The Radio Afterglow produced by the Giant
Flare from the Magnetar SGR 1806-20
Greg Taylor (NRAO/KIPAC)
UCSC/SCIPP - 4/26/2005
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. M. Palmer, N. Gehrels,
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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
An early gamma ray-burst
Vela satellite
A Gamma Ray Burst Sampler
Bursts of all sorts
(Woods &
Thompson
2004)
Radio Light Curves from long GRBs
GRB 970508
• First VLBI detection of a
GRB Afterglow
• absolute position to < 1 mas
• Size < 10**19 cm
• Distance > 3 kpc
Relativistic
Expansion v ~ 0.96c
E ~ 10**53 ergs
(isotropic equivalent)
R ~ (E/n)**1/8
astro-ph/0412483
Long GRBs clearly connected to Supernovae
Hjorth et al 2003
SGR Light Curves & Durations:
t ~ 0.2 s
(Woods & Thompson 2004)
From Pulsed quiescent X-ray emission:
Woods & Thompson 2004
The Magnetar Model for SGRs
• Lquiescent ~ a few 1035 erg/s
• The energy release in a single giant flare is of
the order of the total rotational energy ~1044.5 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 ~ 1015 G ~ 1048 erg
Adapted from
Duncan and Thompson
1992
Giant Flares from SGRs
• Initial spike: t ~ 0.3 s , Eiso ~ a few1044 erg
– hard spectrum
– ~ ms rise time
• Pulsating tail
The 1998 August 27
giant flare from SGR
1900+14
– 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)
SGR 1806-20
on 2004 Dec 27
Rise time: < 1 ms, te-folding ~ 0.3 ms
The rise is
resolved for
the first time
Swift
(Palmer et al. 2005)
Sudden Ionospheric Disturbance (SID)
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Washington, USA to Alberta, CA
Cambell et al. 2005
The 2004 Dec. 27 Giant Flare
• was ~5o from the sun
• It’s distance ≈ 15 kpc
RHESSI
• Eiso ~ (2-9)1046 erg
• Eiso,spike / Eiso,tail ~ 300
Swift
(Hurley et al.
2005)
(Palmer et al. 2005)
Aperture Synthesis – Basic Concept
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 subapertures, there will be
N(N-1)/2 pairs to combine.
This approach is the basis of
aperture synthesis.
The VLA
27 antennas each 25 m in diameter
Synthesised aperature after 45 minutes.
Raphaeli 2001
B ~ 0.3 mG
Source Size, Shape & Polarization:
From Gaensler et al. 2005 (accepted to Nature)
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.
From Cameron et al. 2005
Special Advertising Supplement: The Long Wavelength Array
Exploring the Transient Universe from 20 - 80 MHz
1 “LWA Station” = 256 antennas
Full LWA: 50 stations spread
across NM
Y
100 m
400 km
State of New Mexico
VLA
Growth of the Radio Afterglow
VLA
8.5 GHz
Velocity to
t + 30 days
~ 0.8 c
Size at
t+7 days
1016 cm
Decrease in vexp
Proper motion of the Flux Centroid:
VLA 8.5 GHz
Image Evolution
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
Observations (Gaensler et al. 2003)
Simulation
(Bucciantini 2002)
• 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 tdec
when significant deceleration occurs
Log(R)
R  t0.4
Rt
Log(t)
tcol~ 5 days
What we
missed
tdec~ 33 days
The observed Linear Polarization:
VLA
8.5 GHz
Image Evolution
VLA
8.5 GHz
Observed Polarization Angle
Polarization of Synchrotron Emission
B
Projection of the magnetic
field on plane of the sky
The direction of
the polarization
B
P
k
e
Cone of
angle 1/e
Plane of the sky
• linear polarization perpendicular to the
projection of B on the plane of the sky
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 Photon emitted
tangled within normal to plane
nph = nsh P
a (shock) plane
P
P=0

P = Pmaxsin2/(1+cos2)
(Laing 1980)
P = Pmax
Photon emitted
along the plane
nph  nsh
Elongated emission region gives
rise to net polarization
Net Pol.
Energetics from R(tdec) & tdec:
• M ~ (4/3)R3 ~ 1026 (nISM / 1 cm-3) gr
• E ~ Mv2 ~ 1046 (nISM / 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 Msun yr-1 Mpc-3
• Milky Way SFR ~ 1.3 Msun 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.