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.