Astrophysicists are redrawing a textbook image of pulsars, the dense, rotating remnants of exploded stars, thanks to the Neutron Inner Composition Investigator (NICER) and an X-ray telescope aboard the International Space Station. Using more accurate data, scientists obtained the first accurate and reliable measurements of both the size of the pulsar and its mass, as well as the first ever map of hotspots on its surface.
The pulsar in question, J0030 + 0451 (abbreviated as J0030), is located in an isolated region of space 1100 light-years away in the constellation Pisces. By measuring the weight and proportions of the pulsar, NICER found that the shapes and locations of "hotspots" on the pulsar's surface look much stranger than they previously thought.
"From his workplace on the space station, NICER is revolutionizing our understanding of pulsars," said Paul Hertz, director of astrophysics at NASA headquarters in Washington DC. "Pulsars were discovered more than 50 years ago as beacons of stars that coiled into dense cores, behaving differently from everything we see on Earth. With NICER, we can explore the nature of these dense remnants in ways that until now seemed impossible." …
A series of articles analyzing NICER's observations of J0030 have been published in The Astrophysical Journal Letters and are now available online.
When a massive star dies, it runs out of fuel, collapses under its own weight and explodes like a supernova. These stellar deaths could lead to a neutron star that has a mass greater than the mass of our Sun and a diameter of about 21 km. Pulsars, which belong to the class of neutron stars, rotate up to hundreds of times per second and send beams of energy towards us with each rotation. J0030 rotates 205 times per second.
For decades, scientists have been trying to figure out exactly how pulsars work. In the simplest model, the pulsar has a powerful magnetic field, very similar to a household bar magnet. The field is so strong that it pulls particles away from the pulsar's surface and accelerates them. Some particles follow the magnetic field and hit the opposite side, heating the surface and creating hot spots at the magnetic poles. The entire pulsar glows faintly in X-rays, but the hotspots are getting brighter. As the object rotates, these spots appear and disappear like rays, creating extremely regular changes in the brightness of the object's X-rays. But new research from NICER J0030 shows that pulsars are not as simple as they seem at first glance.
Using NICER observations from July 2017 to December 2018, two teams of scientists mapped the hotspots of J0030, using independent methods, and converged on similar results about its mass and size. A team led by Thomas Riley, a doctoral student in computational astrophysics and his supervisor Anna Watts, professor of astrophysics at the University of Amsterdam, determined that the pulsar is 1.3 times the mass of the Sun and 25.4 km wide. Cole Miller, professor of astronomy at the University of Maryland (UMD) who led the second team, found that J0030 is about 1.4 times the mass of the Sun and slightly more, about 26 kilometers wide.
“When we first started working with J0030, our understanding of how to model a pulsar was incomplete and still exists,” Riley said. "But thanks to NICER's detailed data, open source tools, high-performance computers and a great team, we now have a structure to develop more realistic models of these objects."
The pulsar is so dense that its gravity distorts nearby space-time - the "fabric" of the universe described by Einstein's theory of general relativity - in much the same way that a bowling ball on a trampoline stretches the surface. Space-time is so distorted that the light from the pulsar facing away from us is "bent" and redirected to us. This makes the star look bigger than it is. The effect also means that hotspots may never completely disappear as they rotate towards the far side of the star. NICER measures the arrival of each X-ray burst from a pulsar to the nearest hundred nanoseconds, with an accuracy of about 20 times that previously available, so scientists can use this effect for the first time.
“NICER's unprecedented X-ray measurements have enabled us to make the most accurate and reliable pulsar size calculations to date with an uncertainty of less than 10%,” Miller said. "The entire NICER team has made important contributions to fundamental physics that cannot be explored in ground-based laboratories."