[Paper Review] Independent discovery of a nulling pulsar with unusual sub-pulse drifting properties with the Murchison Widefield Array

S. J. McSweeney, N. D. R. Bhat+11 more|arXiv (Cornell University)|Jun 1, 2022

Pulsars and Gravitational Waves Research63 references11 citations

TL;DR

This paper reports the independent discovery of the nulling pulsar PSR J0027−1956 using the Murchison Widefield Array (MWA), revealing unusual sub-pulse drifting behavior with both rapid mode switching and slow, systematic drift rate evolution. The pulsar exhibits a 77% nulling fraction and complex drifting patterns, including phase-dependent interruptions, making it a critical test case for refining the carousel model of pulsar emission physics through long-duration, single-pulse analysis at low radio frequencies.

ABSTRACT

We report the independent discovery of PSR J0027-1956 with the Murchison Widefield Array (MWA) in the ongoing Southern-sky MWA Rapid Two-meter (SMART) pulsar survey. J0027-1956 has a period of ~1.306 s, a dispersion measure (DM) of ~20.869 pc cm^-3 , and a nulling fraction of ~77%. This pulsar highlights the advantages of the survey's long dwell times (~80 min), which, when fully searched, will be sensitive to the expected population of similarly bright, intermittent pulsars with long nulls. A single-pulse analysis in the MWA's 140-170 MHz band also reveals a complex sub-pulse drifting behavior, including both rapid changes of the drift rate characteristic of mode switching pulsars, as well as a slow, consistent evolution of the drift rate within modes. In some longer drift sequences, interruptions in the otherwise smooth drift rate evolution occur preferentially at a particular phase, typically lasting a few pulses. These properties make this pulsar an ideal test bed for prevailing models of drifting behavior such as the carousel model.

Motivation & Objective

• To investigate the single-pulse and nulling behavior of PSR J0027−1956, a previously known but poorly studied nulling pulsar.
• To analyze its sub-pulse drifting properties in detail, particularly the coexistence of rapid mode switching and slow drift rate evolution.
• To assess the implications of its complex drifting behavior for the theoretical carousel model of pulsar emission.
• To demonstrate the effectiveness of long dwell-time surveys like SMART in detecting intermittent, bright pulsars with long nulls.

Proposed method

• Conducted single-pulse analysis on MWA observations at 140–170 MHz, focusing on the full 80-minute dwell time per pointing.
• Identified and characterized sub-pulse drift bands by stacking pulses over multiple rotations, measuring drift rates in degrees per pulse period.
• Fitted drift sequences to extract relaxation times (τr) and model the evolution of drift rate over time.
• Classified drifting behavior into distinct modes (A and B), with mode A showing slow drift rate evolution and mode B showing chaotic behavior.
• Analyzed null sequences and their relationship to mode transitions, particularly phase-dependent interruptions in drift sequences.
• Used archival MWA data and follow-up observations to validate and expand on initial detections.

Experimental results

Research questions

• RQ1 How does the drift rate of PSR J0027−1956 evolve over time, and what does this imply for the stability of the carousel model?
• RQ2 What is the relationship between null sequences and sub-pulse drifting behavior in this pulsar?
• RQ3 Why do interruptions in the drift rate evolution occur preferentially at specific pulse phases?
• RQ4 Can the observed complex drifting behavior be explained by extensions to the standard carousel model?
• RQ5 How do the relaxation times of drift sequences compare across different modes, and what does this suggest about the underlying emission mechanisms?

Key findings

• PSR J0027−1956 has a period of 1.306 s, a dispersion measure of 20.869 pc cm⁻³, and a nulling fraction of 77%, indicating strong intermittency.
• The pulsar exhibits complex sub-pulse drifting with both rapid mode switching (between modes A and B) and slow, systematic drift rate evolution within modes.
• In mode A1, the drift rate evolves from ~12°/P to ~40°/P over 285 pulses, with a relaxation time τr ≈ 31 P, indicating a slow stabilization process.
• Interruptions in the smooth drift rate evolution occur preferentially at a specific pulse phase, suggesting phase-locked dynamics or transient mode excursions.
• The drift band structure shows smooth connections across mode switches in some cases, implying continuity in the underlying emission mechanism.
• The pulsar's behavior—especially the coexistence of slow drift rate evolution and phase-dependent interruptions—presents a strong challenge to the standard carousel model and calls for extended theoretical frameworks.