•• GRB Prompt radiation mechanismsGRB Prompt radiation mechanisms

Outline†

New Scenarios & Developments for Long New Scenarios & Developments for Long GRBs Prompt Emission ModelsGRBs Prompt Emission Models

New developments led by:New developments led by: GRBs 080319B and 080916CGRBs 080319B and 080916C

Alexandria, Egypt, April 2, 2009Alexandria, Egypt, April 2, 2009

†† Work done with Ramesh Narayan & Rodolfo Burniol-DuranWork done with Ramesh Narayan & Rodolfo Burniol-Duran

Understanding GRB central engine

central engine

relativistic outflow

Emission region

Mechanisms for prompt -ray emission

Internal/external shocks, magnetic reconnection etc.Internal/external shocks, magnetic reconnection etc.

Conversion of jet energy to thermal energy Conversion of jet energy to thermal energy

Radiation mechanismRadiation mechanism

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

Piran et al. ; Rees & Meszaros (1994); Thompson; Lyubarsky; Blandford, Spruit…

Papathanassiou & Meszaros, 1996; Sari & Piran, 1997;Ghisellini et al. 2000; Thompson (1994); Lazzati et al. (2000); Medvedev (2000); Meszaros & Rees (1992-2007)….

Internal-External Fireball ParadigmInternal-External Fireball Paradigm

InternalShocks

-rays

10101313-10-101515cmcm

ExternalShock

Afterglow

10101616-10-101818cmcm

Piran et al. 1993; Rees & Meszaros 1994; Paczynski & Xu 1994

Relativistic Outflow

InnerEngine

101066cmcm

Delete this slide?

My strategy: -rays jet central engine

From From -ray observations identify the-ray observations identify theradiation mechanism and determine radiation mechanism and determine the source properties such as Nthe source properties such as Nee, , , , ee, , ee……

Use this information to determine the jet Use this information to determine the jet energy dissipation mechanism & (hopefully) energy dissipation mechanism & (hopefully) jet composition & central engine properties.jet composition & central engine properties.

In the remaining time we will apply this to two In the remaining time we will apply this to two bright bursts: GRBs 080319B & 080916C.bright bursts: GRBs 080319B & 080916C.

GRB 080319BGRB 080319B ( (naked eye burstnaked eye burst))

Burst detected by Swift and Konus satellites ; duration 50s. Burst detected by Swift and Konus satellites ; duration 50s.

Z = 0.937 (Vreeswijk et al. 2008; Cucchiara & Fox 2008).Z = 0.937 (Vreeswijk et al. 2008; Cucchiara & Fox 2008).

20keV — 7 MeV fluence = 5.7x1020keV — 7 MeV fluence = 5.7x10-4-4 erg cm erg cm-2-2; isotropic; isotropicequivalent energy = 1.3x10equivalent energy = 1.3x105454 erg (Golenetskii et al. 2008). erg (Golenetskii et al. 2008).

Spectrum peaked at 650 keV and flux at the peak = 3 mJy.Spectrum peaked at 650 keV and flux at the peak = 3 mJy.

Time averaged Time averaged -ray spectrum: f-ray spectrum: f 0.180.180.010.01 < 650 keV < 650 keV

(Racusin et al. 2008) (Racusin et al. 2008) -2.87-2.870.440.44 > 650 keV > 650 keV

The low energy spectrum was The low energy spectrum was 0.500.500.040.04 for t<8s. for t<8s.

Optical peaked at 5.4 mag or 20 Jy (Karpov et al. 2008).Optical peaked at 5.4 mag or 20 Jy (Karpov et al. 2008).

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Synchrotron solution is ruled out as f+0.18

Synchrotron peak at 650 kev Bi2 = 7x1014

Electron cooling:

ttcoolcool= ~ (2x10= ~ (2x10-9-9 s) s) i3i333 3 3 « « t ~ 0.1st ~ 0.1s

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•66mmeec(1+z)c(1+z)——————————TTBB22i i

f -1/2 and NOT 0.18 as observed.

