In August 2017, astronomers observed a gravitational wave (GW) signal resulting from the merger of two neutron stars – known as the “kilonova” event. The consequences of this event (GW170817) were studied by 70 ground and space observatories in several wavelengths. This was the first time astronomers had observed a binary neutron star merger in terms of electromagnetic radiation (specifically gamma rays) and GW. The energy released by this merger was comparable to that of a supernova, leading astronomers to theorize that it must have resulted in a black hole.
Two years later, the The Hubble Space Telescope observed the remnant and noted the powerful afterglow and gamma-ray bursts (GRBs) created by the merger, which was consistent with a black hole. However, it would take several more years of analysis before scientists could paint a full picture of what resulted from this explosive event. Use of data from Hubble and several radio observatories, a team of researchers detected a rapidly rotating disc of matter around the black hole and a structured relativistic jet emanating from it.
The team consisted of Kunal P. Mooley, Jay Anderson, and Wenbin Lu, specialists in the emerging field of time domain and multi-messenger astrophysics (TD-MMA). Mooley is an astrophysicist at the National Radio Astronomy Observatory (NRAO) and Caltech, while Anderson is an AURA Observatory Scientist at the Space Telescope Science Institute (STScI). Lu is an assistant professor at UC Berkeley and a Spitzer Fellow at Princeton University. The article describing their findings was recently published in the October 13 edition of Nature.
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The TD-MMA process involves the use of multiple “messengers” (such as certain wavelengths of light and GWs) to study the Universe as it evolves through time. To this end, Mooley, Anderson and Lu relied on optical data from Hubble and radio observations from multiple National Science Foundation (NSF) telescopes taken 75 and 230 days after the explosion. Finally, they consulted the astrometric data of the ESA Gaia satellite, which produced highly constrained estimates of the black hole’s position and proper motion.
This allowed them to combine radio measurements of very long baseline interferometry (VLBI) with the light curve of the afterglow and measurements of its relative motion since it was first detected. The combined data allowed them to locate the site of the explosion and the debris created by the explosion was sucked into the black hole. This material was swept into a rapidly rotating disk which caused superbright jets to emanate from its poles which crashed down and swept the material from the expanding debris cloud.
“I am amazed that Hubble could give us such a precise measurement, which rivals the precision achieved by the powerful VLBI radio telescopes distributed around the world,” Mooley said. While the Hubble data indicated the hot jet had an apparent speed of seven times the speed of light, radio observations showed the jet later decelerated to an apparent speed of four times the speed of light. Consistent with “superluminous” motion, this was an illusion caused by the fact that when a jet approaches Earth at nearly the speed of light, light emitted later has a shorter distance to travel. .
In short, a relativistic jet is chasing its own light, and more time has elapsed between the emission of light from the jet than the observer perceives. This causes the speed of the object to appear to be moving faster than the speed of light, which is physically impossible. Correcting this, Mooley and his colleagues obtained accurate estimates of the jet’s speed that showed it was moving at near the speed of light. “Our result indicates that the jet was traveling at least 99.97% of the speed of light when launched,” Lu said.
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These findings greatly strengthen the long-presumed link between neutron star mergers and short-lived GRBs, which require the emergence of a fast jet. They also pave the way for future precision studies of neutron star mergers and the resulting superluminous jets. These studies could help solve one of the biggest mysteries in astrophysics today: the rate at which the Universe is expanding (i.e. the Hubble constant).
Basically, there is a discrepancy between the estimated values obtained from the measurements of the local Universe and those obtained from the primordial Universe. The first value is based on extremely accurate observations from Hubble and other Type Ia supernova observatories, which is a very accurate method of keeping distance. The latter value is based on Planck satellite measurements of the cosmic microwave background (CMB) – the relic radiation left over from the Big Bang.
With more relativistic jets available for study using a combination of gravitational waves, VLBI radio data and theoretical modeling, astronomers could turn neutron star mergers into a new method of measuring cosmic expansion. . These methods will be particularly useful for next-generation telescopes like the Nancy Grace Roman Space Telescope (RST), Hubblethe most direct successor to, slated for launch by 2026.
Further reading: NASA
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