TITLE: GCN CIRCULAR NUMBER: 11303 SUBJECT: GRB100925A/MAXI J1659-152: Galactic origin from SED analysis DATE: 10/09/26 00:44:23 GMT FROM: Dong Xu at Weizmann Inst Dong Xu (Weizmann Institute) reported on a larger collaboration: GRB100925A happened in the error circle of MAXI J1659-152 (Negoro et al., ATel 2873; Magano et al., GCN 11296). Spectroscopy or a broadband SED is critical to pin down its origin. The Swift UVOT and XRT data were analyzed, covering 2000-1x10^4 s since the BAT trigger. Generally, UVOT light curves are rather flat as the X-ray does. The flux densities in the six UVOT filters without Galactic extinction correction are rather low compared with the X-ray flux density. The UVW2 flux density is almost comparable to that of the X-ray. Assuming the source being extragalactic and correction of E(B-V)~0.61 (Magano et al., GCN 11296) applied, then UVOT flux densities are too high compared with the downward extrapolation of the X-ray, if interpreted with synchrotron radiation mechanism. Additionally, the UVOT spectral index has \beta~2 (F_\nu \propto \nu^\beta) in this case, which is unusual for GRB afterglows. To make UVOT-XRT SED explained by one single mechanism, a moderate extinction is required. We found E(B-V)~0.35 would naturally give rise to a good SED modeling: the synchrotron radiation lies in the slow cooling phase with the minimum frequency ~3x10^15 Hz, above which X-ray has \beta_X~-(p-1)/2~0.7 (i.e., p~2.4) and below which UVOT has \beta_opt~1/3. Therefore, we suggest this source is of Galactic origin. If the above synchrotron mechanism governs the broadband SED from radio to X-ray, then MAXI J1659-152 is likely not detectable at radio, especially when the synchrotron-self absorption frequency is above the radio band. Therefore, radio follow-up is encouraged. [GCN OPS NOTE(28sep10): Per auther's request, "100525A" was changed to "100925A" in both the Subject line and text.]