The many mysteries of gamma-ray bursts
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The many mysteries of gamma-ray bursts R. Mochkovitch (IAP-Paris) All starts as a James Bond movie… Moscow: August 5, 1963. Treaty Banning Nuclear Weapon Tests in the Atmosphere, in Outer Space and under Water

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The many mysteries of gamma-ray bursts R. Mochkovitch (IAP-Paris)

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The many mysteries of gamma-ray bursts

R. Mochkovitch (IAP-Paris)

All starts as a James Bond movie…

Moscow: August 5, 1963

Advanced Topics in Astrophysics - Llafranc 4-6 May, 2011

Treaty Banning Nuclear Weapon Tests in the Atmosphere, in Outer Space and under Water

Signed by the Original Parties, the Union of Soviet Socialist Republics, the United Kingdom of Great Britain and Northern Ireland and the United States of America at Moscow: 5 August 1963 The Governments of the United States of America, the United Kingdom of Great Britain and Northern Ireland, and the Union of Soviet Socialist Republics, hereinafter referred to as the "Original Parties,"Proclaiming as their principal aim the speediest possible achievement of an agreement on general and complete disarmament under strict international control in accordance with the objectives of the United Nations which would put an end to the armaments race and eliminate the incentive to the production and testing of all kinds of weapons, including nuclear weapons,Seeking to achieve the discontinuance of all test explosions of nuclear weapons for all time, determined to continue negotiations to this end, and desiring to put an end to the contamination of man's environment by radioactive substances,Have agreed as follows:

Article I

1. Each of the Parties to this Treaty undertakes to prohibit, to prevent, and not to carry out any nuclear weapon test explosion, or any other nuclear explosion, at any place under its jurisdiction or control:(a) in the atmosphere; beyond its limits, including outer space; or under water, including territorial waters or high seas; or

How to detect a nuclear explosion in the atmosphere?

→ with gamma rays !

To sign a treatyis fine but one wants to be sure itisrespected …

The « VELA » project(3 pairs of satellites launchedin 1963, 64 and 65)

The first gamma ray burst

A great diversity of light curves

20 – 1000 keV

Duration distribution

Simple broken power-law spectra

synchrotron ?


1991-1999: the BATSE revolution


● Isotropy: both short and long GRBs are uniformlydistributed on the sky

Spatial distribution

First hint for a cosmological distance scale

Beppo-SAX: the afterglowera (1997)

Gamma ray error boxes are too large

→ Beppo-SAX : GRB monitor + X-ray cameras

on Feb. 28, 1997

opticalafterglow !

X-ray afterglow

GRB 970508 : z = 0.835

Afterglowsallow to measure the redshift

● directly, fromthe afterglowitself

● undirectly, via the host galaxy (more uncertain)

the first redshift !

the first redshift !

Fe and Mg lines

z = 0.225 (short burst)

Confirmation: GRBs are locatedatcosmological distances !

The afterglow: spectral and temporal evolution

● progressive shift of the emission to lowerenergies :

X→ visible→ radio


● approximative power lawdecay (with accidents: breaks, bumps)

2000: HETE 2 and the progenitors

● long bursts

spectroscopic or photometric signature of an underlying supernova

z = 0.17

Supernova bump at late times

Type Ic SN: exploding WR star

But not all SNe Ic give a GRB !

→ Additional ingredients required:

- Very massive ?

- Very metal poor ?

- Very rapidly rotating ?

● short bursts

severalSHBsassociated to elliptical (non star forming) galaxies

GRB 050724

z = 0.26

● long delaybetweenobject formation and GRB

● no SN bump→stronglimit on underlyingsupernova

Coalescence of compact objects

● The duration of source activity

1 ms – 1000 s


and long-termactivity

GRB 041219A

5 mn

5 h

2004: Swift and the earlyafterglow

● More sensitive thanBeppo/HETE

● Earlyfollow-up in X and V → more redshifts ( z = 2.8; zmax = 8.2 !)

a window opens on the earlyafterglowevolution

Surprises in X-rays !


Initial steep decay Extended plateaus Flares









Which model(s) for gamma ray bursts ?