1. Jitter does not work for this burst (Y » 103).

2. Small pitch angle radiation ( < i-1) can give f+ve

(Lloyd & Petrosian, 2000); but shock accelerations don’t produce small distribution.

This is basically Ghisellini et al. (2000) argument; Sari & Piran 97This is basically Ghisellini et al. (2000) argument; Sari & Piran 97

Note:

angle between e- momentum and B

Synchrotron-self-Compton solution

Synchrotron Synchrotron optical optical andand IC IC -rays-rays•

22. . a a < < ii/25 (because spectrum between 20 & 650 keV /25 (because spectrum between 20 & 650 keV

was was 0.18 0.18 not not +1+1))3. The peak of IC at 650 keV and the flux is 3mJy3. The peak of IC at 650 keV and the flux is 3mJy

Observational constraints:•1. mean optical flux 10 Jy; Synchrotron peak (1. mean optical flux 10 Jy; Synchrotron peak (ii) unknown) unknown

We use all these observational constraints to search for We use all these observational constraints to search for self-consistent solutions (we will make so assumption self-consistent solutions (we will make so assumption regarding the dissipation mechanism for jet energy).regarding the dissipation mechanism for jet energy).

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fficic≈ (8x10≈ (8x10-3-3mJy) mJy) -3-3 f fop,1op,1 ic5.8ic5.8 tt0.90.9 R R16160.640.64

A straightforward (although tedious) calculation finds:A straightforward (although tedious) calculation finds:

Where i/ a & t R/2c2

No solution if one insists on taking t ≈ 0.5s (the observed variability time), except if the source distanceR » 1018cm (much larger than the deceleration radius!).

The IC flux is too small by a factor ~ 10The IC flux is too small by a factor ~ 1022

Solution is possible if we take Solution is possible if we take t ≈ 50s (burst duration), t ≈ 50s (burst duration), and R and R ≈ ≈ 2x102x101717cm; how to produce variability though?cm; how to produce variability though?

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(Clearly NOT internal shocks)(Clearly NOT internal shocks)

0.250.25 22

In general, if In general, if -rays are produced via the SSC process then-rays are produced via the SSC process thenthe jet energy dissipation mechanism is highly unlikely tothe jet energy dissipation mechanism is highly unlikely tobe internal shocks.be internal shocks.

The reason is that for SSC solutions EThe reason is that for SSC solutions EeeRR3 3 and Eand EBB R R-4-4

all internal shocks must take place within a narrow all internal shocks must take place within a narrow range of R (factor ~2) and that seems unlikely. range of R (factor ~2) and that seems unlikely.

Kumar & Narayan, 2009Kumar & Narayan, 2009

An aside:

IC solution when optical and -rays are decoupled?

1. Optical flux is > a few Jy if 1. Optical flux is > a few Jy if -rays are produced via SSC. -rays are produced via SSC.

optical shell

-ray shell

2. Optical photons IC scattered in a different shell?2. Optical photons IC scattered in a different shell?

Causality ensures that separation between shells Causality ensures that separation between shells R/2R/222

Moreover, it can be shown that electron LF in the twoMoreover, it can be shown that electron LF in the twoshells should be the same (to within a factor ~2).shells should be the same (to within a factor ~2).

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This suggests physically related shells.This suggests physically related shells.

Line of Sight

1/

Rt

Relativistic Turbulence Model(Narayan & Kumar, 2009; Lajar, Nakar and Piran, 2009)(Narayan & Kumar, 2009; Lajar, Nakar and Piran, 2009)

Variability time = R(1+z)

————(2c2) t

2

Synchrotron Synchrotron optical (less variable) optical (less variable)

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

(Lyutikov & Blandford, 2003)

The correlation between The correlation between -rays-raysand optical lightcurves suggestsand optical lightcurves suggeststhat they were produced in the that they were produced in the same source. same source.

Larger variability of Larger variability of -ray LC-ray LCRacusin et al. (2008)

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

The ratio of IC & The ratio of IC & synchrotron energies synchrotron energies is ~10, i.e. Y~ 10.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 2The total energy in 2ndnd

IC component is of order IC component is of order the 1the 1stst IC scattering. 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.

