No abstract

The magnetar Swift J1818.0-1607 was discovered in 2020 March when Swift detected a 9 ms hard X-ray burst and a long-lived outburst. Prompt X-ray observations revealed a spin period of 1.36 s, soon confirmed by the discovery of radio pulsations. We report here on the analysis of the Swift burst and follow-up X-ray and radio observations. The burst average luminosity was L burst ∼2×10 39 erg s −1 (at 4.8 kpc). Simultaneous observations with XMM-Newton and NuSTAR three days after the burst provided a source spectrum well fit by an absorbed blackbody N H = (1.13±0.03)×10 23 cm −2 and kT=1.16±0.03 keV) plus a power law (Γ=0.0±1.3) in the 1-20 keV band, with a luminosity of ∼8×10 34 erg s −1 , dominated by the blackbody emission. From our timing analysis, we derive a dipolar magnetic field B∼7×10 14 G, spin-down luminosity~É 1.4 10 rot 36  erg s −1 , and characteristic age of 240yr, the shortest currently known. Archival observations led to an upper limit on the quiescent luminosity <5.5×10 33 erg s −1 , lower than the value expected from magnetar cooling models at the source characteristic age. A 1 hr radio observation with the Sardinia Radio Telescope taken about 1 week after the X-ray burst detected a number of strong and short radio pulses at 1.5 GHz, in addition to regular pulsed emission; they were emitted at an average rate 0.9 min −1 and accounted for ∼50% of the total pulsed radio fluence. We conclude that Swift J1818.0-1607 is a peculiar magnetar belonging to the small, diverse group of young neutron stars with properties straddling those of rotationally and magnetically powered pulsars. Future observations will make a better estimation of the age possible by measuring the spin-down rate in quiescence.

Neutron star models with maximum mass close to 2 M⊙ reach high central densities, which may activate nucleonic and hyperon direct Urca neutrino emission. To alleviate the tension between fast theoretical cooling rates and thermal luminosity observations of moderately magnetized, isolated thermally-emitting stars (with Lγ ≳ 1031 erg s−1 at t ≳ 105.3 yr), some internal heating source is required. The power supplied by the internal heater is estimated for both a phenomenological source in the inner crust and Joule heating due to magnetic field decay, assuming different superfluidity models and compositions of the outer stellar envelope. It is found that a thermal power of W(t) ≈ 1034 erg s−1 allows neutron star models to match observations of moderately magnetized, isolated stars with ages t ≳ 105.3 yr. The requisite W(t) can be supplied by Joule heating due to crust-confined initial magnetic configurations with (i) mixed poloidal-toroidal fields, with surface strength Bdip = 1013 G at the pole of the dipolar poloidal component and ∼90 per cent of the magnetic energy stored in the toroidal component; and (ii) poloidal-only configurations with Bdip = 1014 G.