When something explodes, you expect a bright flash to disappear.
This is what astrophysicists thought would happen when they observed the collision between two neutron stars last August, but, contrary to expectations, it continues to illuminate. months after the event, leaving the scientists stunned.
According to data from NASA's Chandra X-ray observatory, the consequences of that collision are much more complex and interesting than anyone would have expected.
Never before have we directly observed a collision between two neutron stars. It was only thanks to the new wave field of detection of gravitational wave astronomy in the fabric of space-time that astronomers from all over the world could point their instruments at the event later called GW170817 on August 17 last year.
The spectacular fusion occurred at 138 million light years from the Solar System.
And we learned a lot – for example, collisions between neutron stars actually produce gamma-ray bursts, some of the brightest and most energetic events in the universe. This confirmed a long-held hypothesis about the origin of these glasses.
The gamma ray burst was called GRB170817A, and it was expected to fade relatively quickly, but that was not what happened.
Two days after the collision, no optical source was visible, so far, so normal. But nine days after the collision, Chandra's data revealed a new source of X-rays at the site of the explosion.
"Generally when we see a short burst of gamma rays, the generated jet emission becomes bright for a short time and vanishes when the system stops injecting energy into the output stream," he explained. the astrophysicist Daryl Haggard. from McGill University, whose research group led the new study.
"This is different, definitely not a simple, simple and narrow jet."
After 16 days of the collision, the position of the object in the sky was too close to the Sun for it to sensitive X-ray measurements will be taken for some time. It was not until 109 days passed, in early December 2017, that the astronomers could renew their data collection.
As soon as they could, the research team took X-ray readings of GRB170817A and those December measurements were brighter than those taken in early September. These were consistent with the radio data, which also showed an increase in brightness at the same speed.
This brightness can be explained if the collision was a bit more complicated than initially thought, say the researchers.
A potential cause would be if the collision between the neutron stars creates a jet or an outflow, which in turn will heat by shock the material surrounding the new object created by the collision (possibly a black hole).
This could then shine on the X-ray and radio spectrum for months after the event. The X-ray light curve coincides with the predictions for this hypothesis, although the origin of the output is still uncertain.
Now astronomers are trying to discover what the cause is actually, and the physics behind it. An additional follow-up of Chandra will help validate or disprove the exit model.
"This neutron star fusion is unlike anything we've seen before," said co-author Melania Nynka, also a postdoctoral researcher at McGill.
"For astrophysicists, it is a gift that seems to continue to give."
GW170817 may have left a mark in history, but it is far from over. It will continue to be one of the most studied objects in the sky for some time to come.
The team's research has been published in Astrophysical Journal Letters .