Evidence of the association of repeating fast-radio-burst sources with fast-spinning super-twisted magnetars

TL;DR

This work tests a geometrical, magnetar-based emission model against two CHIME/FRB repeater sources to infer neutron-star spin and magnetic geometry from spectro-temporal burst morphologies. By jointly fitting all bursts per repeater using a seven-parameter global model and many local sub-burst parameters, the authors recover fast spins on the order of $P_*$ in the range $\sim$0.8–2.3 s and a dominant toroidal magnetic component decaying as $B_\phi\propto r^{-p}$ with $p\approx4$. Emission regions are found at altitudes of roughly $\sim100R_*$ with small transverse sizes ($a\lesssim10^{-2}r$) and modest beaming ($\Omega\sim5\times10^{-2}$ rad), implying a minimum Lorentz factor $\gamma\gtrsim20$ and spin-down ages potentially in the centuries to decades range. The results point to young, highly twisted magnetars (possibly low-field magnetars) as repeat FRB progenitors, bridging burst morphologies with global magnetospheric structure and offering a framework for constraining FRB emission physics with future broad-band and polarization data.

Abstract

Context: Fast radio bursts (FRBs) are bright millisecond radio events of unknown extragalactic origin. Magnetars are among the main contenders. Some sources, the repeaters, produce multiple events but so far generally without the characteristic periodicity that one could associate with the spin of a neutron star. Aims: Assuming that the bursts originate from a magnetar magnetosphere, we aim to fit our geometrical model to the two main repeaters of the CHIME/FRB catalogue, namely FRB 20180814A and FRB 20180916B, and thus characterise the star. Methods: The model can generate dynamic spectra that can be directly compared to FRBs. We applied nested sampling in order to evaluate the main parameters of the model. These parameters being common to all bursts from a given repeater, they were fitted together as a single dataset. Results: We constrained the spin and magnetic parameters of the star, which were encoded into burst spectro-temporal morphologies. We estimate that a very strong toroidal magnetic component together with spin periods of, respectively, $2.3\_{-0.5}^{+0.5} ~ \rm s$ and $0.8\_{-0.2}^{+0.1} ~ \rm s$ best explain the data. We argue that this points towards young magnetars with super-twisted magnetospheres, and possibly low-field magnetars.

Figures (10)

• Figure 1: Illustration of the model, from voisin_geometrical_2023. The emitter travels from left to right, emitting in a forward cone (the cone is illustrative, in practice we use a smooth angular profile in this work). Emission frequency varies by $\Delta f$ as it travels radially by $\Delta r$. Dotted lines show the observer's direction, with $\gamma$ the angle with respect to the path. The two thick ticks delimit the visible segment beyond which no emission can be seen from within the cone. The characteristic emission frequency, $f_c$, maps to the radius, $r_c$, at the centre of the segment. The visible segment being in a rotating frame, it varies with time until it vanishes.
• Figure 2: Residuals of a particular event from B, FRB20190519B, using best-fit parameters. Left-hand side: CHIME/FRB dynamic spectrum (middle), intensity curve (top), and spectrum (left). The one-$\Omega$ envelope (or emission angle iso-contour) of the emission beam is shown in blue, with a dashed purple line showing its characteristic frequency as a function of time. The vertical dashed red lines mark the injection time for each sub-burst. In the spectrum and intensity plots the blue lines represent the data, and the orange lines the model. Right-hand side: Same but showing the residuals between the data and the model.
• Figure 3: Diagram of the two-stage fitting process. Global parameters are iterated using nested sampling. For a fixed set of global parameters, the $n_e$ envelopes (one per event) each have a set of local parameters comprising two envelope parameters, $f_0, T_0$ (see main text), and parameters for each of the $n(k)$ sub-bursts (also denoted as $n_{b/e}$ in the main text), with $k$ the envelope index. Local parameters were fitted independently to the dynamic spectrum of each event using least-square minimisation. Initial guesses were obtained thanks to pre-processing.
• Figure 4: Posterior distribution of $\chi^2$ for source A (left) and B (right). As a reference, the orange lines represent the theoretical $\chi^2$ distribution for a Gaussian posterior with 7 degrees of freedom.
• Figure 5: Correlation plot of the posterior sample of source A (a) and B (b) marginalised over all but three parameters: the surface emission frequency, $\log_{10}(f_*)$, the surface toroidal-to-poloidal magnetic-field ratio, $\log_{10}(\alpha)$, and the radial power-law exponent of the toroidal field, $p$. A Gaussian mixture model has been fitted to this marginalised distribution. On the 2D correlation plots, grey iso-contours of the model are shown together with coloured ellipses representing the one-standard-deviation area of each mode. The same colours are used on the 1D plots to represent the parts of the histogram attributed to each mode, and the line represents the model. In particular, the low mode is blue, the medium mode is orange, and the high mode is green.