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Register a new userStructure in the radio counterpart to the 2004 Dec 27 giant flare from SGR 1806-20
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R.P. Fender
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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.
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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 2004 Dec. 27, the soft gamma repeater (SGR) 1806-20 emitted the brightest giant flare (GF) ever detected from an SGR, with an (isotropic) energy release $sim 100$ times greater than the only two other known SGR GFs. It was followed by a very bright, fading radio afterglow. Extensive follow-up radio observations provided a wealth of information with unprecedented astrometric precision, revealing the temporal evolution of the source size, along with densely sampled light curves and spectra. Here we expand on our previous work on this source, by explaining these observations within one self-consistent dynamical model. In this scenario, the early radio emission is due to the outflow ejected during the GF energizing a thin shell surrounding a pre-existing cavity, where the observed steep temporal decay of the radio emission seen beginning on day 9 is attributed to the adiabatic cooling of the shocked shell. The shocked ejecta and external shell move outward together, driving a forward shock into the ambient medium, and are eventually decelerated by a reverse shock. As we show in Gelfand et al. (2005), the radio emission from the shocked external medium naturally peaks when significant deceleration occurs, and then decays relatively slowly. The dynamical modeling of the collision between the ejecta and the external shell together with the observed evolution of the source size (which is nicely reproduced in our model) suggest that most of the energy in the outflow was in mildly relativistic material, with an initial expansion velocity $v/c lesssim 0.7d_{15}$, for a distance of $15d_{15}$ kpc to SGR 1806-20. An initially highly relativistic outflow would not have produced a long coasting phase at a mildly relativistic expansion velocity, as was observed.
We report on the study of 14 XMM-Newton observations of the magnetar SGR 1806-20 spread over a period of 8 years, starting in 2003 and extending to 2011. We find that in mid 2005, a year and a half after a giant flare (GF), the torques on the star increased to the largest value yet seen, with a long term average rate between 2005 and 2011 of $lvertdot{ u}rvertapprox1.35times10^{-11}$ Hz s$^{-1}$, an order of magnitude larger than its historical level measured in 1995. The pulse morphology of the source is complex in the observations following the GF, while its pulsed-fraction remained constant at about $7%$ in all observations. Spectrally, the combination of a black-body (BB) and power-law (PL) components is an excellent fit to all observations. The BB and PL fluxes increased by a factor of 2.5 and 4, respectively, while the spectra hardened, in concordance with the 2004 major outburst that preceded the GF. The fluxes decayed exponentially back to quiescence with a characteristic time-scale of $tausim1.5$ yrs, although they did not reach a constant value until at least 3.5 years later (2009). The long-term timing and spectral behavior of the source point to a decoupling between the mechanisms responsible for their respective behavior. We argue that low level seismic activity causing small twists in the open field lines can explain the long lasting large torques on the star, while the spectral behavior is due to a twist imparted onto closed field lines after the 2004 large outburst.
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.
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