GRB physics and cosmology with the E p,i – E iso correlation Lorenzo Amati INAF – IASF Bologna (Italy) Third Stueckelberg Workshop (July 8th to 19th, Pescara, Italy)

Outline  Observations  Implications for GRB physics and origin  Tests and debates  Cosmology  Conclusions and future perspectives

Observations

 GRB spectra typically described by the empirical Band function with parameters  = low-energy index,  = high-energy index, E 0 =break energy  E p = E 0 x (2 +  ) = observed peak energy of the F spectrum The Ep,i – Eiso correlation

 since 1997 GRB redshift estimates through optical spectroscopy of afterglow emission and/or host galaxies  all GRBs with measured redshift (~100) lie at cosmological distances (z = – 6.4) (except for the peculiar GRB980425, z=0.0085)  the pre-Swift GRB z distribution and the Swift GRB z distribution differ

 from redshift, fluence and spectrum, it is possible to estimate the cosmological-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 and Eiso distributions for a sample of 41 long GRBs (Amati 2006) E p,i = E p x (1 + z) log(Ep,i )= 2.52,  = 0.43 log(Eiso)= 1.0,  = 0.9

 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 (  = 0.949, chance prob %) despite the low number of GRBs included in the sample E p,i = kE iso (0.52+/-0.06) Amati et al., A&A, 2002

 HETE-2 data confirm the Ep,i – Eiso correlation and show that it extends to XRFs, thus spanning 5 orders of magnitude in Eiso and 3 orders of magnitude in Ep,i Lamb et al., ApJ, 2004  90% c.l. Ep of XRF from refined analysis of HETE-2 WXM + FREGATE spectrum (Sakamoto et al. 2004) fully consistent with the Ep,i – Eiso correlation Amati, ChJAA, 2003  by adding data from BATSE and HETE- 2 of 10 more GRBs the correlation was confirmed and its significance increased

 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 ~ 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  (logEp,i) ~ 0.2 ( ~0.17 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 (GRB debated) Amati et al. 2008

 the “extra-statistical scatter” of the data was quantified by performing a fit with a method (D’Agostini 2005) which accounts for sample variance  the “intrinsic” dispersion results to be  int (logEp,i) = 0.17 (-0.02,+0.03)  with this method, the power-law index and normalization turn out to be ~0.5 and ~100, respectively (the commonly assumed values !) Amati (2006)

 the E p,i -E iso correlation becomes tighter when adding a third observable: jet opening angle (  jet -> E  = [1cos(  jet )]*E iso (Ghirlanda et al. 2004), break time in optical afterglow decay (Liang & Zhang 2005) or “high signal time” T 0.45 (Firmani et al. 2006)  jet angle inferred from break time in optical afterglow decay, while E p,i -E iso -T 0.45 correlation based on prompt emission properties only 3-parameters spectrum-energy correlations

 3-parameters spectral energy correlation less dispersed than Ep,i-Eiso correlation  but based on lower number of events (~20 against more than 60) -> need more events to be confirmed  addition of a third observable introduces further uncertainties  E p -E  correlation requires modeling; both E p -E  and E p -E iso -t b correlations requires afterglow detection and fine sampling  E p -L p -T 0.45 based only on prompt emission properties and requires no modelization E p,i – E iso correlation vs. 3-param correlations E p,i – E iso correlation vs. 3-param correlations

 Recent debate on Swift outliers to the Ep-E  correlation (including both GRB with no break and a few GRB with achromatic break)  different conclusions based on light curve modeling and considering early or late break Campana et al. 2007Ghirlanda et al. 2007

 Recent evidence, based on BeppoSAX and Swift GRBs that the dispersion of the Lp-Ep-T 0.45 correlation is significantly higher than thought before Rossi et al. 2008

The genealogy and nomenclature of spectrum-energy correlations Ep,i – Eiso “Amati” 02 Ep,i – Liso 04 Ep,i – Lp,iso “Yonetoku”04 Ep,i – E  “Ghirlanda” 04 Ep,i – Eiso-tb “Liang-Zhang” 05 Ep,i – Lp,iso- T0.45 “Firmani” 06 Eiso LisoEiso Lp,iso tb,opt + jet model tb,optT0.45 =

Implications for GRB physics and origin

 Ep is a fundamental parameter in prompt emission mdels, e.g., syncrotron shock emission models (SSM)  it may correspond to a characteristic frequency (possibly m in fast cooling regime) or to the temperature of the Maxwellian distribution of the e- Tavani, ApJ, 1995Sari et al., ApJ, 1998  Origin 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  -2 L 1/2 t -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  Tpk  2 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 Lamb et al., ApJ, 2004, Yonetoku et al.,ApJ, 2004

