Chandra monitoring of the Galactic Centre magnetar SGR J1745-2900 during the initial 3.5 years of outburst decay
Campana, S.; Tiengo, A.; Mereghetti, S.; Ponti, G.; Rea, N.; Torres, D. F.; Esposito, P.; Israel, G. L.; Baganoff, F. K.; Papitto, A.; Mignani, R. P.; Zane, S.; Turolla, R.; Coti Zelati, F.; Pons, J. A.; Haggard, D.; Borghese, A.; Elfritz, J.
Italy, Netherlands, Spain, United Kingdom, Poland, United States, Canada, Germany
Abstract
We report on 3.5 yr of Chandra monitoring of the Galactic Centre magnetar SGR J1745-2900 since its outburst onset in 2013 April. The magnetar spin-down has shown at least two episodes of period derivative increases so far, and it has slowed down regularly in the past year or so. We observed a slightly increasing trend in the time evolution of the pulsed fraction, up to ∼55 per cent in the most recent observations. SGR J1745-2900 has not reached the quiescent level yet, and so far the overall outburst evolution can be interpreted in terms of a cooling hot region on the star surface. We discuss possible scenarios, showing in particular how the presence of a shrinking hotspot in this source is hardly reconcilable with internal crustal cooling and favours the untwisting bundle model for this outburst. Moreover, we also show how the emission from a single uniform hotspot is incompatible with the observed pulsed fraction evolution for any pair of viewing angles, suggesting an anisotropic emission pattern.