Long Term Variability in the pre - Super Giant Flare Epoch

The low energy (< 10 keV) X-ray emission of the Soft Gamma-ray Repeater SGR 1806–20 has been studied by means of four XMM-Newton observations carried out in the last two years, the latter performed in response to a strong sequence of hard X-ray bursts observed on 2004 October 5. The source was caught – 2 – in different states of activity: over the 2003-2004 period the 2-10 keV flux doubled with respect to the historical level observed previously. The long term raise in luminosity was accompanied by a gradual hardening of the spectrum, with the power law photon index decreasing from 2.2 to 1.5, and by a growth of the bursting activity. The pulse period measurements obtained in the four observations are consistent with an average spin-down rate of 5.5×10 −10 s s −1 , higher than the values observed in the previous years. The long-term behavior of SGR 1806–20 exhibits the correlation between spectral hardness and spin-down rate previously found only by comparing the properties of different sources (both SGRs and Anomalous X-ray Pulsars). The best quality spectrum (obtained on 6 Septem-ber 2003) cannot be fitted by a single power law, but it requires an additional blackbody component (kT BB =0.79 keV, R BB = 1.9 (d/15 kpc) 2 km), similar to the spectra observed in other SGRs and in Anomalous X-ray Pulsars. No evidence for spectral lines was found in the persistent emission, with equivalent width upper limits in the range 30-110 eV. However, a ∼3σ deviation at 4.2 keV is present in the cumulative spectrum of 69 bursts detected in September-October 2004. The multi-wavelength spectral energy distribution of SGR 1806–20, extending from the IR band to 150 keV as obtained from simultaneous data from the INTEGRAL satellite and the ESO Very Large Telescope, is also presented.