Yizhong Fan (Niels Bohr International Academy, Denmark Purple Mountain Observatory, China)

Outline General views Fireball model and the observational evidence Two kinds of fireballs—baryonic and magnetic Observational signatures (theoretical predictions) Constraint on the models by current data A possible spectrum problem of the magnetic fireball model

lon g short GRBs: general views

GRB spectrum (Preece et al. 2000; Band et al. 1993)

Outline General views Fireball model and the observational evidence Two kinds of fireballs—baryonic and magnetic Observational signatures (theoretical predictions) Constraint on the models by current data A possible spectrum problem of the magnetic fireball model

GRB fireball model However, this is inconsistent with the observed non- thermal spectrum. Shemi & Piran (1990) pointed out that this puzzle could be solved if the relativistic outflow contained a small amount of baryon material

The main predications of the fireball model 1. GRBs are at a typical redshift z~1 2. The outflows are ultra-relativistic 3. The existence of the low frequency counterpart (i.e., the long-lasting afterglow)

3. The GRB outflow is relativistic GRB : direct evidence for the ultra-relativistic movement

Outline General views Fireball model and the observational evidence Two kinds of fireballs—baryonic and magnetic Observational signatures (theoretical predictions) Constraint on the models by current data A possible spectrum problem of the magnetic fireball model

Two types of Progenitors (Woosley 1992 for collapsar; Eichler et al. 1989, Narayan et al for NS-NS or NS-BH merger)

Two kinds of fireballs Baryonic/Hot fireball Magnetic/Cold fireball

Baryonic fireball model central photosphere internal external shocks engine (shocks) (reverse) (forward) 1E17cm for ISM 1E7cm 1E11cm 1E12-1E14cm 1E15cm for wind gamma-ray UV/opt/IR/radio gamma-ray X-ray UV/optical IR mm radio (see Piran 1999; Meszaros 2002; Zhang & Meszaros 2004 for reviews) prompt promptemission afterglow

(Lyutikov & Blandford 2003) Poynting flux dominated outflow model

Outline General view Fireball model and the observational evidence Two kinds of fireballs—baryonic and magnetic Observational signatures (theoretical predictions) Constraint on the models by current data A possible spectrum problem of the magnetic fireball model

Thermal emission component Baryonic fireball Magnetic fireball (Daigne & Mochkovitch 2002; Lyutikov & Usov 2000; Zhang & Pe’er 2009; Photosphere non-thermal emission model: Thompson 94; Meszaros & Rees 00; Rees & Meszaros 05; Pe’er et al. 06; Giannios 07; Ioka et al. 07)

Prompt high energy gamma-ray emission Baryonic fireball Magnetic fireball synchrotron SSC ( Pe’er & Waxman 2004; Pilla & Loeb 1998; Gupta & Zhang 2007 ) synchrotron SSC

Baryonic fireball: 1E20 eV cosmic rays ( Vietri 1995; Waxman 1995 ) For the magnetic fireball, the particle acceleration mechanism is poorly understood.

Baryonic fireball: prompt PeV neutrinos central photosphere internal external shocks engine (shocks) (reverse) (forward) 1E17cm for ISM 1E7cm 1E11cm 1E12-1E14cm 1E15cm for wind gamma-ray UV/opt/IR/radio gamma-ray X-ray UV/optical IR mm radio For the magnetic fireball, the PeV neutrinos may be too rare to be detected because of the huge radius of the prompt emission

Reverse/forward shock emission central photosphere internal external shocks engine (shocks) (reverse) (forward) 1E17cm for ISM 1E7cm 1E11cm 1E12-1E14cm 1E15cm for wind gamma-ray X-ray UV/optical IR mm radio (Meszaros & Rees 1997, 1999; Sari & Piran 1999; Kobayashi et al. 1999)

Reverse shock emission: optical flash Baryonic fireball (Kobayashi 2000; Kobayashi & Zhang 2003; Wu et al. 2003; c.f. Nakar & Piran 2004) Magnetic fireball (Fan et al. 2004; Zhang & Kobayashi 2005; Fan 2008; Mimica et al. 2009)

Polarization properties Baryonic fireball (b) Magnetic fireball (a) (Waxman 2003; Shaviv & Dar 1995; Lyutikov et al. 2003; Granot 2003; Toma et al. 2009; Fan 2009)

