13 th july 2005poonam chandra the most violent bomb-blast in our galaxy in 100 years sgr 1806-20...

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


27th December 2004 at 4:30:26.65 pm EST

Courtesy: NASA


Saturated all spacecraft detectors

(INTEGRAL, SWIFT etc.)


Disturbed earth’s ionosphere


SGR 1806-20


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


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)


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


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


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


(peak is ~5 km overhead on this scale!)

15-25 keV

25-50 keV

50-100 keV

100-350 keV

SGR 1806-20


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.


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


(Marsdon & Higdon, 2001, Taylor & Cordes 1993)

Sun


2005

SGR 1806-20 was 100 times larger in energy than any other previous busrt.


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


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.”


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)


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.


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


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


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


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(


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


HOW A MAGNETAR IS FORMED?


Supernova explosion leaves neutron star as a remnant in the center.


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


Global changes in the magnetic field geometry“Interchange instability” (Energy released (Bext

2/8)R3)

Flowers & Ruderman (1977)


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


Ic

dlB 4

.

Inside twist => magnetic field lines outside the star also get twisted because they are anchored to the crust (cracking 5 km?).



Tail emission:from the trapped fireball oscillating with the neutron star rotation period.



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


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


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


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.


SGR

FoV of SGR before the giant burst


GMRT map of SGR 1806-20 in 235 MHz band


LBV source

Fading SGR 1806-20

6 January 2005 16 January 2005


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


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


Radio emission described by two components:

1: Rapidly decaying component.

2: Slowly decaying component


Gaensler et al 2005, resolved source fainting with time


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


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


Radio spectra of SGR 1806-20

4.2127.0

F


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


Distance estimation of SGR 1806-20 from the HI absorption spectra

HI emission spectrum


Source

HI absorption spectrum


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


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


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.


Association with the heavy mass cluster and Luminous Blue Variable?

What kind of stars produce magnetars which forms SGRs?


Can Extragalactic SGRs be the candidates for short Gamma Ray Bursts observed in other galaxies?


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?)


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


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


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.


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.


Acknowledgements

• Brian Cameron• Alak Ray• Shri Kulkarni• Dail Frail• M. Wieranga• GMRT staff• VLA staff


SGR 1806-20

THANKSTHANKS

13 th july 2005poonam chandra the most violent bomb-blast in our galaxy in 100 years sgr 1806-20...

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