http://arxiv.org/abs/2004.04083
The magnetar Swift J1818.0-1607 was discovered in March 2020 when Swift revealed a 9 ms hard X-ray burst and long-lived outburst. Prompt X-ray observations revealed a spin period of $1.36$ s, soon confirmed by the detection 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 $L_{\rm burst} \sim2\times 10^{39}$ 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 ($N_{\rm H} =(1.22\pm0.03) \times 10^{23}$ cm$^{-2}$ and $kT = 1.19\pm0.02$ keV) plus a power-law ($\Gamma=0.1_{-1.4}^{+1.2}$) in the 0.3-20 keV band, with a luminosity of $\sim$$7\times10^{34}$ erg s$^{-1}$, dominated by the blackbody emission. From our timing analysis, we derive a dipolar magnetic field $B \sim 7\times10^{14}$ G, spin-down luminosity $\dot{E}_{\rm rot} \sim 1.4\times10^{36}$ erg s$^{-1}$ and characteristic age of 240 yr, the shortest currently known. Archival observations led to an upper limit on the quiescent luminosity $< 6.5\times10^{33}$ erg s$^{-1}$, lower than the value expected from magnetar cooling models. A 1 hr radio observation with the Sardinia Radio Telescope 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.
P. Esposito, N. N.Rea, A. Borghese, et. al.
Thu, 9 Apr 20
37/54
Comments: 9 pages, 4 figures, 1 table; submitted to ApJL
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