•• -ray radiation mechanism – Fermi data

Gamma-ray bursts in the Fermi Era

† Work done with Ramesh Narayan & Rodolfo Barniol-Duran

Pawan Kumar

Outline†

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Problems with the current paradigm and possible solutions

Brief historical introduction

For about 30 years we didnot even know how far away are these explosionsfrom us – and thereforewe were ignorant aboutthe energy release in GRBs..

We have come a long way since the accidental discovery ofGamma-ray bursts (GRBs) by the Vela satellites about 40 year ago.

It is now firmly established – as a result of observations carriedout by BeppoSAX, HETE II & Swift – that these explosions are at large (cosmological) distances,

Swift launch - Nov 20, 2004

HETE II Oct 2000

Stanek et al., Chornock et al. Eracleous et al., Hjorth et al., Kawabata et al.

SN 1998bw:local, energetic,core-collapsedType Ic

GRB 030329: z=0.171.8 billion light-year(afterglow-subtracted)

Emission lines of CII, OII and OIII

and that at least some GRBs are produced when a massive star undergoes collapse

(Woosley and Paczynski)

Short burstLong burst

Kouveliotou et al. (1993)

Gamma-ray bursts last for ~10ms – 102s.

And the true amount of γ-energy release in theseexplosions is ~1048erg – 1052 ergs.

These explosions are observed for a much longer duration of time – days to months – in x-ray, optical & radio bands.

The long lasting emission – following the brief period of γ-ray emission – is called the afterglow.

Afterglows are produced when the GRB jet shock-heats medium in the vicinityof the explosion, and accelerates e-s which radiate via synchrotron process.

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Gehrels, Piro & Leonard: Scientific American, Dec 2002

Pan

aite

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2001

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Afterglow theory provides a very good fitto the multi-wavelength data for t>0.5 day

Afterglow radiation was predicted well before its discovery by Paczynski & Rhoads (1993) and Rees & Meszaros (1992), and extensively investigated by Piran, Sari, Narayan….

Prompt -ray generation mechanism

O’Brien et al., 2006

Factor ~ 104 drop in flux!

Data from the Swift satellite shows very clearly that the engineturns off quickly at the end of γ-ray burst – a few x 10s afterthe first photons are detected.

If the rapid turn-off is due to a fast decline of accretion rateonto the newly formed black-hole, then we can “invert” theobserved x-ray lightcurve and determine progenitor star structure.

time

flu

x

steep fall off

prompt GRB emission

rapid decline

X-ray plateau

r ≈ 9 109 cm

r ~ 1.5

1011 cm

fΩ ~

2

fΩ 2 3

1010 cm

r -2.5

f k

Progenitor Star PropertiesKumar, Narayan & Johnson (2008)

6/11/2008Fermi8 KeV to 300 GeV

One of the goals for Fermi is to understand γ-ray burst prompt radiation mechanismby observing high energy photons from GRBs.

Let us see how Fermi hasdone…

How are γ-rays generated?

MeV emission lasts for a shorter time duration than x-ray optical & radio.

Radiation Mechanism

Prompt γ-rays & x-ray, optical and radio (AG) photons are produced by two different sources.

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And it turns out that ~MeV emission is shorter lived than GeV!

(CGRO/EGRET had already noticed this for a few burts, and Fermi has shown this to be true for just about every burst it has detected.)

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This suggests different sources for ~MeV and GeV photons.

γ-rays not ES

IC of sub-MeV seed photons in internal Shock, RS or FS, or hadronic collision or photo-pion process...

