Lightest black hole or heaviest neutron star? MeerKAT uncovers a mysterious object at the boundary between black holes and neutron stars

An international team of astronomers, led by researchers from the Max Planck Institute for Radio Astronomy, have used the MeerKAT radio telescope to discover an intriguing object of an unknown nature in the globular cluster NGC 1851. The massive object is heavier than the heaviest neutron stars known and yet simultaneously lighter than the lightest black holes known and is in orbit around a rapidly spinning millisecond pulsar. This could be the first discovery of the much-coveted radio pulsar – black hole binary; a stellar pairing that would allow new tests of Einstein’s general relativity.

The research was published today in the journal Science.

Fig. 1: An artist’s impression of the system assuming that the massive companion star is a black hole. The brightest background star is its orbital companion, the radio pulsar PSR J0514-4002E. The two stars are separated by 8 million km and circle each other every 7 days.

Credit: Daniëlle Futselaar (artsource.nl)

Neutron stars, the ultra-dense remains of a supernova explosion, can only be so heavy. Once they’ve acquired too much mass, perhaps by consuming another star or maybe by colliding with another of their kind, they will collapse. What exactly they become once they collapse is the cause of much speculation, with various wild and wonderful flavours of exotic stars being proposed. The prevailing opinion, however, is that neutron stars collapse to become black holes, objects so gravitationally attractive that even light cannot escape them. Theory, backed by observation, tells us that the lightest black holes that can be created by collapsing stars are about 5 times more massive than the Sun. This is considerably larger than the 2.2 times the mass of the sun required for neutron star collapse, giving rise to what is known as the black hole mass gap. The nature of compact objects in this mass gap is unknown and detailed study has thus far proved challenging due to only fleeting glimpses of such objects being caught in observations of gravitational wave merger events in the distant universe.

The discovery of an object in this mass-gap in our own galaxy by a team of astronomers from the international Transients and Pulsars with MeerKAT (TRAPUM) collaboration may help finally understand these objects. Their work, published this week in the journal Science, reports on a massive pair of compact stars in the globular cluster NGC 1851 in the southern constellation Columba (the dove). By using the sensitive MeerKAT radio telescope in South Africa, in combination with powerful instrumentation built by engineers at the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, they 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. This pulsar, designated PSR J0514-4002E, spins more than 170 times a second, with every rotation producing a rhythmic pulse, like the ticking of a clock. By observing small changes in this ticking over time, 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.”, says Ewan Barr, who led the study together with MPIfR colleague and PhD candidate 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 – it is invisible in Hubble Space Telescope images of NGC 1851 – it is therefore an extremely dense remnant of a collapsed star. Furthermore, the observed change with time of the closest point of approach between the two stars (the periastron) 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.

“Whatever this object is, it is exciting news” – says Paulo Freire, of the MPIfR. “If it is a black hole, it will be the first pulsar – black hole system known, which has been a Holy Grail of pulsar astronomy for decades! If it is a neutron star, this will have fundamental implications for our understanding of the unknown state of matter at these incredible densities!”

The team proposes that the formation of the massive object, and its subsequent pairing with the fast-spinning radio pulsar in a tight orbit, is the result of a rather exotic formation history (Fig. 3) only possible due to its particular local environment. The system is found in the globular cluster NGC 1851, a dense collection of old stars that are much more tightly packed than the stars in the rest of the Galaxy. Here, it is so crowded that the stars can interact with each other, disrupting orbits and in the most extreme cases colliding. It is one such collision between two neutron stars that is proposed to have created the massive object that now orbits the radio pulsar. However, before the present binary was created, the radio pulsar must have first acquired material from a donor star in a so-called low-mass X-ray binary. Such a “recycling” process is needed to spin up the pulsar to its current rotation rate. The team believes that this donor star was then replaced by the current massive object in a so-called exchange encounter. “This is the most exotic binary pulsar discovered yet,” says Thomas Tauris from Aalborg University, Denmark. “Its long and complex formation history pushes at the limits of our imagination”.

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 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,“ says Arunima Dutta. She concludes, “uncovering the true nature of the companion will a turning point in our understanding of neutron stars, black holes, and whatever else might be lurking in the black hole mass gap!”.

Fig. 2: The team used the sensitive MeerKAT radio telescope, located in the Karoo desert of South Africa.
Credit: SARAO


Additional Information

MeerKAT: Operated by the South African Radio Astronomy Observatory (SARAO), MeerKAT is the largest radio telescope in the Southern hemisphere and one of two precursor instruments of the SKA Project based in South Africa. Located in the Karoo semi-desert, the radio telescope will soon be expanded with an additional number of dishes, in the context of the “MeerKAT+” project, jointly funded by SARAO and the Max-Planck-Gesellschaft (MPG) in Germany. The telescope will later be gradually integrated into SKAO’s Mid telescope in South Africa.

