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The most violent bomb-blast in
our Galaxy in 100 years
SGR 1806-20
Poonam Chandra
TIFR, Mumbai
13th July 2005
Poonam Chandra
27th December 2004 at 4:30:26.65 pm EST
Courtesy: NASA
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Poonam Chandra
Saturated all
spacecraft detectors
(INTEGRAL, SWIFT etc.)
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Disturbed earth’s
ionosphere
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Poonam Chandra
SGR 1806-20
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 Introduction
Plan of the talk
•
Giant flare from SGR 1806-20
•
What are SGRs?
•
Comparison with other known SGRs
•
Source of SGR giant flare
•
Mechanisms for various SGR flare emissions
 Radio observations of SGR giant flare afterglow
•
Radio emission from Afterglow
•
Observations and results
•
Distance estimations
•
Comparison with other radio observations
 Short GRBs vs SGRs?

Extragalactic SGRs- possible candidates for short GRBs!
13th July 2005
Poonam Chandra
SGR 1806-20
Giant flare on Dec 27, 2004
Detected by INTEGRAL, RHESSI, Wind
Spacecraft, SWIFT, GMRT, VLA, ATCA etc.
80,000 counts/sec (RHESSI)
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SGR stands for
Soft Gamma-ray Repeater
Gives repeated flares, whose energy fall
in soft-gamma rays or hard X-rays in the
electromagnetic spectrum
1806-20
RA
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Dec
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Burst profile of Dec 27, 2004 giant flare
Hurley et al (2005), Nature
Giant flare for 0.2 sec,
tail for 382 sec, 1 sec
precursor before 142
sec giant flare.
Spike
99.7% of the
total energy
Pulsed tail emission
Precursor
Tail
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Precursor Spike
Tail
Duration
1 sec
0.2 sec
382 sec
Temp
15 keV
175 keV 3-100 keV
Fluence 1.8x10-4
(erg/cm2)
Energy
(ergs)
2.4x1042
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1.36
4.6x10-3
1.8x1046 1.2x1044
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SGR 1806-20
15-25 keV
25-50 keV
50-100 keV
100-350 keV
(peak is ~5 km overhead on this scale!)
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In 1/10 of a second as much
energy as sun emits in
100,000 years continuously.
1000 times more bright than
combining all the stars of
Milky Way together.
100 times more energetic than
any previous giant bursts.
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Other Soft Gamma Ray Repeaters
SGR 1900+14
SGR 1627-41
SGR 1806-20
SGR 0526-66
Yellow points- cousins Anamalous Xray Pulsars
also considered to be of same origin as SGRs
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(Marsdon & Higdon, 2001, Taylor & Cordes 1993)
Sun
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2005
SGR 1806-20 was 100 times larger in
energy than any other previous busrt.
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The high energy of the giant
burst implies rarity of the
the such busrts. Since
dN/dE a E-1.6
Such giant flares happen
once in a century
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WHAT COULD BE THE SOURCE
OF SUCH A HUGE ENERGY?
``When you have eliminated all other
possibilities, Sherlock Holmes instructed,
whatever remains, however improbable,
must be the answer to the puzzle.”
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Three candidates for SGRs
Powered
by magnetic
Rotation energy
of pulsar field
Accretion due to binary
 Cannot explain the initial spike.
(MAGNETAR)
 The maximum luminosity obtained is 10
33
 Difficult to explain pure form of energy least
ergs/s
contaminated by baryons.
 Very slow rotating objects, cannot explain
such
No binary
associations found.
huge energy
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Most accepted model
MAGNETAR
SGRs are magnetars occasionally emitting energetic
bursts in the early phase of their life times.
A young neutron star with age <10,000 years.
Extremely high magnetic field (~ 1015 Gauss)
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Comparison of magnetic field strengths
Earth
Strong sunspots
Strongest lab mag. field
Radio pulsar
Magnetar
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0.6 Gauss
4000 Gauss
5x105 Gauss
1012 Gauss
1015 Gauss
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Kouveliotou (Nature, 1998) found that
SGR 1806-20 is oscillating with
7.56 sec
period and slowing down at a speed of
1 sec/ 300 years
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..
2 2
2 2 4
dE
  I   3 
dt
3c
3c 3
as   0 Sin t
2
Since  
; and  0  BR 3
.
P
Magnetic field
B  P.dP / dt
The magnetic field required for SGR
1806-20 slow down rate is ~1015 Gauss!!
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Poonam Chandra
.
.
  k  P  (2 )
n
n 1
2 n
kP
Characteri stic Age
P

