Gamma Ray Bursts Poonam Chandra - PowerPoint Presentation

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Gamma Ray Bursts

Poonam Chandra

National Centre for Radio Astrophysics

Tata Institute of Fundamental Research

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Poonam Chandra

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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Gamma Ray Bursts

Detectable at high

redshift

because of their extreme luminosities.

Ionized

f(HI

) ~ 0

Neutral

f(HI

) ~ 1

Reionized

f(HI

) ~ 1e-5

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DEATH OF MASSIVE STARS

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Evolution of stars

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Massive Stars, 25 M



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Nuclear reactions inside a heavy star

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Evolution of stars

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8M

Θ

≤ M ≤ 30M

Θ

Supernova

M ≥ 30MΘGamma Ray BurstPoonam Chandra

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Gamma Ray Bursts

Indicative of massive star formation

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First stars in the high-

z

universe

Barkana

and Loeb (2007)

Initially formed from dark matter mini-halos at

z

=20-30

before

galaxies

Pop III: M~100

Msun L~10

5 Lsun T~105 K, Lifetime~2-3 Myrs

Dominant mode of star formation below 10

-3.5

Z

solar

Can be found only via stellar deaths

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Gamma Ray Bursts

Meszaros

and Rees 1997

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Challenges (Gamma Ray Bursts)

Localization

Galactic or Cosmological

Central Engine, fireball model?

Origin (massive star?)

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GRB Missions

BATSE

BeppoSAX

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Major breakthrough

BeppoSAX

: first detection of X-ray counterpart of GRB 970228.

Optical detection after 20 hours.

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Afterglows: GRB 970508,

(z=0.83)

Frail et al. 2000, 1997, Waxman et al. 1998

Diffractive scintillation size constraint (<10

17

cm).

Energetics from long lived afterglow E

0

=5 x 10

50

ergs.

Density ~0.5 cm

-2,

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GRB 980425/ SN 1998bw

first GRB/SN association

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Crisis: GRB 990123

Assuming isotropy

, the

g

-

ray isotropic energy ~ 3×1054 ergCentral engine energy requirements??

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The GRB Energy Crisis circa 1999

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Stan Woosley says “I’m a very troubled theorist.”

Piran,

Science,

08 Feb 2002

ApJ 519, L7, 1999

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Jet Break due to collimation: GRB 990123

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GRBs: Jets and Geometry

GRB emission is not spherical but in relativistic jets

Due to relativistic beaming, only small fraction of jet.

As jet slows down, lateral expansion.

Jet break, geometrical effect.Simultaneous in all electromagnetic bands.

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The GRB Energy Crisis Resolved

Frail et al (2001

)

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That was then…

The GRB energy crisis was resolved

GRB outflows are highly beamed (

θ

~ 1-10 degrees)

Geometry measured from jet break signature in light curves

Beaming-corrected radiated energies are narrowly distributed around a “standard” value of ~1051 erg

A host of other measurements (X-ray afterglows, broadband modeling,

calorimetry

) support this energy scale

This energy scale is consistent with models of GRB central engines

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SWIFT

AVERAGE REDSHIFT = 2.7

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11-09-13

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FERMI

AGILE

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This is now… POST-SWIFT

The mystery of the missing/chromatic jets in the Swift era.

The emerging population of hyper-energetic events.

The established class of sub-energetic gamma-ray bursts.

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GRBs: Energetics

E

rel

=

Egamma+Einj

+Erad+EkinTwo models collapsar, magnetar, upper limit ~1E+52 ergs

Cenko et al. 2011

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Gamma Ray Bursts

Meszaros

and Rees 1997

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Multiwaveband

modeling

Long lived afterglow with

powerlaw

decays

Spectrum broadly consistent with the synchrotron. Measure F

m, nm, na, nc and obtain Ek (Kinetic energy), n

(density),

e

e

,

eb (micro parameters), theta (jet break), p (electron spectral index).

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Radio Observations

Late time follow up- accurate

calorimetry

Scintillation- constraint on size

VLBI- fireball expansion

Density structure- wind-type versus constant

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RADIO TELESCOPES

VLA

GMRT

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Energetics from Radio

Radio long lived afterglow emission

The outflow reaches sub-relativistic regime

Quasi-spherical geometry

Energetics are independent of uncertain beaming angles

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Afterglows: GRB 970508,

(z=0.83)

Frail et al. 2000, 1997, Waxman et al. 1998

Energetics from long lived afterglow E

0

=5 x 10

50

ergs.

