GRB physics and cosmology with spectrum-energy correlation Lorenzo Amati INAF – IASF Bologna (Italy)

Outline GRB: standard scenarios and open issues Spectrum-energy correlation and its implications GRB cosmology with spectrum-energy correlation Future perspectives

Standard scenarios for GRB origin LONG SHORT energy budget up to >1054 erg long duration GRBs metal rich (Fe, Ni, Co) circum-burst environment GRBs occur in star forming regions GRBs are associated with SNe likelycollimated emission energy budget up to 1051 - 1052 erg short duration (< 5 s) clean circum-burst environment old stellar population

Standard scenarios for GRB phisics ms time variability + huge energy + detection of GeV photons -> plasma occurring ultra-relativistic (G > 100) expansion (fireball or firejet) non thermal spectra -> shocks synchrotron emission (SSM) fireball internal shocks -> prompt emission fireball external shock with ISM -> afterglow emission

Open issues (several, despite obs. progress) GRB prompt emission physics most time averaged spectra of GRBs are well fit by synchrotron shock models at early times, some spectra inconsistent with optically thin synchrotron: possible contribution of IC component and/or thermal emission from the fireball photosphere

Afterglow emission physics consistency with synchrotron predictions found e.g. for GRB 970508 deviations in the X-ray band from synchrotron: found for GRB000926 (Harrison et al. 2001) and GRB010222 (in ‘t Zand et al. 2001). they are consistent with relevant Inverse Compton on the low energy photons

Early afterglow ( 1 min ≤ t ≤ hours ) new features seen by Swift in X-ray early afterglow light curves (initial very steep decay, early breaks, flares) mostly unpredicted and unexplained initial steep decay: continuation of prompt emission, mini break due to patchy shell, IC up-scatter of the reverse shock sinchrotron emission ? flat decay: probably “refreshed shocks” (due either to long duration ejection or short ejection but with wide range of G) ? flares: could be due to: refreshed shocks, IC from reverse shock, external density bumps, continued central engine activity, late internal shocks… ~ -3 ~ -0.7 ~ - 1.3 ~ -2 10^2 – 10^3 s 10^4 – 10^5 s 10^5 – 10^6 s ( 1 min ≤ t ≤ hours )

GeV emission CGRO/EGRET detected VHE (from 30 MeV up to 18 GeV) photons for a few GRBs VHE emission can last up to thousends of s after GRB onset average spectrum of 4 events well described by a simple power-law with index ~2 , consistent with extension of low energy spectra GRB 941017,measured by EGRET-TASC shows a high energy component inconsistent with synchrotron shock model Recent AGILE detection of 2-3 >500 MeV photons

Prompt optical emission GRB990123: first detection (by ROTSE) of prompt optical emission optical light curve does not follow high energy light curve: evidence for a different origin (reverse shock ?) GRB041219 (RAPTOR) contrary to GRB990123, the optical light curve follows the high energy light curve: evidence for same origin (internal shocks ?)r

the challenging “naked eye” GRB080319B: complexity and energetics Bloom et al. 2008

Circum-burst environment evidence of overdense and metal enriched circum-burst environment from absorption and emission features emission lines in afterglow spectrum detected by BeppoSAX but not by Swift Swift detects intrinsic NH for many GRB afterglows optical (Lya) NH often inconsistent with X-ray NH prompt afterglow Amati et al.,Science, 2000 Antonelli et al., ApJ, 2001

Collimated or isotropic ? lack of jet breaks in several Swift X-ray afterglow light curves, in some cases, evidence of achromatic break challenging evidences for Jet interpretation of break in afterglow light curves or due to present inadequate sampling of optical light curves w/r to X-ray ones and to lack of satisfactory modeling of jets ?