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

RS emission

GRB 080319B jet: a schematic sketch

Poynting outflowPoynting outflowThompson 1994 & 06; Meszaros & Rees 97’;

Lyutikov & Blandford(2003); Spruit et al. 2001

Swept up stellar gasSwept up stellar gas

Some problems with the relativistic turbulence model

1. Not clear how to get asymmetric pulses in GRB lightcurve.1. Not clear how to get asymmetric pulses in GRB lightcurve.

2. Correlation between pulse width and the gap preceding it (reported by Nakar and Piran).

These problems and a few others (less serious) are These problems and a few others (less serious) are discussed in Lazar, Nakar and Piran (2009).discussed in Lazar, Nakar and Piran (2009).

(Narayan and Piran might have a solution)

GRB 080916C (2nd Fermi LAT burst)

Burst detected by Fermi GBM & LAT; duration 55s. Burst detected by Fermi GBM & LAT; duration 55s.

Z = 4.3 (Greiner et al. 2009).Z = 4.3 (Greiner et al. 2009).

10keV — 10 GeV fluence = 2.4x1010keV — 10 GeV fluence = 2.4x10-4-4 erg cm erg cm-2-2; isotropic; isotropicequivalent energy = 8.3x10equivalent energy = 8.3x105454 erg (Abdo et al. 2009). erg (Abdo et al. 2009).

Spectrum peaked at 500 keV and flux at the peak = 2 mJy.Spectrum peaked at 500 keV and flux at the peak = 2 mJy.

Time averaged Time averaged -ray spectrum: f-ray spectrum: f 0.020.020.020.02 < 500 keV < 500 keV

(Abdo et al. 2008) (Abdo et al. 2008) -1.2-1.20.03 0.03 > 500 keV > 500 keV

The low energy spectrum was The low energy spectrum was 0.420.420.040.04 for t<4s. for t<4s.

Highest energy photon detected was 13GeV Highest energy photon detected was 13GeV > 10 > 1033. . The flux above 100 MeV was zero during the first 5s.The flux above 100 MeV was zero during the first 5s.

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LAT flux declined as tLAT flux declined as t-1.2-1.20.20.2 for for >100MeV and 5s<t<1400s>100MeV and 5s<t<1400s•

To say: 1) exceptionally bright burst at z=4.3 seen by GBM and LAT.2) Highest energy photon detected was 13 GeV and 3) spectral properties very similar to the naked eye burst. 4) (optional) >100MeV photons lagged MeV Photons by about 5s.

Possible solutions

1. SSC would work only if t≈R/(2c2), i.e. relativistic turbulence, and bright prompt optical flux >10Jy (5-mag).turbulence, and bright prompt optical flux >10Jy (5-mag).

For this solutionFor this solution: ~ I ~ 103 and the 2nd IC peaks at ~ 2 TeV;(fluence in TeV photons is a factor ~5 larger than MeV photons)

2. Synchrotron process? 2. Synchrotron process?

Rapid cooling is a serious problem (Rapid cooling is a serious problem (same as naked eye burstsame as naked eye burst))

A clever way out of this problem was suggested by A clever way out of this problem was suggested by Nakar (2008) who pointed out that dnNakar (2008) who pointed out that dnee/d/dee ee

-1 -1 when when cooling is dominated by IC scattering in K-N regime. cooling is dominated by IC scattering in K-N regime.

The steady state solution ofThe steady state solution of ne + = Q= Qt—

•(ene)e

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is nis nee ee ee-1-1 • • -1-1 ff 00 in this regime in this regime

Wang, Li, Dai & Meszaros (2009) invoke this mechanism for 080916C.

Synchrotron solution continued…Synchrotron solution continued…

However, this requires tHowever, this requires tcoolcool « t « tcoolcoolic synic syn

, and that leads to problems:, and that leads to problems:

1. R < 3x1013cm and in that case Thomson optical depth is sufficiently large so that the IC flux at 2GeV exceeds the observed value by a factor ~ 3; also Y~10.