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Density estimation GRB 070125

N=50 cm

-3

or A*=2.5 (n=3E35A*r

-2

), Chandra et al. 2008

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GRB 070125: Chromatic jet break

Chandra, P. et al. 2008

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Scintillation puts constraints (Goodman 1997)

GRB 970508. Limit of 3

microarcsec

on the angular size. R~1E17cm

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GRB 070125: Scintillation

theta=~2.8+/-0.5

m

arcsec

R~2E17cm

Chandra et al2008

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Other Inputs in radio bands

Radio VLBI. Direct constraints on fireball size

Confirmation on relativistic expansion

GRB 030329: Size 0.07mas (0.2pc) 25 days and 0.17mas (0.5p) 83 days.

Confirmation of relativistic fireball expansion (Taylor et al. 2004)

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Other Inputs in radio bands:

Detectability at high redshift

Chandra et al. 2012,

ApJ

746, 156

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GRBs detected at high redshift

GRB 090423 (Chandra et al. 2010)

GRB 050904 (Frail et al. 2009)

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Swift

had expected to find many RS

At most, 1:25 optical AG have RS

Favored explanation

Ejecta are magnetized (i.e.

σ>1).

Do not need to be fully Poynting-flux dominated Suppresses RS emissionDoes not explain why prompt radio emission is seen more frequently.About 1:4 radio AG may be RSPossible Explanation: The RS spectral peak is shifted out of the optical band to lower frequencies

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Kulkarni et al. (1999)

Reverse shock in radio

GRBs

Chandra et al. 2010b

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Reverse shocks in radio afterglows

Only 990123 has a confirmed optical and radio reverse shock.

Low incidence of optical reverse shocks, i.e. < 4% (

Gomboc et al. 2009). Radio RS is 1 every 4 bursts, i.e. 6 times more than optical.

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Reverse shocks in Radio GRBs

: :

Chandra et al., 2013-2014, to be submitted soon (hopefully

)

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Reverse shock emission from GRB 090423 (Chandra et al. 2010)

Reverse shock seen in GRB 050904 (

z

=6.26) too

RS seen in

PdBI

data too on day 1.87

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GRB 130427A: Evidence of RS

Laskar

et al. 2013

Observations with VLA, GMRT, CARMA and combined with optical/IR/UV and X-ray bands.

Most detailed modeling of RS. Wind medium with low density

prefered.

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Radio Detection Statistics

95 out of 304 GRBs detected in radio – 31%

Pre-Swift radio detection 42/123 – 34%

Post-Swift radio detection 53/181 – 29% X-ray detection rate 42% (pre-Swift) to 93% (post-Swift) . Optical detection rate 48% (pre-Swift) to

75% (post-Swift) .Chandra et al. 2012,

ApJ

746, 156

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Radio Detection Biases

Chandra et al. 2012,

ApJ

746, 156

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Sample bias or different population?

(Hancock et al. 2013)

Chandra et al. (2012) sensitivity limited.

Hancock et al. (2013):visibility stacking- two different populations. No

more than 70% of GRB afterglows are truly radio-bright. Radio quiet GRBs are intrinsically weak GRBs at all wavelengths. Gamma-ray efficiency of the prompt emission is responsible for the difference between the two populations. One magnetar-driven, and one black-hole-driven, as gamma ray efficiency inversely proportional to magnetic field.

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A seismic shift in radio afterglow studies

The VLA got a makeover!

More bandwidth, better receivers, frequency coverage

20-fold increase in sensitivity

Capabilities started in 2010

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Future of GRB Physics

11-09-13

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In meter wavebands

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Atacama Large Millimeter Array

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Future: Atacama Large Millimeter Array (ALMA)

Accurate determination of kinetic energy

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Future: ALMA

Debate between wind versus ISM solved

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Future: ALMA

Reverse Shock at high

redshifts

mm emission from RS is bright,

redshift

-independent (no extinction or scintillation) (Inoue et al. 2007). ALMA will be ideal.

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