Gravitational Wave Signal Points to Impossible Neutron Stars While Telescope Captures Supernova-Like Explosion
On August 18, 2025, at precisely 01:20:06 UTC, gravitational wave detectors picked up ripples in spacetime from a cosmic collision unlike anything scientists had seen before. The signal, designated S250818k, suggested something that shouldn’t exist according to current physics: neutron stars weighing less than our Sun merging together 1.3 billion light-years away.
Within hours, the Zwicky Transient Facility telescope in California began scanning the region of sky where the gravitational waves came from. What they found sparked intense debate: a brilliant explosion named ZTF 25abjmnps appeared exactly where and when the gravitational wave signal predicted. Either this is an extraordinary coincidence, or researchers witnessed something entirely new.
Neutron stars are the crushed cores left behind when massive stars explode. They’re so dense that a piece the size of a sugar cube would weigh more than Mount Everest. These dead stars typically weigh between 1.2 and 2.5 times the mass of our Sun. Physics says they can’t be lighter than 1.2 solar masses because that’s how much the iron core of a dying star weighs before it collapses.
The gravitational wave signal told a different story. Analysis showed a mass of just 0.87 solar masses, with 99% confidence that at least one object weighed less than the Sun. Modern computer simulations have never produced neutron stars this light. The physics simply doesn’t allow it under normal stellar evolution.
The Zwicky Transient Facility started observing just 2.7 hours after the gravitational wave detection. Over 36 hours, telescopes scanned 370 square degrees of sky, sorting through 30,061 alerts. Most were quickly eliminated as known stars, distant galaxies, or telescope glitches. Only ZTF 25abjmnps remained interesting.
The object appeared as a new point of light in a galaxy 399 million light-years away. Its position fell right in the center of where the gravitational wave probably came from, and the distance matched perfectly. First observations showed a blue, featureless glow that started fading and turning red, exactly like the famous kilonova AT2017gfo from 2017 when two neutron stars merged and created gold, platinum, and other heavy elements.
Telescopes around the world jumped into action. Keck Observatory in Hawaii, Gemini North, Liverpool Telescope, Canada-France-Hawaii Telescope, and facilities in India, Germany, and Spain all turned toward this tiny point of light. They collected spectroscopy, multi-wavelength photographs, infrared data, and searched for radio and X-ray signals.
The first spectra showed smooth light without distinct features, exactly what scientists expected from a kilonova in its early days. The blue color meant hot material expanding at tremendous speeds. The brightness dropped fast, consistent with radioactive decay of heavy elements created in a neutron star merger. Computer models fit the early data perfectly, suggesting the merger threw out material at 20% the speed of light, plus more from an accretion disk moving at 3% light speed.
For several days, everything pointed to a kilonova. This could be only the second kilonova ever detected through both gravitational waves and light, and the first linked to impossibly light neutron stars. The location was nearly perfect, appearing almost exactly where the gravitational wave map predicted. The timing matched within hours.
Then everything changed. Seven days after discovery, Keck Observatory spotted something unexpected in the spectrum. A feature appeared showing a P Cygni profile, an absorption dip with an emission peak. This signature comes from exploding material in supernovae, not kilonovae. The feature got stronger over the next days. By day nine and ten, it was unmistakable. If this was hydrogen, the material was moving at 17,000 kilometers per second, more than 500 times faster than Earth orbits the Sun.
The absorption feature had a weird W-shape instead of the normal U-shape seen in most supernovae, suggesting either two overlapping lines or complicated geometry in the explosion. After fading for five days, the light curve stopped dropping and started brightening again. By day 13, it had risen nearly back to its original brightness. This double peak is the hallmark of Type IIb supernovae, where the first peak comes from shock heating and the second from radioactive nickel decay.
By day 16, spectra showed helium features. The day 31 spectrum revealed hydrogen, helium, and calcium all moving between 10,000 and 20,000 kilometers per second. The hydrogen presence but weak helium pointed to Type IIb classification. When researchers compared the spectra to libraries of known supernovae, the best matches came from Type IIb events. But the hydrogen line seemed weaker than expected, and some helium features appeared delayed. ZTF 25abjmnps wasn’t quite a textbook Type IIb supernova.
The colors also acted strange. Between peak brightness and the second rise, ZTF 25abjmnps appeared almost one magnitude redder than other Type IIb supernovae at the same stage. This represented a temperature difference of several thousand degrees. Even maximum possible dust from its host galaxy couldn’t explain it fully.
Three explanations emerged. First, the object sits much farther away than most studied Type IIb supernovae, potentially causing systematic effects as light gets stretched. Second, the dust estimate might be wrong. Third, and most interesting, the weird colors might come from heavy elements created by neutron star merger mixing into the supernova debris. Even tiny amounts of these elements drastically change how light passes through expanding gas.
Computer simulations helped determine explosion parameters. The best model required energy of 1.6 foe (a standard unit in astrophysics), higher than typical Type IIb events. The explosion threw out 3.0 solar masses of material and created 0.1 solar masses of radioactive nickel. The progenitor star had a compact envelope just 140 solar radii across with only 0.06 solar masses, much smaller than other Type IIb progenitors. This small envelope explained why the first peak was so brief.