Outline General views Fireball model and the observational evidence Two kinds of fireballs—baryonic and magnetic Observational signatures (theoretical predictions) Constraint on the models by current data A possible spectrum problem of the magnetic fireball model

Fit to the afterglow of GRB (Panaitescu & Kumar 2001) Weakly magnetized reverse shock found in optical flash modeling

Magnetized reverse shock

(Zhang, Kobayashi & Meszaros 2003)

Bursts ObservationTheoretical interpretation Magnetized Reverse shock? GRB Akerlof et al. 99Fan et al. 02; Zhang et al. 03; Panaitescu & Kumar 04 Yes GRB Fox et al. 03; Li et al. 03 Zhang et al. 03; Kumar & Panaitescu 03 Yes GRB A Blake et al. 05Fan et al. 05 Yes GRB Blustin et al. 06 Yes GRB Boer et al. 06Wei et al. 06 Yes GRB B Klotz et al. 06 Yes GRB Gomboc et al. 08 Yes GRB B Racusin et al. 08Wu et al. 09 Yes GRB Covino et al. 09 Very likely =magnetic energy density/particle energy density

Thermal emission component significant thermal emission seen in dozens of bursts (Ryde 2005; Ryde et al. 2006)

Thermal emission component ( not seen in most events: Poynting-flux dominated fireball [Daigne & Mochkovitch 2002; Zhang & Pe’er 2009 ] ? ) (Abdo et al. 2009)

The absence of optical flash in most GRBs (e.g., Roming et al. 2006; Meldandri et al. 2008; Klotz et al. 2009) Magnetic outflow (Fan et al. 2004; Zhang & Kobayashi 2005; Fan 2008; Mimica et al. 2009)?

The polarimetry of prompt emission GRB ( Coburn & Boggs 2003: 80+/-20%; see however Rutledge & Fox: null result ) GRB and GRB ( Willis et al. 2005: >35% and >50%, respectively ) GRB A ( McGlynn et al. 2007: 60+/-30%; Gotz et al. 2009: 43+/-25% ) GRB ( McGlynn et al. 2009: <60% )

The reverse shock was too weak to outshine the forward shock optical emission (Jin & Fan 2007)!

Fermi GRBs with GeV emission (six detections among ~70 observations) GRB C (Abdo et al. 2009) GRB B (Omodei et al. 2008) GRB (Ohno et al. 2009a) GRB (Ohno et al. 2009b) GRB (Cutini et al. 2009) GRB (Ohno & Pelassa 2009c) One detection once a month, as expected (assuming a Band spectrum, i.e., no GeV excess )

Interpretation of the non-detection of GeV spectrum excess by Fermi in almost all GRBs (Fan 2009; arXiv: ) synchrotron SSC (magnetized fireball)

phasephenomena Implication Prompt emission Highly polarized gamma-rays? Ordered magnetic field in the emitting region? Prompt emission Absence of thermal emission component Poynting flux dominated outflow? Prompt emission Non-detection of the GeV Spectrum excess A strong magnetic field component? Very early afterglow Bright flash outshining the forward shock optical emission Weakly magnetized reverse shock region? Very early afterglow Absence of bright optical flashes Mildly magnetized reverse shock region? =magnetic energy density/particle energy density Prompt emission Reverse shock emission? A short summary

Outline General views Fireball model and the observational evidence Two kinds of fireballs—baryonic and magnetic Observational signatures (theoretical predictions) Constraint on the models by current data A possible spectrum problem of the magnetic fireball model

Magnetic fireball: spectrum problem (Giannios 2007: gradual magnetic dissipation) GRB C

(Lyutikov & Blandford 2003: sudden magnetic dissipation) Poynting flux dominated outflow model

Synchrotron radiation: sudden magnetic dissipation (Fan et al in preparation)

A possible solution in the case of hot fireball (Derishev et al. 2001) It, however, does not work for a magnetic fireball unless >99% magnetic energy of the outflow has been dissipated ! For typical GRB parameters, this model works (in an upcoming paper)!

My opinion: N

Thank you!

magnetic fireball magnetized reverse shock the absence of thermal component the lack of reverse shock emission the non-detection of GeV spectrum excess