The Sleeping Monster: NuSTAR Observations of SGR 1806-20, 11 Years after the Giant Flare

We 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 ≠ = (-1.25 ± 0.03) × 10^-12 Hz s^-1, implying a surface dipole equatorial magnetic field B ≈ 7.7 × 10¹⁴ 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 ∼8% with no indication of energy dependence. The NuSTAR spectra are well fit with an absorbed blackbody kT = 0.62 ± 0.06 keV, plus a power law γ = 1.33 ± 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 τ = 543 ± 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.