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Discovery of two new millisecond pulsars towards the Galactic bulge

Through deep radio follow-up observations of X-ray candidates using MeerKAT, Murriyang, and Green Bank telescopes, researchers discovered two new millisecond pulsars in the Galactic bulge, including a black widow candidate, thereby doubling the number of such pulsars found within the innermost 2 degrees of the Galactic center.

Original authors: J. Berteaud, F. Calore, M. Clavel, S. Dai, J. S. Deneva, S. Hyman, F. K. Schinzel, A. Ridolfi, S. M. Ransom, F. Abbate, C. J. Clark, M. Kramer, T. Thongmeearkom, B. W. Stappers, E. D. Barr, R. P. Bret

Published 2026-03-18

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Original authors: J. Berteaud, F. Calore, M. Clavel, S. Dai, J. S. Deneva, S. Hyman, F. K. Schinzel, A. Ridolfi, S. M. Ransom, F. Abbate, C. J. Clark, M. Kramer, T. Thongmeearkom, B. W. Stappers, E. D. Barr, R. P. Breton

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the center of our galaxy, the Milky Way, as a bustling, crowded city at night. For over 15 years, astronomers have been staring at a strange, glowing "fog" of high-energy light (gamma rays) coming from this city center. They call it the Galactic Center Excess.

For a long time, they had two main theories about what was causing this glow:

The Dark Matter Theory: Invisible, mysterious particles are colliding and exploding, creating light.
The "Hidden Crowd" Theory: The glow isn't from one big explosion, but from thousands of tiny, faint, spinning stars called Millisecond Pulsars (MSPs) that are too dim to be seen individually.

Think of it like trying to see a single firefly in a dark forest. If you have one firefly, it's easy to spot. But if you have a million fireflies blinking rapidly in the distance, they just look like a single, fuzzy patch of light. Astronomers suspected the "fog" was actually a swarm of these hidden fireflies (pulsars) that previous telescopes were too "shallow" (not sensitive enough) to spot individually.

The Mission: A Deep Dive Search

The authors of this paper decided to play detective. They didn't just look randomly; they used a clever new strategy. They looked at a list of "suspects"—X-ray sources in the galactic center that might be these hidden pulsars.

They pointed three massive radio telescopes (MeerKAT in South Africa, Murriyang in Australia, and the Green Bank Telescope in the US) at these suspects. It was like using a super-powered night-vision camera to scan the forest for those specific fireflies.

The Surprise: Finding Two New Fireflies

While hunting for pulsars associated with their specific X-ray suspects, they found something unexpected. They didn't find the pulsar they were looking for at the exact spot of their target. Instead, they found two brand new millisecond pulsars right next door in the same general area.

PSR J1740–28: A star spinning 65 times a second. It's like a lighthouse with a very long, slow blink. It's in a wide, slow dance with a small companion star.
PSR J1740–2805: A much faster star, spinning 133 times a second. This one is a "Black Widow." In the universe, a Black Widow pulsar is a voracious star that is slowly eating its small companion star, like a spider consuming its mate. It spins incredibly fast and has a very tight, fast orbit.

Why This Matters

Doubling the Count: Before this discovery, only two such pulsars were known in the very innermost part of the galaxy. This discovery doubles that number. It's like finding two new species of rare birds in a forest where you thought there were only two left.
The "Serendipity" Factor: The discovery was accidental. They were looking at one X-ray source, but the pulsars they found weren't actually connected to it. They were just hiding nearby. This proves that the galactic center is full of these hidden objects, and we just need to look deeper to find them.
Solving the Mystery: While these two pulsars don't solve the "Galactic Center Excess" mystery on their own, they prove that the "Hidden Crowd" theory is plausible. If we can find these two, there are likely thousands more hiding in the dark, potentially explaining that mysterious gamma-ray fog.

The Takeaway

This paper is a story about persistence and better tools. By using deeper, more sensitive searches, astronomers found two new cosmic speedsters. It's a reminder that in the universe, the most interesting things are often the ones we can't see with our eyes, hiding in plain sight until we build the right "flashlight" to find them.

In short: We were looking for a specific ghost in a haunted house, but instead, we found two new ghosts hiding in the hallway. This proves the house is full of ghosts, and we are getting better at finding them.

1. Problem Statement

The Galactic Center Excess (GCE) is a mysterious excess of gamma-ray emission observed by the Fermi Large Area Telescope (LAT) originating from the Galactic Center. Two primary hypotheses exist to explain its origin:

Dark Matter Annihilation: Self-annihilation of dark matter particles.
Unresolved Millisecond Pulsars (MSPs): A large population of faint, fast-rotating pulsars (periods < 30 ms) that are too dim to be detected individually by Fermi and too dispersed to be found in shallow radio surveys.

While the MSP hypothesis is supported by the GCE's bulge-like morphology and photon statistics, direct evidence in the form of individual radio detections of MSPs within the innermost Galactic bulge has been lacking. Existing radio surveys in the GHz band are often too shallow to detect distant, highly dispersed MSPs associated with the bulge, and distance estimates based on Dispersion Measure (DM) are uncertain due to complex electron density models.

