SGR 1806-20: X-ray and IR post-Giant Flare monitoring

ATel #645; Nanda Rea (SRON), GianLuca Israel (OAR), Stefano Covino (OAB), Andrea Tiengo (IASF-MI), Elisa Nichelli (OAR), Roberto Turolla (Univ. Padua), Silvia Zane (MSSL), Sandro Mereghetti (IASF-MI), Paolo Esposito (IASF-MI) and Luigi Stella (OAR)
on 28 Oct 2005; 12:18 UT
Credential Certification: Nanda Rea (N.Rea@sron.nl)

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We report on the status of our IR and X-ray monitoring the SGR 1806-20 after the 2004 December 27th Giant Flare.

The IR counterpart of SGR1806-20 (Israel et al. 2005, A&A, 438, L1; Kosugi et al. 2005, ApJ, 623, L125) was observed on 11-12th September 2005 at the VLT-UT4 equipped with the IR adaptive optics instrument NAOS-CONICA in the Ks band (2400s of effective exposure time) under good sky conditions. The source was detected at Ks = 20.2 +\- 0.2 mag (all reported uncertainties are 90% c.l.), a level similar to that measured in March 2004 and significantly lower than the pre-Giant Flare value observed in October 2004 (Ks = 19.32 +/- 0.16 mag; Israel et al. 2005).

XMM-Newton observed SGR 1806-20 for 34 ks on 2005 October 4 (23 days after the VLT run). The source spin period was P=7.56687(3) (all reported uncertainties are 90% c.l.) with a double peaked pulse profile (similar to the previous Chandra and XMM post-flare observations: Rea et al. 2005, ApJ 627, L1; Tiengo et al. 2005, A&A, 440, L63). Pulsed fractions of 7.4% +/- 1.4% and 5.7% +/- 1.3% are found for the fundamental and the first harmonic, respectively (0.5-10 keV energy range). We note that the pulsed fraction, which dropped soon after the Giant Flare to about 3% (Rea et al. 2005), is now increasing smoothly towards the pre- Giant Flare value of about 10% (Mereghetti et al. 2005, ApJ 628, 938).

Despite the source is still in a bursting state (Mereghetti et al. 2005, GCN 4168), the X-ray flux has decreased and the spectrum softened with respect to the previous observations, following the intensity/hardness correlation observed in the monitoring before and after the giant flare (Mereghetti et al. 2005; Rea et al. 2005; Tiengo et al. 2005). The flux on October 4 was 1.34x10^-11 erg/s/cm^2 (absorbed in the 2-10 keV range). The spectrum is well fitted by an absorbed (Nh=6.4+/-0.4 x10^22 cm^-2) blackbody plus a power-law component (kT = 0.69+/-0.12 keV and Gamma = 1.4+/-0.3). The blackbody accounts for 10% of the total absorbed flux.

We thank the XMM-Newton Project Scientist, Norbert Schartel, for promptly according us the XMM-Newton Target of Opportunity observation of SGR 1806-20 and the VLT personnel for their continuous help in optimising and performing the NACO observations.

Preliminary results