Presentation on theme: "The Radio Afterglow produced by the Giant Flare from the Magnetar SGR 1806-20 Greg Taylor (NRAO/KIPAC) with: J. Granot, B. M. Gaensler, C. Kouveliotou,"— Presentation transcript:
The Radio Afterglow produced by the Giant Flare from the Magnetar SGR 1806-20 Greg Taylor (NRAO/KIPAC) with: J. Granot, B. M. Gaensler, C. Kouveliotou, J. D. Gelfand, D. Eichler, E. Ramirez-Ruiz, R. A. M. J. Wijers, Y. E. Lyubarsky, R. W. Hunstead, D. Campbell-Wilson, A. J. van der Host, M. A. McLaughlin, R. P. Fender, M. A. Garrett, K. J. Newton-McGee, D. Campbell-Wilson, A. J. van der Host, M. A. McLaughlin, R. P. Fender, M. A. Garrett, K. J. Newton-McGee, D. M. Palmer, N. Gehrels, UCSC/SCIPP - 4/26/2005
Outline The Mystery of Gamma Ray Bursts (GRBs) Short overview of soft gamma repeaters (SGRs) The 2004 Dec. 27 Giant Flare from SGR 1806-20 The Radio Afterglow produced by the giant flare (astro-ph/0504363) A dynamical model for the radio observations Implications for short gamma-ray bursts
GRB 970508 First VLBI detection of a GRB Afterglow absolute position to < 1 mas Size < 10**19 cm Distance > 3 kpc
R ~ (E/n)**1/8 Relativistic Expansion v ~ 0.96c E ~ 10**53 ergs (isotropic equivalent) astro-ph/0412483
Long GRBs clearly connected to Supernovae Hjorth et al 2003
SGR Light Curves & Durations: (Woods & Thompson 2004) t ~ 0.2 s
From Pulsed quiescent X-ray emission: Woods & Thompson 2004
The Magnetar Model for SGRs L quiescent ~ a few 10 35 erg/s The energy release in a single giant flare is of the order of the total rotational energy ~10 44.5 erg another energy source is required Main competing model for the energy source: accretion - does not work well (no binary companion or quiescent IR emission) The measurement of the period and its time derivative was considered a confirmation of the magnetar model: B ~ 10 15 G ~ 10 48 erg
Giant Flares from SGRs Initial spike: t ~ 0.3 s, E iso ~ a few10 44 erg –hard spectrum –~ ms rise time Pulsating tail –Lasts a few min. –Modulated at the NS rotation period –Softer spectrum Only 2 previous events ever recorded: in 1979 (SGR 0526-66 in LMC) & 1998 (SGR 1900-14) The 1998 August 27 giant flare from SGR 1900+14
Rise time: < 1 ms, t e-folding ~ 0.3 ms The rise is resolved for the first time Swift (Palmer et al. 2005)
Sudden Ionospheric Disturbance (SID) Cambell et al. 2005 Washington, USA to Alberta, CA
The 2004 Dec. 27 Giant Flare RHESSI Swift was ~ 5 o from the sun It’s distance ≈ 15 kpc E iso ~ (2-9) 10 46 erg E iso,spike / E iso,tail ~ 300 (Palmer et al. 2005) (Hurley et al. 2005)
If the source emission is unchanging, there is no need to collect all of the incoming rays at one time. One could imagine sequentially combining pairs of signals. If we break the aperture into N sub- apertures, there will be N(N 1)/2 pairs to combine. This approach is the basis of aperture synthesis. Aperture Synthesis – Basic Concept
The VLA 27 antennas each 25 m in diameter Synthesised aperature after 45 minutes.
Source Size, Shape & Polarization: From Gaensler et al. 2005 (accepted to Nature)
From Cameron et al. 2005 Radio Afterglow has a Steep Spectrum ~ -0.6 at t+7 days down to 220 MHz Flux > 1 Jy at early times and low frequencies.
400 km 1 “LWA Station” = 256 antennas Full LWA: 50 stations spread across NM 100 m State of New Mexico Special Advertising Supplement: The Long Wavelength Array Y VLA Exploring the Transient Universe from 20 - 80 MHz
Growth of the Radio Afterglow VLA 8.5 GHz Size at t+7 days 10 16 cm Velocity to t + 30 days ~ 0.8 c Decrease in v exp
Proper motion of the Flux Centroid: VLA 8.5 GHz
Theoretical Interpretation: The supersonic motion of the SGR in the ISM creates a bow shock & a thin shell of shocked wind and shocked ISM, surrounding a cavity Simulation (Bucciantini 2002) Observations (Gaensler et al. 2003)
The outflowing material that was ejected from the magnetar during the giant flare collides with the bow shock shell and “lights up” The merged shocked shell continues to coast outward & the shock accelerated electrons cool adiabatically: reproduces the observed fast decay and constant expansion velocity ~ 0.3c A shock is driven into the ISM that eventually slows down the shell causing a bump in the light curve which naturally peaks at the time t dec when significant deceleration occurs
Log(R) Log(t) t col ~ 5 days t dec ~ 33 days R t 0.4 R t What we missed
Polarization of Synchrotron Emission linear polarization perpendicular to the projection of B on the plane of the sky B e Plane of the sky Projection of the magnetic field on plane of the sky The direction of the polarization B P Cone of angle 1/ e
P = 0 P = P max Shock Produced Magnetic Field: A magnetic field that is produced at a relativistic collisionless shock, due to the two-stream instability, is expected to be tangled within the plane of the shock (Medvedev & Loeb 1999) Magnetic field tangled within a (shock) plane Photon emitted normal to plane n ph = n sh Photon emitted along the plane n ph n sh P = P max sin 2 /(1+cos 2 ) (Laing 1980) P P
Elongated emission region gives rise to net polarization Net Pol.
Energetics from R(t dec ) & t dec : M ~ (4 /3) R 3 ~ 10 26 (n ISM / 1 cm -3 ) gr E ~ Mv 2 ~ 10 46 (n ISM / 1 cm -3 ) erg
Implications for Short GRBs BATSE detection rate ~ 150 yr -1 Rate of Giant Flares in our galaxy ~ 0.03 yr -1 Giant Flares can be detected to 40 Mpc Assume SGRs proportional to star formation Local (z=0) SFR ~ 0.013 M sun yr -1 Mpc -3 Milky Way SFR ~ 1.3 M sun yr -1 Expected Giant Flares within 40 Mpc ~ 80 yr -1 But where is Virgo concentration?
Conclusions : The radio afterglow of the SGR 1806-20 giant flare is a unique opportunity to study a nearby relativistic outflow. Giant flares from extragalactic SGRs might explain short duration GRBs. After 35 years we have a fair start on understanding the origin of GRBs. Low frequency observations of the transient universe could dramatically improve our understanding and may open up entirely new puzzles.
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