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Register a new userHigh Energy Afterglow Emission from Giant Flares of Soft Gamma-Ray Repeaters: The Case of the 2004 December 27 Event from SGR 1806-20
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We discuss the high enegry afterglow emission (including high energy photons, neutrinos and cosmic rays) following the 2004 December 27 Giant Flare from SGR 1806-20. If the initial outflow is relativistic with a bulk Lorentz factor Gamma_0sim {rm tens}, the high-energy tail of the synchrotron emission from electrons in the forward shock region gives rise to a prominent sub-GeV emission, if the electron spectrum is hard enough and if the intial Lorentz factor is high enough. This signal could serve as a diagnosis of the initial Lorentz factor of the giant flare outflow. This component is potentially detectable by GLAST if a similar giant flare occurs in the GLAST era. With the available 10 MeV data, we constrain that Gamma_0 < 50 if the electron distribution is a single power law. For a broken power law distribution of electrons, a higher Gamma_0 is allowed. At energies higher than 1 GeV, the flux is lower because of a high energy cut off of the synchrotron emission component. The synchrotron self-Compton emission component and the inverse Compton scattering component off the photons in the giant flare oscillation tail are also considered, but they are found not significant given a moderate Gamma_0 (e.g. leq 10). The forward shock also accelerates cosmic rays to the maximum energy 10^{17}eV, and generate neutrinos with a typical energy 10^{14}eV through photomeson interaction with the X-ray tail photons. However, they are too weak to be detectable.
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The 2004 Dec. 27 giant Gamma-ray flare detected from the magnetar SGR 1806-20 created an expanding radio nebula which we have monitored with the Australia Telescope Compact Array and the Very Large Array. These data indicate that there was an increase in the observed flux ~25 days after the initial flare that lasted for ~8 days, which we believe is the result of ambient material swept-up and shocked by this radio nebula. For a distance to SGR 1806-20 of 15 kpc, using the properties of this rebrightening we infer that the initial blast wave was dominated by baryonic material of mass M>10^{24.5} g. For an initial expansion velocity v~0.7c (as derived in an accompanying paper), we infer this material had an initial kinetic energy E>10^{44.5} ergs. If this material originated from the magnetar itself, it may have emitted a burst of ultra-high energy (E > 1 TeV) neutrinos far brighter than that expected from other astrophysical sources.
On Dec 27, 2004, the magnetar SGR 1806-20 underwent an enormous outburst resulting in the formation of an expanding, moving, and fading radio source. We report observations of this radio source with the Multi-Element Radio-Linked Interferometer Network (MERLIN) and the Very Long Baseline Array (VLBA). The observations confirm the elongation and expansion already reported based on observations at lower angular resolutions, but suggest that at early epochs the structure is not consistent with the very simplest models such as a smooth flux distribution. In particular there appears to be significant structure on small angular scales, with ~10% of the radio flux arising on angular scales <100 milliarcsec. This structure may correspond to localised sites of particle acceleration during the early phases of expansion and interaction with the ambient medium.
XMM-Newton observed the soft gamma repeater SGR 1806-20 about two months after its 2004 December 27 giant flare. A comparison with the previous observations taken with the same instrument in 2003-2004 shows that the pulsed fraction and the spin-down rate have significantly decreased and that the spectrum slightly softened. These changes may indicate a global reconfiguration of the neutron star magnetosphere. The spectral analysis confirms that the presence of a blackbody component in addition to the power-law is required. Since this additional component is consistent with being constant with respect to the earlier observations, we explore the possibility of describing the long-term spectral evolution as only due to the power-law variations. In this case, the slope of the power-law does not significantly change and the spectral softening following the giant flare is caused by the increase of the relative contribution of the blackbody over the power-law component.
The extraordinary giant flare (GF) of 2004 December 27 from the soft gamma repeater (SGR) 1806-20 was followed by a bright radio afterglow. We present an analysis of VLA observations of this radio afterglow from SGR 1806-20, consisting of previously reported 8.5 GHz data covering days 7 to 20 after the GF, plus new observations at 8.5 and 22 GHz from day 24 to 81. We detect motion in the flux centroid of the afterglow, at an average velocity of 0.26 +/- 0.03 c (assuming a distance of 15 kpc) at a position angle of -45 degrees. This motion, in combination with the growth and polarization measurements, suggests an asymmetric outflow, mainly from one side of the magnetar. We find a deceleration in the expansion, from ~9 mas/day to <5 mas/day. The time of deceleration is roughly coincident with the rebrightening in the radio light curve, as expected to result when the ejecta from the GF sweeps up enough of the external medium, and transitions from a coasting phase to the Sedov-Taylor regime. The radio afterglow is elongated and maintains a 2:1 axis ratio with an average position angle of -40 degrees (north through east), oriented perpendicular to the average intrinsic linear polarization angle.
We present CO(J=1-0) observations in the direction of the Soft Gamma Repeater SGR 1806-20 with the SEST telescope. We detected several molecular clouds, and we discuss in this paper the implications of these observations for the distance to the X-ray counterpart AX 1805.7-2025, the supernova remnant G10.0-0.3 and the very luminous O9-B2 star detected in the line of sight. The distance of SGR 1806-20 is estimated to be 14.5 +/- 1.4 kpc and this Soft Gamma Repeater is very likely associated with one of the brightest HII regions in the Galaxy, W31. The large size of G10.0-0.3 (25 x 38 pc) for a young supernova remnant possibly powered by a central pulsar (AX 1805.7-2025) indicates that G10.0-0.3 could be expanding in the very low density region produced by the wind of the blue star.
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