Flux Relaxation after two outbursts of the magnetar SGR 1627$-$41 and possible hard X-ray emission

TL;DR

This study tracks the long-term flux decay of magnetar SGR 1627-41 after its 2008 outburst, models crustal cooling, and finds evidence for hard X-ray emission, providing insights into the star's thermal and emission properties.

Contribution

It presents new flux measurements up to 2000 days post-outburst, applies crustal cooling models to interpret the relaxation, and reports evidence of hard X-ray excess in SGR 1627-41.

Findings

Flux has continued to decline since 2011, similar to 1998 outburst relaxation.

Crustal cooling models suggest energy deposition into the inner crust and a low core temperature.

Evidence for hard X-ray emission correlates with previous magnetar observations.

Abstract

We report on the long-term flux relaxation of the magnetar SGR 1627-41 after its 2008 outburst, and evidence for hard X-ray excess measured with NuSTAR. We use new observations made with Chandra and XMM-Newton, and an archival NuSTAR observation which add flux measurements at ~2000 days into quiescence after the 2008 outburst. We find that the source flux has further declined since the last measurement made in 2011, ~1000 days after the outburst in 2008. This trend is similar to the relaxation after the source's 1998 outburst. We use crustal cooling models to reproduce the flux relaxation; if the whole surface of the star is heated in the outbursts, the modeling suggests that the 2008 outburst of SGR 1627-41 deposited energy into the inner crust and that the core temperature of SGR 1627-41 is low (T_c<10^8 K) as previously suggested. On the other hand, if only a small fraction of the surface is heated or the temperature in the crust reached the melting temperature, relaxation at early times requires another emission mechanism. Finally, we report on evidence for hard X-ray emission in SGR 1627-41 which follows the observational correlation suggested by Kaspi & Boydstun (2010) in magnetars.