A survey for radio pulsars and transients in the 10 pc region around Sgr A*

Here we report on a new survey for pulsars and transients in the 10 pc region around Sgr A* using the Effelsberg radio telescope at frequencies between 4 to 8 GHz. Our calibrated full-Stokes data were searched for pulsars and transients using PulsarX, TransientX and PRESTO. Polarisation information is used in the scoring of the candidates. Our periodicity acceleration and jerk searches allowed us to maintain good sensitivity towards binary pulsars in $\gtrsim$ 10-hr orbits. In addition we performed a dedicated search in linear polarisation for slow transients. While our searches yielded no new discovery beyond the redetection of the magnetar SGR J1745-2900, we report on a faint single pulse candidate in addition to several weak periodicity search candidates. After thoroughly assessing our survey’s sensitivity, we determined that it is still not sensitive to a population of millisecond pulsars. Next generation radio interferometers can overcome the limitations of traditional single-dish pulsar searches of the Galactic Centre.

💡 Research Summary

This paper presents a dedicated search for radio pulsars and transient radio sources within a 10 pc radius of the Galactic Centre (GC) black hole Sgr A*, using the 100‑m Effelsberg telescope at frequencies between 4 and 8 GHz. The authors recorded full‑Stokes data with the PSRIX backend, covering two contiguous 2‑GHz sub‑bands centred at 5 GHz and 7 GHz, each split into 2048 frequency channels and sampled at 131 µs. The observing strategy tiled the inner 10 pc region with 37 pointings arranged in three concentric rings, each pointing having a half‑power beam width of 1.55 arcmin at 8 GHz, yielding a total sky coverage of ~0.025 deg². Each pointing was observed for one hour, and a 2‑minute noise‑diode scan was taken before each observation for polarimetric calibration; monthly observations of NGC 7027 were used to monitor system sensitivity.

Data processing began with manual RFI excision (≈15 % of the band was flagged), followed by full‑Stokes calibration using the measured Mueller matrix for each frequency channel. The calibrated data were stored as 32‑bit floats to avoid quantisation loss. The authors implemented three complementary search pipelines:

1. Periodicity search in total intensity (Stokes I). Using PulsarX for dedispersion (DM 0–5000 pc cm⁻³, step 2 pc cm⁻³) and PRESTO for Fourier‑domain searches, they performed a low‑acceleration pass (max drift 50 Fourier bins, 16 harmonics summed) and a high‑acceleration/jerk pass (up to 200 drift bins and 600 jerk bins, 8 harmonics summed). Candidates were sifted to remove harmonics and non‑contiguous DM detections, then folded with PulsarX, which now supports full‑Stokes folding and incorporates jerk information. For each folded candidate, a Faraday rotation measure (RM) synthesis was performed over –100 000 to +100 000 rad m⁻² (step 5 rad m⁻²), and the RM that maximised linear polarisation was recorded. Up to 210 candidates per pass were folded due to memory constraints, and all were visually inspected.

2. Single‑pulse search in total intensity. TransientX was adapted to handle full‑Stokes data and to perform RM synthesis on each detected pulse. Events with S/N > 6 were saved for visual inspection.

3. Slow‑transient search in linear polarisation. To mitigate red‑noise that dominates total‑intensity time series, the calibrated data were time‑scrunched by a factor of 128 (≈16.7 ms resolution). The data were then dedispersed with coarse DM steps (200 pc cm⁻³) up to 5000 pc cm⁻³, and Faraday‑corrected over a very wide RM range (–500 000 to +500 000 rad m⁻², step 40 rad m⁻²). The magnitude of the complex linear polarisation L = Q + iU was summed across frequency and searched with PRESTO’s single‑pulse routine using box‑car widths up to 300 bins and a detection threshold of S/N = 7. Simulated injections of linearly polarised pulses of widths 0.2, 1.5 and 3 s demonstrated that the linear‑polarisation pipeline could recover wide pulses without spurious detections, whereas the total‑intensity pipeline failed due to red‑noise.

Sensitivity was estimated using the modified radiometer equation (Cordes & Chernoff 1997), incorporating system temperature, gain (G = 1.5 K Jy⁻¹), effective bandwidth (~3.4 GHz after RFI removal), integration time, number of summed polarisations (2), and harmonic summing factors. Pulse broadening from sampling, intra‑channel dispersion, and interstellar scattering (τ₁GHz ≈ 1.3 s, scaling as ν⁻³·⁸) was included to compute the effective duty cycle. The analysis shows that at 5–8 GHz the scattering time drops to ≤ 7 ms, still insufficient to detect canonical millisecond pulsars (P ≈ 1–5 ms) with the current setup.

The survey redetected the known magnetar SGR J1745‑2900 with the expected dispersion (DM ≈ 1778 pc cm⁻³) and rotation measure (RM ≈ –66 960 rad m⁻²). Beyond this, a handful of weak periodicity candidates (S/N ≈ 6–8) and a faint single‑pulse candidate were identified, but none reached the significance required for a confident discovery. The authors conclude that, despite the sophisticated acceleration/jerk searches and the novel linear‑polarisation transient pipeline, the Effelsberg survey is not yet sensitive enough to detect the predicted population of ~1000 recycled millisecond pulsars in the central parsec.

In the discussion, the authors argue that the primary limitations are residual interstellar scattering, red‑noise in total intensity, and the modest collecting area of a single 100‑m dish. They advocate for next‑generation interferometric arrays (e.g., SKA‑Mid, ngVLA) that can provide larger effective apertures, wider instantaneous bandwidths, and the ability to form multiple tied‑array beams, thereby overcoming scattering and sensitivity constraints. Moreover, they highlight that linear‑polarisation searches, especially when combined with high‑RM de‑rotation, can reveal slow, highly polarised transients that would be missed in conventional total‑intensity pipelines.

Overall, this work demonstrates the feasibility of full‑Stokes, acceleration‑aware pulsar searches in the GC, introduces a promising linear‑polarisation transient detection technique, and underscores the need for more sensitive, wide‑band interferometric facilities to finally uncover the elusive millisecond pulsar population orbiting Sgr A*.