GRB/SN connection are all long GRB produced by a type Ibc SN progenitor ? which fraction of type Ibc SN produces a GRB, and what are their peculiarities ? are the properties (e.g., energetics) of the GRB linked to those of the SN ?

short / long classification and physics Swift GRB 060614: a long GRB with a very high lower limit to the magnitude of an associated SN -> association with a GRB/SN is excluded high lower limit to SN also for GRB 060505 (and, less stringently, XRF 040701) In the spectral lag – peak luminosity plane, GRB060614 lies in the short GRBs region -> need for a new GRB classification scheme ? Gehrels et al., Nature, 2006

Alternative scenarios EMBH / fireshell model (Ruffini et al.) CannonBall (CB) (Dar et al.) Precessing jet (Fargion’s talk), stochastic wakefield plasma acceleration (Barbiellini et al.), ns-quark star transition, … Short GRB as SGR giant outburst at high z

The Ep,i – Eiso correlation GRB spectra typically described by the empirical Band function with parameters a= low-energy index, b= high-energy index, E0=break energy Ep = E0 x (2 + a) = observed peak energy of the nFn spectrum

from redshift, fluence and spectrum, it is possible to estimate the cosmologica-rest frame peak energy, Ep,i, and the radiated energy assuming isotropic emission, Eiso isotropic luminosities and radiated energy are huge; both Ep,i and Eiso and span several orders of magnitude Ep,i = Ep x (1 + z) log(Eiso)= 1.0 , s = 0.9 log(Ep,i )= 2.52 , s = 0.43 (Amati, MNRAS, 2006)

Amati et al. (2002) analyzed a sample of 12 BeppoSAX events with known redshift we found evidence of a strong correlation between Ep,i and Eiso , highly significant (r = 0.949, chance prob. 0.005%) despite the low number of GRBs included in the sample Ep,i = kEiso (0.52+/-0.06) Amati et al. , A&A, 2002

only firm outlier the local peculiar GRB 980425 (GRB 031203 debated) analysis of an updated sample of long GRBs/XRFs with firm estimates of z and Ep,i (41 events) gives a chance probability for the Ep,i-Eiso correlation of ~10-15 and a slope of 0.57+/-0.02 the scatter of the data around the best fit power-law can be fitted with a Gaussian with s(logEp,i) ~ 0.2 (~0.15 extra-poissonian) confirmed by the most recent analysis (more than 70 events, Ghirlanda et al. 2008, Amati et al. 2008) only firm outlier the local peculiar GRB 980425 (GRB 031203 debated) Amati et al. 2008

all long Swift GRBs with known z and published estimates or limits to Ep,i are consistent with the correlation the parameters (index, normalization,dispersion) obatined with Swift GRBs only are fully consistent with what found before Swift allows reduction of selection effects in the sample of GRB with known z -> the Ep,i-Eiso correlation is passing the more reliable test: observations ! Amati 2006, Amati et al. 2008

Implications of the Ep,i – Eiso correlation physics of prompt emission still not settled, various scenarios: SSM internal shocks, IC-dominated internal shocks, external shocks, photospheric emission dominated models, kinetic energy dominated fireball , poynting flux dominated fireball) e.g., Ep,i  G-2 L1/2 tn-1 for syncrotron emission from a power-law distribution of electrons generated in an internal shock (Zhang & Meszaros 2002, Ryde 2005) e.g., Ep,i  G Tpk  G2 L-1/4 in scenarios in whch for comptonized thermal emission from the photosphere dominates (e.g. Rees & Meszaros 2005, Thomson et al. 2006)

jet geometry and structure XRF-GRB unification models viewing angle effects Uniform/variable jet PL-structured /universal jet Uniform/variable jet PL-structured /universal jet

