Is the black-widow pulsar PSR J1555-2908 in a hierarchical triple system?

TL;DR

This study analyzes long-term gamma-ray data of the black-widow pulsar PSR J1555-2908, suggesting its irregular spin variations may be due to a third object in a hierarchical triple system rather than intrinsic noise.

Contribution

It proposes a hierarchical triple system model to explain pulsar timing variations, offering an alternative to traditional timing noise explanations.

Findings

Timing variations are better explained by a third object in a wide orbit.

Hierarchical triple system model fits the data slightly better than timing noise models.

Future observations are needed to confirm the model.

Abstract

The 559 Hz black-widow pulsar PSR J1555-2908, originally discovered in radio, is also a bright gamma-ray pulsar. Timing its pulsations using 12 yr of Fermi-LAT gamma-ray data reveals long-term variations in its spin frequency that are much larger than is observed from other millisecond pulsars. While this variability in the pulsar rotation rate could be intrinsic "timing noise", here we consider an alternative explanation: the variations arise from the presence of a very-low-mass third object in a wide multi-year orbit around the neutron star and its low-mass companion. With current data, this hierarchical-triple-system model describes the pulsar's rotation slightly more accurately than the best-fitting timing-noise model. Future observations will show if this alternative explanation is correct.