Gamma Ray Bursts: open issues  Brief history  Power  Short history of the paradigm: internal vs external shocks  Afterglows: external shocks  The spectral-energy relations  GRBs for cosmology Gabriele Ghisellini – Osservatorio di Brera

Gamma-Ray Bursts: The story begins Treates banning nuclear tests between USA and USSR in early 60s VELA Satellites: X and soft  -ray detectors Klebesadel R.W., Strong I.B., Olson R., 1973, Astrophysical Journal, 182, L85 `Observations of Gamma-Ray Bursts of Cosmic Origin’ Brief, intense flashes of  -rays

Shortest 6 ms GRB Longest ~2000 s GRB Paciesas et al (2002) Briggs et al (2002) Koveliotou (2002) SHORT LONG Short – Hard Long - Soft Two flavours, long and short

Spectra Non thermal spectra featureless continuum power-laws - peak in F power-laws - peak in F F ~ E  F ~ E  E peak

1997: The BeppoSAX satellite Slewing in several hours Italian-Dutch “Satellite per l’ Astronomia X” Instruments Wide Field Cameras: 5% of sky – positioning ~ 4’ 5% of sky – positioning ~ 4’ + Narrow Field Instruments arcmin resolution NFI

Discovery of first afterglow! GRB March 28 February

Optical id. host galaxy: redshifts Cosmological origin ! ~120 / 3000 with z: <0.1 – 6.3 (Batse, SAX, HETE-II, Integral, Swift, …)

Huge isotropic equivalent energy! 119 GRBs with z GRB typical Fluence (i.e. time int. flux) is – erg/cm 2 (1keV – 10 MeV) Assume Isotropy Energy and Power

GRB are powerful AGN: L < erg/sAGN: L < erg/s SN: L < erg/s (in photons)SN: L < erg/s (in photons) GRB: L < erg/sGRB: L < erg/s Planck power: Mc 2 c 5 R g /c G = = 3.6x10 59 erg/s

“first light” & PopIII chemical evolution large scale structures cover the epoch of re-ionization Star Formation Rate Probes of far universe SNIa

Huge energy Small Volume Fireball Invented even before knowing that GRBs are cosmological….

A short history of fireballs 1978 Cavallo & Rees: fireball: photons trapped by their own pairs 1978 Rees: internal shocks in M87 to transport energy along the jet 1986 Paczynski: Cosmological GRB  L=10 51 erg/s and T~1 MeV 1986 Goodman: T obs remains T during expansion. Doppler balances adiabatic cooling 1992 Pure fireball made by   e+e-. Focussing by gravitation

NS e+e-

A short history of fireballs 1978 Cavallo & Rees: fireball: photons trapped by their own pairs 1978 Rees: internal shocks in M87 to transport energy along the jet 1986 Paczynski: Cosmological GRB  L=10 51 erg/s and T~1 MeV 1986 Goodman: T obs remains T during expansion. Doppler balances adiabatic cooling 1992 Pure fireball made by   e+e-. Focussing by gravitation 1992 Dirty fireball polluted by baryons. Re-conversion of bulk kinetic into radiation through shocks with external medium 1994 Internal shocks due to shells moving with different 

Why internal shocks? Spikes have same duration A process that repeats itself

Relativ. e - + B: synchrotron?? Relativ. e - + B: synchrotron Shell still opaque Shell still opaque “The” model: Internal/External Shocks Rees-Meszaros-Piran

Progenitors Host galaxies Faint (m R ~ 25 ) galaxies Sites of star formation Low metallicities Bloom et al GRBs associated with SN (Ib,c) SN afterglow Matheson et al Afterglow re-brightening A few spectroscopic ident. (underluminous?)

Progenitors core collapse of massive stars (M > 30 M sun ) long GRBs Collapsar or Hypernova (MacFadyen & Woosley 1999) GRB simultaneous with SN Supranova – two-step collapse (Vietri & Stella 1998) GRB delayed by few months-years Discriminants: host galaxies, location within host, duration, environment, redshift distribution,... compact object mergers (NS-NS, NS-BH) short GRBs ?

The engine Accreting torus Formation of a spinning BH + dense torus, sustaining B ~ G Extraction BH spin energy (0.29 M BH c 2 ) Extract E > erg t GRB ~ 10 4 t dyn

Jets

Jet half opening angle Jet effect  , Surf.  Relativistc beaming: emitting surface  1/    1/   1/ Log(t) Log(F) Jet break  >> 1/

Israel et al GRB Jet measure “Jet break” Jet break time t break Jet opening angle

“True” energetics Frail et al Isotropic equivalent energy E true = E iso (1 – cos ) Bloom et al E p e a k w a s n o t c o n s i d e r e d …

Amati et al BeppoSAX GRBs E peak  E iso 0.5 Peak energy – Isotropic energy Correlation E peak (1+z)

Nava et al. 2006; Ghirlanda et al “Amati” (62) “Ghirlanda” (25) 1- cos  jet

Ghirlanda, Ghisellini, Lazzati & Firmani 2004 Luminosity distance redshift GRBs can be used as cosmological RULERS ! Supernovae GRBs

Problems: 1: Efficiency

Efficiency=Radiated/total energy Only the RELATIVE kinetic energy can be used! Shells of equal masses Shells of equal energies  final ~ (  1  2 ) 1/2  final ~ (  1  2 ) 1/2 Dynamical efficiency (%) 5%

Piro astro-ph/ A lot of kinetic energy should remain to power the afterglow A lot of kinetic energy should remain to power the afterglow SAX X-ray afterglow light curve Prompt

Willingale et al SWIFT

E afterglow < E prompt E afterglow ~ 0.1 E prompt

Problems: 2: Early “afterglow”

Good old times Piro astro-ph/

Now: a mess GRB z=6.29

Panaitescu 2006 X Opt.