TRAPUM: Transients and Pulsars with MeerKAT (TRAPUM) is an ongoing survey project using the MeerKAT radio telescope. The project’s members form an international collaboration representing universities and research institutes in 10 countries, led by Prof. Michael Kramer (Max Planck Institute for Radio Astronomy, Germany) and Prof. Benjamin Stappers (University of Manchester, UK). The goals of the TRAPUM survey and collaboration are to observe targets of special interest in search of radio pulsars. Observations focus on Fermi-LAT detected gamma-ray sources, globular clusters, supernova remnants and nearby galaxies. To date, TRAPUM and its sister projects have discovered more than 200 pulsars with MeerKAT.

NGC 1851E formation history: A possible evolutionary history for the NGC 1851E system is captured in figure 3. On the left of the figure it is shown how the millisecond pulsar (MSP), PSR J0514-4002E, was spun up via the capture of matter from a stellar companion in a low-mass x-ray binary (LMXB). What remains after the low-mass x-ray binary stage, is a rapidly spinning pulsar and a white dwarf that orbit each other – a typical configuration seen throughout the Galaxy. On the right the formation of the massive companion object is shown. Here two neutron stars in orbit (NS + NS) can be seen. The loss of energy by gravitational wave emission causes this orbit to shrink with time, eventually resulting in an explosive neutron star merger. The outcome of the merger is an isolated low-mass black hole (BH) or possibly a supermassive neutron star. At some later time the black hole and the pulsar – white dwarf binary meet, resulting in an exchange encounter in which the lightest of the three stars (in this case the white dwarf) is kicked out of orbit. The result is a stable pulsar – black hole system.

Fig. 3: Potential formation history of the radio pulsar NGC 1851E and its exotic companion star (see text for details).
Credit: Thomas Tauris (Aalborg University / MPIfR)

The authors of the paper are Ewan D. Barr, Arunima Dutta, Paulo C. C. Freire, Mario Cadelano, Tasha Gautam, Michael Kramer, Cristina Pallanca, Scott M. Ransom, Alessandro Ridolfi, Benjamin W. Stappers, Thomas M. Tauris, Vivek Venkatraman Krishnan, Norbert Wex, Matthew Bailes, Jan Behrend, Sarah Buchner, Marta Burgay, Weiwei Chen, David J. Champion, C. -H. Rosie Chen, Alessandro Corongiu, Marisa Geyer, Y. P. Men, Prajwal V. Padmanabh, Andrea Possenti. Ewan Barr, Arunima Dutta, Paulo Freire, Tasha Gautam, Michael Kramer, Alessandro Ridolfi, Thomas Tauris, Vivek Venkatraman Krishnan, Norbert Wex, Jan Behrend, Weiwei Chen, David Champion, Rosie Chen, Y. P. Men and Prajval Padmanabh are affiliated with the MPIfR.


Original Paper

E. Barr et al.: A pulsar in a binary with a compact object in the mass gap between neutron stars and black holes, Science 2024 (DOI: xxx)

https://www.science.org/

More information, including a copy of the paper, can be found online at the Science Press Package at https://www.eurekalert.org/press/scipak/ Original Paper (preprint): https://cloud.mpifr-bonn.mpg.de/index.php/s/zeR646H8Z2d9CZz

 

Further Information/Links:

Max Planck Institute for Radio Astronomy (MPIfR)

http://www.mpifr-bonn.mpg.de/2169/en

Fundamental Physics in Radio Astronomy. Research Department at MPIfR

https://www.mpifr-bonn.mpg.de/research/fundamental

MeerKAT, South Africa

https://www.sarao.ac.za/science/meerkat/

MeerKAT+ The MeerKAT Extension Project

https://www.mpifr-bonn.mpg.de/pressreleases/2020/9

Transients and Pulsars with MeerKAT (TRAPUM)

http://www.trapum.org/

Max-Planck-Gesellschaft (MPG)

https://www.mpg.de/en

SKAO Observatory

https://www.skao.int/

Video / Swinburne Animation

A zoom into the globular cluster NGC 1851 followed by an orbital simulation showing the original pulsar – white dwarf binary being disrupted by the arrival of a massive third body of unknown nature. The new arrival kicks the white dwarf out of orbit and captures the pulsar for itself, forming a new binary system with a pulsar in orbit around, most likely, either a light black hole or a supermassive neutron star.

Video: https://www.youtube.com/watch?v=0LrIiHLO5Yc

Credit: OzGrav, Swinburne University of Technology


Local Contact:

Dr. Ewan Barr
Group Leader: Electronics, Software Development
Max-Planck-Institut für Radioastronomie, Bonn
Fon: +49 228 525-535
E-mail: ebarr@mpifr-bonn.mpg.de

Arunima Dutta
Max-Planck-Institut für Radioastronomie, Bonn
Fon: +49 228 525-122
E-mail: adutta@mpifr-bonn.mpg.de

Dr. Norbert Junkes,
Press and Public Outreach
Max-Planck-Institut für Radioastronomie, Bonn.
Fon: +49 228 525-399
E-mail: njunkes@mpifr-bonn.mpg.de

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