Basic constraints :

● temporal variability : ~1ms→ compact objects

● GRBswithknownz→Egiso = 1048 (shortestbursts)→1054 erg = M c2 !

butcorrectedfrombeaming:EgL-GRB≲1051 erg

● the compactnessproblem:→ large Thomson opticaldepth

solution:emissionfromrelativisticallymovingmaterial : G > 100

→Lorentz boost/relativisticbeamingsupress pair creation

● progenitors and rate

long GRBs: RGRB ~ 0.1 - 1 Gpc-3yr-1extremeSNIc

short GRBsRSHB ~ ? coalescence of compact objects

A model in threesteps(both for long and short GRBs)

1) The central engine and the production of a relativisticoutflow

2) The origin of the gamma ray emission

3) The production of the afterglow

1) Verydifficult and poorlyunderstood :

● GR, relativistic hydro, superstrongmagneticfields

2) From the thermal/magnetic/kineticenergy of the flow

●  a wellstudied scenario: internalshocks

3)The best understood (maybe):

● relativisticshockpropagating in the source environment


The “standard” model

  • acceleration of the flow

  • coasting phase G> 100

  • transparency radius → thermal precursor ?

  • internal shocks → gamma-ray emission

  • reverse shock → ?

  • surface of discontinuity

  • forward shock → afterglow

  • break in the light curve : 1/G > θ

Log R (m)

Collapse of a massive rapidly rotating star (long GRBs)

Coalescence of two compact objects

(NS/BH) (short GRBs)

The central engine

• A black holewith an accretion torus

Mochkovitch, Hernanz, Isern & Martin, 1993

Relativistic jets : a lot of energyinjectedintovery few baryons (E/Mc2 >> 1)

“ baryonic pollution problem ”

• Milli-second proto-magnetar

  • ● The ultimate source(s) of energy

  • accretion of diskmaterial by the black hole

  • rotationalenergy of the black hole

    ● The extraction of energy

  • annihilation

magnetic extraction

● not very efficient:

●from the BH: Blandford-Znajek process

● from the disk: magnetized wind

● The acceleration of the flow

Thermal (fireball model)adiabatic expansion of a radiativelydominated plasma: g, e+e- pairs + a few baryons (baryon load: h = E/Mc2 )initiallyconfinedwithin R0 ~ 10 – 100 km

thermal energy→ kineticenergy of the baryons

Radiative phase: Gα R and T α R-1

Matterdominated phase: G→ h(for R ≳ h R0)

No strong thermal component at transparency

→ hint for magnetic acceleration ?

(Daigne & Mochkovitch, 2002)

E = 1053erg

h= 400

  • Magnetic

  • uncertainefficiencys ∞ =Emag/Ekin|∞



G1 > G2




The prompt gamma-ray emission

Internal shocks

Nice results(Daigne & Mochkovitch, 1998)

But: no shock if s > 1 at the location of IS (RIS~ 1015 cm)

and low efficiency < 5%

→ Alternative to internal shocks:

magnetic reconnection

comptonization at the photosphere

One big mystery: why so many different gamma-ray light curves ?

simple FRED

complex, very spiky

with a pause

  • Where are the fluctuations generated ?

  • at the origin of the flow ?

  • during its propagation through the progenitor star ?

  • later, as a result of flow instabilities ?

  • Why is short time scale variability sometimes present and sometimes not ?

  • Why does the burst sometimes make a pause ?

  • The early afterglow

  • Plateau and flares : a late activity of the central engine ?

  • (steep rise and decay incompatible with a forward shock origin)

  • Up to one day after the burst, even in short burst (GRB 050724: duration 2.4 s)

  • steady/flaring activity: powered by fall back material ? a magnetar ?

  • An alternative to late activity: a long-lived reverse shock realizes

  • a “tomography” of the ejecta(Genet, Daigne & Mochkovitch, 2007)

  • It supposes that: the reverse shock is present (s < 1).

  • the forward shock is radiatively inefficient

  • Conclusion

  • What we know :

  • Most (if not all) observed GRBs are located are cosmological distances

  • (but a large population of nearby sub-luminous GRBs may exist)

  • GRBs are formed in beamed relativistic outflows

  • The afterglow results from the deceleration of the flow

  • Progenitors: L-GRBs “extreme” exploding WR stars, S-GRBs: compact object mergers

  • But we still don’t know…

  • What is the ultimate source of energy (accretion, BH rotation) ?

  • How is the flow accelerated (thermal/magnetic acceleration) ?

  • For how long can the source be active ?

  • Which process makes the prompt emission (IS, reconnection, comptonization) ?

  • What is the role of the reverse shock (if present) ?

  • Swift and Fermi, rather than solving old questions, have raised new ones !

  • However a tremendous progress in our knowledge of GRBs in the past 20 years

  • → should ultimately lead to more understanding…

2470: c !

2290: vs

2210: 1m/s

2064: 1 m/day

Do you know what it is ?

Peu de cabra comú



Entenmuschel (Klette?)


דג השבלול


Goose neck barnacle

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