X-ray and optical behave differently X-rays: steep-flat-steep TATA Is this “real” afterglow? i.e. external shock?

Early (normal) prompt: Early (normal) prompt:  >>1 /  j Late prompt: Late prompt:  >1 /  j Late prompt: Late prompt:  =1 /  j Late prompt: Late prompt:  <1 /  j ”real” after- glow Ghisellini et al. 2007

Long lasting engine?? R s /c ~ s (for a 10 solar mass BH)R s /c ~ s (for a 10 solar mass BH) Even 10 s are 10 5 dynamical timesEven 10 s are 10 5 dynamical times Two-phase accretion?Two-phase accretion?

Conclusions “Paradigm”: internal+external shocks, synchrotron for both: it helps, but it is limiting Efficiency is an issue Efficiency is an issue Progenitors for long: done. For short: not yet Progenitors for long: done. For short: not yet Central engine? How long does it live? Central engine? How long does it live? GRBs as probes of the far universe (continue…) GRBs as probes of the far universe (continue…)

There can be a Black Body … but Time resolved spectra Time integrated spectrum The same occurs for ALL GRBs detected by BATSE and with WFC Ghirlanda et al. 2007b

Memory

E peak =509 keV E peak =503 keV E peak = 416 keV Time [sec] cts/sec E peak = 390 keV EF(E) GRB spectrum evolves with time within single bursts Ghirlanda PhD thesis

phot /cm^2 sec Hard to Soft evolution E pea k   E peak,  t),  t) Decrease independent of the rise and decay of the flux

E pea k   time Tracking evolution Photon flux Correlated with E peak (t),  (t),  (t)

By construction, internal shocks should all be equal. Then, why does the spectrum evolve?

Spectra Spectra Fishman & Meegan 1995   E peak

Prompt radiation: Synchrotron?

Energy spectrum of a cooling electron Fast cooling + synchro: E( ) -1/2 N( ) -3/2

Typical synchrotron frequency syn = 3.6x10 6 B  2  /(1+z)  Hz syn = 3.6x10 6 B  2  /(1+z)  Hz Magnetic field from: L B =  B L kin R 2  2 B 2 c =  B L shell Size from: R ~ R 0  2 (internal shock) Electron energy from:  m e c 2 =  e m p c 2 (  ’-1) ~  e m p c 2  B L shell   R 0 B B ~ 1/21/2  ~  e m p /m e

Synchrotron  -ray emission?  B  e L kin,shell,50 h syn ~ 400 keV 1/221/2  3 R 0,7 (1+z) 2 t cool ~  e (  /100) MeV MeV 23sec  Extremely short - No way to make it longer  t cool << t dynamical ~ sec  It must be short, if not, how can the flux vary?

0.2 ms More than exponential

Low energy photon power law index N(E) = kE -  Preece et al. F ~ 1/3 F ~ -1/2

Can it be rescued by:  Reacceleration? No, in ISS e- are accelerated only once  Adiabatic losses? No, too small regions would be involved, too much IC  Self absorption? No, lots of e- needed, too much IC  Self Compton? No, t cool too small even in this case

Clustering of the optical luminosities

Flux vs observed time  =0.48 Nardini et al. 2006

Luminosity vs rest frame time  =0.28 Nardini et al. 2006

Swift GRBs

Pre-Swift +Swift Dark??

12 hours pre-Swift (19) Including Swift (30)

G1=100, G2=200

Thompson, Meszaros & Rees 2007 At R ~ R star the fireball dissipates part of its energy  BB L ~  2 L iso   2 ~ L/L iso L iso ~ R 2   2 T’ 4 ~ ( R/  2  T 4 ~ ( R/  2  T 4  L iso ~ R 2 (L /L iso )T 4  E peak ~ L iso 1/2 L -1/4

A short history of fireballs

Short Bursts

The spectrum of short bursts harder softer

Log Epeak Log  short long Ghirlanda et al because harder because  is harder, E peak is the same or even smaller

Density

Star forming regions are dense

GRB – Afterglow – Temporal Properties GRB emission in X, Optical Panaitescu & Kumar

Why densities are so small?

Firmani et al L

No corr.

L x > L opt L opt more clustered than L x v c between optical and X-rays ~same values of  B,  e, E k,iso moderate cooling (small  B ) large Comptonization y parameter different p

Universal energy reservoir? Bloom et al Frail et al Best n from fits Frail et al. 2001

Same energy, different angles?

Structured jets view  view jet  jet E(  )=const E (  )= E 0  -2  L) = kL -2

Universal E peak ? Preece et al. ~200 keV, observer frame BATSE

HETE II

X-ray flashes Amati et al. +Lamb et al. The “Amati et al.” relation E peak = 100 keV E iso,52 + E iso = E true /  2 E peak = 1/  1/2