Gamma-Ray Bursts II : Physics and Phenomenology of the Afterglow Andrea Melandri INAF – Astronomical Observatory of Brera

Setting the Stage Lecture 1 Lecture 2

The Puzzling Enigma 1960 : discovery “by chance”, reported only on : self-similar solution of the relativistic blast waves 1986 : cosmological models NS-NS mergers failed SN or collapsar millisecond pulsar internal and external shock scenario 1990 : Fireball Model synchrotron radiation radio and optical afterglow suggested predictions: power law decays and optical flashes

The Puzzling Enigma 1991 : BATSE on CGRO provided first statistically useful sample isotropic spatial distribution variety of y-rays profiles distance still unknown 2-classes of GRBs 1997 : Beppo-SAX X-ray afterglow (GRB )  optical and radio detection  fireball model found to be successful first redshifts  cosmological distances

Doors open 1998 : redshifts measurements (cosmological problem solved!) and LC observations 1998 : GRB-SN association ( GRB SN1998bw )  GRB associated with star forming regions  collapsar and supranova models 1999 : prompt optical flash and radio flare for GRB  reverse shock emission studies (Energy catastrophe!) 2000 : achromatic breaks in the afterglow LC  collimated emission, jets 2000 : X-ray spectral features in several GRBs ( Fe X-ray lines, GRB ) 2002 : strong polarization of the prompt emission ( GRB ) 2003 – now : INTEGRAL, Swift, Fermi……more to come!

Brief pulses of gamma rays Irregular light curves Wide range of individual pulses, symmetric or asymmetric Duration (T 90 ): 5 order of magnitude (from to 10 3 sec) = 20 sec for long bursts = 0.2 sec for short bursts The energy spectra of short bursts are harder than those of long bursts Continuum spectrum is non-thermal (Band Function) Low significance spectral features Prompt : facts

NS-NS ( BH-NS & BH-WD ) travel far from their formation sites before producing GRBs => “clean environment” Hypernovae/collapsar evolve much faster, going off in their formation site (SN bump needed, no Fe X-ray lines) => “mass-rich environment” SupraNova : the GRB is preceded by a SN explosion leaving a dense shell of matter of many solar masses in the nearby (Fe X- ray lines, no SN bump) => “mass-rich environment” 2 classes = 2 population ? The answer in the next lecture……

Energy Budget  keV  rays: 65% keV X-rays: 7% 3Optical: 0.1% 4Radio ? 5MeV/GeV/TeV, ? >10%? 6Gravitational radiation ?  keV  rays: 7% keV X-rays: 9% 3Optical: 2% 4Radio: 0.05% Prompt Emission (The ‘Burst’) Afterglow Emission

Standard Scenario from F. Daigne’s lecture – GDRE School

  ISM INTERNAL SHOCK  RAYS EXTERNAL SHOCK X-RAYS OPTICAL RADIO 20 km 1-6 AU AU The picture FSRS

Forward shock : Dynamics (Blandford &McKee 1976); Syncrhotron radiation (Sari, Piran & Narayan 1998); Stellar wind (Chevalier &li 2000); Jet vs Spherical outflow (Rhoads 1997) 6 57 from F. Daigne’s lecture – GDRE School

β = v/c Γ = 1/ √(1-β 2 ) β = v/c Γ = 1/ √(1-β 2 ) Energy ≈ Γ -p ; Flux ≈ ν -β t -α from F. Daigne’s lecture – GDRE School

SpectrumLight curve Sari, Piran & Narayan, 1998, ApJ, 497, 17

vava Synchrotron self-absorption frequency = ν a Injection frequency = ν m (synchrotron emission) Cooling frequency = ν c (it moves from high to low energies!)

