Magnetic White Dwarf - M Dwarf Binaries in Pre-mCV Phase as Special Population of Long-Period Radio Transients

TL;DR

This work addresses the origin of long-period radio transients (LPTs) by proposing that a significant subset arises from magnetic WD–MD binaries in the detached, pre-mCV phase, where asynchronism drives magnetospheric interactions that power emission. The authors systematically evaluate accretion, unipolar-inductor, and magnetosphere-interaction scenarios, arguing that high-accretion states suppress coherent emission and that two magnetospheric channels (UI and MI) naturally produce geometically beamed, beat-modulated radiation via loss-cone-driven maser (LCDM). They derive observationally testable predictions, including a period distribution $f_P(P) \propto P^{(1.67-2.33)}$ and a luminosity function $f_L(L) \propto L^{-(1.80-2.67)}$, as well as propagation-induced polarization changes from Faraday conversion. The results connect LPTs to the broader WD–MD binary population and offer concrete avenues to probe WD magnetic-field origins, the pre-mCV phase, and the magnetic interactions governing these exotic radio transients.

Abstract

Long-period radio transients (LPTs) are a new class of coherent radio sources with periods ranging from minutes to hours. Recently, two LPT sources, ILT J1101+5521 and GLEAM-X J0704-37, with periods of 2-3 hours has been confirmed to originate from white dwarf (WD) -- M dwarf (MD) binaries. In this work, we propose that at least some LPTs originate from the magnetic WD -- MD binaries in the pre-magnetic cataclysmic variables (pre-mCV) phase. The asynchronism between the WD's rotation and the binary's orbital motion allows for the unipolar-inductor mechanism or magnetosphere interaction to operate and accelerate radiating particles, with the dominant process depending on the magnetic moment ratio of the two stars. Under asynchronism condition, both the peak flux and the polarization of radio pulses will be modulated by the beat period. The pre-mCV phase characterized by an extremely low accretion rate provides the relatively clean magnetospheric environment necessary for a loss-cone-driven maser (LCDM) mechanism to operate, producing the LPT emission. The observed pulse duty cycle of $10^{-3}-10^{-1}$ is attributed to a beaming effect modulated by the binary's magnetic geometry. Furthermore, the magnetized environment of a WD--MD binary is conducive to Faraday conversion with weak coupling, which implies that the polarization state of LPTs should vary significantly at different periods. Finally, we predict that LPTs from WD--MD binaries should exhibit a period distribution following $f_P(P)dP \propto P^{(1.67-2.33)}dP$ and a luminosity function described by $f_L(L)dL \propto L^{-(1.80-2.67)}dL$, which can be tested by the future large sample.

Figures (8)

• Figure 1: Period distributions of LPTs shown alongside populations of CVs and mCVs (including Polars, LARPs, and IPs). The population data are compiled as follows: CVs from the SDSS I--IV archives Inight23; Polars and LARPs from the PolarCat catalog Schwope25; IPs from the online catalog maintained by Koji Mukai's "The Intermediate Polars" homepage (https://asd.gsfc.nasa.gov/Koji.Mukai/iphome/iphome.html, also see Mukai17). The data of LPTs is from Table 1 of Qu25. The vertical dashed and dotted lines mark the periods of the confirmed WD--MD binary LPTs, ILT J1101+5521 (125.5 min) and GLEAM-X J0704-37 (174 min), respectively. The grey shaded region indicates periods below the canonical $\sim$80-minute orbital minimum for CVs/mCVs.
• Figure 2: Schematic illustration of the three primary evolutionary stages in a magnetic white dwarf (WD)--M dwarf (MD) binary. The panels depict: (a) the accretion phase, characterized by an accretion stream or an accretion disk around the WD; (b) the unipolar-inductor phase; and (c) the magnetosphere-interaction phase, where direct interaction between the two magnetospheres leads to magnetic reconnection (highlighted in orange).
• Figure 3: Schematic evolution of the mass transfer rate ($\dot{M}$) as a function of orbital period for a WD--MD binary. Top panel: Evolution of the mass transfer rate assuming the WD magnetic field originates from the fossil field at birth Braithwaite04 or the dynamo action during common-envelope evolution (CEE) Tout08. The pre-mCV phase includes the post-common-envelope binary (PCEB) phase. Bottom panel: Evolution of the mass transfer rate assuming the WD magnetic field originates from the crystallization- and rotation-driven dynamo, adapted from Schreiber21. In this scenario, the WD acquires its magnetic field during the pre-mCV phase, marking the evolutionary transition from a non-magnetic CV to a fully magnetic CV. The evolutionary sequence proceeds from a PCEB, non-magnetic CV, pre-mCV to mCV. In the pre-mCV phase of each possible scenario, the accretion rate can be significantly lower than the critical value $\dot{M}_{\rm cr}$ for typical parameters (Eq.(\ref{['dotMc']})), as indicated by the horizontal dashed line.
• Figure 4: Schematic diagram of the unipolar-inductor mechanism applied to a WD -- MD binary. The magnetic WD is assumed to have a dipole field, threading the non-magnetic MD. In the frame of the non-magnetic MD, there is an induced electric field of $\bm{E}'=(\bm{v}_{\rm rel}\times\bm{B})/c$. $\theta$ denotes the polar angle, $\psi$ denotes the angle between the field line and the magnetic moment $\bm\mu_s$, and $\delta\psi_{\max}$ denotes the angle difference of $\psi$ that can cover the MD companion.
• Figure 5: Schematic diagram of the magnetosphere interaction applied to the WD -- MD binary. Both the magnetic WD and the magnetic MD are assumed to have a dipole field with magnetic moments of $\bm{\mu_s}$ and $\bm{\mu}_c$. The magnetic reconnetion region has a length scale of $L$, a thickness of $H$ and is at $R_{\rm rec}$ from the WD center. Particles are accelerated at the magnetic reconnection region and move to the stars along the field lines.