(ORDO NEWS) — What do Albert Einstein, the Global Positioning System (GPS), and a pair of stars 29,000 light-years from Earth have in common?
The answer lies in the effect of Einstein’s General Theory of Relativity, called “gravitational redshift”, in which light shifts to redder colors due to gravity. Using the Chandra X-ray Observatory, astronomers have discovered the phenomenon in two stars orbiting each other in our galaxy about 29,000 light years (200,000 trillion miles) from Earth. Although these stars are very distant, gravitational redshift has a tangible impact on modern life, as scientists and engineers must take them into account in order to provide an accurate position for GPS.
Although scientists have found compelling evidence of gravitational redshifts in the solar system, it was difficult to observe them in more distant objects in space. Chandra’s new findings are convincing evidence of the effects of gravitational redshift in a new space environment.
The intriguing system 4U 1916-053 contains two stars in remarkably close orbits. One of them is the core of a star, without outer layers, as a result of which a star remains, which is much denser than the Sun. Another is a neutron star, an even denser object created when a massive star collapses in a supernova explosion. A neutron star (gray) is shown in the artist’s print in the center of a disk of hot gas blowing away from its companion (white star on the left).
These two compact stars are only 215,000 miles apart, roughly equal to the distance between the Earth and the Moon. While the Moon orbits our planet once a month, the dense companion star in 4U 1916-053 orbits a neutron star and orbits a neutron star in just 50 minutes.
In a new work on 4U 1916-053, the team analyzed X-ray spectra, that is, the number of X-rays at different wavelengths, with Chandra. They found a characteristic sign of absorption of X-ray light by iron and silicon in the spectra. In three separate observations with Chandra, the data show a sharp drop in the detected amount of X-rays, close to the wavelengths at which iron or silicon atoms are expected to absorb X-rays. One of the absorption spectra of iron (dips on the left and right) is included in the main graph.
The supplementary graph shows the absorption spectrum of silicon. In both spectra, the data are shown in gray and the computer model is shown in red.
The wavelengths of these signatures of iron and silicon have been shifted towards longer or redder wavelengths compared to laboratory values found on Earth (shown by the blue vertical line for each absorption signature). The researchers found that the shift in absorption characteristics was the same in each of Chandra’s three observations, and that it was too large to be explained by movement away from us. Instead, they concluded that it was caused by gravitational redshift.
How does this relate to General Relativity and GPS? According to Einstein’s theory, a clock, under the influence of gravity, runs slower than a clock observed from a distant region experiencing weaker gravity. This means that clocks on Earth seen from orbiting satellites are running slower. To ensure the high accuracy required for GPS, this effect must be taken into account, otherwise there will be small differences in time that quickly accumulate, calculating inaccurate positions.
All types of light, including X-rays, are also affected by gravity. An analogy can be drawn with a person who climbs a descending escalator. In this case, the person loses more energy than if the escalator was stationary or rising up. Gravity has a similar effect on light, with the loss of energy giving a lower frequency. Since light always travels at the same speed in a vacuum, the loss of energy and lower frequency means that light, including the features of iron and silicon, is shifted towards longer wavelengths.
This is the first conclusive evidence that the absorption signatures are shifting towards longer wavelengths due to gravity in a pair of stars that have either a neutron star or a black hole. Strong evidence of gravitational absorption redshifts has previously been observed from the surface of white dwarfs, with wavelength shifts usually only about 15% of those for 4U 1916-053.
The gaseous atmosphere surrounding the disk next to the neutron star (shown in blue) absorbed X-rays, the scientists said, giving these results. (This atmosphere is not related to the bulge of red gas in the outer part of the disk, which blocks light from the inner part of the disk once per orbit.) The size of the shift in the spectra allowed the team to calculate how far this atmosphere is from the neutron star using General Relativity and assuming the standard mass of a neutron star.
They found that the atmosphere is 1500 miles from the neutron star, equivalent to only 0.7% of the distance from the neutron star to the satellite. It probably extends several hundred miles from the neutron star.
In two of the three spectra, there is also evidence that the absorption signatures have been shifted towards even redder wavelengths, corresponding to a distance of only 0.04% of the distance from the neutron star to the satellite. But these signatures are less likely to be detected than signatures farther from the neutron star.
Scientists were given additional time to observe Chandra the following year to study the system in more detail.
A paper describing these results was published in the August 10, 2020 issue of the Astrophysical Journal.
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