13 th july 2005poonam chandra the most violent bomb-blast in our galaxy in 100 years sgr 1806-20...
TRANSCRIPT
13th July 2005 Poonam Chandra
The most violent bomb-blast in The most violent bomb-blast in our Galaxy in 100 yearsour 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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Saturated all spacecraft detectors
(INTEGRAL, SWIFT etc.)
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Disturbed earth’s ionosphere
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SGR 1806-20
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Introduction• 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!
Plan of the talk
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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
1806-20RA Dec
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Precursor
Spike
Tail
Pulsed tail emission
Giant flare for 0.2 sec, tail for 382 sec, 1 sec precursor before 142 sec giant flare.
99.7% of the total energy
Burst profile of Dec 27, 2004 giant flare Hurley et al (2005), Nature
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Precursor Spike Tail
Duration 1 sec 0.2 sec 382 sec
Temp 15 keV 175 keV 3-100 keV
Fluence (erg/cm2)
1.8x10-4 1.36 4.6x10-3
Energy (ergs)
2.4x1042 1.8x1046 1.2x1044
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(peak is ~5 km overhead on this scale!)
15-25 keV
25-50 keV
50-100 keV
100-350 keV
SGR 1806-20
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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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SGR 0526-66
SGR 1627-41SGR 1900+14
SGR 1806-20
Other Soft Gamma Ray Repeaters
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 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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Accretion due to binary Cannot explain the initial spike.
Difficult to explain pure form of energy least contaminated by baryons.
No binary associations found.
Three candidates for SGRs
Rotation energy of pulsar
The maximum luminosity obtained is 1033
ergs/s Very slow rotating objects, cannot explain such huge energy
Powered by magnetic field
(MAGNETAR)
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MAGNETAR
Most accepted model
A young neutron star with age <10,000 years.
Extremely high magnetic field (~ 1015 Gauss)
SGRs are magnetars occasionally emitting energetic bursts in the early phase of their life times.
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Comparison of magnetic field strengths
Earth 0.6 Gauss
Strong sunspots 4000 Gauss
Strongest lab mag. field 5x105 Gauss
Radio pulsar 1012 Gauss
Magnetar 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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dtdPPB /.
field Magnetic
The magnetic field required for SGR 1806-20 slow down rate is ~1015 Gauss!!
tcc
Idt
dE
Sin as3
2
3
2
0
3
42
3
..2.
30 and ;
2 Since BR
P
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dtdP
P
/2
Age sticCharacteri
The characteristic age of the SGRs estimated are 10,000 years in contrast to 1 million years of age of neutron stars.
nnn kPPk 21..
)2(
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Why high 1015G magnetic field?
1: Required for such high spin down observed.
2: Required to explain the energy of the explosion
3: To explain the trigger of SGR activity in 104 years.
4: Explain super eddington luminosity
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).
215262
22
)G10/(keV) 10/(10)/(~/
field magnetichigh in on crosssectiThompson
m.equilibriu radiativeor
chydrostatisustain toNeeded
44
force nalgravitatio pressureRadiation
BeBm
cGMmL
R
GMm
cR
L
T
Edd
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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Giant Flares:Sudden Large-scale re-arrangement of the magnetic field
223 46
154 10
8 10core coreB BR erg
G
Magnetar burst emissionThompson & Duncan 1996
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Global changes in the magnetic field geometry“Interchange instability” (Energy released (Bext
2/8)R3)
Flowers & Ruderman (1977)
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22 241 0 max
15 310
10 1 10SGR
B lE erg
G km
Small Bursts (SGR events):
Cracking of crust leads to small displacements of magnetic field
Thompson & Duncan 1996
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Ic
dlB 4
.
Inside twist => magnetic field lines outside the star also get twisted because they are anchored to the crust (cracking 5 km?).
Thompson & Duncan 1996
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Tail emission:from the trapped fireball oscillating with the neutron star rotation period.
Thompson & Duncan 1996
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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
1420
MHz
1420
MHz
610 MHz
610 MHz
50 M
Hz
50 M
Hz
150 MHz
150 MHz
235
MHz
235
MHz 325 M
Hz
325 MH
z
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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.
3: Could resolve 12” away LBV 1806-20 source close by at low frequencies.
Negative declination
Positive 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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SGR
FoV of SGR before the giant burst
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GMRT map of SGR 1806-20 in 235 MHz band
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LBV source
Fading SGR 1806-20
6 January 2005 16 January 2005
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Light curve from day 5 to day 50
tF
Freq (GHz)
C
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
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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.
Features of the light curve
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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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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.
Gaensler et al 2005
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Measurement of parameters using Equipartition assumption
674
11104
RFU
RFU
pprel
ppB
If we assume that the 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
6R
R
11R
U
BUrelU
relB UU
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Radio spectra of SGR 1806-20
4.2127.0
F
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Implications and interpretations
Hence Equipartition Magnetic field
Bmin=13 mG
and
Equipartition energy density (when UB=Urel)
Umin=1043 erg
)132/(4)132/(2min
assumptionion equipartitUnder DFB p
Umin<U
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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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1928 Oort model for distance estimation
0
0R
lo90
R
minR
o90
sin R
d
o90
Sun
Star
and
as and constants sOort' Define
)(
i.e. oodneighbourhsolar For
cos)(
cossin velTangential
sin)(
sincos velRadial
00
0
)/ and / Here(
00
0
00
0
0
000
BA
RRdR
d
Rd
dlR
lV
lR
lV
R
RR
t
r
0AB
2
0
0
RdR
dRA
)2 cos ( BlAdVt lAdVr 2sin
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SGR 1806-20
Flu
x de
nsity
(Jy
)
d (k
pc)
Flu
x de
nsity
(Jy
)
Brig
htne
ss te
mp
(K)
100
20
40
60
80
Velocity (km/s)- -50 0 50 100 150
0.2
0
0.4
0.6
0.8
0.08
0.04
20
10
Lower limit d=6.4 kpc
Upper limit d=9.8 kpc
km/s 220
kpc 8
0
0
R
21cm HI spectrum
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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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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
Hurley et al. 2005
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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%
Hurley et al. 2005
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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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ConclusionsMost energetic burst observed, energy 100
times more than any previous burst.
Energy powered by high magnetic field
Not associated with the massive star cluster containing the LBV source.
Rare event, probability once in a century
Chromatic decay in the radio flux density
Unlikely to be the major candidates for short duration GRBs.
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Acknowledgements
• Brian Cameron• Alak Ray• Shri Kulkarni• Dail Frail• M. Wieranga• GMRT staff• VLA staff
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SGR 1806-20
THANKSTHANKS