2.2. Moreover, at this small R, rapid production of eMoreover, at this small R, rapid production of e modifies the low energy spectrum to fmodifies the low energy spectrum to f -1/2-1/2!!

Therefore, this very attractive solutionTherefore, this very attractive solutioncan be ruled out for GRB 080916C. can be ruled out for GRB 080916C.

It can be shown that there are only two synchrotron solutions It can be shown that there are only two synchrotron solutions that are self-consistent:that are self-consistent:

1. 1. >10 >1044, , ii>10>1044, R>10, R>101515cm & Ecm & EBB ~ 5x10 ~ 5x1054 54 ergerg

2. Electrons are continuously accelerated so that the 2. Electrons are continuously accelerated so that the distribution below distribution below ii is actively maintained as is actively maintained as ee

-1-1..

The bottom line is that the Fermi burst is theThe bottom line is that the Fermi burst is the22ndnd burst for which internal shocks are ruled burst for which internal shocks are ruledout, and a magnetic outflow is implicated (theout, and a magnetic outflow is implicated (the11stst burst was 080319B). burst was 080319B).

Synchrotron solution continued…Synchrotron solution continued…

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(cooling unimportant)

2. For these bursts a relativistic magnetic outflow2. For these bursts a relativistic magnetic outflow is indicated & is indicated & -ray source -ray source distance (from the distance (from the center of explosion) is found to be » 10center of explosion) is found to be » 1015 15 cm.cm.

Summary

1. The 6-decades of frequency coverage for GRBs 1. The 6-decades of frequency coverage for GRBs 080319B & 080916C has severely constrained the 080319B & 080916C has severely constrained the established paradigm for GRB jet energy dissipation established paradigm for GRB jet energy dissipation mechanism and the emission process. mechanism and the emission process.

3. Prompt Optical data together with Fermi GBM and 3. Prompt Optical data together with Fermi GBM and LAT provide unprecedented 1—10LAT provide unprecedented 1—101010 eV coverage, eV coverage, and that should finally solve the mystery of and that should finally solve the mystery of -ray -ray emission process in GRBs and the jet composition.emission process in GRBs and the jet composition.

Synchrotron peak: Synchrotron peak: ii = B = B´́ee2 2 /1.8x10/1.8x1088 ( (ii is in eV) is in eV)

Synchrotron peak flux: fSynchrotron peak flux: fpp = 59 B = 59 B´́NNe,55 e,55 mJy mJy

NNe,55e,55 = N = Nee/10/105555

Self-absorption frequency:Self-absorption frequency:

aa= (18 eV) f= (18 eV) fp4p45/85/8 ( (t/3s)t/3s)-1/4-1/4 ii

-1/8-1/8 Y Y-1/8-1/8 R R1515-1-1

(The deceleration radius, Rd, is about 1017 cm)

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RR1515 = R = R /10 /101515 cm cm

(z=0.94)(z=0.94)

GRB 080319B (continued)GRB 080319B (continued)Analysis of optical data

Optical radiation, if from a thermal source, requires:Optical radiation, if from a thermal source, requires: ≈1.2x107 (t)1

-1 T5-1/2 ; (t)1= t/10s; T5=T/105K

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• This suggests a nonthermal origin (likely synchrotron)This suggests a nonthermal origin (likely synchrotron)

ffp4 p4 = = f fpp/10/10 JyJy

How is the jet energy dissipated? How is the jet energy dissipated? Jet is launched at ~10Jet is launched at ~1077cm and thecm and the

energy is dissipated at R>10energy is dissipated at R>101111cm.cm.

How are How are -rays produced? -rays produced?

