The prompt optical emission in the Naked Eye Burst

R. Hascoet with F. Daigne & R. Mochkovitch (Institut d’Astrophysique de Paris)

Kyoto − Deciphering then Ancient Universe with Gamma-Ray Bursts

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Modeling the « Naked Eye Burst »

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Observations : a cosmological naked eye burst- For the first time, optical light curve during the whole prompt emission high temporal resolution.- huge radiated energy : Eg,iso = 1.3×1054 erg (20 keV – 7 MeV)

- redshift : z = 0.937- V magnitude peak : mV,max = 5.3 (bright as 107 galaxies)

Light curves (gamma & optical)

Huge optical brightness – big challenge for the different models –

optical✕

g-ray spectrum

(Racusin et al. 2008)

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Different scenarios already proposed

Scenario 1 (single zone) : Synchrotron-Self Compton radiation from a single electron population

✕• Optical : synchrotron• Gamma : first IC scattering on the synchrotron photons(Racusin et al. 2008)(Kumar & Panaitescu 2008)(Kumar & Narayan 2009)

Scenario 2 (single zone) : Synchrotron radiation from two electron populations

• Optical : synchrotron – mildly relativistic electron pop. • Gamma : synchrotron – highly relativistic electron pop.

✕

These two scenarios face big problems : energy crisis, ….(Zou, Piran & Sari 2008)

No self-absSelf-abs

No self-absSelf-abs

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Scenario 3 : Huge optical brightness due to a highly variable jet

Internal Shock model

Huge optical brightness due to a highly variable jet ( Lorentz Factor : Gmax/Gmin ≈ 5 - 10)

Synchrotron radiation from shock-accelerated electrons in multi-shocked regions- gamma component : violent shocks- optical component : mild shocks

Log(

R) [m

]

• Variability during the ejection : “fast” shells catch up with “slow” shells ( ≈ 100G )• Shocks : magnetic field amplification particle acceleration (relativistic electrons)• Radiation (g-rays) from the electrons : Synchrotron – IC

We use a multi-shell model as proposed by Daigne & Mochkovitch 1998

(see also Yu, Wang, Dai 2009)

Proposed scenario : 1 electron population in multiple regions – Synchrotron emission- optical component : mild shocks - gamma component : violent shocks

Huge optical brightness due to a highly variable jetInternal Shock model framework

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Characteristic photon energy vs. radius Spectrum – Asymptotic Synch.(Sari, Piran & Narayan 1998)

initial profile

Optical light curve Gamma light curve

mild shockcontribution

violent shockcontribution

✕

Ekin,iso = 5 10⋅ 55 ergee = 1/3eB = 1/3z = 10-2

No self-absSelf-abs

Proposed scenario : 1 electron population in multiple regions – Synchrotron emission- optical component : mild shocks - gamma component : violent shocks

Huge optical brightness due to a highly variable jetInternal Shock model framework

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Characteristic photon energy vs. radius Spectrum – Ad hocinitial profile

Optical light curve Gamma light curve

mild shockcontribution

violent shockcontribution

✕

Ekin,iso = 5 10⋅ 55 ergee = 1/3eB = 1/3z = 10-2

• The high optical brightness of the Naked Eye Burst is very challenging for GRB models.

• Proposed scenario : the initial outflow is highly variable.

A potential problem : the shape of the gamma-ray spectrum in some cases.Due to a high dispersion in the characteristic energies of the emitted photons

Reproduced observational features (with a fair probability : Monte Carlo analysis) : 1. High optical flux :

- mainly built up by the milder shocks2. The optical light curve is less variable than the gamma-ray one : - G of the shocked material is smaller for mild shocks (Dtobs ≈ R/2G2c)

3. The optical light curve begins after the gamma-ray one : - the optical synchrotron emission of the shocks with smaller radii is self-absorbed

4. The optical light curve ends after the gamma-ray one : - same reason as for (2.) - late shocks enhance the delay, in some cases

Summary

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The Naked Eye Burst : why is it so bright in the optical domain ?

(gamma & optical)

(Racusin et al. 2008)

The precise predicted fraction of optically bright bursts depends on the unknown central engine exact properties

What would be the probability of an event such as the Naked Eye Burst ?

What is the probability of having a burst such as the

“naked eye burst” ?

– the physics of the central engine is still unclear –

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Statistical approach – Monte Carlo Simulation

G varies on timescales 0.5s andis forced to be either 200 or 800

(with equal probability)

G values are uniformly distributed between 200 or 800

Cumulative fraction

66% cases brighter than GRB080319B

16% cases brighter than GRB080319B

Cumulative fraction

Series of 500 runs

Example of the − optical mean flux −

NN

Modeling Internal Shocks

23/4/10

- Discretisation of the jet in N shells- Successive collisions between these shells mimic the propagation of shock waves- We follow the evolution of the physical conditions in shocked regions

- Discretisation of the jet in N shells- Successive collisions between these shells mimic the propagation of shock waves- We follow the evolution of the physical conditions in shocked regions

23/4/10

Shock1 Shock2

Modeling Internal Shocks

- Discretisation of the jet in N shells- Successive collisions between these shells mimic the propagation of shock waves- We follow the evolution of the physical conditions in shocked regions

23/4/10

Modeling Internal Shocks

Shock1 Shock2

- Discretisation of the jet in N shells- Successive collisions between these shells mimic the propagation of shock waves- We follow the evolution of the physical conditions in shocked regions

23/4/10

Modeling Internal Shocks

Shock1 Shock2

- Discretisation of the jet in N shells- Successive collisions between these shells mimic the propagation of shock waves- We follow the evolution of the physical conditions in shocked regions

23/4/10

Modeling Internal Shocks

Shock1 Shock2

- Discretisation of the jet in N shells- Successive collisions between these shells mimic the propagation of shock waves- We follow the evolution of the physical conditions in shocked regions

23/4/10

Modeling Internal Shocks

Shock1 Shock2

- Discretisation of the jet in N shells- Successive collisions between these shells mimic the propagation of shock waves- We follow the evolution of the physical conditions in shocked regions

23/4/10

Modeling Internal Shocks

Shock1

- Discretisation of the jet in N shells- Successive collisions between these shells mimic the propagation of shock waves- We follow the evolution of the physical conditions in shocked regions

23/4/10

Modeling Internal Shocks

Shock1

- Discretisation of the jet in N shells- Successive collisions between these shells mimic the propagation of shock waves- We follow the evolution of the physical conditions in shocked regions

23/4/10

Modeling Internal Shocks

Shock1

- Discretisation of the jet in N shells- Successive collisions between these shells mimic the propagation of shock waves- We follow the evolution of the physical conditions in shocked regions

23/4/10

Modeling Internal Shocks

Shock1

23/4/10

Modeling Internal Shocks- Discretisation of the jet in N shells- Successive collisions between these shells mimic the propagation of shock waves- We follow the evolution of the physical conditions in shocked regions