Sci-Tech
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Mooley, K.P., Anderson, J. & Lu, W. Optical superluminal motion measurement in the neutron-star merger GW170817. Nature 610, 273–276 (2022). https://doi.org/10.1038/s41586-022-05145-7
In 2017, astrophysicists observed an unusual feat among the stars. The Laser Interferometer Gravitational-Wave (LIGO) observatories recorded a signal which indicated that two massive and dense stellar bodies had merged to form a third body, likely a black hole. In the process they gave off vibrations that quite literally shook the universe and its very fabric of space-time. For the very first time, scientists noted that this observation of the LIGO observatories coincided with the measurements made by other telescopes that measured visual and electromagnetic signals. Was this light given off by the merging bodies? Evidence seemed to suggest that it was. From this, scientists, piecing together evidence from complementary measurements, surmised that the event they had observed was of two neutron stars merging and forming a black hole and, in the process, giving off light. An unusual jet of matter was observed that gave an illusion of travelling faster than light. These were all exciting phenomena observed for the very first time by telescopes and observatories on earth.
Now, using data that had been recorded by the Global Astrometric Interferometer for Astrophysics (GAIA) spacecraft and Hubble Space Telescope instruments, scientists have confirmed that the above picture is correct. They have made it more precise and descriptive.
In a paper published in Nature, they describe measuring the “apparent speed” of the jet to be about seven times the speed of light. They have also measured more accurately a factor called the Lorenz factor which scales with the actual speed of the particles in the jet. Unlike earlier estimates which placed this factor at about 4, the present paper estimates this factor to be over 40. This is because they measure the speed of the relativistic jet to be close to 0.9997c, where “c” is the speed of light. This resolves the earlier fuzziness about what the source was and puts the source clearly as massive neutron stars merging to give a black hole and throwing off relativistic jets of particles in the process.
Neutron stars are stellar corpses, left behind after a star has undergone a supernova explosion and reached the end of its lifetime. They are extremely dense, containing more mass than the sun in a sphere that is a few tens of kilometre wide.
The observation of particles moving at seven times the speed of light is an illusion. “This happens in cases where a source moves (towards us) with a velocity that is very close to light's velocity. This phenomenon is known to astrophysicists earlier,” says Resmi Lekshmi, a scientist with the Department of Earth and Space Sciences, Indian Institute of Space Science and Technology, Thiruvananthapuram, who has worked in this area. This has been seen in many active galactic nuclei — galaxy centres that harbour black holes — and binary star systems within our galaxy, where one of the stars is a black hole. “Mostly, black holes are responsible for producing such fast-moving material,” she explains.
The present measurements and observations made with GAIA data are extremely challenging. They amount to measuring the position of an object in sky co-ordinates. “These authors measured a change in sky position one millionth the span of the full moon,” says Dr. Lekshmi. Normally, if one were making these measurements from earth-based telescopes, it would require data from radio telescopes spaced apart by intercontinental distances. This technique is called Very Long Baseline Interferometry (VLBI) and was used in the earlier papers. “Here, the authors could beat VLBI in precision because they calibrated Hubble Space Telescope data with GAIA, which is a precision astrometry mission,” she says.
However, the researchers used both their Hubble Space Telescope and GAIA optical position measurement along with the earlier VLBI position measurement to get a better estimate of the speed of the source and angle (viewing angle) with which it is travelling with respect to us on earth. Dr. Lekshmi clarifies that this estimate requires plugging in equations of the special theory of relativity. “So, it is an estimate as opposed to a measurement,” she says.
The significance of the paper is that now, we have learnt that neutron star mergers can result in material moving with speeds as high as 0.9997c. Earlier results using Very Long Baseline Interferometry had pegged this value at about 0.938c. And with the new results this lower limit has been improved. Even earlier, with VLBI, it was understood that it was a neutron-star merger that produced such ultra-relativistic material.
Before the VLBI results, there were several models that could replicate the observations. “The observations could be explained both by ultra-relativistic material and non-relativistic material, with some differences in assumptions,” says Dr. Lekshmi. That study indicated that the observed gamma ray bursts were produced along with the ultra-relativistic material. This paper, in turn, strengthens the hypothesis that such neutron star mergers are responsible for a class of gamma-ray bursts. Gamma-ray bursts are flashes of extreme gamma ray photons that release a huge amount of energy — nearly ten-raised-to-47 joules. They come from different galaxies in the universe and are observed here quite frequently.
