Dark star in deep space has scientists intrigued
Astronomers have uncovered a mysterious object in the Milky Way; is it a lightweight black hole, or a heavy neutron star?
THERE'S something strange lurking out there in the cold, dark reaches of outer space, but this isn't the beginnings of a blurb for some sci-fi space opera.
No, there really is something strange out there in the Milky Way, and scientists can't quite decide what it is. This new and mysterious object is heavier than the heaviest neutron stars known, yet at the same time it is lighter than even the lightest of black holes.
So, what the Hell is it? It's all a bit of a cosmic enigma. That this object exists, we know that much.
An international teams of astronomers from the University of Manchester, UK, and the Max Planck Institute for Radio Astronomy in Germany, discovered the object using the MeerKAT Radio Telescope situated in South Africa.
Whatever this thing is, it is located around 40,000 light years away from Earth, orbiting a rapidly spinning millisecond pulsar in a dense group of stars known as a globular cluster. Using the clock-like ticks from the pulsar they showed that the massive object lies in the so-called black hole mass gap.
Publishing their findings in the journal Science, the scientists have suggested it could be the first discovery of a radio pulsar-black hole binary system; a much-coveted discovery, if correct. The stellar pairing could allow new tests of Einstein's general relativity and open doors to the study of black holes.
“Either possibility for the nature of the companion is exciting,” said Manchester's Ben Stappers, professor of astrophysics, and the UK project lead. “A pulsar-black hole system will be an important target for testing theories of gravity, and a heavy neutron star will provide new insights in nuclear physics at very high densities.”
Collision density
When neutron stars – the ultra-dense remains of a dead star – acquire too much mass, usually by consuming or colliding with another star, they will collapse.
What they become after that collapse is the cause of much speculation, the scientists say, but it is believed that they could become black holes – objects so gravitationally attractive that even light cannot escape them.
Astronomers believe that the total mass required for a neutron star to collapse is 2.2 times the mass of the sun. Theory, backed by observation, tells us that the lightest black holes created by these stars are much larger, at about five times more massive than the Sun, giving rise to what is known as the ‘black hole mass gap’.
The nature of compact objects in this mass gap is unknown, the scientists say, and detailed study has so far proved “challenging”. The discovery of the object may help finally understand these objects.
Stappers added: “The ability of the extremely sensitive MeerKAT telescope to reveal and study these objects is a enabling a great step forward, and provides us with a glimpse of what will be possible with the Square Kilometre Array.”
The globular cluster NGC 1851 is a dense collection of old stars that are much more tightly packed than the stars in the rest of the galaxy. Here, the scientists explain, it is so crowded that the stars can interact with each other, disrupting orbits, and in the most extreme cases colliding.
The astronomers, part of the international Transients and Pulsars with MeerKAT (TRAPUM) collaboration, believe that it is one such collision between two neutron stars that is proposed to have created the massive object that now orbits the radio pulsar.
Cosmic clockwork
The team were able to detect faint pulses from one of the stars, identifying it as a radio pulsar – a type of neutron star that spins rapidly and shines beams of radio light into the universe like a cosmic lighthouse.
The pulsar spins more than 170 times a second, with every rotation producing a rhythmic pulse, like the ticking of a clock. The ticking of these pulses is incredibly regular, and by observing how the times of the ticks change, using a technique called pulsar timing, they were able to make extremely precise measurements of its orbital motion.
“Think of it like being able to drop an almost perfect stopwatch into orbit around a star almost 40,000 light years away, and then being able to time those orbits with microsecond precision,” said Ewan Barr, from the Max Planck Institute for Radio Astronomy, who led the study with his colleague Arunima Dutta
The regular timing also allowed a very precise measurement of the system’s location, showing that the object in orbit with the pulsar was no regular star, but an extremely dense remnant of a collapsed star.
Observations also showed that the companion has a mass that was simultaneously bigger than that of any known neutron star, and yet smaller than that of any known black hole, placing it squarely in the black-hole mass gap.
While the team cannot conclusively say whether they have discovered the most massive neutron star known, the lightest black hole known, or even some new exotic star variant, what they say is certain is that they have uncovered a “unique laboratory” for probing the properties of matter under the most extreme conditions in the universe.
"We're not done with this system yet,” said Dutta. “Uncovering the true nature of the companion will be a turning point in our understanding of neutron stars, black holes, and whatever else might be lurking in the black hole mass gap.”
MC