The Ep,i – Eiso correlation and sub-energetic GRB GRB980425 not only prototype event of GRB/SN connection but closest GRB (z = 0.0085) and sub-energetic event (Eiso ~ 1048 erg, Ek,aft ~ 1050 erg) GRB031203: the most similar case to GRB980425/SN1998bw: very close (z = 0.105), SN2003lw, sub-energetic

d=[g(1 - bcos(qv - Dq))]-1 , DEp  d , DEiso  d(1+a) the most common explanations for the (apparent ?) sub-energetic nature of GRB980425 and GRB031203 and their violation of the Ep,i – Eiso correlation assume that they are NORMAL events seen very off-axis (e.g. Yamazaki et al. 2003, Ramirez-Ruiz et al. 2005) d=[g(1 - bcos(qv - Dq))]-1 , DEp  d , DEiso  d(1+a) a=1÷2.3 -> DEiso  d(2 ÷ 3.3) Yamazaki et al., ApJ, 2003 Ramirez-Ruiz et al., ApJ, 2004

GRB 060218, a very close (z = 0.033, second only to GRB9809425), with a prominent association with SN2006aj, and very low Eiso (6 x 1049 erg) and Ek,aft -> very similar to GRB980425 and GRB031203 but, contrary to GRB980425 and (possibly) GRB031203, GRB060218 is consistent with the Ep,i-Eiso correlation -> evidence that it is a truly sub-energetic GRB -> likely existence of a population of under-luminous GRB detectable in the local universe also XRF 020903 is very weak and soft (sub-energetic GRB prompt emission) and is consistent with the Ep-Eiso correlation Amati et al., 2007

GRB060218 was a very long event (~3000 s) and without XRT mesurement (0.3-10 keV) Ep,i would have been over-estimated and found to be inconsistent with the Ep,i-Eiso correlation Ghisellini et al. (2006) found that a spectral evolution model based on GRB060218 can be applied to GRB980425 and GRB031203, showing that these two events may be also consistent with the Ep,i-Eiso correlation sub-energetic GRB consistent with the correlation; apparent outliers(s) GRB 980425 (GRB 031203) could be due to viewing angle or instrumental effect

Recent Swift detection of an X-ray transient associated with SN 2008D at z = 0.0064, showing a light curve and duration similar to GRB 060218 Peak energy limits and energetics consistent with a very-low energy extension of the Ep,i-Eiso correlation Evidence that this transient may be a very soft and weak GRB (XRF 080109), thus confirming the existence of a population of sub-energetic GRB ? Modjaz et al., ApJ, 2008 Li, MNRAS, 2008

Ep,i – Eiso correlation and short GRBs only very recently, redshift estimates for short GRBs all SHORT Swift GRBs with known redshift and lower limits to Ep.i are inconsistent with the Ep,i-Eiso correlation intriguingly, the soft tail of GRB050724 is consistent with the correlation GRB 060614: no SN, first pulse inconsistent with correlation, sfto/long tail consistent: evidence that two different emission mechanisms are at work in both short and long GRB, with different relative efficiency in the two classes (thus generating also intermediate) Amati, NCimB, 2006

GRB-SN connection and the Ep,i-Eiso correlation GRB 060218 and other GRB with firmest evidence of association with a SN are consistent with the Ep,i-Eiso correlation GRB 060614: the long GRB with a very deep lower limit to the magnitude of an associated SN is consistent with the correlation too GRB 060505: stringent lower limit to SN magnitude, inconsistent with correlation, but it is likely short XRF 08019 / SN 2008D: possible very low energy GRB, possibly consistent with with the correlation Further evidence that GRB properties are independent on those of the SN Amati et al. A&A, 2007

GRB cosmology with Ep,i – Eiso correlation GRB have huge luminosity, a redshift distribution extending far beyond SN Ia high energy emission -> no extinction problems unfortunately, they are not standard candles ! but need to investigate their properties to find ways to standardize them (if possible)

Standardizing GRB with 3-parameters spectrum-energy correlations the Ep,i-Eiso correlation becomes tighter when adding a third observable: jet opening angle (qjet -> Eg = [1-cos(qjet)]*Eiso (Ghirlanda et al. 2004) or “high signal time” T0.45 (Firmani et al. 2006) the logarithmic dispersion of these correlations is very low: can they be used to standardize GRB ? jet angle inferred from break time in optical afterglow decay, while Ep,i-Eiso-T0.45 correlation based on prompt emission properties only