 GRB not only prototype event of GRB/SN connection but closest GRB (z = ) and sub-energetic event (Eiso ~ erg, Ek,aft ~ erg)  GRB031203: the most similar case to GRB980425/SN1998bw: very close (z = 0.105), SN2003lw, sub-energetic  The Ep,i – Eiso correlation and sub-energetic GRB Soderberg et al., Nature, 2003Ghirlanda et al., 2007

 the most common explanations for the (apparent ?) sub-energetic nature of GRB and GRB 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)   =[  (1 -  cos(  v -  ))] -1,  Ep  Eiso   )  =1÷2.3 ->  Eiso   ÷  ) Yamazaki et al., ApJ, 2003 Ramirez-Ruiz et al., ApJ, 2004

 but, contrary to GRB and (possibly) GRB031203, GRB is consistent with the Ep,i-Eiso correlation -> evidence that it is a truly sub- energetic GRB  also XRF is very weak and soft (sub-energetic GRB prompt emission) and is consistent with the Ep-Eiso correlation Amati et al., A&A, 2007  GRB , a very close (z = 0.033, second only to GRB ), with a prominent association with SN2006aj, and very low Eiso (6 x erg) and Ek,aft -> very similar to GRB and GRB031203

 GRB was a very long event (~3000 s) and without XRT mesurement ( 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 GRB can be applied to GRB 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 (GRB ) could be due to viewing angle or instrumental effect

 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 GRB is consistent with the correlation  Ep,i – Eiso correlation and short GRBs Amati, NCimB, 2006

 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  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)

 GRB-SN connection and the Ep,i-Eiso correlation  GRBs with firmest evidence of association with a SN are consistent with the Ep,i-Eiso correlation (except for peculiar )  GRB : the long GRB with a very deep lower limit to the magnitude of an associated SN is consistent with the correlation too  GRB : stringent lower limit to SN magnitude, inconsistent with correlation, but it is likely short  Evidence that GRB properties are independent on those of the SN ? Amati et al. A&A, 2007

 Recent Swift detection of an X-ray transient associated with SN 2008D at z = , showing a light curve and duration similar to GRB  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 ), thus confirming the existence of a population of sub-energetic GRB ?  XRF / SN2008D: are soft X-ray flashes due to SN shock break-out ? How they connect to “normal” GRBs ? Modjaz et al., ApJ, 2008 Li, MNRAS, 2008

 Ep,i-Eiso correlation in the fireshell model (Ruffini et al.)  By assuming CBM profile from a real GRB and varying Etot, the correlation is obtained, with a slope of 0.45+/+0.01 (consistent with obs.)  no correlation when assuming constant CBM profile (Guida et al. 2008) CBM profile as GRB CBM constant (n=1cm -3 )

 Natural explanation of the deviation of short GRB from the correlation  extrinsic scatter of the correlation mostly due to the inclusion of P-GRB, the computation of Ep based only on the “prompt” spectrum, cosmology Piranomonte et al. (2008) Ruffini et al. (2008)

Tests and debates

 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  Debate based on BATSE GRBs without known redshift

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

 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.: 50% of Swift GRB are inconsistent with the pre-Swift Ep,i-Eiso correlation  but Swift/BAT has a narrow energy band: 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 , missing the soft part of GRB emission leads to overestimate of Ep

Cosmology with spectrum-energy correlations

 GRB have huge luminosity, a redshift distribution extending far beyond SN Ia  high energy emission -> no extinction problems  but need to investigate their properties to find ways to standardize them (if possible)

 redshift estimates available only for a small fraction of GRB occurred in the last 10 years based on optical spectroscopy  pseudo-redshift estimates for the large amount of GRB without measured redshift -> GRB luminosity function, star formation rate evolution up to z > 6, etc.  use of the Ep,i – Eiso correlation for pseudo-redshift: most simple method is to study the track in the Ep,i - Eiso plane ad a function of z  not precise z estimates and possible degeneracy for z > 1.4  anyway useful for low –z GRB and in general when combined with optical  a first step: using Ep,i – Eiso correlation for z estimates

 the E p,i -E iso correlation becomes tighter when adding a third observable: jet opening angle (  jet -> E  = [1-cos(  jet )]*E iso (Ghirlanda et al. 2004) or “high signal time” T 0.45 (Firmani et al. 2006)  the logarithmic dispersion of these correlations is very low: they can be used to standardize GRB ?  jet angle inferred from break time in optical afterglow decay, while E p,i -E iso - T 0.45 correlation based on prompt emission properties only  a step forward: standardizing GRB with 3-parameters spectrum-energy correlations

 general purpouse: estimate c.l. contours in 2-param surface (e.g.  M -   )  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  Methods (e.g., Ghirlanda et al, Firmani et al., Dai et al., Zhang et al.) : E p,i = E p,obs x (1 + z) D l = D l (z, H 0,  M,  , …)

