High-energy radiation from gamma-ray bursts Zigao Dai Nanjing University Xiamen, August 2011

Astrophysical implications of high-energy emission from GRBs  Central engines  Bulk Lorentz factor of fireballs  Composition of fireballs  Acceleration of particles  Radiation mechanisms of particles  Quantum gravity  Extragalactic infrared background radiation  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

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 Credit: N. Omedi

Four important cases (1) GRB080916C: z= , ~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 long GRBs: E iso >10 54 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 synchrotron 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 expanding 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 t fp 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 n GMC density jump n fw  R -2 R r sh R f sh R CD n sw  R -2 Dai & Wu (2003): a weak wind for GRB030226

Feng & Dai (2011)

  ~ , consistent with the popular collaspar model.  E K,iso ~10 54 ergs, indicating highly collimated outflows.   0 ~  ~ , consistent with the opacity constraints.  A *,35.5 ~ , 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 (~10 12 G): cyclotron absorption  Stellar magnetic fields (~1-100 G): Zeeman effect  Galactic magnetic fields (~ G): Faraday rotation  Intergalactic magnetic fields (primordial): Lorentz deflection

 A cascade from propagation of high-energy photons of GRBs ： γ >100GeV + γ IR → e ±, e － (e + ) + γ CMB → e － (e + ) + γ GeV  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 B IG 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: 1ES

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.