2dP / dt
The characteristic age of the SGRs
estimated are 10,000 years in contrast to
1 million years of age of neutron stars.
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Radiation pressure  gravitatio nal force
Why high 10L15
field?
 TG magnetic
GMm
4R c
2

R2
4cGMm
 spin down observed.
1: Required for suchLEdd
high

Needed
to sustainofhydrostati
c
2: Required to explain
the energy
the explosion
or radiative
equilibriu
m. in 104 years.
3: To explain the trigger
of SGR
activity
Thompson crosssecti on in high magnetic field
4: Explain super eddington luminosity
 /  T ~ (m / eB) 2  10 6 ( / 10 keV) 2 ( B / 1015 G )  2
5: Explains quiescent X-ray emission through mag. field
decay.
6: Explains the initial spike, comparable to Alfven wave
time crossing in magnetosphere of a neutron star (R*/t).
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HOW A MAGNETAR IS FORMED?
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Supernova explosion
leaves neutron star as a
remnant in the center.
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Magnetar burst emission
Thompson & Duncan 1996
Giant Flares:
Sudden Large-scale re-arrangement of the
magnetic field
2
core
B
3
46
R  4 10 erg
8
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 Bcore 
 15 
 10 G 
2
Poonam Chandra
Global changes in the magnetic field geometry
“Interchange instability” (Energy released (Bext2/8)R3)
Flowers & Ruderman (1977)
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Thompson & Duncan 1996
Small Bursts (SGR events):
Cracking of crust leads to small displacements
of magnetic field
ESGR
 B0 
 10 erg  15 
 10 G 
41
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2
2
 l    max 

  3 
 1 km   10 
2
Poonam Chandra
Thompson & Duncan 1996
Inside twist => magnetic field lines outside the star also
get twisted because they are anchored to the crust
(cracking 5 km?).
 B.dl 
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4
I
c
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Thompson & Duncan 1996
Tail emission:
from the trapped fireball oscillating with
the neutron star rotation period.
WIND
HARD EMISSION
TRAPPED
PLASMA
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SOFT
EMISSION
Poonam Chandra
Radio emission from SGR 1806-20 afterglow
The ejected particles moving with very high speed hit the
surrounding matter and generate synchrotron emission due
to the relativistic electrons moving in a magnetic field.
GMRT + VLA + ATCA
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Radio observations with the
Giant Meterwave Radio Telescope
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Advantages of GMRT
1: UV coverage is
provided by the rotation of
the earth.
2: Very high sensitivity at
very low frequencies
unlike WSRT and MOST.
Positive
declination
3: Could resolve 12” away
LBV 1806-20 source close
by at low frequencies.
Negative
declination
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GMRT observations of SGR
1806-20
•From January 4, 2005 to
February 24, 2005
•Initially very frequently, almost
everyday
•Snapshots, 40-60 minutes.
•Mostly in 240 and 610 MHz and
in 1060 MHz at some occasions.
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FoV
of
SGR
before
the
giant
burst
SGR
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GMRT map of SGR 1806-20 in 235 MHz band
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Fading SGR 1806-20
LBV source
6 January 2005
13 July 2005
th
16 January 2005
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Light curve from day 5 to day 50
F  t a
Freq aA
(GHz)
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aB
aC
0.24
-1.7
0.61
-1.9
1.4
-2.0
-4.1
-0.85
2.4
-0.95
-3.5
-0.95
4.9
-1.55
-3.1
-0.65
6.1
-2.3
8.5
-2.0
-2.8
-0.64
Poonam Chandra
Features of the light curve
Chromatic decay of the
light curve between day 8
and day 18.
Low frequencies
decaying slower and high
frequencies faster.
Steepening of high
frequencies between day 8
to 18.
Flattening in some
frequencies after 18 days.
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Radio emission
described by two
components:
1: Rapidly decaying
component.
2: Slowly decaying
component
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Gaensler et al 2005, resolved source fainting with time
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Gaensler et al 2005
Steepening from day 8.8 onwards.
Chromatic decay is not apparent because of the lack
of low frequency coverage in this paper, which does
not include GMRT data.
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Measurement of parameters using
Equipartition assumption
If we assume that the
U rel
R
UB
6
R11
U
total energy available
for radio emission is
equally divided between
relativistic electrons and
magnetic energy density
i.e.
equipartition between
magnetic energy density
and relativistic energy
density
U B  U rel
UB  F  R
R
U rel  F 
4
p
4
p
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10
p
7
p
11
R
6
Poonam Chandra
Radio spectra of SGR 1806-20
F   a
a  0.7    2a  1  2.4
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Poonam Chandra
Implications and interpretations
Under equipartit ion assumption
Bmin  Fp2 /(2 13) D 4 /(2 13)
Hence Equipartition Magnetic field
Bmin=13 mG
and
Equipartition energy density (when UB=Urel)
Umin=1043 erg
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Umin<U
Poonam Chandra
Distance estimation of SGR 1806-20
from the HI absorption spectra
HI emission
spectrum
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Source
HI absorption spectrum
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Poonam Chandra
1928 Oort model for distance estimation
Radial vel Vr  cosa  0 sin l
0
Sun
 R0 (  0 )sin l
Vr  Ad sin 2l R (   )cosl  d
Tangential vel Vt  sin a  0 cosl
d
0
0
Vt  d ( AForcos
2l  Bood) i.e. d  R
solar neighbourh
( Here    / R and  0   0 / R 0 )
R0
90  a
o
Star
90 o  a
a
0

