The magnetar Swift J1818.0–1607 was discovered in March 2020 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 } \sim 2 \times 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 \pm 0.03 ) \times 10 ^ { 23 } cm ^ { -2 } and kT = 1.16 \pm 0.03 keV ) plus a power-law ( \Gamma = 0.0 \pm 1.3 ) in the 1–20 keV band , with a luminosity of \sim 8 \times 10 ^ { 34 } erg s ^ { -1 } , dominated by the blackbody emission .
From our timing analysis , we derive a dipolar magnetic field B \sim 7 \times 10 ^ { 14 } G , spin-down luminosity \dot { E } _ { rot } \sim 1.4 \times 10 ^ { 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 \times 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 \sim 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 .