Methods (e. g. , Ghirlanda et al, Firmani et al. , Dai et al Methods (e.g., Ghirlanda et al, Firmani et al., Dai et al., Zhang et al.): Ep,i = Ep,obs x (1 + z) Dl = Dl (z, H0, WM, WL, …) general purpouse: estimate c.l. contours in 2-param surface (e.g. WM-WL) general method: construct a chi-square statistics for a given correlation as a function of a couple cosmological parameters method 1 – luminosity distance: fit the correlation and construct an Hubble diagram for each couple of cosmological parameters -> derive c.l. contours based on chi-square

method 2 – minimum correlation scatter: for each couple of cosm method 2 – minimum correlation scatter: for each couple of cosm.parameters compute Ep,i and Eiso (or Eg), fit the points with a pl and compute the chi-square -> derive c.l. contours based on chi-square surface method 3: bayesian method assuming that the correlation exists and is unique Ghirlanda et al., 2004 Firmani et al. 2007

What can be obtained with 150 GRB with known z and Ep and complementarity with other probes (SN Ia, CMB) complementary to SN Ia: extension to much higher z even when considering the future sample of SNAP (z < 1.7), cross check of results with different probes Ghirlanda, Ghisellini et al. 2005, 2006,2007

Drawbacks: lack of solid physical explanation physics of prompt emission still not settled, various scenarios: SSM internal shocks, IC-dominated internal shocks, external shocks, photospheric emission dominated models, kinetic energy dominated fireball , poynting flux dominated fireball) e.g., Ep,i  G-2 L1/2 tn-1 for syncrotron emission from a power-law distribution of electrons generated in an internal shock (Zhang & Meszaros 2002, Ryde 2005); for Comptonized thermal emission geometry of the jet (if assuming collimated emission) and viewing angle effects also may play a relevant role

Drawbacks: lack of calibration differently to SN Ia, there are no low-redshift GRB (only 1 at z < 0.1) -> correlations cannot be calibrated in a “cosmology independent” way would need calibration with a good number of events at z < 0.01 or within a small range of redshift -> neeed to substantial increase the number of GRB with estimates of redshift and Ep Bayesian methods have been proposed to “cure” the circularity problem (e.g., Firmani et al., 2006), resulting in slightly reduced contours w/r to simple (and circularity free) scatter method (using Lp,iso-Ep,i-T0.45 corr.)

“Crisis” of 3-parameters spectrum-energy correlations Recent debate on Swift outliers to the Ep-Eg correlation (including both GRB with no break and a few GRB with chromatic break) Recent evidence that the dispersion of the Lp-Ep-T0.45 correlation is significantly higher than thought before and comparable to the Ep,i-Eiso corr. Campana et al. 2007 Rossi et al. 2008

Using the simple Ep,i-Eiso correlation for cosmology Based on only 2 observables: a) much higher number of GRB that can be used b) reduction of systematics Evidence that a fraction of the extrinsic scatter of the Ep,i-Eiso correlation is due to choice of cosmological parameters used to compute Eiso Simple PL fit 70 GRB Amati et al. 2008

By quantifying the extrinsic scatter with a proper log-likelihood method, WM can be constrained to 0.04-0.40 (68%) for a flat LCDM universe (WM = 1 excluded at 99.9% c.l.) releasing assumption of flat universe still provides evidence of low WM, with a low sensitivity to WL significant constraints on both WM and WL (and likely on dark energy EOS) expected from sample enrichment and z extension by present and next GRB experiments (e.g., Swift, Konus_WIND, GLAST, SVOM) completely independent on other cosmological probes (e.g., CMB, type Ia SN, BAO; clusters…) and free of circularity problems 70 REAL 70 REAL + 150 SIM. Amati et al. 2008

