The Sleeping Monster: NuSTAR observations of SGR 1806-20, 11 years after the Giant Flare
George Younes, Matthew G. Baring, Chryssa Kouveliotou, Alice Harding,, Sophia Donovan, Ersin Gogus, Victoria Kaspi, Jonathan Granot

TL;DR
This study analyzes NuSTAR observations of SGR 1806-20 over a year, revealing its return to a quiescent state with stable X-ray flux and consistent spectral properties, 11 years after its giant flare, providing insights into magnetar behavior.
Contribution
First detailed long-term X-ray monitoring of SGR 1806-20 post-giant flare, showing persistent quiescent emission and stable spectral characteristics.
Findings
SGR 1806-20's spin frequency follows a linear trend with a magnetic field of ~7.7×10^14 G.
The source's X-ray flux has stabilized at a lower level than historical averages.
No short bursts or transient activity detected in NuSTAR data.
Abstract
We report the analysis of 5 NuSTAR observations of SGR 1806-20 spread over a year from April 2015 to April 2016, more than 11 years following its Giant Flare (GF) of 2004. The source spin frequency during the NuSTAR observations follows a linear trend with a frequency derivative Hz s, implying a surface dipole equatorial magnetic field G. Thus, SGR 1806-20 has finally returned to its historical minimum torque level measured between 1993 and 1998. The source showed strong timing noise for at least 12 years starting in 2000, with increasing one order of magnitude between 2005 and 2011, following its 2004 major bursting episode and GF. SGR 1806-20 has not shown strong transient activity since 2009 and we do not find short bursts in the NuSTAR data. The pulse profile is complex with a pulsed fraction of…
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