Discovery of a 62-min long-period transient with near-orthogonal rotator geometry

The discovery is J155543.7−563102 (LPT J1555−5631), a 62.2-minute long-period transient found serendipitously in MeerKAT and ASKAP survey data. It emitted bright (10–30 mJy), steep-spectrum (α ∼ −1.5), highly polarized radio pulses during a 15-hour active window in May 2022, with no other quiescent or pulsed emission detected before or after over about 5 years.

Its mean light curve shows two alternating pulses per cycle: a main pulse (MP) and an interpulse (IP) separated by Δφ = 165.4°, with similar amplitudes but distinct polarization. The wider MP (3.9%) is partially linearly and circularly polarized and has a constant polarization position angle, whereas the narrow IP (1.1%) is almost 100% circularly polarized.

The authors classify LPT J1555−5631 as a near-orthogonal rotator: emission from both magnetic poles is visible during one rotation, and the magnetic and rotation axes are nearly perpendicular. The 14.6° deviation from an antipodal separation and the nearly constant polarization position angle suggest the emission does not arise from the standard dipolar polar-cap geometry of ordinary pulsars, but instead comes from a broader, more complex magnetosphere.

These observed properties can be accommodated by extended emission regions and favor coherent emission mechanisms such as the electron cyclotron maser instability, though they do not uniquely determine the progenitor or emission physics. If these inferences apply more broadly to the long-period transient population, they could explain why their beam widths are decoupled from rotation periods and the prevalence of flat polarization position angles, and may favor an elevated incidence of orthogonal rotators.