The X-ray outburst of the Galactic Centre magnetar SGR J1745-2900 during the first 1.5 year

In 2013 April a new magnetar, SGR 1745-2900, was discovered as it entered an outburst, at only 2.4 arcsec angular distance from the supermassive black hole at the centre of the Milky Way, Sagittarius A*. SGR 1745-2900 has a surface dipolar magnetic field of similar to 2 x 10(14) G, and it is the neutron star closest to a black hole ever observed. The new source was detected both in the radio and X-ray bands, with a peak X-ray luminosity L-X similar to 5 x 10(35) erg s(-1). Here we report on the long-term Chandra (25 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 unprecedented data set allows us to refine the timing properties of the source, as well as to study the outburst spectral evolution as a function of time and rotational phase. Our timing analysis confirms the increase in the spin period derivative by a factor of similar to 2 around 2013 June, and reveals that a further increase occurred between 2013 October 30 and 2014 February 21. We find that the period derivative changed from 6.6 x 10(-12) to 3.3 x 10(-11) s s(-1) in 1.5 yr. On the other hand, this magnetar shows a slow flux decay compared to other magnetars and a rather inefficient surface cooling. In particular, starquake-induced crustal cooling models alone have difficulty in explaining the high luminosity of the source for the first similar to 200 d of its outburst, and additional heating of the star surface from currents flowing in a twisted magnetic bundle is probably playing an important role in the outburst evolution.