Standard Scenario from F. Daigne’s lecture – GDRE School

Jets Anisotropic emission (beaming) invoked Achromatic break in afterglow light curve – Jet ‘break’ F ~ t  ;  steepens by  ~1 – For  j, jet radiates into f b = (1-cos  j ) ~  j 2 /2 – Break at t j when  < 1/  j ; on seeing edge of jet (e.g. Band et al. 2003; Klose et al. 2004) Woosley

We now know so much about the “standard” model : - Long lasting emission - Light curves behaviours - Jets ………so let’s make some predictions We now know so much about the “standard” model : - Long lasting emission - Light curves behaviours - Jets ………so let’s make some predictions

Closure Relations Energy ≈ Γ -p ; Flux ≈ ν -β t -α Zhang & Meszaros, 2006

Predictions Optical counterparts to most GRBs Many bright optical flashes at early time Smooth light curves - jet breaks easy to spot High-energy spectral turnover High-z GRBs easily identified Short bursts understood

Early-Time Light Curves (X-rays) ~ -3 ~ -0.5 ~ ~ – 10 3 s10 3 – 10 4 s 10 4 – 10 5 s Prompt Emission High-latitude Emission Late internal shocks Prolonged Central engine activity Standard forward shock emission Jet break Zhang et al 2006

Wide range of observed brightness Deep, fast observations vital ~40-50% of optical afterglows remain undetected The class of Dark GRBs (next week talk….) Early-Time Light Curves (Optical) Melandri et al. 2008, ApJ, 686,

Predictions Optical counterparts to most GRBs Many bright optical flashes at early time Smooth light curves - jet breaks easy to spot High-energy spectral turnover High-z GRBs easily identified Short bursts understood ~~

Early optical LC : expected GRB , Akerlof et al. ‘99 GRB , Gomboc et al. ‘08 Kobayashi & Zhang ‘03, Gomboc et al. ‘09

α rise ~ 1.6 ± 0.7 α decay,1 = ± 0.09 t peak,1 ~ 480 s α decay,2 = ± 0.12 α decay,3 = ± 0.17 Melandri et al. ’09, MNRAS, 395, 1941 Fine tuned central engine activity or possibly two-component jet ( Zheng & Deng ‘09 ) No sign of RS, not easily explained! GRB A

GRB Melandri et al. ’09, in prep. ISM, no wind, no sign of RS ; no passage of ν m ; magnetized fireball ? α rise = 1.72 ± 0.41 α decay = ± 0.08 t peak ~ 1000 s Sharp bump at ~ 2 × 10 4 s Underlying object (t > 10 5 s)

GRB  -ray duration Mundell et al. ’09, in prep. RS/FS + refresh shocks ?

Predictions Optical counterparts to most GRBs Many bright optical flashes at early time Smooth light curves - jet breaks easy to spot High-energy spectral turnover High-z GRBs easily identified Short bursts understood ~~ no

GRB Blue: Swift/XRT WT mode Red: Swift/XRT PC mode Green: Chandra/ACIS Time since BAT trigger (s) Chandra X-rays Obs : 8 GRBs (3 have late-time breaks in LC, 5 do not) Burrows et al., ‘09

Predictions Optical counterparts to most GRBs Many bright optical flashes at early time Smooth light curves - jet breaks easy to spot High-energy spectral turnover High-z GRBs easily identified Short bursts understood ~~ no ~ not yet next

GRB Salvaterra et al., ‘09 The high-z population: GRB z=6.3 GRB z=6.7 GRB z= % of all Swift GRBs at z > 6 for P lim =0.4 ph/s/cm 2 Nothing spectacularly different from the high-z population

Summary : Answers or open questions ? Not many high-z events (and the are not special) : why? Progenitors? S-GRBs vs L-GRBs : emission? Hosts? Redshift distribution Lack of optical flashes (peaks might have different explanations) and where are the jets breaks (geometry)? Different behaviour at different wavelength : complex LCs odd cases make you think about the right physics energy injection, long lasting central engine, micro-physical params Challenge to the fireball model (still valid for ~70% of the cases) Multi-wavelength analysis is critical : same regions? Same behaviours?