High-energy radiation from gamma-ray bursts

Zigao DaiNanjing University

Xiamen, 23-24 August 2011

Astrophysical implications of high-energy emission from GRBs

Central enginesBulk Lorentz factor of fireballsComposition of fireballsAcceleration of particles Radiation mechanisms of particlesQuantum gravityExtragalactic infrared background radiationIntergalactic magnetic fields

Outline

1. High-energy emission mechanisms before Fermi

2. Fermi/LAT observations

3. Models of high-energy emission

4. Constraints on intergalactic magnetic fields

Outline

1. High-energy emission mechanisms before Fermi

2. Fermi/LAT observations

3. Models of high-energy emission

4. Constraints on intergalactic magnetic fields

The standard fireball + shock model

From T. Piran

High-energy emission mechanisms in GRBs before Fermi

1. Leptonic models:

① Synchrotron self-Compton (prompt phase, reverse-shock phase, afterglow phase)

② External inverse-Compton (prompt photons, reverse shock photons)

③ Flare inverse-Compton (late internal shock photons)

2. Hadronic models:

① proton synchrotron radiation,

② proton + photon high-energy photons and neutrinos,

③ proton + proton high-energy photons and neutrinos.

(Gupta & Zhang 2007)

Interactions: which electrons (or protons)? + which photons?

Meszaros & Rees (1994); Sari & Esin (2001); Wang, Dai & Lu (2001); Zhang & Meszaros (2001); Dai & Lu (2002), and so on

Wang, Dai & Lu (2001); Beloborodov (2005); Fan & Piran (2006); Fan et al. (2008)

Burrows et al. (2005)

Wang, Li & Meszaros (2006); Fan et al. (2008); Galli & Piro (2007)

Fan et al. (2008)

Outline

1. High-energy emission mechanisms before Fermi

2. Fermi/LAT observations

3. Models of high-energy emission

4. Constraints on intergalactic magnetic fields

530 GBM GRB (since Aug 2008)

22 LAT GRB (>100 MeV)

Fermi detections as of 2011-01-20

Credit: N. Omedi

Four important cases(1) GRB080916C: z=4.35+-0.15 , ~13 GeV at t=16.54 s,

(1) Time delay of high-E gamma-ray emission. (2) A steep power-law decay

Zhang et al. 2011

Implications

Synchrotron radiation from internal shocks or forward shocks (Wang, Li, Dai & Meszaros 2009; Wang, He, Li, Wu & Dai 2010)

Limit on the fireball’s Lorentz factor (Abdo et al. 2009, Science)

Limit on the Extragalactic Background Light.

Limit on Lorentz invariance violation

(2) Short burst GRB090510: z=0.903,

~31 GeV (Abdo et al. 2009, Nature)

Light from this GRB backs up a key prediction of Albert Einstein’s theory of relativity — that photon speed is the same regardless of energy.

Zhang et al. 2011

(3) GRB090902B: z=1.822,

~33 GeV (Abdo et al. 2009, ApJL)

Zhang et al. 2011

(4) GRB090926A: z=2.106,

~20 GeV (Uehara et al. 2009)

Zhang et al. 2011

3 long GRBs: Eiso>1054 ergs,

indicating most energetic

explosive events.

Features of high-energy emission

Sub-MeV and GeV photons observed by GBM and LAT respectively behave with distinctive temporal properties.

Spectral slopes of the GBM and LAT emissions are often different.

LAT emission usually lags behind the GBM emission from a fraction of a second to a few seconds.

High-energy emission and low-energy emission detected by LAT and GBM seem to have different origins.

Outline

1. High-energy emission mechanisms before Fermi

2. Fermi/LAT observations

3. Models of high-energy emission

4. Constraints on intergalactic magnetic fields

Afterglow (forward shock) synchrotron scenario(Kumar & Barniol Duran 2009, 2010)

Can easily explain the simple decay.

Can explain the delayed onset as the onset of the HE afterglow.

