The MPIfR-MeerKAT Galactic Plane Survey (MMGPS)

© MMGPS (MPIfR/TLS/SARAO)

© MMGPS (MPIfR/TLS/SARAO)

© MMGPS (MPIfR/TLS/SARAO)

The MPIfR-MeerKAT Galactic Plane Survey (MMGPS) is an on-going project involving 3,000 hr of observational data from the MeerKAT radio telescope, and is a joint effort between three international research institutes – the MPIfR, the Thuringian State Observatory (TLS), and the South African Radio Astronomy Observatory (SARAO). The excellent data from the commensal observations in beamforming and interferometric modes will simultaneously address our two key science goals – (I) the discovery and characterisation of new pulsars, and (II) the study of the (magnetised) interstellar medium (ISM) of the Milky Way.

Science Topic I: Pulsar search

Our foremost science objective is to search for new pulsars along the southern Galactic Plane. The southern Galactic plane is the region with the highest star density in the Milky Way, and therefore it has historically resulted in hundreds of discoveries with southern telescopes like Parkes “Murriyang” and the Molongo Observatory Synthesis Telescope. The superior sensitivity of MeerKAT provides an excellent opportunity to revisit this region and discover new faint and/or distant pulsars missed by previous surveys. The following science cases are covered by the MMGPS:

• Population census of the Milky Way. New discoveries improve the statistics on the spin and luminosity distribution of the pulsar population as a whole. The MMGPS has unveiled tens of previously undiscovered millisecond pulsars and canonical pulsars with a large diversity in emission properties, including intermittent pulsars.
• Probing the Galactic ISM and spatial distribution of pulsars. The MMGPS discoveries have a large array of distance and DM values, which provide crucial data points for electron density and magnetic field models in the Milky Way.
• Binary pulsars and tests of gravity. Millisecond pulsars in binary orbits are great probes of gravity, as well as opportunities to measure neutron star and stellar companion masses with timing. These measurements contribute to the probing of compact matter via the neutron star mass distribution, and can also unveil new binary types with exotic binary evolution. The MMGPS implements an acceleration search on short observations to be sensitive to short orbital periods.
• Pulsar Timing Arrays. The discovery of new millisecond pulsars, known for their rotational stability, provides new probes for the search of low-frequency gravitational waves signals with pulsar timing. 

These surveys take advantage of the UHF-, L-, and S-bands receivers installed at MeerKAT, searching for pulsars at 0.8, 1.3, and 2.4 GHz. Each frequency provides its set of advantages. The L-band survey, the original one, has already discovered 78 new pulsars, is a reproduction of the older Parkes surveys but at the higher sensitivity provided by MeerKAT, and taking advantage of the multi-beam nature of the observations. The high-frequency survey at S-band bypasses pulse smearing due to DM and scattering that pulsars deep in the Galactic plane may suffer from, enabling discoveries that would be impossible at lower frequencies. The UHF survey, on the other hand, is expected to unveil many nearby, low-DM pulsars with its highly efficient spatial coverage.

MeerKAT observations are beamformed by the on-site Filterbanking Beamformer user-supplied equipment (FBUSE), and stored and processed in the in-house built Accelerated Pulsar Search User-Supplied Equipment (APSUSE) with a time-domain acceleration pulsar search in a quasi-real time fashion. A dedicated team of observers inspects the pulsar candidates in a weekly fashion, leading to more than 80 discoveries, including 17 binaries. A frequently updated list of all MMGPS discoveries can be consulted here.

The first pulsar discoveries of MMGPS. PSR J1306–6043 is a recycled MSP in a wide orbit with a light WD companion found with the L-band receiver.

The first pulsar discoveries of MMGPS. PSR J1239–6307 is a distant, high-DM and highly scattered young pulsar found with the S-band receiver. It would have been impossible to detect in L-band.

The first pulsar discoveries of MMGPS. PSR 1217–7021 is a very bright, aged discovery, found at low frequency with the UHF receiver.

Science Topic II: ISM studies

The secondary science goal of MMGPS is to further our understanding of the physics governing the ISM, hinging on MeerKAT’s capability to deliver excellent image data products (in terms of the sensitivity to diffuse emission, and the angular resolution) across a wide frequency range (544-2844 MHz). Therefore, in addition to the pulsar search data described above, we concurrently collect visibility data using the standard MeerKAT FX correlator to produce our imaging data products. These images will enable us to address the following interlinked science sub-topics:

1. Galactic magnetism (continuum, polarimetry)

2. Nebulae in the Milky Way such as supernova remnants, HII regions, and planetary nebulae (continuum, polarimetry)

3. Structures and dynamics in the Milky Way (continuum, spectral lines)

4. Astrochemistry (spectral lines)

5. Star formation processes (continuum, spectral lines, polarimetry)

Observation specifications

The MMGPS survey is divided into five distinct components – the snapshot-style observations in UHF-, L-, and S-bands conducted commensally for pulsar search and continuum images, the deep integration of Sgr A* in S-band again for both pulsar search and continuum, and the dedicated CH/HI/OH spectral line observations. The sky coverage and the detailed specifications of the MMGPS observations are shown below.

Sky coverage of the UHF-, L-, and S-band components of MMGPS (adopted from Padmanabh et al. 2023).

For the UHF-, L-, and S-band observations along the southern Galactic Plane, the wide area is covered with a densely packed hexagonal grid, with pointing separation of just FWHM/5 (with FWHM being the full-width at half-maximum of the primary beam). This strategy benefits both the beam forming for pulsar search, as well as the image fidelity of the imaging component of MMGPS.

Technology and innovation

The S-band components of MMGPS utilise the MeerKAT Max-Planck S-band System (MP-SbS) developed by the MPIfR, which is crucial for our survey:

• Pulsar search: enables the detection of high-DM and/or highly scattered pulsars which would have its signal smeared at lower frequencies. This enables, for example, the discovery for the most distant pulsars in the Galactic plane with the MMGPS-S, or the search for pulsars in the Galactic Center, a region known for its high scattering properties. 

• Polarimetry: Enable studies of highly turbulent and magnetised regions of the Milky Way that are heavily depolarised at lower frequencies.

• Continuum: Grant a long lever arm in frequency domain for accurate determination of spectral indices (when combined with the UHF- and L-band data). The higher angular resolution also enables us to study fine-scale structures in star-forming complexes and supernova remnants.

• Spectral lines: Coverage of the CH spectral lines.

In addition, as mentioned above, the MMGPS observations are highly commensal, with most being conducted in both pulsar search and imaging modes simultaneously. Our survey can therefore be seen as a demonstrator for how to maximise the observation efficiency of future radio telescopes, such as the Square Kilometre Array (SKA).

The MMGPS is also one of the reasons for the building and installation of the Accelerated Pulsar Search User-Supplied Equipment (APSUSE) on the MeerKAT site. This cluster has the capacity to store multiple TB of filterbank data arising from multibeam MeerKAT pulsar observations, and process them at high speed in multiple, parallel-running GPUs. This machine has not only enabled the MMGPS, but also by other pulsar surveys on Globular Clusters, unidentified Fermi sources, supernova remnants, and dwarf galaxies.

More information

For a full description of the survey, please refer to the MMGPS survey paper (Padmanabh et al. 2023).