An Energetic Blast Wave from the 2004 December 27 Giant Flare of the Soft Gamma-Ray Repeater SGR 1806-20

Abstract

Recent follow-up observations of the 2004 December 27 giant flare of SGR 1806-20 have detected a multiple-frequency radio afterglow from 240 MHz to 8.46 GHz, extending in time from a week to about a month after the flare. The angular size of the source has also been measured for the first time. Here we show that this radio afterglow provides the first clear evidence of an energetic blast wave sweeping up its surrounding medium and producing a synchrotron afterglow, the same mechanism as has been established for gamma-ray burst afterglows. The optical afterglow is expected to have been intrinsically as bright as m_R 13 at t 0.1 days after the flare, but very heavy extinction due to the low Galactic latitude of the source would have made detection difficult. Rapid infrared follow-up observations of giant flares are therefore crucial for low-latitude soft gamma-ray repeaters (SGRs), while for high-latitude SGRs (e.g., SGR 0526-66), rapid follow-up should result in the identification of possible optical afterglows. Rapid multiwavelength follow-up will also provide more detailed information on the early evolution of the fireball, as well as its composition.