Chandra monitoring of the Galactic Centre magnetar SGR J1745-2900 during the initial 3.5 years of outburst decay

TL;DR

This study presents a 3.5-year Chandra observation of the Galactic Centre magnetar SGR J1745-2900, revealing its outburst decay, spin-down episodes, and surface cooling, supporting the untwisting bundle model over crustal cooling.

Contribution

It provides detailed long-term monitoring data and interprets the outburst evolution, favoring the untwisting bundle model and highlighting anisotropic emission patterns.

Findings

Multiple spin-down episodes observed.

Outburst decay consistent with a shrinking hot spot.

Emission pattern likely anisotropic.

Abstract

We report on 3.5 years of Chandra monitoring of the Galactic Centre magnetar SGR J1745-2900 since its outburst onset in April 2013. 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 about 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 hot spot in this source is hardly reconcilable with internal crustal cooling and favors the untwisting bundle model for this outburst. Moreover, we also show how the emission from a single uniform hot spot is incompatible with the observed pulsed fraction evolution for any pair of viewing angles, suggesting an anisotropic emission pattern.