Ghirlanda et al., 2004  method 2 – minimum correlation scatter: for each couple of cosm.parameters compute Ep,i and Eiso (or E  ), 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 Firmani et al. 2007

Ghirlanda, Ghisellini et al. 2005, 2006,2007  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

 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  -2 L 1/2 t -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 solid physical explanation

 Lack of calibration  differently to SN Ia, there are no low-redshift GRB (only 1 at z correlations cannot be calibrated in a “cosmology independent” way  would need calibration with a good number of events at z neeed to substantial increase the number of GRB with estimates of redshift and Ep  Very recently (Kodama et al., 2008; Liang et al., 2008) calibrated GRB spectrum—energy correlation at z < 1.7 by using the cosmology independent luminosity distance – redshift relation derived for SN Ia

 “Crisis” of 3-parameters spectrum-energy correlations  Recent debate on Swift outliers to the Ep-E  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. 2007Rossi et al. 2008

 Using the simple E p,i -E iso 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 E p,i -E iso correlation is due to choice of cosmological parameters used to compute E iso Amati et al Simple PL fit 70 GRB

 By using a maximum likelihood method the extrinsic scatter can be parametrized and quantified (e.g., D’Agostini 2005)   M can be constrained to (68%) and (90%) for a flat  CDM universe (  M = 1 excluded at 99.9% c.l.) Amati et al. 2008

 releasing assumption of flat universe still provides evidence of low  M, with a low sensitivity to    significant constraints on both  M and   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 Amati et al REAL SIMUL

 possible further improvements on cosmological parameter estimates by exploiting self-calibration with GRB at similar redshift or solid phyisical model for the correlation Amati et al REAL SIMUL 70 REAL SIMUL 70 REAL

 given their redshift distribution ( up to now), GRB are potentially the best-suited probes to study properties and evolution of “dark energy” Amati et al REAL (flat,  m=0.27) 70 REAL SIMUL (flat) (e.g.,Chevalier & Polarski, Linder & Utherer)

Complementarity to other probes: the case of SN Ia  Several possible systematics may affect the estimate of cosmological parameters with SN Ia, e.g.:  different explosion mechanism and progenitor systems ? May depend on z ?  light curve shape correction for the luminosity normalisation may depend on z  signatures of evolution in the colours  correction for dust extinction  anomalous luminosity-color relation  contaminations of the Hubble Diagram by no-standard SNe-Ia and/or bright SNe-Ibc (e.g. HNe) Kowalski et al. 2008

 The Hubble diagram for type Ia SNe may be significantly affected by systematics -> need to carry out independent measurement of   and    GRBs allow us today to change the “experimental methodology” and provide an independent measurement of the cosmological parameters:  GRBs are extremely bright and detectable out of cosmological distances (z=6.3 Kuwai et al. 2005, Tagliaferri et al. 2005) -> interesting objects for cosmology  SNe-Ia are currently observed at z<1.7: GRBs appear to be (in principle) the only class of objects capable to study the evolution of the dark energy from the beginning (say from z~7-8)  No need of correction for reddening  Different orientation of the contours

Conclusions and future perspectives

 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 locations of GRB in the Ep,i-Eiso plane help in indentifying and understanding sub-classes of GRB (short, sub-energetic, GRB-SN connection) Conclusions - I

 Given their huge luminosities and redshift distribution extending up to at least 6.3, GRB are a powerful tool for cosmology and complementary to other probes (CMB, SN Ia, BAO, clusters, etc.)  The use of Ep,i – Eiso correlation to this purpouse is promising (already significant constraints on  m, in agreement with “concordance cosmology), 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,…)  auspicable solid physical interpretation  identification and understanding of possible sub- classes of GRB not following correlations Conclusions - II

The future: what is needed ? 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 = 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 time frame a significant step forward expected from SVOM:  spectral study of prompt emission in 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 in the sample of GRB with known z  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

End of the talk