The ghostly relic of a massive star, that has just gone screaming into oblivion as a result of the dazzling, colorful fireworks of a supernova explosion, pulsars are city-sized, newborn neutron stars. Spinning rapidly, these stellar relics send beams of light out into the space between stars with a regularity that has often been compared to the brilliant beacons sent out by lighthouses on Earth. In February 2017, a team of scientists announced their discovery of a new record holder for the brightest pulsar ever detected. However, the astronomers are still trying to determine how this stellar ghost can manage to shine so brightly. This newly discovered dense relic of a massive-star-that-was is now a member of a small and exclusive class of mysteriously brilliant pulsars that are forcing astronomers to rethink how pulsars accumulate new stellar material, in a process termed accretion.
Whirling wildly, a pulsar is a magnetized baby neutron star that sends its regular pulses of radiation out in two symmetrical beams across the Universe. If aligned well enough with our own planet, these beacons appear to flash on and off as the pulsar rotates.
A neutron star is approximately 20 kilometers in diameter, and sports a mass that is equivalent to about 1.4 times that of our Sun. This indicates that the stellar ghost is so extremely dense that, on Earth, one teaspoon full of neutron-star-stuff would weigh as much as a thundering herd of wild horses. Because of its relatively small size–similar to that of a city like Seattle–this stellar relic possesses a surface gravitational field that is approximately 2 X 10 to the 11th power times that of Earth. Furthermore, the magnetic field of a neutron star is a million times more powerful than the strongest magnetic fields formed on our planet.
Neutron stars are only one of several possible fates that herald the end of a star’s “life” on the hydrogen-burning main-sequence of the Hertzsprung-Russell Diagram of Stellar Evolution. After a star, of any mass, has finished burning its necessary supply of hydrogen fuel into heavier atomic elements (stellar nucleosynthesis)–by way of the process of nuclear-fusion–it has reached its inevitable and tragic grand finale. A neutron star emerges from the catastrophic wreckage of a massive star that, during its glory days, sported a mass greater than 4 to 8 times that of our Sun. After these massive stars have finished burning their nuclear-fusing fuel, they blast themselves to pieces in a violent, brilliant, and blazing supernova explosion. The blast sends the outer gaseous layers of the doomed star fleeing into space–and a terrible beauty is born. The outer gaseous layers of the erstwhile massive star create a dazzling, multicolored supernova remnant. The core of the dying star collapses under the merciless pull of its own gravity, and it collapses to such an extent that protons and electrons merge together to create neutrons.
Neutron stars may reveal themselves haunting the centers of supernova remnants. However, they may also appear as lonely, isolated objects, or may even dwell in close company with another star or stellar relic in a binary system. Four known neutron stars are generally thought to be orbited by exoplanets. Indeed, the discovery of the very first exoplanets were announced back in 1992 by the Polish astronomer Dr. Aleksander Wolszczan and the Canadian astronomer Dr. Dale Frail. Dr. Wolszczan discovered the first pulsar planet on February 9, 1990, using the Arecibo radio telescope. The first exoplanet discovered circling a main-sequence star like our own Sun was announced in 1995 by a different team of astronomers.
When a neutron star dwells in a binary system with a stellar companion, astronomers are able to take advantage of the situation because they can then measure the stellar ghost’s mass. For binary systems that host an unknown object, this information helps astronomers determine whether the mysterious object is a neutron star or a stellar mass black hole. Stellar mass black holes emerge from the ruins of a progenitor star that was even more massive than the stellar progenitors of neutron stars. Hence, stellar mass black holes are even more massive than neutron stars.
The first pulsar was discovered on November 28, 1967 by then-doctoral-student Dr. Jocelyn Bell Burnell and professor Dr. Antony Hewish of the University of Cambridge in the UK. The two astronomers spotted mysterious pulses separated by 1.33 seconds that apparently originated from the same region in space, and also kept sidereal time. The strange radio sources winked off and on at a remarkably regular frequency. While attempting to determine the origins of these bizarre pulses, their very brief period ruled out most known astrophysical sources that could explain them. To make matters even more confusing, because the pulses followed sidereal time, they could not be produced by intelligent alien beings.
