Flux variability from the likely IR counterpart of SGR 1806-20
ATel #439; G. L. Israel (INAF AO Roma), S. Covino (INAF AO Milano), S. Mereghetti (INAF IASF Milano), R. Mignani (ESO), L. Stella (AO Roma), V. Testa (AO Roma), G. Marconi (ESO), on behalf of a larger team
on 10 Mar 2005; 21:08 UT
Credential Certification: GianLuca Israel (gianluca@mporzio.astro.it)
Subjects: Infra-Red, Soft Gamma-ray Repeater
Following the refined radio position of SGR 1806-20 (Gaensler et al., astro-ph/0502393; Cameron et al., astro-ph/0502428), about 0.5" away from the preliminary one (Kouveliotou et al. 2005, GCN \#2929), we note that the possible counterpart we tentatively identified (Israel et al., ATel \#378) is not anymore included in the radio positional uncertainty region.
Only one relatively faint object falls within the refined radio position regions, while two additional objects are just outside. The former object is consistent with the Chandra error circle (radius of 0.3") reported by Kaplan et al. 2002 (ApJ, 564, 935). Moreover, it is the only one to show a constant brightening during 2004, with the highest magnitude recorded just after the intense SGR 1806-20 burst occurred on 5th October 2004. The likely counterpart of SGR 1806-20 has an average Ks magnitude of about 19.01+\-0.07 (1sigma uncertainty is reported) and displays a Ks variation of about 0.7 mags on a 7 months baseline. The IR flux variation is roughly of the same order of that shown during the same period by the persistent X-ray emission of SGR 1806-20 (Mereghetti et al., astro-ph/0411695).
We note that the candidate counterpart plus the two closest objects (Ks=18.0 and Ks=18.3, respectively) are all within a radius of less than 0.2" and consistent with being unresolved components of candidate B in the IR images of Eikenberry et al. (2001, ApJ, 563, L133).
