release. This is the first time that a neutron star collision has been spotted in such a setting.
Neutron stars are the ultra-dense remnants left behind after massive stars collapse. When neutron stars occasionally collide with one another, they can produce important elements like gold and platinum and generate gravitational waves that ripple across space. This latest discovery may help solve open questions as to how those precious elements are sometimes outside galaxies as well as how some gamma-ray bursts mysteriously do not appear to be associated with a known galaxy.
Two artist’s illustrations — one in the main panel and the other on the bottom left — depict what astronomers think is happening in the event. Known as GRB 230906A, this event was first picked up by NASA’s Fermi Gamma-ray Space Telescope in September 2023. Astronomers then used the Neil Gehrels Swift Observatory to provide a more accurate position followed by observations with Chandra and the Hubble Space Telescope.
The Chandra data, shown in the inset to the upper left of the graphic, gave the researchers an even more accurate position for the GRB, and once Chandra told them exactly where to look, the researchers then used Hubble to reveal a tiny, extremely faint galaxy at that position.
The tiny galaxy that hosted this neutron star collision is located about 4.7 billion light-years away, embedded within a stream of gas that stretches some 600,000 light-years long. (For context, our Milky Way galaxy is about 100,000 light-years across.) This stream was likely created when a group of galaxies collided hundreds of millions of years ago, stripping gas and dust from the galaxies and tossing it into intergalactic space. The artist’s illustration in the main panel shows members of the galaxy group in yellow and orange and tidal streams around the galaxies in blue.
Once these galaxies collided, it likely triggered a wave of star formation that, over hundreds of millions of years, led to the birth and eventual collision of these neutron stars. The artist’s illustration in the inset to the lower left shows a view from the side of what the aftermath of a neutron star collision might look like. The GRB was detected by viewing it down the barrel of the jet.
The unusual location of GRB 230906A may also help explain how astronomers have spotted elements like gold and platinum in stars at relatively large distances from the centers of galaxies. Such stars are generally expected to be older and to have formed from gas that had less time to be enriched in heavy elements from supernova explosions.
Through a chain of nuclear reactions, a collision between two neutron stars can produce heavy elements like gold and platinum, which astronomers witnessed in a much closer collision seen in 2017. Events like GRB 230906A could generate elements like these and spread them throughout the outskirts of galaxies, eventually appearing in future generations of stars.
An alternative identity for the explosion is that it is in a much more distant galaxy that is behind the galaxy group. The team considers this to be a less likely explanation than the tiny galaxy idea.
A paper describing these results has been accepted in The Astrophysical Journal Letters. The authors of the paper are Simone Dichiara (Penn State University), Elena Troja (University of Rome, Italy), Brendan O’Connor (Carnegie Mellon University), Yu-Han Yang (University of Rome), Paz Beniamini (University of Israel), Antonio Galvan-Gamez (National Autonomous University of Mexico), Takanori Sakamoto (Aoyama Gakuin University, Japan), Yuta Kawakubo (Aoyama Gakuin), and Jane Charlton (Penn State).
NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.
This release features two artist's concepts and a composite image depicting two cosmic collisions that began hundreds of millions of years ago.
At the center of the large artist's concept is a brilliant glowing ball with a nearly white core, and golden orange outer layers. This brilliant ball represents the brightest galaxy in a collision between two groups of galaxies, which began hundreds of millions of years ago. Gas and dust from that collision were tossed into intergalactic space in long tidal streams. In the illustration, the tidal streams resemble swooping blue streaks shooting off the brilliant ball. Near the end of each swooping tidal stream is a glowing orange streak, or ellipse. These glowing shapes are smaller individual galaxies, some of which are revealed to have spiraling arms when examined closely.
One of the tidal streams shoots toward our upper left, then begins to hook back down, passing two glowing orange galaxies along its path. Near the end of this tidal stream is a tiny galaxy and an X-ray source presented in the middle of a close-up insert. In the center of the composite insert, Hubble observations in orange reveal the tiny, faint galaxy buried in the tidal stream. A pool of neon blue haze shows X-rays detected by Chandra from the collision of two ultra-dense neutron stars.
Astronomers believe that the tiny galaxy was born from gas and dust along the 600,000 light-year-long tidal stream, created by the initial collision of the galaxy groups. Over hundreds of millions of years, that material contributed to the birth of many stars within the tiny galaxy. Two of those stars collapsed into neutron stars, and ultimately collided, producing important elements like gold and platinum, and gravitational waves that rippled across space.
The artist's concept in the other insert shows a close-up view from the side of what the aftermath of a neutron star collision might look like. A burst of gamma rays was originally detected by viewing it down the barrel of the jet, which triggered follow-up X-ray observations with Chandra and other X-ray telescopes.
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