In August 2017, astronomers were treated to one of the most spectacular stellar light shows ever seen – a collision between two neutron stars. The smashup was so powerful it sent gravitational ripples through the very fabric of spacetime, and produced flares in visible light, radio waves, x-rays and a gamma ray burst. Now that things have quietened down, astronomers have studied the strange object created in the cosmic collision.
The LIGO facility was the first to notice something big was happening. On August 17 last year, the instrument detected gravitational waves coming from a source now officially known as GW170817, which lies about 138 million light-years away. Gravitational waves alone are old news, but there was something different about this one – it wasn't caused by invisible black holes merging, but the very-visible crash of two neutron stars.
About 70 observatories around the world quickly trained their sights on the location, and weren't disappointed. Across the various instruments, signals were detected in visible light, radio waves, x-rays and a short gamma ray burst. The fireworks were expected to be short-lived, but to make things even weirder, the afterglow actually seemed to get brighter over the next few months.
The big question is, what kind of object was created in the collision? The two leading theories were that the neutron stars would merge to form either a black hole or a denser neutron star. Whatever it is, it has a mass of about 2.7 times that of the Sun, according to LIGO data.
That figure just raises further questions. If it's a neutron star, it's the most massive one ever detected, but if it's a black hole, it has almost half the mass of the previous smallest known black hole.
To find out either way, the new study has analyzed data gathered by NASA's Chandra X-ray Observatory in the days, weeks and months after the event. Chandra detected no x-ray signals coming from the object two or three days after the explosion, but observations made nine, 15 and 16 days afterwards all picked up signals. Unfortunately, soon after that the Sun passed between Earth and the object, halting attempts to track it.
When the sky was finally clear again, Chandra made more observations about 110 days after the collision, when it detected brighter signals, and 50 days after that the x-rays became more intense.
The team compared these observations to some made by the Very Large Array, and were able to explain the x-ray emissions as being the result of the shock wave of the explosion smashing into the surrounding gas. The signal suggests the object isn't a dense neutron star, but more likely a black hole.
"We may have answered one of the most basic questions about this dazzling event: what did it make?" says Pawan Kumar, co-author of the study. "Astronomers have long suspected that neutron star mergers would form a black hole and produce bursts of radiation, but we lacked a strong case for it until now."
Still, they can't be completely sure just yet, and future observations should confirm the object's identity either way. If it's a black hole, the signal should gradually fade away as the shock wave weakens. However, if the source gets brighter at x-ray and radio wavelengths over the next few years, it could turn out to be a neutron star after all.
"GW170817 is the astronomical event that keeps on giving," says J. Craig Wheeler, a co-author on the study. "We are learning so much about the astrophysics of the densest known objects from this one event. If follow-up observations find that a heavy neutron star has survived, such a discovery would challenge theories for the structure of neutron stars and how massive they can get."
Te study was published in The Astrophysical Journal Letters.
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