Shocks in!Gamma-Ray Bursts:!

from theory to observations Lara Nava!

Marie Skłodowska-Curie Fe$ow! !

INAF - OABrera!INAF - OATrieste

GRBs: the standard model

PROMPT

or magnetic

reconnection ???

Rdiss=1013-1014cm

Rdiss=1015-1016cm

AFTERGLOW

Rdiss=1016-1017cm

PROMPTAFTERGLOW

Shocks take place in two different sites

•10 keV- 10 MeV!!

•< 1s to 103 s!!

•non-thermal ! spectra

• softX, OT, radio!!

• weeks, months!!• flux PL decay in ! time

!’

Prompt !— internal shocks —!

mildly relativistic

electrons magnetic field

γ

N(γ)

γmin

-p

PROMPT spectra:!synchrotron theory vs observations

�c �m �

F� �

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Typical prompt spectrumTheoretical synchro spectrum

PROMPT spectra:!synchrotron theory vs observations

�c �m �

F� �

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Typical prompt spectrumTheoretical synchro spectruminformation on !

the electron spectrum

depends on !B, Lk, εe, ξe

�m �

F� �

+1

Black: typical observed !prompt spectrum

Colours: predicted !synchrotron spectrum!

Inconsistency!!Observed spectra!

are harder than predicted

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-(p-2)/2

�c

PROMPT spectra:!synchrotron theory vs observations

Prompt emission !Light curves

Oganesyan, Nava, Ghirlanda, Celotti, 2017, ApJ

red = XRT 0.3 -10 keV!green = BAT 15 - 150 keV

Oganesyan, Nava, Ghirlanda, Celotti, submitted

Comparison !between observed spectral shape and synchrotron spectrum

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�c �m �

F� �

Ebreak Epeak

α1

βα2

Spectral breaks in Fermi bursts?!GRB 160625BRavasio,…LN et al., 2018, in press

GBM GRB 160625B !results from time-resolved analysis

Spectral indices

¡3:5¡3:0¡2:5¡2:0¡1:5¡1:0¡0:5

Index

0

1

2

3

4

5

6

7

8

9

N

®1

®2

¯

®syn1=¡0:67

®syn2=¡1:5

¡3:5 ¡3:0 ¡2:5 ¡2:0 ¡1:5 ¡1:0 ¡0:5

Index

0

1

2

3

4

5

6

7

8

9

N

®1

®2

¯

®syn1 = ¡0:67

®syn2 = ¡1:5

1.5 1 0.5 0 -0.5 -2 -3

- - - - Synchrotron predictions

Are we observing synchrotron radiation in marginally fast cooling regime?

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�c �m �

F� �

1

Theoretical models !marginally fast cooling:!

�c~�mDerishev 2007!Kumar & McMahon 2008!Daigne et al. 2011!Beniamini & Piran 2013,2014!Uhm & Zhang 2014

Implications: small B’~10 Glarge Γ > 500 large γmin > 104-5

!

Afterglow !— external shocks —!

ultra-relativistic

electrons magnetic field

γ

N(γ)

γmin

-p

!’ !

Prompt !— internal shocks —!

mildly relativistic

Afterglow !— external shocks —!

ultra-relativistic

B’ ~ 20 G!γe ~ 104B’ ~ 104 G!

γe ~ 400

Afterglow !observations/modeling

Six (seven) free parameters in the model! εe, εB, (ξe), p, Ek, n = n0 R-s!

non simultaneous observations at different ν!radio observations available only for a small sample and at late time!observed features not included in the model (bumps, plateaus, flares,…)!

some of the parameters are fixed to what is believed is their typical value

Six (seven) free parameters in the model! εe = 0.1, εB= 0.01 (ξe) p =2.3, Ek (from prompt), n=n0 R-s!

non simultaneous observations at different ν!radio observations available only for a small sample and at late time!observed features not included in the model (bumps, plateaus, flares,…)!

some of the parameters are fixed to what is believed is their typical value

Afterglow !observations/modeling

same origin as the X-ray and Optical afterglow:!-- External Shocks --

Kumar &Barniol Duran 2009, 2010; Gao et al. 2009; !Corsi, Guetta & Piro 2010; De Pasquale et al. 2010

Ghisellini, Ghirlanda, Nava & Celotti 2010 — Ghirlanda, Ghisellini & Nava 2010

GeV AFTERGLOW!around 50 GRBs with GeV long lasting

emission

Fermi-LAT!0.1 - 10 GeV

optical !afterglow

"i "c

SSC suppressed

"i "c

Slow Cooling

Density

uppe

r lim

its to

# B

Upper limits on εB!always smaller that the canonical value εB=10-1-10-2!

(Kumar & Barniol Duran 2009, Lemoine 2013a/b — Santana et al., 2014)

X-ray X-rayGeV GeV

ConclusionsSmall B…? What is the value of B relevant for particle cooling?!

Maximum energy of accelerated electrons?!

Efficiency of the acceleration mechanism (= fraction of particles injected in the acceleration process)?

FutureSKA: radio observations at late time, when fireball isotropic, give direct estimate of the true energy!

CTA: cutoff in the afterglow synchrotron spectrum (=maximal energy of accelerated particles)? Inverse Compton component?