Mystery Objects Uncovered in Globular Clusters

Globular clusters are ancient, dense balls of stars that festoon the outskirts of our galaxy. Inside them is a stellar melee, with objects catching and releasing each other or colliding and building something bigger.

This raucous environment is a fertile spot for the creation of interesting objects — things like pulsars spinning thousands of times per second, or neutron stars and small black holes slurping gas off companion stars and skirting themselves in hot, glowing disks.

Last week, two teams announced independent results fingering the presence of a strange object in two different globular clusters in the Milky Way: NGC 1851, in the southern constellation Columba, and 47 Tucanae, the second brightest globular in the sky. Both discoveries have important implications for what we do and don’t know about neutron stars and black holes.

Mysterious Mass Gap

Ewan Barr (Max Planck Institute for Radio Astronomy, Germany) and others turned up the first object during a survey of globular clusters with the MeerKAT radio telescope array in South Africa.

The team was looking for millisecond pulsars, the aforementioned rapidly rotating pulsars which have been spun up by the transfer of gas from a donor star. Among their discoveries in NGC 1851 was PSR J0514−4002E.

The researchers combined two dozen MeerKAT observations, taken over 1½ years, with archival observations from the Green Bank Telescope in West Virginia that had been gathered more than a decade earlier. Together, the data enabled them to precisely determine how long PSR J0514−4002E takes to complete an orbit with its unseen companion (7.44 days) and what the total mass of the pair is: 3.887 solar masses. (Note the three decimal places — that’s thanks to the precision pulsar’s clocklike signals enable.)

Based on the pulsars that astronomers have found elsewhere, the team subtracted the pulsar’s likely contribution to this total mass and inferred a range of masses for the unseen companion: 2.09 to 2.71 Suns. The researchers found no star when they looked with Hubble, so they conclude that this companion is either a neutron star or a black hole.

This mass range is eye-catching: It lies in the gap between the largest neutron stars and smallest black holes that astronomers have detected. The largest pulsars fall below 2.1 solar masses, and only a smattering of black holes have popped up with fewer than 5 solar masses. But the enigmatic no-man’s land between these bookends should be replete with black holes, given that stars (and the remnants they leave behind) are more abundant at lower masses.

The researchers can find nothing that would help them distinguish whether the object is a neutron star or black hole, they write in the January 19th Science. “We therefore cannot determine whether the companion is a massive [neutron star] or a low-mass [black hole].”

The discovery has “fascinating implications,” writes Maya Fishbach (University of Toronto) in an accompanying Perspective piece. “If a neutron star, it is probably the heaviest one known to date, with lessons for the uncertain physics of extremely dense nuclear matter. If a black hole, it may be the lightest known, which could affect the understanding of supernova explosions or dynamical interactions . . . inside globular clusters.”

The object’s orbit with PSR J0514−4002E is highly elongated, which suggests the two didn’t start life paired. Instead, they probably caught each other in the helter-skelter dance inside the cluster.  

The team suggests that the object might have been created by the merger of two neutron stars, since it’s similar in size to the remnant created in the GW170817 event. However, mergers involving neutron stars are thought to be rare inside globular clusters, Fishbach notes.

Is That a Black Hole?

The second mystery object has implications for a very different mass gap: the long-sought intermediate-mass black holes. These are objects between hundreds and hundreds of thousands of solar masses. Astronomers have found plenty of black holes below and above this range, but they’re only beginning to flesh out this in-between space — mostly thanks to puny supermassive black holes found in the hearts of dwarf galaxies.

One promising place to search is globular clusters, where these middling black holes might form thanks to stellar collisions. But searches have turned up nothing conclusive.

Alessandro Paduano (Curtin University, Australia) and colleagues may have finally succeeded.

The team undertook a huge radio survey of 50 globular clusters, using the Very Large Array and the Australia Telescope Compact Array. Black holes light up only if they’re eating gas, and although there’s not much gas in globulars, any that does make it to the black hole would glow faintly in radio and X-rays, at a level determined by the black hole’s mass.

One of the black hole candidates the researchers identified lies in 47 Tucanae. Follow-up observations uncovered an X-ray source in a similar location as the radio one, but no signs of a star.

Given the various possibilities, the object is likely either a millisecond pulsar or a weakly accreting black hole with a mass somewhere between 50 and 6,000 Suns, the team concludes in the January 20th Astrophysical Journal.

The cluster has plenty of pulsars — more have been tallied in 47 Tuc than in any other globular, save for Terzan 5. And various studies of stars’ motions inside 47 Tuc have disfavored the presence of a big black hole.

But how big is key, says Philip Kaaret (NASA Marshall Space Flight Center), who wasn’t involved with the study. The best lower limit is 600 solar masses. That still leaves plenty of room for the new putative black hole, so long as it weighs less than 500 Suns.

Kaaret himself thinks an intermediate-mass black hole is a plausible interpretation. The technique the team used is a long-advocated approach for this search.

To confirm that the object really is a middleweight black hole and not a pulsar, astronomers would need to look deep into the cluster with high-resolution optical or infrared observations and find evidence of an unseen object messing with stars’ orbits. That will require large ground-based telescopes fitted with adaptive optics, or the James Webb Space Telescope.

References:

Ewan D. Barr et al. “A Pulsar in a Binary with a Compact Object in the Mass Gap Between Neutron Stars and Black Holes.” Science. January 19, 2024.

Maya Fishbach. “Mystery in the ‘Mass Gap.’” Science. January 19, 2024.

Alessandro Paduano et al. “Ultradeep ATCA Imaging of 47 Tucanae Reveals a Central Compact Radio Source.” Astrophysical Journal. January 20, 2024.