The future: what is needed ? increase the number of z estimates, reduce selection effects and optimize coverage of the fluence-Ep plane in the sample of GRBs with known redshift more accurate estimates of Ep,i by means of sensitive spectroscopy of GRB prompt emission from a few keV (or even below) and up to at least ~1 MeV Swift is doing greatly the first job but cannot provide a high number of firm Ep estimates, due to BAT ‘narrow’ energy band (sensitive spectral analysis only from 15 up to ~200 keV) Ep estimates for some Swift GRBs from Konus (from 15 keV to several MeV) ant, to minor extent, RHESSI and SUZAKU NARROW BAND BROAD BAND

2008(-2011 ?): GLAST (AGILE) + Swift: accurate Ep (GLAST/GBM = 15-5000 keV) and z estimate (plus study of GeV emission) for simultaneously detected events by assuming that Swift will follow-up ALL GLAST GRB, about 80 GRB with Ep and z in 3 years AGILE and GLAST: second peak at E > 100 MeV ? (e.g., IC like in Blazars)

In the 2011-2015 time frame a significant step forward expected from SVOM: spectral study of prompt emission in 1-5000 keV -> accurate estimates of Ep and reduction of systematics (through optimal continuum shape determination and measurement of the spectral evolution down to X-rays) fast and accurate localization of optical counterpart and prompt dissemination to optical telescopes -> increase in number of z estimates and reduction of selection effects optimized for detection of XRFs, short GRB, sub-energetic GRB substantial increase of the number of GRB with known z and Ep -> test of correlations and calibration for their cosmological use

GRB as cosmological beacons: EDGE/XENIA (under study) GRB can be used as cosmological beacons for study of the IGM up to z > 6 Because of the association with the death of massive stars GRB allow the study the evolution of massive star formation and the evolution of their host galaxy ISM back to the early epochs of the Universe (z > 6) EDGE Team

End of the talk

Backup slides

Gamma-Ray Bursts most of the flux detected from 10-20 keV up to 1-2 MeV measured rate (by an all-sky experiment on a LEO satellite): ~0.8 / day bimodal duration distribution fluences (= av.flux * duration) typically of ~10-7 – 10-4 erg/cm2 short long

The GRB phenomenon: VHE CGRO/EGRET detected VHE (from 30 MeV up to 18 GeV) photons for a few GRBs VHE emission can last up to thousends of s after GRB onset average spectrum of 4 events well described by a simple power-law with index ~2 , consistent with extension of low energy spectra

afterglow emission (> 1997): X, optical, radio counterparts

… and host galaxies Long Short GRB 050509b host galaxies long GRBs: blue, usually regular and high star forming, GRB located in star forming regions host galaxies of short GRBs (very recent): elliptical, irregular galaxies, away from star forming region (but still unclear) Long Short GRB 050509b

in 1997 discovery of afterglow emission by BeppoSAX first X, optical, radio counterparts, redshifts, energetics GRB 970508, Metzger et al., Nature, 1997

1998: GRB/SN connection GRB 980425, a normal GRB detected and localized by WFC and NFI, but in temporal/spatial coincidence with a type Ic SN at z = 0.008 (chance prob. 0.0001) further evidences of a GRB/SN connection: bumps in optical afterglow light curves and optical spectra resembling that of GRB980425

circum-burst environment evidence of overdense and metal enriched circum-burst environment from absorption and emission features found in a few cases

>2005 prompt – afterglow Swift era BeppoSAX era prompt afterglow

Conclusions - I The Ep,i-Eiso correlation is the most firm GRB correlation followed by all normal GRB and XRF Swift results and recent analysis show that it is not an artifact of selection effects The existence, slope and extrinsic scatter of the correlation allow to test models for GRB prompt emission physics The study of the locatios of GRB in the Ep,i-Eiso plane help in indentifying and understanding sub-classes of GRB (short, sub-energetic, GRB-SN connection)

