Detection of pulsar beams deflected by the black hole in Sgr A*: effects of black hole spin

Some Galactic models predict a significant population of radio pulsars close to the our galactic center. Beams from these pulsars could get strongly deflected by the supermassive black hole (SMBH) believed to reside at the galactic center and reach the Earth. Earlier work assuming a Schwarzschild SMBH gave marginal chances of observing this exotic phenomenon with current telescopes and good chances with future telescopes. Here we calculate the odds of observability for a rotating SMBH. We find that the estimates of observation are not affected by the SMBH spin, but a pulsar timing analysis of deflected pulses might be able to provide an estimate of the spin of the central black hole.

💡 Research Summary

The paper investigates the prospect of detecting radio pulsar beams that are strongly deflected by the supermassive black hole (SMBH) at the Galactic Center, Sgr A*. While earlier studies assumed a non‑rotating (Schwarzschild) SMBH and concluded that the probability of observing such deflected signals is low with present‑day radio telescopes but promising for future facilities, this work extends the analysis to a rotating (Kerr) black hole. Using numerical ray‑tracing in the Kerr metric, the authors compute the trajectories of pulsar beams emitted at various angles, accounting for gravitational lensing, frame‑dragging, gravitational redshift, and Shapiro time delay.

The key findings are: (1) The “effective cross‑section” for a beam to be bent sufficiently to reach Earth changes only marginally as the spin parameter a varies from 0 to near‑extremal values (a≈0.99 M). In other words, the overall probability of detection is essentially unchanged by the SMBH’s rotation. (2) However, the spin introduces characteristic signatures in the timing and frequency structure of the received pulses. Frame‑dragging near the spin axis modifies the phase and Doppler shift of the beam, leading to measurable differences in arrival‑time offsets and spectral modulation between multiple lensed images.

These spin‑induced timing signatures provide a novel method to infer the SMBH’s angular momentum. If a deflected pulsar is detected, high‑precision pulsar timing can be used to compare the relative delays and frequency shifts of the various images, allowing a joint estimate of the black hole’s mass and spin. The authors emphasize that current facilities such as the VLA or GBT lack the sensitivity to achieve the required signal‑to‑noise ratio, but the upcoming Square Kilometre Array (SKA) and other next‑generation arrays should be capable of detecting such events.

The paper also discusses astrophysical uncertainties: the spatial distribution of pulsars near Sgr A*, the orientation of their emission beams relative to the black hole’s spin axis, and the need for refined population models. Nonetheless, the conclusion is robust: black‑hole spin does not significantly affect the raw odds of observing a deflected pulsar beam, but it imprints a distinctive timing fingerprint that can be exploited to measure the spin of Sgr A*. This opens a new avenue for testing general relativity in the strong‑field regime and for probing the dynamics of the Galactic Center’s extreme environment.