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Katz 1994; Derishev et al. 1999; Bahcall & Meszaros 2000 Dermer & Atoyan 2004; Razzaque & Meszaros 2006 Fan & Piran 2008; Gupta & Zhang 2008; Granot et al. 08…

Meszaros & Rees 1994; Pilla & Loeb 1996; Dermer et al. 2000 Wang et al. 2001 & 06; Zhang & Meszaros 2001; Sari & Esin 01’Granot & Guetta 2003; Piran et al. 2004; Fan et al. 2005 & 08 Beloborodov 2005; Fan & Piran 2006; Galli & Guetta 2008Pe’er et al. 06; Granot et al. 08; Bosnjak, Daigne & Dubus 09

(~102 man-years of work has gone into it)

Radiation mechanism (high energy -rays)

Fermi/LAT data offers a surprising answer…

Abdo et al. (2009)Delayed high energy emission;

Abdo et al. 2009

(GRB 080916C)

Long lived lightcurve for >102MeV (Abdo et al. 2009)

fν α ν -1.2 t -1.2

α = 1.5β – 0.5 (FS)

>102MeV data expected ES flux in the X-ray and optical band (GRB 080916C)

We can then compare it with the available X-ray and optical data.

Abdo et al. 2009, Greiner et al. 2009, Evans et al. 2009

Long lived lightcurve for >102MeV (Abdo et al. 2009)

Ku

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Or we can go in the reverse direction…

Assuming that the late (>1day) X-ray and optical flux are from ES, calculate the expected flux at 100 MeV at early times

And that compares well with the available Fermi data.

X-ray

Optical

> 100MeV

50 - 300keV

Abdo et al. 2009, Greiner et al. 2009, Evans et al. 2009

Ku

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GRB 090902B: LAT data expected flux at late times

Bissaldi et al. 2009

fν α ν -1.2 t -1.2

α = 1.5β – 0.5 (FS)

We can then compare it with the available X-ray and optical data.

Bissaldi et al. 2009Bissaldi et al. 2009; Swenson et al. 2009; Guidorzi et al. 2009; Evans et al. 2007, 2009

GRB 090902B: LAT data expected flux at late times

Using the parameter space determined from the LAT data: What is the expected ES flux in the X-ray and optical band?

Ku

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Assuming the X-ray, optical and radio flux are from ES: what is the expected flux at > 100 MeV at early time?

Swenson et al. 2009; Guidorzi et al. 2009; Evans et al. 2007, 2009

GRB 090902B: Late time data expected LAT flux

We can then compare it with the available Fermi data.

Swenson et al. 2009; Guidorzi et al. 2009; Evans et al. 2007, 2009

Assuming the X-ray, optical and radio flux are from ES: what is the expected flux at > 100 MeV at early time?

Bissaldi et al. 2009; Swenson et al. 2009; Guidorzi et al. 2009; Evans et al. 2007, 2009

GRB 090902B: Late time data expected LAT flux

Ku

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Ghisellini, Ghirlanda and Nava (2009)

t-(3p-2)/4 ≈ t-1.3

(independent of n, ε )B

Using only >100MeV Fermi data

Par

amet

er s

earc

h a

t t

= 5

0 se

c.Magnetic field in shocked fluid: GRB 090902B

30 μG

5 μG

only late time x-ray, optical & radio data

Par

amet

er s

earc

h a

t t

= 0

.5 d

ay.

30 μG

5 μG

εB is consistent with shock-compressed magnetic field of CSM of a fewtens of μG; Similar conclusions found for GRB 080916c & GRB 090510.

(Kumar & Barniol Duran, 2009)

This suggests either a weak dynamoor no dynamo in relativistic shocks.

Similar conclusion found for SNaremnants (Thompson et al. 2009)

central engine

relativistic outflow

Jet energy dissipation

and γ-ray generation

External shock

radiation

central engine jet -rays

I hope you see the compelling evidence we have that >102MeV photons – and x-ray, optical & radio photons – from GRBs are generated in external shock.

How about photons of energy between ~10kev & 0.1 Gev?

Emission in this band lasts for <102s, however it carries a good fraction of the total energy release in GRBs.

And it offers the best link to the GRB central engine.

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Internal/external shocks, magnetic reconnection etc.