Conclusions - II Given their huge luminosities and redshift distribution extending up to at least 6.3, GRB are potentially a very powerful tool for cosmology and complementary to other probes (CMB, SN Ia, clusters, etc.) The use of spectrum-energy correlations to this purpouse is promising, but: need to substantial increase of the # of GRB with known z and Ep (which will be realistically allowed by next GRB experiments: Swift+GLAST/GBM, SVOM,…) need calibration with a good number of events at z < 0.01 or within a small range of redshift need solid physical interpretation identification and understanding of possible sub-classes of GRB not following correlations

Tests and debates

Debate based on BATSE GRBs without known redshift Nakar & Piran and Band & Preece 2005: a substantial fraction (50-90%) of BATSE GRBs without known redshift are potentially inconsistent with the Ep,i-Eiso correlation for any redshift value they suggest that the correlation is an artifact of selection effects introduced by the steps leading to z estimates: we are measuring the redshift only of those GRBs which follow the correlation they predicted that Swift will detect several GRBs with Ep,i and Eiso inconsistent with the Ep,i-Eiso correlation Ghirlanda et al. (2005), Bosnjak et al. (2005), Pizzichini et al. (2005): most BATSE GRB with unknown redshift are consistent with the Ep,i-Eiso correlation different conclusions mostly due to the accounting or not for the dispersion of the correlation

Ghirlanda et al., MNRAS, (2008) Swift GRBs and selection effects Swift / BAT sensitivity better than BATSE for Ep < ~100 keV, slightly worse than BATSE for Ep > ~100 keV but better than BeppoSAX/GRBM and HETE-2/FREGATE -> more complete coverage of the Ep-Fluence plane CGRO/BATSE Swift/BAT Ghirlanda et al., MNRAS, (2008) Band, ApJ, (2003, 2006)

fast (~1 min) and accurate localization (few arcesc) of GRBs -> prompt optical follow-up with large telescopes -> substantial increase of redshift estimates and reduction of selection effects in the sample of GRBs with known redshift fast slew -> observation of a part (or most, for very long GRBs) of prompt emission down to 0.2 keV with unprecedented sensitivity –> following complete spectra evolution, detection and modelization of low-energy absorption/emission features -> better estimate of Ep for soft GRBs drawback: BAT “narrow” energy band allow to estimate Ep only for ~15-20% of GRBs (but for some of them Ep from HETE-2 and/or Konus GRB060124, Romano et al., A&A, 2006

all long Swift GRBs with known z and published estimates or limits to Ep,i are consistent with the correlation the parameters (index, normalization,dispersion) obatined with Swift GRBs only are fully consistent with what found before Swift allows reduction of selection effects in the sample of GRB with known z -> the Ep,i-Eiso correlation is passing the more reliable test: observations ! Amati 2006, Amati et al. 2008

very recent claim by Butler et al very recent claim by Butler et al.: 50% of Swift GRB are inconsistent with the pre-Swift Ep,i-Eiso correlation but Swift/BAT has a narrow energy band: 15-150 keV, nealy unesuseful for Ep estimates, possible only when Ep is in (or close to the bounds of ) the passband (15-20%) and with low accuracy comparison of Ep derived by them from BAT spectra using Bayesian method and those MEASURED by Konus/Wind show they are unreliable as shown by the case of GRB 060218, missing the soft part of GRB emission leads to overestimate of Ep

Full testing and exploitation of spectral-energy correlations may be provided by EDGE in the > 2015 time frame in 3 years of operations, EDGE will detect perform sensitive spectral analysis in 8-2500 keV for ~150-180 GRB redshift will be provided by optical follow-up and EDGE X-ray absorption spectroscopy (use of microcalorimeters, GRB afterglow as bkg source) substantial improvement in number and accuracy of the estimates of Ep , which are critical for ultimately testing the reliability of spectral-energy correlations and their use for the estimates of cosmological parameters