R
90  l  a
o
a
Rmin
R sin a
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 d 
  0  
 ( R  R0 )
 dR  R0
Define Oort' s constants A and B as
and B  A  0
R0  d 
A 

2  dR  R0
Poonam Chandra
0  220 km/s
Brightness temp (K)
d (kpc) Flux density (Jy)
R0  8 kpc
21cm HI spectrum
100
80
60
40
20
0.08
-
Lower limit d=6.4 kpc
0.04
0
10
20
Upper limit d=9.8 kpc
0.8
Flux density (Jy)
SGR 1806-20
0.6
0.4
0.2
-50
0
50 100 150
Velocity
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2005 (km/s)
Poonam Chandra
In contrast to previously
estimated distance of 15 kpc
(Gaensler et al), the SGR 1806-20
lies between d=6.4 kpc - 9.8
kpc.
Much closer.
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Association with the heavy
mass cluster and Luminous
Blue Variable?
What kind of stars produce
magnetars which forms
SGRs?
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Can Extragalactic SGRs be
the candidates for short
Gamma Ray Bursts
observed in other galaxies?
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What are Gamma Ray bursts (GRBs)?
Most energetic events in the universe
Long duration GRBs (t>2s) (Massive star explosions?)
Short duration GRBs(t<2s) (NS-NS merger, SGRs?)
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Hurley et al. 2005
Three ways to identify:
1: SGRs can be detected only if they
are close by (because of lower energy
scales), hence associated with bright
galaxies.
2: SGRs should produce periodic tail
following the giant bursts
3: SGRs having thermal Black body
spectrum vs GRBs having powerlaw
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Hurley et al. 2005
With BATSE sensitivity, it should have
detected SGRs within 30 Mpc, i.e. 19
SGRs per year. Account for 40% of the
total short GRBs.
(With the given sensitivity, SWIFT can
detect 53 SGRs per year.)
However, no association with bright
galaxies.
Not found having thermal blackbody
spectrum.
Final probability reduces to 5%
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Inputs from our radio measurements:
1: Our revised distance estimate reduces
the probability further.
2: Umin/Egamma <1, whereas in GRB radio
afterglow models, Umin/Egamma=1
3: Decay rate of radio emission
incompatible with GRB afterglow model.
The extragalactic SGRs being short GRB
candidates is highly improbable.
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Conclusions
Most energetic burst observed, energy 100
times more than any previous burst.
Rare event, probability once in a century
Energy powered by high magnetic field
Chromatic decay in the radio flux density
Not associated with the massive star
cluster containing the LBV source.
Unlikely to be the major candidates
for short duration GRBs.
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Poonam Chandra
Acknowledgements
•
•
•
•
•
•
•
Brian Cameron
Alak Ray
Shri Kulkarni
Dail Frail
M. Wieranga
GMRT staff
VLA staff
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Poonam Chandra
THANKS
SGR 1806-20
13th July 2005
Poonam Chandra