The X-ray outburst of the Galactic Centre magnetar during the first 1.5 year

In 2013 April a new magnetar, SGR1745−2900, was discovered as it entered an outburst, at an angular separation of only 2.4 arcsec from the supermassive black hole at the centre of the Milky Way, Sagittarius A*. The new source was detected both in the radio and X-ray bands, with a peak X-ray luminosity L_X ∼ 5 × 10³⁵ erg s⁻¹, and it has a spin-down magnetic field of ∼ 2 × 10¹⁴ G. Here we report on the long-term Chandra (twenty-six observations) and XMM–Newton (eight observations) X-ray monitoring campaign of SGR 1745−2900 from the onset of the outburst in 2013 April until 2014 September. This magnetar shows a slow flux decay compared to other magnetars and a rather inefficient surface cooling. Starquake-induced crustal cooling models alone have difficulty in explaining the high luminosity of the source for the first ∼200 d of its outburst. Additional heating of the star surface from currents flowing in a twisted magnetic bundle is probably playing an important role in the outburst evolution.