The New Magnetar Sgr J1830-0645 In Outburst

The detection of a short hard X-ray burst and an associated bright soft X-ray source by the Swift satellite in 2020 October heralded a new magnetar in outburst, SGR J1830-0645. Pulsations at a period of similar to 10.4 s were detected in prompt follow-up X-ray observations. We present here the analysis of the Swift/Burst Alert Telescope burst, of XMM-Newton and the Nuclear Spectroscopic Telescope Array observations performed at the outburst peak, and of a Swift/X-ray Telescope monitoring campaign over the subsequent month. The burst was single-peaked, lasted similar to 6 ms, and released a fluence of 5 x 10(-9) erg cm(-2) (15-50 keV). The spectrum of the X-ray source at the outburst peak was well described by an absorbed double-blackbody model plus a power-law component detectable up to similar to 25 keV. The unabsorbed X-ray flux decreased from similar to 5 x 10(-11) to similar to 2.5 x 10(-11) erg cm(-2) s(-1) one month later (0.3-10 keV). Based on our timing analysis, we estimate a dipolar magnetic field 5.5 x 10(14) G at pole, a spin-down luminosity 2.4 x 10(32) erg s(-1), and a characteristic age 24 kyr. The spin modulation pattern appears highly pulsed in the soft X-ray band, and becomes smoother at higher energies. Several short X-ray bursts were detected during our campaign. No evidence for periodic or single-pulse emission was found at radio frequencies in observations performed with the Sardinia Radio Telescope and Parkes. According to magneto-thermal evolutionary models, the real age of SGR J1830-0645 is close to the characteristic age, and the dipolar magnetic field at birth was slightly larger, similar to 10(15) G.