Browsing by Author "Kaspi, Victoria"

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    The Sleeping Monster: NuSTAR Observations of SGR 1806–20, 11 Years After the Giant Flare
    (IOP Publishing, 2017) Younes, George; Baring, Matthew G.; Kouveliotou, Chryssa; Harding, Alice; Donovan, Sophia; Göğüş, Ersin; Kaspi, Victoria; Granot, Jonathan
    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 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.