These numbers pointed to a binary star system where one star stripped the other’s outer layers before the explosion. Isolated massive stars don’t lose enough mass to create such compact progenitors. If ZTF 25abjmnps was really a neutron star merger, radio and X-ray telescopes should have detected radiation from jets. They found nothing. The Very Large Array and Australian Telescope Compact Array saw no radio emission. Swift satellite X-ray observations came back empty. Chandra X-ray Observatory also detected nothing.
These non-detections created problems for the kilonova idea. Models that fit the visible light predicted radio and X-ray brightness way above the actual limits. Even assuming very dense material around the merger couldn’t make the numbers work. The clear hydrogen features in spectra provided more evidence against kilonovae. AT2017gfo and theoretical models produce relatively smooth spectra, never the distinct lines seen in ZTF 25abjmnps.
So the evidence favored calling this a Type IIb supernova. But the gravitational wave coincidence remained compelling. Could both be true? A speculative model called a superkilonova might reconcile the contradiction.
Here’s how it might work. When a massive star’s core collapses, it normally forms one neutron star or black hole weighing at least 1.2 solar masses. But if the core spins really fast, material might form a disk instead of falling straight in. If this disk gets massive enough, it can become unstable, just like protoplanetary disks that fragment to form planets. As the disk cools, pieces can collapse under their own gravity. In neutron-rich environments, the mass limit drops, potentially creating neutron stars lighter than the Sun.
Recent computer simulations showed disks can fragment into multiple objects weighing 0.01 to 1 solar mass. These fragments might pair into binary systems and crash together within hours of the original collapse, producing gravitational waves. Alternatively, the collapsing core itself might split into two neutron stars instead of one, which then rapidly merge.
Either way, you’d get gravitational waves from low-mass neutron star merger happening at nearly the same time as a core-collapse supernova. Most visible light would come from the supernova itself: shock heating followed by nickel decay. The merger would add heavy elements that might affect colors. Any jet from the merger might not break through the supernova debris, explaining the lack of gamma-rays and X-rays.
This model explained several puzzles. The Type IIb classification meant a stripped binary progenitor, exactly the high angular momentum system needed. The unusual colors might reflect merger elements mixed into supernova gas. The star-forming host galaxy supported recent massive star birth. The location and timing matched the gravitational wave perfectly.
But could this just be coincidence? Researchers calculated the odds of a random Type IIb supernova exploding in the search volume and time window. Using a Type IIb rate of 12% of all Type II supernovae, and searching 2.3 million cubic megaparsecs over one day, the chance came to roughly 3 to 5 percent. Not impossible, but not trivial either.
The team checked six other candidate supernovae in the search region. Two were Type Ia supernovae from white dwarf explosions, unrelated to neutron stars. One was Type II with a thick hydrogen envelope, inconsistent with the model. Another was indeed Type IIb, but its light curve showed it exploded 2.27 days after the gravitational wave, ruling it out since the supernova must come first. Two others were either too far away or too old.
The case remains unresolved. Evidence favors Type IIb supernova, but the gravitational wave association awaits confirmation. Offline analysis of the gravitational wave data might improve significance and refine mass measurements. Late observations with James Webb Space Telescope could directly measure whether merger elements exist in the debris. Continued radio and X-ray monitoring might eventually detect faint emission, though supernova material could hide it for months.
The superkilonova idea makes testable predictions. If real, future gravitational wave detections with better localization should show similar patterns. The LIGO-Virgo-KAGRA network now detects neutron star mergers weekly instead of yearly. Some might show subsolar masses if this channel exists. Follow-up strategies need improvement. Visible light alone can’t distinguish kilonovae from supernovae. Spectroscopy must start within hours to catch key features. Radio and X-ray observations provide crucial clues about jets. Infrared monitoring can see through dust better than visible light.
Current simulations explore disk fragmentation but haven’t produced full predictions for what we’d actually see. Mixing of merger debris with supernova material remains poorly understood. Whether jets can escape through supernova envelopes needs detailed study. If these light neutron stars form and merge, their properties would probe extreme physics inaccessible any other way. The merger rate would inform models of massive star evolution and angular momentum in collapsing cores.
The next generation of telescopes promises breakthroughs. The Vera Rubin Observatory will survey the visible sky every few nights, reaching much fainter than current surveys. Nancy Grace Roman Space Telescope will provide infrared imaging with incredible sensitivity. Deep Synoptic Array will enable real-time radio follow-up. ULTRASAT and UVEX satellites will monitor ultraviolet emission. Antarctica’s Cryoscope will observe from a unique location. These facilities will detect counterparts at distances where current surveys fail and increase sample sizes from individual detections to populations that reveal the true nature of these events.
For now, ZTF 25abjmnps remains a puzzle at the intersection of supernova physics, neutron star mergers, and gravitational waves. The coincidence with S250818k could be exactly that, a chance alignment of a Type IIb supernova with a false gravitational wave alarm. Or it could be the first glimpse of superkilonovae, where core collapse, neutron star formation, and merger blend into a single event spanning gravitational waves and the entire electromagnetic spectrum. Future observations will reveal which story is correct.
Source:
Research published in The Astrophysical Journal Letters (https://doi.org/10.3847/2041-8213/ae2000).