Today, astronomers observe the best and the brightest pulsars at almost every wavelength of light. These neonatal neutron stars spin wildly, sending forth enormous jets of particles traveling almost at the speed of light, shooting out above their magnetic poles. These jets are responsible for producing extremely strong beams of light. For a similar reason, the “magnetic north” and the “true north” are different on our own planet–the magnetic and rotational axes of a pulsar are also misaligned. This is the reason why the light that streams out from a pulsar resembles the spotlight in a lighthouse on Earth. Just like passengers in a ship on the ocean can observe only regular blinks of light from a lighthouse, astronomers can only observe pulsars blinking off and on as their beam sails over the Earth. Pulsars are sometimes referred to as spin-powered pulsars, suggesting that the source of their energy is the rotation of the baby neutron star.
The astronomers Walter Baade and Fritz Zwicky were the first to suggest the existence of neutron stars back in 1934, when they proposed that a small, very dense stellar relic could be made up mainly of neutrons–left lingering in the wreckage of a massive star that went noisily into that good night in the catastrophic blast of a supernova conflagration. The “core” of the progenitor massive star–that had collapsed under the merciless weight of its own gravitational pull–would be smashed to the point that its protons and electrons merged into neutrons. Therefore, these city-sized stellar ghosts are really one enormous atomic nucleus.
A newborn pulsar keeps most of the angular momentum of its progenitor star, and because it possesses only a small fraction of its massive progenitor star’s radius, it is born with a very high rate of rotation.
The theory that defines pulsars as wildly whirling neonatal neutron stars is generally accepted among astronomers. However, not everyone is in complete agreement. This is because the theory explaining how pulsars emit their radiation is still in its infancy–even after almost half a century of work.
The Best And The Brightest Pulsar
The brightest known pulsar, as described in the journal Science (2017), is officially dubbed NGC 5907 ULX. In a mere second, this bright stellar ghost emits the same quantity of energy as our Sun does in about three and a half years. The European Space Agency’s (ESA’s) XMM-Newton satellite is responsible for discovering the pulsar and, independently, NASA’s NuSTAR (Nuclear Spectroscopic Telescope Array) mission also spotted the signal. The pulsar resides 50 million light years from Earth. This means that its traveling light, that we now see, was first sent forth into space–to make its long and treacherous journey–long before human beings had evolved on Earth. It is also the most distant of all known neutron stars.
“This object is really challenging our current understanding of the accretion process for high-luminosity pulsars. It is 1,000 times more luminous than the maximum thought possible for an accreting neutron star, so something else is needed in our models in order to account for the enormous amount of energy released by the object,” explained Dr. Gian Luca Israel in a February 28, 2017 Jet Propulsion Laboratory (JPL) Press Release. Dr. Israel is of the ONAF-Observatorio Astronomica di Roma, Italy, and lead author of the February 2017 Science research paper. The JPL is in Pasadena, California.
The previous record holder for the brightest known pulsar was reported in October 2014. NuSTAR detected this brilliant pulsar, dubbed M82 X-2, approximately 12 million light-years from Earth in the “Cigar Galaxy” (Messier 82, or M82, for short). M82 was ultimately identified as a pulsar instead of a black hole. NGC 5907 ULX is 10 times brighter than M82.
NGR 7793 P13 is the third brightest known pulsar. One group of astronomers, using a combination of XMM Newton and NuSTAR, reported their discovery of NGR 7793 P13 in the Astrophysical Journal Letters, while another used XMM-Newton to report it in the Monthly Notices of the Royal Astronomical Society (UK). Both studies were published in October 2016. Astronomers term these three extremely bright pulsars “ultraluminous X-ray sources” (ULXs). Prior to the 2014 discovery, many astronomers believed that the brightest ULXs were black holes.
“They are brighter than what you would expect from an accreting black hole of 10 solar masses,” noted Dr. Felix Fuerst in the February 28, 2017 JPL Press Release. Dr. Fuerst is lead author of the Astrophysical Journal Letters paper, and he is based at the European Space Astronomy Center in Madrid, Spain. Dr. Fuerst did this research while at Caltech.
But the reason why these objects shine so brilliantly remains a mystery. The most widely favored theory is that this bright trio of pulsars possess powerful and complex magnetic fields closer to their surfaces. A magnetic field would distort the flow of incoming material close to the neutron star, and in this way the neutron star could continue to accrete material while still emitting high levels of brightness.
The astronomers think that it is possible that many more ULXs are neutron stars.
Dr. Israel commented in the February 28, 2017 JPL Press Release that “These discoveries of ‘light,’ compact objects that shine so brightly, is revolutionizing the field.”