Goliath Neutron Star Detected –“Most Hostile Environment in the Universe”

 

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Using a pioneering method, researchers from the Astronomy and Astrophysics Group of the Universitat Politècnica de Catalunya (UPC) and the Canary Islands Institute of Astrophysics (IAC) have found a neutron star of about 2.3 solar masses—one of the most massive ever detected. The study opens a new path of knowledge in many fields of astrophysics and nuclear physics.


Neutron stars (often called pulsars) are stellar remnants that have reached the end of their evolutionary life: they result from the death of a star of between 10 and 30 solar masses. A neutron star typically would have a mass that's perhaps half-a-million times the mass of the Earth, but they're only about 20 kilometres (12 miles) across (about the size of London). A handful of material from this star would weigh as much as Mount Everest.

They are very hot, perhaps a million degrees, they are highly radioactive, they have incredibly intense magnetic fields… They are arguably the most hostile environments in the Universe today.

Researchers from the (UPC) and the Canary Islands Institute of Astrophysics (IAC) used an innovative method to measure the mass of one of the heaviest neutron stars known to date. Discovered in 2011 and called PSR J2215+5135, with about 2.3 solar masses it is one of the most massive of the more than 2,000 neutron stars known to date. Although a study published in 2011 reported evidence of a neutron star with 2.4 solar masses, the most massive neutron stars that had previously achieved a consensus among scientists, reported in 2010 and 2013, have 2 solar masses.

The researchers used data obtained from the Gran Telescopio Canarias (GTC), the largest optical and infrared telescope in the world, the William Herschel Telescope (WHT), the Isaac Newton Telescope Group (ING) and the IAC-80 telescope, in combination with dynamical models of binary stars with irradiation. An article reporting on the results of the study, entitled "Peering into the dark side: magnesium lines establish a massive neutron star in PSR J2215+5135", was published in the Astrophysical Journal.

The team developed a more accurate method than those used to date to measure the mass of neutron stars in compact binaries. PSR J2215+5135 is part of a binary system, in which two stars orbit around a common center of mass: a "normal" star (like the sun) "accompanies" the neutron star. The secondary or companion star is strongly irradiated by the neutron star.

The more massive the neutron star is, the faster the companion star moves in its orbit. The novel method uses spectral lines of hydrogen and magnesium to measure the speed at which the companion star moves. This allowed the team to measure for the first time the speed of both sides of the companion star (the irradiated side and the shaded side), and to show that a neutron star can have more than twice the sun's mass.

This new method can also be applied to the rest of this growing population of neutron stars: over the last 10 years, the Fermi-LAT NASA gamma ray telescope has revealed dozens of pulsars similar to PSR J2215+5135. In principle, the method can also be used to measure the mass of black holes and white dwarfs (remnants of stars that die with more than 30 or fewer than 10 solar masses, respectively) when they are found in similar binary systems in which irradiation is important.

Being able to determine the maximum mass of a neutron star has very important consequences for many fields of astrophysics, as well as for nuclear physics. The interactions between nucleons (the neutrons and protons that make up the nucleus of an atom) at high densities are one of the great mysteries of physics today. Neutron stars are a natural laboratory for studying the densest and most exotic states of matter that can be imagined.

The results of the project also suggest that in order to support the weight of 2.3 solar masses, the repulsion between particles in the nucleus of the neutron star must be sufficiently strong. This would indicate that we are unlikely to find free quarks or other exotic forms of matter in the centre of the neutron star.

Astronomers discovered a special kind of neutron star for the first time outside of the Milky Way galaxy shown at the top of the page, using data from NASA's Chandra X-ray Observatory and the European Southern Observatory's Very Large Telescope (VLT) in Chile. This newly identified neutron star is a rare variety that has both a low magnetic field and no stellar companion.

The neutron star is located within the remains of a supernova – known as 1E 0102.2-7219 (E0102 for short) – in the Small Magellanic Cloud, located 200,000 light years from Earth. In the image, X-rays from Chandra are blue and purple, and visible light data from VLT’s Multi Unit Spectroscopic Explorer (MUSE) instrument are bright red. Additional data from the Hubble Space Telescope are dark red and green.

Oxygen-rich supernova remnants like E0102 are important for understanding how massive stars fuse lighter elements into heavier ones before they explode. Seen up to a few thousand years after the original explosion, oxygen-rich remnants contain the debris ejected from the dead star’s interior. This debris (visible as a green filamentary structure in the combined image) is observed today hurtling through space after being expelled at millions of miles per hour.

Chandra observations of E0102 show that the supernova remnant is dominated by a large ring-shaped structure in X-rays, associated with the blast wave of the supernova. The new MUSE data revealed a smaller ring of gas (in bright red) that is expanding more slowly than the blast wave. At the center of this ring is a blue point-like source of X-rays. Together, the small ring and point source act like a celestial bull’s eye.

The combined Chandra and MUSE data suggest that this source is an isolated neutron star, created in the supernova explosion about two millennia ago. The X-ray energy signature, or “spectrum,” of this source is very similar to that of the neutron stars located at the center of two other famous oxygen-rich supernova remnants: Cassiopeia A (Cas A) and Puppis A. These two neutron stars also do not have companion stars.

The lack of evidence for extended radio emission or pulsed X-ray radiation, typically associated with rapidly rotating highly-magnetized neutron stars, indicates that the astronomers have detected the X-radiation from the hot surface of an isolated neutron star with low magnetic fields. About ten such objects have been detected in the Milky Way galaxy, but this is the first one detected outside our galaxy.

But how did this neutron star end up in its current position, seemingly offset from the center of the circular shell of X-ray emission produced by the blast wave of the supernova? One possibility is that the supernova explosion did occur near the middle of the remnant, but the neutron star was kicked away from the site in an asymmetric explosion, at a high speed of about two million miles per hour. However, in this scenario, it is difficult to explain why the neutron star is, today, so neatly encircled by the recently discovered ring of gas seen at optical wavelengths.

Another possible explanation is that the neutron star is moving slowly and its current position is roughly where the supernova explosion happened. In this case, the material in the optical ring may have been ejected either during the supernova explosion, or by the doomed progenitor star up to a few thousand years before.

A challenge for this second scenario is that the explosion site would be located well away from the center of the remnant as determined by the extended X-ray emission. This would imply a special set of circumstances for the surroundings of E0102: for example, a cavity carved by winds from the progenitor star before the supernova explosion, and variations in the density of the interstellar gas and dust surrounding the remnant.

Future observations of E0102 at X-ray, optical, and radio wavelengths should help astronomers solve this exciting new puzzle posed by the lonely neutron star.

The Daily Galaxy via Universitat Politècnica de Catalunya (UPC) and NASA/Chandra

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