Thanks to NASA’s NICER (Neutron Star Interior Composition Explorer) mission, scientists have taken a major step forward in understanding pulsars, the extremely dense remnants of stars that have exploded into supernova. The mission’s X-ray telescope, installed on the International Space Station, provided the first accurate and reliable measurements of a pulsar’s mass and radius, and mapped the surface of these mysterious objects for the first time.
The pulsar in question, J0030+0451 (also known as J0030) is located in an isolated region of our Galaxy, 1100 light years in the direction of the Pisces constellation. While measuring the mass and size of this pulsar, NICER also determined that the size and position of hot spots of about a million degrees on the surface of the pulsar were much stranger than previously thought.
“From its perch on the space station, NICER is revolutionizing our understanding of pulsars,” said Paul Hertz, director of NASA’s Astrophysics Division. “Pulsars, the remnants of collapsed stars, were discovered more than 50 years ago because their light acts as a beacon for an observer on Earth. While pulsars have a unique behavior, we can now probe the nature of these dead stars, something that was impossible before NICER arrived.”
A series of papers presenting analyses of J0030 observations with NICER has just been published in a special issue of The Astrophysical Journal Letters, which is available online.
A massive star dies when it has used up all the fuel in its core. It then collapses under its own weight and explodes into a supernova. The cores of these dead stars can then give birth to a neutron star, in which the mass of our sun is compacted into a sphere about the size of a large city like Toulouse. Pulsars, which are a type of neutron star, spin on themselves up to several hundred times per second and sweep the space with their beam of energy with each rotation, like a lighthouse. Pulsar J0030 makes 205 rotations per second.
For decades, scientists have been trying to understand how pulsars work. According to simple models, a pulsar has a magnetic field with a north and a south pole, like a magnet. For pulsars, the magnetic field is so powerful that it can pull particles from the surface and accelerate them to high speeds. Some of these particles then follow magnetic field lines to the opposite pole and then bombard the star’s surface, heating it. This creates hot spots at the magnetic poles. The entire surface of the pulsar emits faint X-rays, but the hot spots are much brighter. When the object rotates, the X-rays from the hot spots sweep across our line of sight, acting like the beams from a lighthouse. This produces the extremely regular variations in the X-ray brightness of pulsars. However, studies of J0030 with NICER show that reality is not as simple as that.
Using NICER observations from J0030 obtained between July 2017 and December 2018, two teams of scientists, including researchers from the Institut de recherche en astrophysique et planétologie (IRAP, Université Toulouse 3 – Paul Sabatier / CNRS / CNES), and the Laboratoire de physique et chimie de l’environnement et de l’espace (LPC2E), Université d’Orléans / CNRS / CNES), the Nançay Radio Astronomy Station (Université d’Orléans / CNRS / Observatoire de Paris) and the Laboratoire Univers et Théories de Meudon (LUTH, CNRS / Observatoire de Paris), have mapped the hot spots of this pulsar, following two independent methods, and have obtained similar results for its mass and radius.
The first team, led by Thomas Riley, PhD student in computational astrophysics, and his thesis supervisor, Dr. Anna Watts, professor at the University of Amsterdam, determined that this pulsar is about 25.4 km in diameter and has a mass of 1.3 times the mass of the Sun. Dr. Cole Miller, Professor of Astronomy at the University of Maryland (UMD), who leads the second team, found that the pulsar is 26 km in diameter and has a mass of 1.4 times the mass of the Sun.
“When we started working on J0030, our understanding of pulsar emission was incomplete, and it still is,” comments Thomas Riley, “but thanks to the detailed data from NICER, open-source tools, high-performance computers and excellent teamwork, we now have a structured framework for developing more realistic models.
A pulsar is so dense that its intense gravity distorts the space-time around it, as described by Einstein’s theory of general relativity, like a bowling ball on a trampoline for example. Space-time is so distorted that the light coming from the opposite side of the pulsar is deflected enough to reach us. This has the effect of making a pulsar appear larger than it really is. The other effect is that the hot spots never really disappear when the star rotates on itself. In fact, the hot spots remain visible even when they are on the opposite side of the pulsar.
