Long Period Transients (LPTs): a comprehensive review

TL;DR

Long Period Transients (LPTs) are a puzzling class of coherent radio emitters with periods spanning minutes to hours and high polarization. The review synthesizes 12 sources, including WD binary systems hosting low-mass companions and magnetar-like objects, and assesses their energy reservoirs, emission mechanisms, and multiwavelength properties. It argues that rotational energy alone is insufficient to power LPTs, pointing to magnetic or binary-interaction energies and coherent processes such as electron cyclotron maser emission or curvature radiation as plausible pathways, with multiple viable options likely coexisting. The work highlights population and survey biases, outlines future observational strategies, and suggests that LPTs may constitute a significant Galactic population revealed through upcoming wide-field, low-frequency surveys and multiwavelength campaigns.

Abstract

Long Period Transients (LPTs) are a recently identified class of sources characterized by periodic radio bursts lasting seconds to minutes, with flux densities that might reach several tens of Jy. These radio bursts repeat with periodicity from minutes to hours, and they exhibit strong polarization and transient activity periods. To date, about 12 such sources have been identified, which might encompass the same or different physical scenarios. Proposed explanations include binary systems with a white dwarf and a low-mass star companion, slow-spinning magnetars, highly magnetized isolated white dwarfs, and other exotic objects. In a few cases the optical counterpart indeed points toward a white dwarf with a low-mass companion, while in other cases, transient X-ray emission was detected, very common in magnetars. However, despite being able to reproduce partially some of the characteristics of LPTs, all the proposed scenarios find difficulty in explaining the exact physical origin of their bright, highly polarized and periodic radio emission. We review here the state-of-the-art in the observations and interpretation of this puzzling class of radio transients.

Figures (12)

• Figure 1: Radio-transient plane including all transient sources. LPTs radio peaks are reported in green, and WD pulsars in yellow. Data collected from: Keane2018Nimmo2022HurleyWalker2022Rea20222025Natur.642..583W.
• Figure 2: Position distribution of LPTs compared to different source classes in Galactic coordinates. LPTs are represented by green crosses, LPTs associated with a WD system as orange crosses, and the two WD pulsars as yellow crosses. See also Table 1.
• Figure 3: Period ($P$) versus period derivative ($\dot{P}$) diagram for different pulsar classes, with LPTs' periodicity overplotted for comparison (note for most LPTs their $\dot{P}$ are probably not due solely to dipolar losses, as for pulsars). We show isolated ATNF radio pulsars 2005AJ....129.1993M (dots), pulsars with magnetar-like X-ray emission ReaDeGrandis2025 (stars), including the long-period magnetar 1E 161348-5055 2006Sci...313..814D2016ApJ...828L..13R, X-ray Dim Isolated NSs (XDINSs; squares) and Central Compact Objects (CCOs; triangles). The three long-period radio pulsars are reported as larger circles Tan-etal20182022NatAs...6..828C44sP_2025. LPTs are represented with green crosses, the LPTs associated with a WD system as orange crosses, and the two WD pulsars as yellow crosses (their period derivatives are estimated from Pelisoli2022Pelisoli2023). Vertical arrows represent $\dot{P}$ upper limits, while only a vertical line is plotted for CHIME J1634$+$44 that has a negative $\dot{P}$ measurement. The oblique lines represent different configurations of the pulsar death lines: the dashed lines correspond to the theoretical death-lines for a pure dipole, dotted lines for a twisted dipole, and the solid lines for the twisted multipole configuration 1993ApJ...402..264C2000ApJ...531L.135Z2023Natur.619..487H2024ApJ...961..214R. The shaded region is the death valley for pulsars.
• Figure 4: Radio Pulses stack of data on the LPT GLEAM-X J1627$-$5235 (from HurleyWalker2022) .
• Figure 5: Absolute rotation measure (RM) against dispersion measure (DM) for the ATNF Pulsar Catalogue v2.6.5 2005AJ....129.1993M, with grey and red points indicating, respectively, isolated and binary pulsars. The LPTs with known RMs and DMs are overplotted with black circles with 1-$\sigma$ error bars (for some sources, these are too small to be seen).