Younes, GeorgeBaring, Matthew G.Kouveliotou, ChryssaHarding, AliceDonovan, SophiaGöğüş, ErsinKaspi, VictoriaGranot, Jonathan2018-07-022018-07-022017Younes, George, Baring, Matthew G., Kouveliotou, Chryssa, et al.. "The Sleeping Monster: NuSTAR Observations of SGR 1806–20, 11 Years After the Giant Flare." <i>The Astrophysical Journal,</i> 851, (2017) IOP Publishing: https://doi.org/10.3847/1538-4357/aa96fd.https://hdl.handle.net/1911/102331We report the analysis of five Nuclear Spectroscopic Telescope Array (NuSTAR) observations of SGR 1806−20 spread over a year from 2015 April to 2016 April, 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 $\dot{\nu }=(-1.25\pm 0.03)\times {10}^{-12}$ Hz s−1, implying a surface dipole equatorial magnetic field $B\approx 7.7\times {10}^{14}$ 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 $\dot{\nu }$ 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 $\sim 8 \% $ with no indication of energy dependence. The NuSTAR spectra are well fit with an absorbed blackbody, ${kT}=0.62\pm 0.06\,\mathrm{keV}$, plus a power law, ${\rm{\Gamma }}=1.33\pm 0.03$. We find no evidence for variability among the five observations, indicating that SGR 1806−20 has reached a persistent and potentially its quiescent X-ray flux level after its 2004 major bursting episode. Extrapolating the NuSTAR model to lower energies, we find that the 0.5–10 keV flux decay follows an exponential form with a characteristic timescale $\tau =543\pm 75$ days. Interestingly, the NuSTAR flux in this energy range is a factor of ~2 weaker than the long-term average measured between 1993 and 2003, a behavior also exhibited in SGR 1900+14. We discuss our findings in the context of the magnetar model.engArticle is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use.Journal articlehttps://doi.org/10.3847/1538-4357/aa96fd