extending to high energies (1-2 MeV): due to the broad spectral curvature of GRBs, a WFM with a limited energy band (<150-200 keV) will allow the determination of Ep for only a small fraction of events and with low accuracy (e.g., Swift/BAT ~15%) an extension up to 1-2 MeV with a good average effective area (~600cm2@600 keV) in the FOV of the WFM is required -> baseline solution: WFM + GRB Detector based on crystal scintillator accuracy of a few % for GRB with 15-150 keV fluence > 10-6 erg cm-2 over a broad range of Ep (20-1500) Pow.law fit Pow.law fit WFM: 2mm thick CZT detector, eff. Area~500 cm2@100 keV, FOV~1.4sr WFM + GRBD (5cm thick NaI, 850cm2 geom.area)

extending to low energies (1 keV) GRB060218 was a very long event (~3000 s) and without XRT mesurement (0.3-10 keV) Ep,i would have been over-estimated and found to be inconsistent with the Ep,i-Eiso correlation Ghisellini et al. (2006) found that a spectral evolution model based on GRB060218 can be applied to GRB980425 and GRB031203, showing that these two events may be also consistent with the Ep,i-Eiso correlation missing the X-ray prompt emission -> overestimate of Ep

In some cases, Ep estimates form different instruments are inconsistent with each other -> particular attention has to be paid to systematics in the estimate of Ep (limited energy band, data truncation effects, detectors sensitivities as a function of energies, etc.) include X-ray flares (and the whole afterglow emission) in the estimate of Ep,i and Eiso

e.g., 060218, 061007, 070110

Swift has shown how fast accurate localization and follow-up is critical in the reduction of selection effects in the sample of GRB with known z increasing the number of GRB with known z and accurate estimate of Ep is fundamental for calibration and verification of spectral energy correlations and their use for estimate of cosmological parameters (sim. by G. Ghirlanda)

consider BATSE 25-2000 keV fluences and spectra for 350 bright GRBs Ep,i = K x Eisom , Ep,i = Ep x (1+z), Eiso=(Sx4pDL2)/(1+z) Ep,i-Eiso correlation re-fitted by computing Eiso from 25*(1+z) to 2000*(1+z) gives K ~100, m ~0.54 , s(logEp,i) ~ 0.2, Kmax,2s ~ 250 -> Smin,ns = min[(1+z)2.85/(4pDL2)] x (Ep/Kmax,ns)1.85 (min. for z = 3.8) 2 s only a small fraction (and with substantial uncertainties in Ep) is below the 2 s limit ! Adapted from Kaneko et al., MNRAS, 2006

Open issues (several, despite recent huge observational advancements) Prompt and afterglow emission mechanisms: still to be settled early afterglow (steep decay, flat decay, flares): to be understood GeV / TeV emission: need for new measurements to test models X/gamma-ray polarization: need of measurements to test models short vs. long events: emission mech. and GRB/SN connection sub-energetic GRB and GRB rate: to be investigated -> SFR evolution collimated emission: yes or no ? spectrum-energy correlations and GRB cosmology: explanations and test … and more !

jet angles derived from the achromatic break time, are of the order of few degrees the collimation-corrected radiated energy spans the range ~5x1040 – 1052 erg-> more clustered but still not standard recent observations put in doubt jet interpretation of optical break

long weak soft emission is indeed observed for some short GRBs confirmation of expectations based on the fact that short GRBs are harder and have a lower fluence spectra of short GRBs consistent with those of long GRBs in the first 1-2 s evidences that long GRBs are produced by the superposition of 2 different emissions ? e.g., in short GRBs only first ~thermal part of the emission and lack or weakness (e.g. due to very high G for internal shocks or low density medium for external shock) of long part long weak soft emission is indeed observed for some short GRBs Ghirlanda et al. (2004)

Polarization no secure detection of polarization of prompt emission (some information from INTEGRAL and BATSE ?) polarization of a few % for some optical / radio afterglows radiation from synchrotron and IC is polarized, but a high degree of polarization can be detected only if magnetic field is uniform and perpendicular to line of sight small degree of polarization detectable if magnetic field is random, emission is collimated (jet) and we are observing only a (particular) portion of the jet or its edge