2. Methodology

The authors employed a multi-wavelength candidate selection strategy followed by deep, targeted radio pulsation searches.

Candidate Selection:
- The team utilized a catalog of X-ray sources identified by Chandra in the inner Galaxy, cross-matched with radio (VLA) and gamma-ray (Fermi) counterparts.
- They specifically targeted 9 X-ray candidates that lacked bright infrared/optical/UV counterparts (a characteristic of distant MSPs) and showed plausible radio or gamma-ray associations.
- This included 5 sources detected only in X-ray and radio, and 4 additional sources cross-matched with Fermi sources (including one in the globular cluster Terzan 1).
Observational Campaigns:
- Initial Surveys: Conducted with the Murriyang (Parkes) telescope (UWL receiver, 704–4032 MHz) and the Green Bank Telescope (GBT) (C-band and S-band) to search for periodic radio pulsations.
- Deep Follow-up: For promising candidates and serendipitous discoveries, deep observations were conducted using MeerKAT (S-band) as part of the TRAPUM Large Science Project.
- Data Processing:
  - Used PRESTO and Pulsar Miner for periodicity searches, including Fourier-domain acceleration searches to detect binary systems.
  - Applied coherent de-dispersion and standard RFI excision.
  - Timing analysis was performed using TEMPO2 to derive phase-connected solutions and orbital parameters.

3. Key Results

The campaign resulted in the discovery of two new millisecond pulsars in the direction of the Galactic bulge, though neither was associated with the original X-ray target they were searching for.

PSR J1740–28

Discovery: First detected with the GBT (S-band) at 13 $\sigma$ significance; confirmed by Murriyang.
Parameters:
- Spin Period ( $P$ ): 15.4 ms.
- Dispersion Measure (DM): 317 cm $^{-3}$ pc.
- Distance: Estimated at 4.58–5.29 kpc (depending on NE2001 or YMW16 electron models), placing it within the Galactic bulge.
Binary System: Low-mass companion (< 0.53 $M_\odot$ ), orbital period of ~191 days, and very low eccentricity ( $e \approx 0.0004$ ).
Association: Not associated with the target X-ray source 2CXO J174007.6–280708.

PSR J1740–2805

Discovery: Detected serendipitously in the upper UWL band during follow-up observations of PSR J1740–28; re-detected in initial GBT data and confirmed by MeerKAT.
Parameters:
- Spin Period ( $P$ ): 7.51 ms.
- Dispersion Measure (DM): 330 cm $^{-3}$ pc.
- Distance: Estimated at 4.74–7.01 kpc, strongly suggesting a bulge origin.
Binary System: Extremely compact orbit ( $P_{orb} \approx 0.48$ days) with a very low-mass companion ( $M_{min} \approx 0.05 M_\odot$ ).
Classification: Meets the criteria for a Black Widow pulsar (a type of "spider" pulsar).
Counterpart: Identified as a continuum, circularly polarized radio source in MeerKAT L-band imaging (Frail et al. 2024) with a flux density of 0.86 mJy.
Association: Located 1.6 arcmin from the original X-ray target; no X-ray counterpart was found.

Non-Detections and Upper Limits

No pulsars were found in the other 7 targeted candidates.
The authors calculated theoretical upper limits on the spin periods for non-detected sources. For two targets, periods longer than ~0.9 ms were ruled out, effectively excluding them as standard MSPs (though young pulsars or highly eclipsing systems remain possible).

4. Significance and Implications

Doubling the Known Population: These discoveries double the number of MSPs known within the innermost $2^\circ$ of the Galactic Center.
Validation of Deep Surveys: The discoveries were serendipitous (not linked to the specific X-ray targets), highlighting that existing shallow surveys miss these objects. It demonstrates the critical necessity of deep, targeted radio observations at high frequencies to overcome scattering and dispersion in the inner Galaxy.
Support for the MSP Hypothesis: The detection of two MSPs with distances consistent with the Galactic bulge supports the hypothesis that a hidden population of such pulsars could contribute to the GCE.
Formation Mechanisms: The properties of these pulsars (particularly PSR J1740–2805 as a black widow and PSR J1740–28 as a wide binary) provide constraints on MSP formation channels in the bulge, potentially involving accretion-induced collapse or disrupted globular clusters.
Methodological Insight: The paper notes that while the X-ray selection method yielded serendipitous discoveries, the low yield (2 out of 9) suggests that while the method is promising, deeper radio imaging of the inner Galaxy is required to formally validate or refine the selection criteria for GCE-associated MSPs.

Conclusion

The paper successfully identifies two new millisecond pulsars in the Galactic bulge using a multi-wavelength candidate approach and deep radio follow-up. While the specific X-ray targets did not yield the pulsars, the serendipitous discovery of PSRs J1740–28 and J1740–2805 underscores the existence of a hidden population of MSPs in the inner Galaxy, reinforcing the stellar origin hypothesis for the Galactic Center Excess.