the radio afterglow produced by the giant flare from the magnetar sgr 1806-20 greg taylor...
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The Radio Afterglow produced by the Giant Flare from the Magnetar SGR 1806-20
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Greg Taylor (NRAO/KIPAC)
with: with: J. Granot, B. M. Gaensler, C. Kouveliotou, J. D. GelfandJ. 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. Eichler, E. Ramirez-Ruiz, R. A. M. J. Wijers, Y. E. Lyubarsky, R. W. Hunstead,
DD. . Campbell-Wilson, A. J. van der Host, M. A. McLaughlin, R. P. Fender, M. A. Garrett, K. J. Newton-McGee, 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,D. M. Palmer, N. Gehrels,
UCSC/SCIPP - 4/26/2005
OutlineOutline
• 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
Vela satellite
An early gamma ray-burst
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
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
• 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 Thompson1992
Giant Flares from SGRs
• Initial spike: t ~ 0.3 s , Eiso ~ a few1044 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
SGR 1806-20on 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)Sudden Ionospheric Disturbance (SID)
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Cambell et al. 2005Washington, USA to Alberta, CA
The 2004 Dec. 27 Giant Flare
RHESSI
Swift
• was ~ 5o from the sun
• It’s distance ≈ 15 kpc
• Eiso ~ (2-9)1046 erg
• Eiso,spike / Eiso,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 breakthe aperture into N sub-apertures, there will be N(N1)/2 pairs to combine.
This approach is the basis of aperture synthesis.
Aperture Synthesis – Basic Concept
The VLA27 antennas each 25 m in diameter
Synthesised aperature after 45 minutes.
Raphaeli 2001
B ~ 0.3 G
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 N
ew M
exicoSpecial Advertising Supplement: The Long Wavelength Array
Y
VLA
Exploring the Transient Universe from 20 - 80 MHz
Growth of the Radio Afterglow VLA8.5 GHz
Size att+7 days1016 cm
Velocity tot + 30 days~ 0.8 c
Decrease in vexp
Proper motion of the Flux Centroid:
VLA 8.5 GHz
Image Evolution
VLA8.5 GHz
Theoretical Interpretation: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 tdec
when significant deceleration occurs
Log
(R)
Log(t)
tcol~ 5 days tdec~ 33 days
R t0.4
R t
What wemissed
The observed Linear Polarization:VLA8.5 GHz
Image Evolution
VLA8.5 GHz
Observed Polarization Angle
Polarization of Synchrotron EmissionPolarization 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 magneticfield on plane of the sky
The direction of the polarization
kB
P
Cone of Cone of angleangle 1/ 1/ee
P = 0P = 0
P = PP = Pmaxmax
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
nnph ph == nnshsh
Photon emitted along the plane
nnph ph nnshsh
P = PP = Pmaxmaxsinsin22/(1+cos/(1+cos22))(Laing 1980)P
P
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.