Late-blooming magnetars: awakening as long period transients after a dormant cooling epoch

Abstract

Long-period transients are an elusive class of compact objects uncovered by radio surveys. While magnetars are a leading candidate for those sources that appear isolated, several observational properties challenge the established evolutionary framework: (i) low quiescent X-ray luminosities, (ii) $\sim$hour-long rotational periods, and (iii) highly-variable radio flux. It is shown via magnetothermal modelling that, if electric currents thread the fluid core at the time of crust freezing, the neutron star remains multiband silent for an initial period of approximately 0.1 Myr while cooling passively. Once the crust becomes cold enough, the Hall effect begins to dominate the magnetic evolution, triggering crustal failures that inject magnetospheric twist that initiates radio pulsing while depleting rotational kinetic energy from an already-slow star. Depending on where electric currents circulate, such 'late-blooming' magnetars manifesting as long-period transients may thus form a distinct branch from soft gamma repeaters and anomalous X-ray pulsars.

Figures (7)

• Figure 1: Evolutionary tracks, in the $T_b$-$B$ plane, for representative CC (solid) and CT (dashed) models. Stellar age is indicated by grey labels along the lines. The background colour contours show the magnetic Reynolds number ($\mathcal{R}_{m}$), calculated at a density of $\rho \approx 5 \times 10^{10}$ g/cm$^3$ and polar latitude, with redder shades indicating greater $\mathcal{R}_{m}$.
• Figure 2: Comparison of simulated (lines) and observed (points) luminosities for magnetars and LPTs. Solid (CC) and dashed (CT) tracks represent the X-ray luminosity curves as a function of the 'real' age as determined by the simulation. For the CC models, results are shown for two initial field strengths of $2 \times 10^{14}$ G (lower) and $2 \times 10^{15}$ G (upper). The color bar indicates the polar magnetic field intensity. Galactic magnetars are represented by circles as a function of their characteristic age. For seven isolated LPTs, upper limits on the X-ray luminosity are marked by the vertical arrows under horizontal lines as reliable age estimates are unavailable for these objects.
• Figure 3: Histograms of waiting times (left), energies (middle), and colatitudes (right) of failure events counted during evolution for the representative CT (top panel) and CC (bottom) models up to ages of 2 Myr and 1.375 Myr, respectively. Maximum-likelihood and method of moments fits, log-normal for both sets of waiting times, are overlaid with uncertainties. For the energy distributions, a gamma (log-normal) fit best represents the simulation output for the CT (CC) model.
• Figure 4: Time evolutions of the normalised ($\bar{\zeta} = 10^{-3}$) cumulative failure (dotted red), magnetic ($E_{\mathrm{mag}}$, dashed green), and rotational ($E_{\mathrm{rot}}$, blue) energies for representative CC (left) and CT (right) cases. The solid (dashed) blue curve corresponds to the spindown law \ref{['eq:braking']} for an orthogonal (aligned) rotator.
• Figure 5: Spindown trajectories for the CT model with various efficiencies spanning the range $7 \leq \bar{\zeta}/(10^{-4}) \leq 14$ in increments of $10^{-4}$ (with $\bar{\zeta}$ increasing from left to right). The spin period of DA J1832 ($P \approx 44.2$ min) is overlaid by the dashed, horizontal line for reference. Models with $\bar{\zeta} \gtrsim 8 \times 10^{-4}$ are sufficiently slow by $t \sim 2$ Myr to match the observed pulse period of DA J1832.