Jet energy Jet energy radiation radiation

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So the optical band is below or close to theself-absorption frequency

The observed peak optical flux of 20 Jy requires:The observed peak optical flux of 20 Jy requires:

RR > [1.4x10> [1.4x1016 16 cm] (cm] (t/3s)t/3s)-1/4-1/4 ii-1/8-1/8 Y Y-1/8-1/8

Substituting this into Substituting this into t ≈ Rt ≈ R/2c /2c 22

~ 375 ~ 375 ((t/3s)t/3s)-5/8-5/8 ii-1/16-1/16 Y Y-1/16-1/16

It can be shown that It can be shown that ee ≈ 120 ≈ 120 ((t/3s)t/3s)-1/8-1/8 ii

3/163/16 Y Y3/163/16

The radiation was produced at a large distance!The radiation was produced at a large distance!

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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 PropertiesProgenitor Star PropertiesKumar, Narayan & Johnson

((ScienceScience, July 2008), July 2008)

The early steep decline occurs when accretionThe early steep decline occurs when accretion drops below ~ 10drops below ~ 10-2-2 .M s-1

1) The neutrino cooled disk is replaced with a hot 1) The neutrino cooled disk is replaced with a hot torus (ADAF) torus (ADAF) 2) The outer part of the stellar core has rapidly2) The outer part of the stellar core has rapidly declining density profile declining density profile steep decline of dM/dt. steep decline of dM/dt.

Accretion rate during a burst ~ 10Accretion rate during a burst ~ 105252 erg/s * 100/c erg/s * 100/c22

~ 0.1 ~ 0.1 ..MM ss-1-1

1% efficiency for jet1% efficiency for jet

The x-ray plateau occurs when the envelope of the star The x-ray plateau occurs when the envelope of the star with with r r-2.5-2.5 is accreted. is accreted.

Observed flux Observed flux accretion rate accretion rate

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Constraints

Flux Flux spectral index below the peak of spectrumspectral index below the peak of spectrum frequency at peak of spectrumfrequency at peak of spectrum burst/pulse durationburst/pulse duration

5 unknowns and 3 constraints gives 5 unknowns and 3 constraints gives 2-D solution surface.2-D solution surface.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Factor ~ 10Factor ~ 1033 drop in flux!drop in flux!

The central engine comes back to life and isThe central engine comes back to life and isactive for hours and days! active for hours and days!

Fall-back time: tfb~ 2(r3/GMr)1/2 = 2/k

Or rOr r1010 ~ 1.5 t ~ 1.5 t222/32/3 M MBH,1BH,1

1/31/3 ; r ; r1010 r/10 r/101010cm, tcm, t2 2 t/100s t/100s

Specific angular momentum: j(r) = (r) r2 f k r2

Accretion time: tacc~ 2/ k (rd); rd: disk radius

ttacc acc ~ ~ -1 -1 ff33 t tfbfb

Progenitor Structure: Basic procedureProgenitor Structure: Basic procedure

(Problem when you have good data!)

O’Brien et al. 2006

New puzzles posed by Swift data

Flares lastingfor hours - shortand long GRBs

Chromatic plateauChromatic plateauIn x-ray LCsIn x-ray LCs

RS emission?

Jet breaks?Do we have theFS AG right?

3. No firm evidence for r-2 density structure (except perhaps in 1 or 2 cases). And very low density found in several cases is puzzling.

1. The nature of the central engine is not understood.

Unsolved Problems

2. Is the energy from the explosion carried outward by magnetic field, e±, or baryonic material?

4. Collisionless shocks, particle acceleration, magnetic field generation etc. poorly understood.

AGILE (an Italian mission) AGILE (an Italian mission) 30 Mev – 30 Gev & 10 – 40 kev 30 Mev – 30 Gev & 10 – 40 kev is expected to launch in 2005is expected to launch in 2005.

ICECUBE, ANTARES will explore ICECUBE, ANTARES will explore Neutrino emission from GRBs: 10 Gev – 10Neutrino emission from GRBs: 10 Gev – 1055TevTev.

Future MissionsFuture Missions

GLAST, due for launch in 2007,GLAST, due for launch in 2007,will cover 10 Kev – 300 Gev, andwill cover 10 Kev – 300 Gev, anddetect > 200 GRBs yrdetect > 200 GRBs yr-1-1..

Gravitational waves from GRBs?

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