The flux level matches the observations:

KN effects in high-energy afterglow emission(Wang, He, Li, Wu, & Dai 2010)

For afterglow electrons in the Thomson scattering,

Y

For high-energy afterglow emission, ( ) is large, inverse Compton scattering with s

ynchrotron peak photons should be in Klein-Nishina regime

Sari & Esin (2001):

<

Values of Compton Y (100 MeV) parameters (Wang et al. 2010)

We need to take into account carefully the KN effect in modeling the high-energy afterglow.

Shortcomings of previous afterglow scenarios

Cannot explain an initial rapid brightening of the high-energy emission.

Cannot explain the late bumps of optical afterglow light curves.

Other models: Anisotropic inverse Compton scattering of an optically-thin exp

anding cocoon (Toma, Wu & Meszaros 2009) Hadronic scenarios (Razzaque et al. 2010; Asano et al. 2009)

Ejecta with energy injection sweeping up a density-jump medium

(Feng & Dai 2011)

Angular momentum of the accreted fall-back matter spins up central compact object.

Earlier-ejected shells suffer from more massive baryon contamination.

where tfp is the apparent energy-injection time.

Reverse shock (S1)

Forward shock (S2)

Contact discontinuity

Ambient gas (GMC or a slow wind)

Fast wind

Shocked stellar wind

Shocked ambient gas

Schematic sketch for a stellar wind bubble

A density-jump medium

log n

log R

nGMC

density jumpnfw R-2

Rrsh

RfshRCD

nsw R-2

Dai & Wu (2003): a weak wind for GRB030226

Feng & Dai (2011)

~0.3-0.8, consistent with the popular collaspar model. EK,iso~1054 ergs, indicating highly collimated outflows.

0~~500-700, consistent with the opacity constraints.

A*,35.5 ~0.02-0.06, indicating a weak wind.

Feng & Dai (2011)

Outline

1. High-energy emission mechanisms before Fermi

2. Fermi/LAT observations

3. Models of high-energy emission

4. Constraints on intergalactic magnetic fields

Measuring astrophysical magnetic fields

Pulsar magnetic fields (~1012 G): cyclotron absorption Stellar magnetic fields (~1-100 G): Zeeman effect Galactic magnetic fields (~10-6-10-9 G): Faraday rotation Intergalactic magnetic fields (primordial): Lorentz deflection

A cascade from propagation of high-energy photons of GRBs ： γ>100GeV + γIR → e±, e － (e +) + γCMB → e － (e +) + γGeV

Plaga (1995) suggested probing intergalactic magnetic fields by measuring a delay in arrival time of the secondary emission.

Dai & Lu (2002) and Dai et al. (2002) first derived the secondary emission spectrum, and proposed that BIG would be detectable by measuring the spectrum with GLAST (now Fermi) in the paper of Dai et al. (2002).

Delayed secondary emission

Inverse-Compton cooling time (fixed frame):

Inverse-Compton cooling time (observer’s frame):

Angular spreading time (observer’s frame):

Magnetic deflection time (observer’s frame):

Observed scattered-photon duration:

Gamma-ray energy spectrum from Mrk 501:

Resulting electron/positron energy spectrum:

Scattered photon energy spectrum:

GeV emission from Mrk 501 (z=0.034): Dai et al. (2002)

Tavecchio et al. (2011)

Tavecchio et al. (2010)

Following Dai & Lu (2002) and Dai et al. (2002) ……

TeV blazar: 1ES0229+200

Summary

Fermi provided new data for high-energy emission from GRBs. In particular, high-energy emission and low-energy emission detected by LAT and GBM seem to have different origins.

The new data (e.g., an initial rising of the light curve) implies an initial energy injection, being consistent with the popular collapsar model.

The dalayed secondary emission in propagation of high-energy photons was detected by Fermi (e.g., for TeV blazars), and its spectrum provided a constraint on intergalactic magnetic fields.