Research results

“We know that globular clusters like Terzan 5 are home to many rapidly rotating neutron stars, and we also know that previous observations of this cluster probably missed some. Nevertheless, we were very excited to discover ten previously unknown millisecond pulsars, including some of them in unusual and extreme binaries,” says Prajwal Voraganti Padmanabh, a postdoctoral researcher at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute; AEI) in Hannover, Germany. “These discoveries and their full characterization were made possible by a combination of highly sensitive MeerKAT observations, archival observations from the NSF Green Bank Telescope spanning nearly two decades, and clever and efficient data analysis techniques.”

Padmanabh is the lead author of the study recently published in Astronomy & Astrophysics. He started his work on pulsars while working at the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn.

Neutron stars are compact remnants of supernova explosions and consist of exotic, extremely dense matter. They are more massive than our Sun, but with a diameter of only about 20 kilometers. Because of their strong magnetic fields and fast rotation they emit beamed radio waves similar to a cosmic lighthouse. When the rotation periodically points these beams towards Earth, the neutron star becomes visible as a pulsating radio source: a radio pulsar. Some of these radio pulsars are spun up to rotation periods of just a few milliseconds by accumulating material from a binary companion star. These are called millisecond pulsars.

The globular cluster Terzan 5 is one of the most crowded places for stars in our Milky Way. In its core, where there are millions of times more stars per unit volume than in the neighbourhood of our Sun, stars meet and interact much more often than elsewhere. This makes it a very efficient “factory” for producing pulsars in extraordinary binary systems. 39 pulsars were already known in Terzan 5 before this study, which added another ten.

The astronomers made their discoveries using data from the MeerKAT radio telescope. MeerKAT is an array of 64 antenna dishes in the Karoo, South Africa with unprecedented sensitivity to sources in the southern celestial hemisphere. As part of the TRansients and Pulsars using MeerKAT (TRAPUM) Large Survey Project, the team observed Terzan 5 twice for several hours with 56 MeerKAT dishes. The hardware for TRAPUM was funded, designed and installed by MPIfR.

“Using special hardware and software, we combined the data from the 56 individual MeerKAT antennas into a virtual telescope that simultaneously observed nearly 300 closely spaced sky positions covering Terzan 5,” said Dr Padmanabh. “Of course, this results in much more data to analyze compared to observations with a single dish. But it also helps us pinpoint the position of each new pulsar much more precisely, and with single dishes that is usually the tricky part, requiring months of additional dedicated observations.”

The research team prepared the data and then searched for pulsars from 45 positions covering the core of Terzan 5. Their workhorse: the Atlas supercomputer at AEI Hannover. This search led to the discovery of ten new millisecond pulsars.

Characterising the new pulsars, a task that can take many years, was done very quickly thanks to decades of archival data taken with NSF’s Green Bank telescope. “Without the NSF Green Bank Telescope’s archive, we wouldn’t have been able to characterize these pulsars and understand their astrophysics,” says Scott Ransom, staff astronomer at the National Radio Astronomy Observatory (NRAO). The team was able to create ‘timing models’ for all of its discoveries. These mathematical descriptions accurately predict the arrival time of each of the several hundred billion pulses over the entire 19-years of observations.

To achieve this level of precision, the timing models must take into account many astrophysical properties that describe each binary pulsar system, including relativistic effects arising from Einstein's theory of general relativity. This, in turn, allowed astronomers to closely and precisely study and monitor the neutron stars, their binary orbits, their companions, and many other properties.

“All ten pulsar discoveries are very special and unusual, helping us to better understand globular clusters and neutron stars, and to test general relativity. But some of them are rare and special even in this group,” says Paulo Freire, scientific staff in the Fundamental Physics in Radio Astronomy research department at MPIfR. “These systems are only the latest examples of the wonderful, exotic systems we have been finding in these dense globular clusters with MeerKAT. This, together with recent examples like the NGC 1851E system, which could be the first pulsar - black hole system, is showing us that globular clusters are a gold mine of opportunities.”

Vivek Venkatraman Krishnan from MPIfR is co-author of the present paper. He is also looking for binary pulsars in Globular Clusters in the framework of the ERC research grant COMPACT (“Understanding gravity using a comprehensive search for fast-spinning pulsars and compact binaries”). “With COMPACT, we will design and use a tailor-made pulsar search system to discover even more extreme versions of these binaries in Globular Clusters,” he says.