A lot of people have worked on this problem

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• Synchrotron, SSC, IC of external photon field, thermal radiation, jitter radiation…

Piran et al. ; Rees & Meszaros; Dermer;Thompson; Lyubarsky; Blandford, Lyutikov;Spruit…

Papathanassiou & Meszaros, 1996; Sari, Narayan & Piran, 1996Liang et al. 1996; Ghisellini et al. 2000; Thompson (1994); Lazzati et al. (2000); Medvedev (2000); Meszaros & Rees 1992-2007Totani 1998; Paczynski & Xu 1994; Zhang & Meszaros 2001…

2. Synchrotron-self-Compton solutionsSSC solutions have Ee R3 and EB R-4

emission must be produced within a narrow range of R (factor ~ 2) and that is highly unlikely for the internal shock model.

sharp minimum of Ee+ EB; Kumar & Narayan (2009)

No reason that jet energy should dissipate at the minimum of Ee+EB

There is another problem with the SSC solution:

Lack of excess flux in the FermiLAT band; Abdo et al. 2009 (also Ando, Nakar & sari 2008; Piran et al. 2008)

1. Synchrotron: e-s cool rapidly fn α ν-1/2 (rarely observed)

External shock ruled out (Sari and Piran’s excellent argument)

It can’t be internal shock either:

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•(<MeV & GeV behave differently)

1. Thermal radiation + IC Thompson (1994 & 06); Liang et al. 1997; Ghisellini & Celloti 1999; Meszaros & Rees (2001); Daigne & Mochkovitch (2002); Pe’er et al. (2006)…(for prompt -rays)

Problem: we don’t see a thermal component in GRB prompt emission – Ryde (2004, 05) claims to find evidence for thermal spectrum, but Ghirlanda et al. 2007 do not.

There are only three possibilities

2. Continuous acceleration of electrons

If electrons can be continuously accelerated (as they lose energyto radiation) then many of the problems mentioned earlier disappear (Kumar & McMahon, 2008).

However, is it possible to accelerate electronscontinuously in shocks (as opposed to impulsivelyat the shock front)? (Transferring energy fromshock heated protons to e- s continuously requiresunlikely fine tuning.)

3. Relativistic turbulenceLyutikov & Blandford 03’; Narayan & Kumar 09’; Lazar, Nakar & Piran 09’

Beloborodov (2009)

Line of Sight

1/

Rs t

2. Relativistic Turbulence Model

Variability time = Rs(1+z)

————(2c2) t

2

Synchrotron emission in quiescent part of shell less variable optical

IC scattering of synchrotron off of blobs -rays (more variable)

Lyutikov & Blandford 03’; Narayan & Kumar 09’; Lazar, Nakar & Piran 09’

Consistent solutions for -rays arefound in this case & Rs~Rd as suggested by observations.

One possibility:

(This is a way to get around the argument of Sari and Piran, 1997)

GRB 080319B: relativistic turbulence model Kumar & Narayan (2009)

The ratio of IC & synchrotron energies is ~10, i.e. Y~ 10.

Optical band lies above the synchrotron peak wheref -2.9 and so the total energy in synchrotron photons is larger than optical emission by a factor ~ 10.

The total energy in 2nd

IC component is of order the 1st IC scattering.

The 2nd IC lies in the K-Nregime since i/ >1, and the peak photon energy in e- rest frame is ~ 2 MeV.

Focus on Y being order 10 and energy in 2nd IC of order 1st IC

Steep decline (~t-5) consistent with LAE (not RS)

RS emission

GR

B 0

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9B:

x-ra

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Relativistic turbulence model: outstanding issues

2. Not clear how to get asymmetric pulses in GRB lightcurve.

3. Correlation between pulse width and the gap preceding it (reported by Nakar and Piran, 2002; Quilligan et al. 2002).

1. How to generate relativistic turbulence?

Relativistic turbulence (Poynting jet) has a number of attractive properties, but there are also a number of difficulties.

Summary

2. Generation of sub-MeV γ-rays remains unclear:

✪ The internal-shock model for GRBs + synchrotron (SSC) radiation mechanisms have severe problems.

1. High energy -ray data for a number of bursts — 080916C, 090510, 090902B — observed by Fermi suggest that >102MeV photons are produced in shock heated CSM; εB ~ shock-compressed CSM field.

✪