Giant flare in SGR 1806-20 and its Compton reflection from the Moon

Abstract

We analyze the data obtained when the Konus-Wind gamma-ray spectrometer detected a giant flare in SGR 1806-20 on December 27, 2004. The flare is similar in appearance to the two known flares in SGR 0526-66 and SGR 1900+14 while exceeding them significantly in intensity. The enormous X-ray and gamma-ray flux in the narrow initial pulse of the flare leads to almost instantaneous deep saturation of the gamma-ray detectors, ruling out the possibility of directly measuring the intensity, time profile, and energy spectrum of the initial pulse. In this situation, the detection of an attenuated signal of Compton back-scattering of the initial pulse emission by the Moon with the Helicon gamma-ray spectrometer onboard the Coronas-F satellite was an extremely favorable circumstance. Analysis of this signal has yielded the most reliable temporal, energy, and spectral characteristics of the pulse. The temporal and spectral characteristics of the pulsating flare tail have been determined from Konus-Wind data. Its soft spectra have been found to contain also a hard power-law component extending to 10 MeV. A weak afterglow of SGR 1806-20 decaying over several hours is traceable up to 1 MeV. We also consider the overall picture of activity of SGR 1806-20 in the emission of recurrent bursts before and after the giant flare.

Echoes of multiple outbursts of Sagittarius A⋆revealed byChandra

“…But most of the energy of a giant flare is released in high energy photons beyond 100 keV which do not effectively produce 6.4 keV line emission. To properly evaluate the effective reflected emission produced by such an event occurring at the Galactic center, we used the 2004-flare spectrum of SGR 1806-20 measured by Frederiks et al (2007): a very hard power-law (photon index Γ = 0.73) with a cutoff at about 660 keV. We modulated it by the iron cross section for photoelectric absorption and applied the parameters of the Bridge 2011 filament discussed in section 4.1.…”

Pulsars and Magnetars

“…They have optically thin bremsstrahlung spectra with temperatures of a few tens of keV, but in the two most recent and better observed giant flares a combination of cooling thermal components and power laws extending into the MeV region was required [74,13,54]. The decaying light curves, lasting several minutes, are strongly modulated at the NS rotation period and show complex pulse profiles which evolve with time.…”

The strongest cosmic magnets: soft gamma-ray repeaters and anomalous X-ray pulsars

“…For example, for the initial spike of SGR 1806−20 a cooling blackbody spectrum, with temperature varying from 230 keV to 170 keV within ∼0.2 s, was derived using charged particle detectors on the Wind and RHESSI spacecrafts [15]. Instead the analysis of the radiation Compton-scattered from the Moon seen with the Coronas-F satellite [73], as well as the results from small particle detectors on other satellites Palmer et al [207], favor an exponentially cut-off power-law, although with poorly constrained parameters (photon index Γ =0.73…”