One discovery from the present paper is a binary system that on one hand might consist of two neutron stars. These double neutron stars are very rare – roughly 20 out of more than 3600 known pulsars belong to this particular class. If future observations confirm these suspicions, the double system would also be a record breaker, with the fastest spinning pulsar and the longest period orbit for such a class of systems. On the other hand, the same system could also be a massive pulsar with a white dwarf companion star. A high mass pulsar could put constraints on the interior composition of neutron stars.

The extremely elliptical orbit of another discovery indicates a number of close stellar encounters in its past. When stars in the crowded centre of Terzan 5 pass by a binary system, their gravity can disrupt its orbits, even possibly ejecting and replacing its component stars.

Having already increased the number of known pulsars in Terzan 5 by more than a quarter, the team is already making plans to find even more. They will search for pulsars in compact binaries whose orbital periods are shorter than any discovered before. The strategy is to complete the analysis of Terzan 5 data obtained with MeerKAT, using the power of the distributed volunteer computing project Einstein@Home. The project, led by scientists at AEI Hannover, has already discovered more than 90 new neutron stars. The astronomers will also once more observe Terzan 5 with MeerKAT at higher radio frequencies, which should further increase the chances of new discoveries.

“From what we know about Terzan 5, we expect it to harbour many more extreme binary systems, each a potential laboratory for testing Einstein's theory of relativity,” concludes Prajwal Voraganti Padmanabh. “Who knows, maybe the next thing we find in this amazing globular cluster is something as exotic as a pair of millisecond pulsars or a millisecond pulsar orbiting a black hole?”

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Background Information

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 desert, the radio telescope will soon be expanded with an additional number of dishes, becoming “MeerKAT+”. This will later be gradually integrated into SKAO's Mid telescope, whose construction has already begun and will continue until 2028. The first scientific observations of MeerKAT+ started in 2023, during the testing phases of the telescope.

TRAPUM (TRAnsients and PUlsars with MeerKAT) is one of the Large Survey Proposals running on MeerKAT and is an international collaboration, led by the University of Manchester and the MPIfR, and includes institutions such as INAF, the National Radio Astronomy Observatory (NRAO) and the South African Radio Astronomy Observatory (SARAO).

The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The Green Bank Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

Authors of the publication include P. V. Padmanabh, S. M. Ransom, P. C. C. Freire, A. Ridolfi, J. D. Taylor, C. Choza, C. J. Clark, F. Abbate, M. Bailes, E. D. Barr, S. Buchner, M. Burgay, M. E. DeCesar, W. Chen, A. Corongiu, D. J. Champion, A. Dutta, M. Geyer, J. W. T. Hessels, M. Kramer, A. Possenti, I. H. Stairs, B. W. Stappers, V. Venkatraman Krishnan, L. Vleeschower and L. Zhang. Paulo Freire, Ewan Barr, Weiwei Chen, David Champion, Arunima Dutta, Michael Kramer and Vivek Venkatraman Krishnan are based at MPIfR. The first author, Prajwal Voraganti Padmanabh, and also Alessandro Ridolfi and Federico Abbate have an affiliation with the MPIfR.

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Wissenschaftlicher Ansprechpartner:
Dr. Paulo Freire
Max-Planck-Institut für Radioastronomie, Bonn.
Fon: +49 228 525-496
E-mail: pfreire@mpifr-bonn.mpg.de

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

Prof. Dr. Michael Kramer
Director and Head of "Fundamental Physics in Radio Astronomy“ Research Dept.
Max-Planck-Institut für Radioastronomie, Bonn.
Fon: +49 228 525-299 (Secretary)
E-mail: mkramer@mpifr-bonn.mpg.de

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Originalpublikation:
Discovery and timing of ten new millisecond pulsars in the globular cluster Terzan 5, P. V. Padmanabh et al., Astronomy & Astrophysics (A&A), Volume 686, A166 (June 2024)

https://www.aanda.org/articles/aa/full_html/2024/06/aa49303-24/aa49303-24.html

ADS: https://ui.adsabs.harvard.edu/abs/2024A%26A...686A.166P/abstract

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Weitere Informationen:
https://www.mpifr-bonn.mpg.de/pressreleases/2024/8