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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, 2008 - Pescara, Italy). Outline Observations Implications for GRB physics and origin Tests and debates Cosmology

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GRB physics and cosmology with the Ep,i – Eiso correlation

Lorenzo Amati

INAF – IASF Bologna (Italy)

Third Stueckelberg Workshop

(July 8th to 19th, 2008 - Pescara, Italy)


  • Outline

  • Observations

  • Implications for GRB physics and origin

  • Tests and debates

  • Cosmology

  • Conclusions and future perspectives



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


  • 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 = 0.033 – 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 = Epx (1 + z)

log(Eiso)= 1.0 , s = 0.9

log(Ep,i )= 2.52 , s = 0.43

Ep,i and Eiso distributions for a sample of 41 long GRBs (Amati 2006)


Ep,i= kEiso

(0.52+/-0.06)

Amati et al. , A&A, 2002


Amati, ChJAA, 2003

  • 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

  • 90% c.l. Ep of XRF020903 fromrefined analysis ofHETE-2 WXM + FREGATE spectrum (Sakamoto et al. 2004)fully consistent with the Ep,i – Eiso correlation

Lamb et al., ApJ, 2004


  • analysis of an updated sample of with known redshiftlongGRBs/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 (~0.17 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 980425 (GRB 031203 debated)

Amati et al. 2008


  • the with known redshift“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 sint(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)


3-parameters spectrum-energy correlations with known redshift

  • the Ep,i-Eiso correlation becomes tighter when adding a third observable: jet opening angle (qjet -> Eg = [1cos(qjet)]*Eiso(Ghirlanda et al. 2004), break time in optical afterglow decay (Liang & Zhang 2005) or “high signal time” T0.45 (Firmani et al. 2006)

  • jet angle inferred from break time in optical afterglow decay, while Ep,i-Eiso-T0.45 correlation based on prompt emission properties only


E with known redshiftp,i – Eiso correlation vs. 3-param 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

  • Ep-Eg correlation requires modeling; both Ep-Eg and Ep-Eiso-tb correlations requires afterglow detection and fine sampling

  • Ep-Lp-T0.45 based only on prompt emission properties and requires no modelization


  • Recent debate on Swift outliers to the Ep-E with known redshiftg 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. 2007

Ghirlanda et al. 2007


Rossi et al. 2008


Eiso<->Liso dispersion of the Lp-Ep-T

Ep,i – Eiso

“Amati” 02

Eiso<->Lp,iso

Ep,i – Liso

04

Ep,i – Lp,iso

“Yonetoku”04

tb,opt + jet model

tb,opt

T0.45

=

Ep,i – Eg

“Ghirlanda” 04

Ep,i – Eiso-tb

“Liang-Zhang” 05

Ep,i – Lp,iso-T0.45

“Firmani” 06

The genealogy and nomenclature of spectrum-energy correlations


Implications for GRB physics and origin dispersion of the Lp-Ep-T


  • Ep is a fundamental parameter in prompt emission mdels, e.g., syncrotron shock emission models (SSM)

  • it may correspond to a characteristic frequency (possibly nm in fast cooling regime) or to the temperature of the Maxwellian distribution of the e-

Sari et al., ApJ, 1998

Tavani, ApJ, 1995


  • 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 scenarios: SSM internal shocks, IC-dominated internal shocks, external shocks, photospheric emission dominated models, kinetic energy dominated fireball , poynting flux dominated fireball)

  • 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


  • 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

Soderberg et al., Nature, 2003

Ghirlanda et al., 2007


  • 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


  • 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

  • also XRF 020903 is very weak and soft (sub-energetic GRB prompt emission) and is consistent with the Ep-Eiso correlation

Amati et al., A&A, 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


  • 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

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 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)


  • GRBs with firmest evidence of association with a SN are consistent with the Ep,i-Eiso correlation (except for peculiar 980425)

  • 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

  • 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 = 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 ?

  • XRF 080109 / 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


  • 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 050315

CBM constant (n=1cm-3)


Piranomonte et al. (2008)

Ruffini et al. (2008)


Tests and debates correlation


  • 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


  • 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.: 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


Cosmology with spectrum-energy correlations inconsistent with the pre-Swift Ep,i-Eiso correlation



  • 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


  • 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: they can 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 spectrum-energy correlations(e.g., Ghirlanda et al, Firmani et al., Dai et al., Zhang et al.):

Ep,i = Ep,obsx (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 spectrum-energy correlations: 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


Ghirlanda, Ghisellini et al. 2005, 2006,2007


  • 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


  • Lack of calibration complementarity with other probes (SN Ia, CMB)

  • 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

  • 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 complementarity with other probes (SN Ia, CMB)

  • 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 E complementarity with other probes (SN Ia, CMB)p,i-Eiso correlation for cosmology

  • Based on only2 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

70 GRB

Simple PL fit

Amati et al. 2008


Amati et al. 2008


  • releasing assumption of flat universe still provides evidence of low WM, with a low sensitivity to WL

  • significant constraints on both WM and WL 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 SIMUL

Amati et al. 2008


70 REAL

+ 150 SIMUL

70 REAL

70 REAL

70 REAL

+ 150 SIMUL

Amati et al. 2008


(e.g.,Chevalier & Polarski, Linder & Utherer)

70 REAL

+ 150 SIMUL (flat)

70 REAL (flat, Wm=0.27)

Amati et al. 2008


Complementarity to other probes: the case of SN Ia GRB are potentially the best-suited probes to study properties and evolution of “dark energy”

  • 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


  • 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 affected by systematics -> need to carry out independent measurement of


Conclusions - I affected by systematics -> need to carry out independent measurement of

  • 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 - II affected by systematics -> need to carry out independent measurement of

  • 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 Wm, 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


The future: what is needed ? affected by systematics -> need to carry out independent measurement of

  • 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: affected by systematics -> need to carry out independent measurement of

  • accurate Ep (GLAST/GBM = 10-5000 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 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 expected from SVOM:


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