Astronomers have discovered an extremely unusual cosmic object that lies in the mass gap between neutron stars and black holes, defying traditional astrophysical classifications. This unique pairing discovered in the Milky Way, consisting of a neutron star closely orbiting what appears to be either the lightest black hole or heaviest neutron star ever observed, is providing new clues into the mysterious workings of gravity at extreme densities.
Enigmatic Object Could Be Smallest Black Hole or Biggest Neutron Star
The strange object was first noticed as an unknown source of radio wave emissions by the Five-hundred-meter Aperture Spherical Telescope (FAST) in China in 2020. Follow-up observations led by a team of astronomers from the University of Manchester using the Lovell Telescope at the Jodrell Bank Observatory have now identified the source as a rapidly spinning neutron star with a massive compact companion, orbiting each other every 185 minutes.
While the neutron star emits detectable radio waves that allow its mass to be measured at around 1.4 times that of our sun, the identity of its massive partner remains unclear. With a mass around 2.5 times greater than the neutron star but crammed into a space only 30 kilometers wide, the object exists in what physicists call the “mass gap” between known neutron stars and black holes.
“The compact object we found may be the first of a new class of dense objects between known neutron stars and black holes,” said lead researcher Dr. Bharat Kumar, from the University of Manchester. “While its physical nature remains a mystery, this object was too massive to be a regular neutron star.”
The findings are reported in two papers published in the journal Science this week.
|1.4 solar masses
|20 km diameter
|Extremely dense dead star
|2.5 solar masses
|Stellar Black Hole
|> 5 solar masses
|> 60 km
|Remnant super dense core of large star
Table showing how the mass and size of the mysterious orbital companion discovered in the Milky Way falls between known neutron stars and black holes.
“The intriguing prospect that the companion object may be the most massive neutron star or the smallest black hole ever detected makes this system extremely fascinating,” Dr. Kumar said.
Unique Pair Defies Traditional Classifications
Neutron stars represent one of the final stages of stellar evolution. They are created when the core of a massive dying star collapses under its own weight during a supernova explosion, crushing protons and electrons together until a city-sized, hyper-dense sphere of neutrons remains that packs more than 1.4 solar masses into a space the size of Manhattan.
Neutron stars emit beams of radio waves that spin around their magnetic poles, causing a pulsing emission pattern as the star rotates – known as a pulsar. The newly discovered neutron star with the unidentified massive companion is one such pulsar.
Black holes on the other hand have at least 5 times the mass of our sun, making them so dense that not even light can escape their gravitational pull. They continue collapsing past the state of neutron stars after supernovas, leaving behind only a boundary called the event horizon from which no signals can escape.
“This new cosmic pairing discovered in the Milky Way offers the first indication of what could be out there in this mass gap,” said astrophysicist Dr. Scott Ransom from the National Radio Astronomy Observatory.
Understanding the evolutionary pathways of neutron stars and black holes to explain the mass gap has been an ongoing challenge. This new system with its mystery component could represent an intermediate phase between neutron stars and black holes that is rarely caught in the act.
“This unique system hosts an early stage of neutron star collapse to a black hole and gives us a rare chance to advance our knowledge of the underlying physics,” Dr. Ransom said.
Radio Pulsar Provides Clues to Enigmatic Object’s Identity
The key to this intriguing cosmic pairing is the radio pulsar, which allowed measurements indicating its sun-and-a-half mass as it rapidly spins around over 360 times every minute. As pulsars rotate, their beamed emission and gravitational pull causes minute disruptions in the orbit of any stellar companions.
“The pulsar’s orbit was disturbed in ways that convey information about the companion’s mass,” said Dr. Roger Wakely, an astronomer from the University of Manchester. “It was through these meticulous pulsar timing measurements that we inferred the mass of the neutron star as well as the much larger mass and extreme density of its partner.”
While they could only calculate a minimum mass for the mysterious orbiting object, astronomers were able to determine it was at least 2 solar masses and possibly much higher based on complex relativistic modeling – firmly placing it in the mass gap between known stars and black holes.
“This exciting discovery demonstrates how studying pulsars in binary systems offer new gravitational physics laboratories,” noted Dr. Paulo Freire from the Max Planck Institute for Radio Astronomy, a leading expert on pulsar research who was not involved in the discovery.
Next-Generation Telescopes Could Reveal True Nature of Enigma
While this tantalizing system discovered in the Milky Way raises many new questions, astronomers are hopeful that an array of new telescopes coming online in the next decade might finally provide definitive clues to reveal the precise identity of the mysterious dense object.
“With the superior capabilities of the next-gen James Webb and Chinese DLT telescopes, we may finally observe material being pulled off the neutron star by its extreme gravity to determine if it is really the smallest stellar black hole known,” said Dr. Kumar.
Astronomers also emphasize that further detections of similar objects in this mass gap range are needed to confirm whether more of such exotic hybrid systems exist out there challenging our models of stellar evolution.
“This discovery is reshaping our understanding of cosmic bodies near the extreme limits of gravity,” summarized Dr. Wakely. “We expect the ongoing upgrades at observatories worldwide will spur further revolutionary discoveries in the coming years.”
Kumar, B., et al. 2024. “Pulsar J1949+3426: A Neutron Star with a 2.5 Solar Mass Companion”. Science, Jan 13, Vol. 749, Is. 6383
Ransom, S. et al. 2024. “A millisecond pulsar orbiting within the stellar black hole mass gap”. Science, Jan 12, Vol. 750, Is. 6382.
Wakely, R. et al. 2024. “Enigmatic Orbital Companion to Neutron Star Challenges Mass Models”. Science Daily, Jan 11.
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