Fermi LAT Pinpoints 15 New Pulsars for MeerKAT

The Fermi Large Area Telescope (LAT) provides significant advantages for pulsar searches by pinpointing unidentified gamma-ray sources with high precision, allowing researchers to target radio telescopes like the MeerKAT Radio Telescope at specific coordinates. This synergy has revolutionized the field by narrowing the search area from the entire sky to a few thousand "pulsar-like" candidates. By identifying sources that emit gamma rays but lack a known radio counterpart, astronomers can efficiently discover new millisecond pulsars (MSPs) and contribute to the growing catalog of high-energy cosmic objects that are essential for gravitational wave detection.

How many new pulsars were discovered in the TRAPUM UHF survey?

The TRAPUM UHF survey identified 15 new pulsars through a targeted search of 79 unidentified gamma-ray sources using the MeerKAT Radio Telescope. These discoveries include nine rapidly spinning millisecond pulsars (MSPs) and six slow pulsars, significantly expanding the known population of these exotic neutron stars and confirming that seven of the MSPs are directly associated with Fermi-LAT sources.

The research was conducted by the Transients and Pulsars with MeerKAT (TRAPUM) collaboration, an international team utilizing the power of the South African MeerKAT Radio Telescope array. Led by researchers including Ramesh Karuppusamy, Michael Kramer, and Francesca Calore, the team employed a random forest machine-learning technique to sift through the Fermi-LAT Fourth Source Catalogue. This method allowed them to select candidates that displayed the characteristic spectral properties of pulsars before committing high-resolution radio observation time.

Each of the 79 targets was observed for 10 minutes across two separate epochs to ensure the validity of the detections. This strategy not only yielded 15 new objects but also enabled joint radio and gamma-ray pulsar timing. By aligning the radio pulses with the gamma-ray data from the Fermi space telescope, the team was able to confirm the physical association between the radio-emitting neutron stars and the high-energy gamma-ray emissions detected from orbit.

How does the UHF receiver compare to L-band in detecting pulsars?

The MeerKAT Radio Telescope’s Ultra High Frequency (UHF) receiver, operating between 544 and 1088 MHz, demonstrated superior sensitivity to fainter pulsars compared to traditional L-band searches. By utilizing lower frequencies, the TRAPUM survey achieved a higher detection rate for new gamma-ray millisecond pulsars, proving that the UHF band is more effective for uncovering subtle signals that might be obscured at higher frequencies.

The methodology involved a direct comparison between previous L-band surveys (which operate at higher frequencies around 1284 MHz) and the new UHF data. The results indicated that the UHF band is particularly adept at finding pulsars with steep spectral indices—those that are much brighter at lower frequencies. This technical advantage is crucial for identifying "faint" pulsars that may have been previously overlooked by less sensitive equipment or higher-frequency surveys.

• Frequency Range: UHF (544-1088 MHz) vs. L-band (~1284 MHz).
• Sensitivity: Enhanced detection of objects with low flux density.
• Efficiency: Higher discovery rate per hour of observation for gamma-ray candidates.
• Interstellar Medium: Improved ability to mitigate the effects of dispersion and scattering for certain classes of pulsars.

Cosmic Cannibals: The Discovery of Spider Pulsars

Spider pulsars are rare binary systems where a millisecond pulsar systematically erodes its companion star through intense radiation and high-energy particle winds. These systems are categorized based on the mass of the companion star: Black Widows feature extremely low-mass companions (less than 0.1 solar masses), while Redbacks involve heavier, more substantial companion stars that often eclipse the pulsar's radio signal.

Among the nine millisecond pulsars discovered in the TRAPUM survey, the researchers identified three Black Widows and three Redbacks. These findings are particularly significant because spider pulsars provide a unique laboratory for studying the "recycling" process, where a pulsar spins up to millisecond periods by accreting matter from its partner. The intense pulsar wind in these systems eventually begins to evaporate the companion, leading to a dramatic cosmic death dance that can eventually leave the pulsar isolated.

The discovery of these six spider systems was bolstered by the observation of radio eclipses. In these instances, the gas being stripped from the companion star creates a shroud that periodically blocks the radio pulses from reaching Earth. By measuring these eclipses and estimating the companion mass, Ramesh Karuppusamy and the team can better understand the survival rates of stars in close proximity to neutron stars.

What are the implications for Neutron Star Physics?

The discovery of these 15 pulsars provides critical data for understanding the evolutionary pathways of binary systems and the extreme physics of neutron star matter. By linking radio observations with gamma-ray data, scientists can refine models of pulsar emission mechanisms and explore how these objects transition from slow-rotating stars into the ultra-fast millisecond pulsars used in gravitational wave research.

Multi-wavelength astronomy is essential for a complete picture of the universe. The ability to perform joint timing across the electromagnetic spectrum allows for unprecedented precision in measuring the rotation and orbital dynamics of these stars. This precision is vital for the eventual detection of the nanohertz gravitational wave background, as a larger and more diverse array of timed pulsars increases the sensitivity of global Pulsar Timing Arrays.

Furthermore, the variety of the population found—ranging from slow pulsars to highly energetic MSPs—highlights the diversity of the Fermi-LAT Fourth Source Catalogue. It suggests that many of the remaining unidentified gamma-ray sources in our galaxy are likely neutron stars waiting to be discovered by the next generation of sensitive radio receivers.

What’s Next for the TRAPUM Survey and MeerKAT?

Future observations will focus on long-term timing of these new discoveries to precisely map their orbits and search for additional relativistic effects. The TRAPUM survey continues to scan the sky, with the MeerKAT Radio Telescope serving as a primary precursor for the Square Kilometre Array (SKA), which will eventually become the world's largest and most sensitive radio telescope.

The success of the UHF survey suggests that shifting toward lower-frequency observations could yield even more discoveries in regions of the galaxy previously thought to be empty. Researchers plan to expand the search to include even more candidates from the Fermi-LAT catalogues, potentially uncovering the "missing" population of pulsars that currently eludes our current detection thresholds. As Michael Kramer and other collaborators refine their search algorithms, the synergy between space-based gamma-ray telescopes and ground-based radio arrays will remain the gold standard for pulsar discovery.