Galactic Centre Pulsars with the SKAO

TL;DR

This paper updates GC pulsar prospects with the SKA, focusing on the SKA-MID design and observing strategies to detect pulsars near Sgr A* and in the inner 100 pc. It outlines how pulsar timing can measure post-Keplerian parameters and SMBH spin/quadrupole with high precision, enabling tests of the no-hair theorem and cosmic censorship, while also addressing GC ISM scattering and DM-related physics. The authors present sensitivity analyses under GC magnetar and strong scattering scenarios, detail FFT/FFA and acceleration-search strategies, and propose targeted beam configurations (around Sgr A* and the inner 100 pc) across multiple frequency bands, including data-archiving and multi-messenger follow-ups with ngVLA and future GW observatories. The work highlights the scientific payoff—from gravity tests to DM constraints and GC evolution—along with practical observing plans, data-management recommendations, and anticipated improvements with SKA2.

Abstract

The detection of a pulsar closely orbiting our Galaxy's supermassive black hole - Sagittarius A* - is one of the ultimate prizes in pulsar astrophysics. The relativistic effects expected in such a system could far exceed those currently observable in compact binaries such as double neutron stars and pulsar white dwarfs. In addition, pulsars offer the opportunity to study the magneto-ionic properties of Earth's nearest galactic nucleus in unprecedented detail. For these reasons, and more, a multitude of pulsar searches of the Galactic Centre have been undertaken, with the outcome of just seven pulsar detections within a projected distance of 100 pc from Sagittarius A*. It is currently understood that a larger underlying population likely exists, but it is not until observations with the SKA have started that this population can be revealed. In this paper, we look at important updates since the publication of the last SKAO science book and offer a focused view of observing strategies and likely outcomes with the updated SKAO design.

Figures (6)

• Figure 1: Evolution of the Faraday RM towards the line-of-sight of PSR J1745$-2$900. Black crosses, red triangles, blue points, and green stars indicate RM values obtained with the Nançay Radio Telescope at 2.5 GHz and Effelsberg at 4.85, 8.35 and 6 GHz, respectively. Adapted from Desvignes+2018.
• Figure 2: Sensitivity of SKA GC surveys for AA4 (left) and AA* (right) configurations, using the GC magnetar scattering timescale. The integration time was assumed to be 4 hr. The central frequency of band 5a, band 5b part1, band 5b part2 are 6.6, 9.55 and 14.15 GHz, respectively, each with a 2.5-GHz bandwidth. This avoids the radio interference within 10.7--12.7 GHz caused by the Starlink satellites. The sensitivity of AA4 at these frequencies are approximately 1250, 1120, 890 m$^2$/K, respectively, and AA* possesses 60% of AA4's sensitivity. For the sensitivity estimate of survey with a 100-m dish, we assumed a system SEFD measured during GC observations with the Effelsberg radio telescope by eatough+2021. For the case of AA4, the survey is anticipated to detect 84%, 79%, 66% of the whole GC pulsar population, and 60%, 59%, 43% of the GC MSP population. For AA*, these are 78%, 71%, 56% for the whole population, and 50%, 46%, 31% for the MSP population.
• Figure 3: Sensitivity of SKA GC surveys for AA4 (left) and AA* (right) configurations, assuming a strong scattering scenario as mentioned in Section \ref{['ssec:GC_ISM']}. The observational setup is the same as in Figure \ref{['fig:weak_scatter']}. The survey is anticipated to detect 64%, 65%, 60% of the whole GC pulsar population when using AA4, and 58%, 60%, 52% when using AA*. As for MSPs, only the survey at Band 5b part 2 would be able to detect a very small fraction of MSPs, namely 8% for AA4 and 6% for AA*.
• Figure 4: $z_{\rm max}$ (upper row) and $w_{\rm max}$ (lower row) values required in presto by a 4-hr search to incorporate the maximum acceleration and jerk for a range of pulsar orbits around the Sgr A*. The spin period of the pulsar was assumed to be 500 and 5 ms for case of an ordinary pulsar and an MSP, respectively. The number of harmonics summed in the search was set to be 16.
• Figure 5: Possible configuration of the beams in a targeted observation around Sgr A*. The center of each of the 16 beams is marked with a black dot. The blue curves show the half-power size of the beams at 9 GHz. For reference we report the position of PSR J1745$-$2900 in green. The dashed circle shows the area where the S-cluster is located and encompasses all the stars in circular orbit around Srg A* with orbital period up to $\sim100$ years. All the pulsars useful for the gravity tests described in Section \ref{['sec:gravity_tests']} would be found in the central beam.