Quasi-steady emission from repeating fast radio bursts can be explained by magnetar wind nebula

TL;DR

This work addresses the quasi-steady radio emission from repeating FRBs with compact PRSs by modeling a young magnetar surrounded by a magnetized wind nebula (MWN) and SN ejecta. It develops a time-dependent framework incorporating rotation- and magnetar-flare energy injection into the MWN, and solves coupled kinetic equations for electrons and photons to predict the MWN synchrotron spectrum, the near-source DM evolution, and the MWN/SNR dynamics. Applying the model to FRB 121102, FRB 190520, and FRB 201124, the authors constrain neutron-star parameters ($B_{\rm dip}$, $P_i$), ages ($t_{age}$), and SN ejecta properties under ultra-stripped and core-collapse progenitor scenarios, finding that rotation energy is typically the dominant reservoir for producing the observed PRS; minimum NS ages are set by DM and absorption considerations. The framework yields testable predictions for high-energy signatures and DM/RM evolution, providing a path to distinguish progenitor types and energy-injection modes with future multi-wavelength observations.

Abstract

Among over 1000 known fast radio bursts (FRBs), only three sources - FRB 121102 (R1), FRB 190520 (R2) and FRB 201124 (R3) - have been linked to persistent radio sources (PRS). The observed quasi-steady emission is consistent with synchrotron radiation from a composite of magnetar wind nebula (MWN) and supernova (SN) ejecta. We compute the synchrotron flux by solving kinetic equations for energized electrons, considering electromagnetic cascades of electron-positron pairs interacting with nebular photons. For rotation-powered model, a young neutron star (NS) with age $t_{\rm age}\approx 20\,{\rm yr}$, dipolar magnetic field $B_{\rm dip}\approx (3-5)\times10^{12}\,{\rm G}$ and spin period $P_i\approx 1.5-3\,{\rm ms}$ in an ultra-stripped SN progenitor can account for emissions from R1 and R2. In contrast, R3 requires $t_{\rm age}\approx 10\,{\rm yr}$, $B_{\rm dip}\approx 5.5\times10^{13}\,{\rm G}$ and $P_i\approx 10\,{\rm ms}$ in a conventional core-collapse SN progenitor. For magnetar-flare-powered model, NS aged $t_{\rm age} \approx 25\,/40\,{\rm yr}$ in a USSN progenitor and $t_{\rm age} \approx 12.5\,{\rm yr}$ in a CCSN progenitor explains the observed flux for R1/R2 and R3, respectively. Finally, we constrain the minimum NS age $t_{\rm age,min} \sim 1-3\,{\rm yr}$ from the near-source plasma contribution to observed DM, and $t_{\rm age,min} \sim 6.5-10\,{\rm yr}$ based on the absence of radio signal attenuation.

Figures (8)

• Figure 1: Schematic picture of a magnetar surrounded by magnetized wind nebula and baryonic SN ejecta. Magnetar flares and/or rotationally powered outflows inject particles and magnetic energy into the nebula. Synchrotron radiation, observed as PRS associated with the FRB, is emitted by energetic electrons gyrating within the magnetized nebula. The radio emission is observable once the system becomes optically thin to various processes.
• Figure 2: Constraints on NS parameters $B_{\rm dip}$ and $P_i$ from the nebula energy requirement (red curves), ejecta and nebula contribution to the source DM (green curves), and the NS spindown luminosity (black curves) are shown for FRB 121102 in top row, FRB 190520 in middle row and FRB 201124 in bottom row. In each panel, the solid curves show the results for ($E_{\rm sn}$, $M_{\rm ej}$) = ($10^{50}\,{\rm erg}$, $0.1\,M_{\odot}$) while the dashed curves show results for ($E_{\rm sn}$, $M_{\rm ej}$) = ($10^{51}\,{\rm erg}$, $3.0\,M_{\odot}$). For the magnetic energy injection, we fix $B_{\rm int} = 10^{16}\,{\rm G}$ and $t_{\rm inj} = 0.6\,{\rm yr}$ for all cases. For FRBs 121102 and 190520, we vary $t_{\rm age} = 10\,{\rm yr}$ (left panel), $20\,{\rm yr}$ (middle panel) and $40\,{\rm yr}$ (right panel) as $t_{\rm obs}>5\,{\rm yr}$. In case of FRB 201124, results are shown for $t_{\rm age} = 5\,{\rm yr}$ (left panel), $10\,{\rm yr}$ (middle panel) and $20\,{\rm yr}$ (right panel).
• Figure 3: Effect of NS age ($t_{\rm age}$) on the spectral energy distribution of persistent radio emission is shown for FRB 121102 (190520) [201124] in the left (center) [right] panel. Data from radio observations of each PRS source is shown with filled circles. In each panel, we show the results for USSN/CCSN progenitors using solid/dashed curves, for a fixed ($B_{\rm dip}$, $P_i$) combination and varying $t_{\rm age}$ -- including the best-fit NS age for the respective source. We assume the rotation-powered model with $\epsilon_B=0.01$, $\gamma_b=10^5$, $q_1=1.5$ and $q_2=2.5$.
• Figure 4: The effect of NS dipolar magnetic field $B_{\rm dip}$ (in top row) and initial spin period $P_i$ (in bottom row) on the spectral energy distribution of persistent radio emission is shown for FRB 121102 (190520) [201124] in the left (center) [right] column panels. In each panel, the data from radio observations is shown with filled circles and results for USSN/CCSN progenitors are shown using solid/dashed curves. For each FRB, we fix $t_{\rm age}$ to the best-fit value obtained from Figure \ref{['fig:SED_Mej_tage']} and vary $B_{\rm dip}$ or $P_i$ -- including their best-fit combinations for the respective source. As in Figure \ref{['fig:SED_Mej_tage']}, we assume microphysical parameters corresponding to the rotation-powered model.
• Figure 5: Light curves for the persistent radio emission associated with FRB 121102 (190520) [201124] are shown in the left (center) [right] panel. The corresponding data at various radio frequencies for each source is shown using unfilled circles. As earlier, the results for USSN and CCSN progenitors are shown with solid and dashed curves, respectively. We fix the NS parameters $B_{\rm dip}$, $P_i$ and $t_{\rm age}$ to their best-fit values obtained from Figures \ref{['fig:SED_Mej_tage']} and \ref{['fig:SED_Mej_Bdip_Pi']} for the rotation-powered model.