Neutron Stars by Katia Moskvitch

Author:Katia Moskvitch [Moskvitch, Katia]
Language: eng
Format: azw3
ISBN: 9780674919358
Publisher: Harvard University Press
Published: 2020-09-15T00:00:00+00:00

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IT ALL STARTED IN 1974, the same year that Anthony Hewish was awarded a Nobel Prize in Physics for the discovery of pulsars. Astronomers Russell Alan Hulse and Joseph Hooton Taylor were in the Puerto Rican rainforest, in a tiny control room next to the 305-meter (thousand-foot) diameter dish in the ground. They registered a pulse, a new neutron star rotating seventeen times per second. After observing the pulsar for a while, they noticed a bizarre yet regular variation in the arrival time of its pulses; at times they arrived slightly sooner than expected, at times slightly later. A bit of math and reasoning led the two astronomers to conclude that there was a companion, orbiting with the pulsar around their common center of mass. It was the first-ever discovery of a pulsar in a binary system, where a neutron star wasn’t isolated but had a companion.

Later, they found out that the companion was a neutron star as well—not a white dwarf, which in later years turned out to be the much more frequent scenario. Hulse and Taylor’s observations also led to the very first accurate measurement of a neutron star’s mass. They found that their binary’s neutron stars had around 1.4 times the mass of the Sun—the pulsar at 1.44 solar masses and the companion neutron star at 1.39 solar masses, both eerily close to the Chandrasekhar limit. As mentioned in Chapter 1, the pulsar now bears its discoverers’ name—the Hulse-Taylor pulsar (PSR 1913 + 16).28

So far, astronomers have used pulsar timing to pin down the masses of some thirty-five neutron stars in the range of 1.17 to slightly more than two solar masses. In the Karoo, MeerKAT has started timing pulsars with a project called MeerTIME. The array’s observations of known pulsars are already blowing other telescopes “out of the water,” says astronomer Fernando Camilo of SARAO (we met him in Chapter 4). MeerKAT and Parkes are both in the Southern Hemisphere, and the Karoo array can focus on those pulsars that have been detected at Parkes over the years, but cannot be seen from the Green Bank Telescope, which is in the north. At Parkes, a neutron star’s mass is often calculated with large uncertainties, says Camilo. But MeerKAT, he adds, because of its sensitivity, can make these measurements much better and faster. “Imagine you observe twenty well-known binaries with MeerKat. And within a year or less it detects two or three really well-measured high masses. That could have an enormous impact.”29

But masses alone are not enough. Researchers are also trying to measure the radii of neutron stars very precisely, to constrain—put limits on—their equation of state. Those are trickier to get, though. A team of researchers led by Michael Kramer, a director of the Max Planck Institute for Radio Astronomy in Germany, has been for years trying to use the data from the only known binary with two pulsars, the Double Pulsar, to constrain the moment of inertia of the neutron star—a combination of mass and radius.

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