NICER measures the arrival of each X-ray of the pulsar with an accuracy of about 100 nanoseconds, an accuracy 20 times better than was possible with previous instruments. So scientists can now measure the effects described above for the first time.
“NICER’s unique X-ray observations have allowed us to obtain the most accurate and reliable measurements of the pulsar size, with an uncertainty of less than 10%,” explains Cole Miller. “The entire NICER mission team has made important contributions to the understanding of fundamental physics that cannot be probed experimentally in the laboratory.”
From Earth, our view of D0030 is oriented towards the northern hemisphere of the pulsar. By mapping the shape and position of the hot spots on the surface of J0030, the scientific teams expected to find something similar to the representations found in astrophysics books. Instead, the researchers identified up to three hot spots, all in the southern hemisphere of the pulsar.
Thomas Riley and his colleagues performed a series of simulations using superimposed circles of different sizes and temperatures to generate the X-ray signals from the hot spots. These analyses took almost a month on the national supercomputer Cartesius in the Netherlands, but would have taken almost 10 years on a modern personal computer. Using these calculations, the team identified two hot spots, one small and circular, and the other in the shape of an elongated crescent.
Cole Miller’s team ran similar simulations, but for oval spots of different sizes and temperatures, on UMD’s Deepthought2 supercomputer. They found two possible solutions with equivalent probabilities. In the first, the spots are oval and resemble those of Thomas Riley’s team. In the second solution, there is a third spot, colder, and located just near the rotational south pole of the pulsar.
Some theoretical predictions suggested that the position and size of the hot spots could vary, but the studies at J0030 were the first to map these surface properties. Scientists are still trying to understand why the hot spots of J0030 are like this, but it is clear at the moment that the magnetic field of the pulsar is much more complicated than the traditional dipole model with a north and a south pole.
The main goal of the NICER mission is to accurately determine the mass and size of several pulsars. With this information, scientists will be able to better understand the state of matter in the cores of neutron stars, an extremely compact material in which the pressure and density is unlike anything that can be reproduced on Earth.
“It is remarkable, and also reassuring, that the two teams, using different approaches, are obtaining similar mass and radius values, and thus similar hot spot shapes for J0030,” said Zaven Arzoumanian, NICER’s chief scientist at NASA’s Goddard Space Flight Center in the state of Maryland. “This confirms that we are on the right track to answer this great question in astrophysics: What shape does matter take in the ultra-dense cores of neutron stars?”
NICER is an Astrophysics Mission of Opportunity under NASA’s Explorers program, which provides frequent launch opportunities for space science missions using innovative approaches in Heliophysics and Astrophysics. NASA’s Space Technology Mission Directorate supports the SEXTANT component of the mission, which demonstrated the feasibility of triangulation-based space navigation using pulsars.
A series of seven articles were published on this topic within the Astrophysical Journal Letters (887).
- T. E. Riley et al. 2019, A NICER View of PSR J0030+0451: Millisecond Pulsar Parameter Estimation
- G. Raaijmakers et al. 2019, A NICER View of PSR J0030+0451: Implications for the Dense Matter Equation of State
- A. V. Bilous et al. 2019, A NICER View of PSR J0030+0451: Evidence for a Global-scale Multipolar Magnetic Field
- M. C. Miller et al. 2019, PSR J0030+0451 Mass and Radius from NICER Data and Implications for the Properties of Neutron Star Matter
- Slavko Bogdanov et al. 2019, Constraining the Neutron Star Mass–Radius Relation and Dense Matter Equation of State with NICER. I. The Millisecond Pulsar X-Ray Data Set
- Slavko Bogdanov et al. 2019, Constraining the Neutron Star Mass–Radius Relation and Dense Matter Equation of State with NICER. II. Emission from Hot Spots on a Rapidly Rotating Neutron Star
- Sebastien Guillot et al. 2019, NICER X-Ray Observations of Seven Nearby Rotation-powered Millisecond Pulsars
- Sébastien Guillot, firstname.lastname@example.org