Unusual pulsars discovered in star cluster Terzan 5

Ten new neutron stars allow new tests of extreme stellar physics and the theory of relativity

Neutron stars are among the most fascinating objects in the universe. A significant mass fraction of the progenitor star is gathering in a sphere with a radius of just over 10 kilometers. In other words, a teaspoon of these stellar corpses weighs over a billion tons. Many of the secrets of these exotic stars are still unexplored. The globular cluster Terzan 5, known for its dense population of stars, is a neutron star hotspot and thus a popular target for telescopes. Thanks to highly sensitive observations with the MeerKAT radio telescope, an international team has now detected ten previously unknown millisecond pulsars in Terzan 5, a special class of such neutron stars. Their investigation could reveal much more about these mysterious objects and help to further test Einstein's theory of relativity. 

An international team led by researchers from the Max Planck Institute for Gravitational Physics (Albert Einstein Institute; AEI), the Max Planck Institute for Radio Astronomy (MPIfR), and the U.S. National Science Foundation National Radio Astronomy Observatory (NRAO) has discovered ten rapidly rotating neutron stars in the globular cluster Terzan 5. Many of them are in unusual and rare binaries, including a potential candidate for a record-breaking double neutron star, a pulsar in an extremely elliptical orbit, and several “spider” systems in which the neutron stars are evaporating their companions. These findings in data from the MeerKAT radio telescope array increase the number of known millisecond pulsars in this very dense stellar cluster by more than a quarter to a total of 49.  

“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.” Prajwal Voraganti Padmanabh is the lead author of the study recently published in Astronomy & Astrophysics.

Neutron stars in one of the most crowded places of our Galaxy

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 they are millions of times denser than in the neighborhood 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.

Leveraging the power of MeerKAT

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.

“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 Prajwal Voraganti 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.”

Atlas supercomputer enables ten new discoveries

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, which provided some 99,000 logical CPU cores in nearly 3,200 servers, as well as 400 graphics cards with nearly one million cores for data analysis. This search led to the discovery of ten new millisecond pulsars.

MeerKAT and the NSF Green Bank Telescope team up

Over the past two decades, the NSF Green Bank Telescope has conducted repeated observing campaigns of Terzan 5, building up a large archive of observations.

For each pulsar found in the MeerKAT data at a well-defined sky position, the astronomers went back to the NSF Green Bank Telescope archival data to see if they could find their discovery there as well. “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.

Of spiders, double neutron stars, and close encounters

“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 group at the Max Planck Institute for Radio Astronomy.

One discovery 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.

The team also discovered a trio of rare “spider” pulsars. In these binaries, the pulsar evaporates its lightweight companion star. The material evaporated from the companion fills the binary system with plasma, which is impenetrable to radio waves and can eclipse the pulsations for large parts of the orbit. Over time, the pulsar “consumes” its companion, taking its name from certain spiders known for this behavior: redbacks and black widows.

“One of our three spider discoveries, a redback pulsar, was confirmed to be associated with an already known object that emits X-rays and radio waves,” says Colin Clark, group leader at AEI Hannover. “This system also has a remarkably unstable orbit: Over the 19 years of observations, the time at which the pulsar passed a given point on its orbit varied unpredictably by more than a minute”. These large variations are thought to be due to a shape-shifting companion star, but the causes of this are unclear. One possibility is that magnetic activity interacts with the plasma inside the companion, deforming it. This in turn changes the companion's gravitational pull and thus the binary's orbit.

Finding even more pulsars? Einstein@Home to the rescue!

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 harbor many more extreme binary systems, each a potential laboratory for testing Einstein's theory of relativity,” explains 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?”