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Gamma-Ray Bursts (GRB) as cosmological probes

Gamma-Ray Bursts (GRB) as cosmological probes. Lorenzo Amati INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Bologna. 43 rd Rencontres de Moriond La Thuile (Val d'Aosta, Italy) March 15 - 22, 2008. Gamma-Ray Bursts. most of the flux detected from 10-20 keV up to 1-2 MeV

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Gamma-Ray Bursts (GRB) as cosmological probes

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  1. Gamma-Ray Bursts (GRB) as cosmological probes Lorenzo Amati INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Bologna 43rd Rencontres de Moriond La Thuile (Val d'Aosta, Italy) March 15 - 22, 2008

  2. 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; estimated true rate ~2 / day • bimodal duration distribution • fluences (= av.flux * duration) typically of ~10-7 – 10-4 erg/cm2 short long

  3. isotropic distribution of GRBs directions • paucity of weak events with respect to homogeneous distribution in euclidean space • hints to cosmological origin of GRBs -> would imply huge luminosity • but a “local” origin could not be excluded until 1997 !

  4. in 1997 discovery of afterglow emission by BeppoSAX • first X, optical, radio counterparts

  5. optical spectroscopy of afterglow and/or host galaxy –> first measurements of GRB redshift • redshifts higher than 0.1 -> GRB are cosmological • their isotropic equivalent radiated energy is huge (up to 1054 erg in a few tens of s !) GRB970228, Van Paradijs et al., Nature, 1997 GRB 970508, Metzger et al., Nature, 1997

  6. Are GRB standard candles ? • all GRBs with measured redshift (~100, including a few short GRB) lie at cosmological distances (z = 0.033 – 6.4) (except for the peculiar GRB980425, z=0.0085) • isotropic luminosities and radiated energy are huge and span several orders of magnitude: GRB are not standard candles (unfortunately) Mean: Eiso = 1053 erg

  7. 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

  8. 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)

  9. log(Ep,i )= 2.52 , s = 0.43 “Standardizing” GRB with spectrum-energy correlations • GRB nFn spectra typically show a peak at photon energy Ep • for GRB with known redshift it is possible to estimate the cosmological rest frame peak energy Ep,i and the radiated energy assuming isotropic emission, Eiso log(Eiso)= 1.0 , s = 0.9

  10. Amati et al. (2002) analyzed a sample of BeppoSAX events with known redshift found evidence of a strong correlation between Ep,i and Eiso, highly significant (r = 0.949, chance prob. 0.005%) • Further analysis with updated samples confirmed the correlation and extended it to X-Ray Flashes (XRF) • 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) Amati et al., 2008

  11. a first step: using Ep,i – Eiso correlation for z estimates • 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

  12. a step forward: standardizing GRB with 3-parameters spectral 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: 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

  13. Methods (e.g., Ghirlanda et al, Firmani et al., Dai et al., Zhang et al.): • general purpouse: estimate c.l. contours in 2-param surface (e.g. WM-WL, WM-w0) • general method: construct a chi-square statistics for a given correlation as a function of a couple cosmological parameters • general assumption: the correlation exists • method 1 – 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 2 – luminosity distance: for each couple of param. fit the correlation, derive Eiso from from Ep for each GRB through the best fit law, derive DL for each GRB from Eiso/fluence, construct chi-square based on difference from these DL and those derived form the z with the assumed couple of cosm. param. -> derive c.l. contours based on chi-square surface • method 3: more sophisticated bayesian method assuming that the correlation exists AND IS UNIQUE (which is reasonable given that it is expected that depends from the physics of GRB emission)

  14. GRB Hubble diagram with Ep-Eg using lum. dist. method: Ghirlanda et al., 2004,2007

  15. Contour plots in Wm – WL plane with Ep-Eg correlation(left) and Ep-Lp-T0.45 correlation (right) with the bayesian method Ghirlanda et al., Firmani et al.

  16. 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

  17. Combining spectrum-energy correlations with other (less tight) Luminosity correlations in GRB (Schaefer 2007)

  18. Drawbacks and circularity problems • 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

  19. 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.)

  20. 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 (RIess et al, 2007) • Obtained significant constraints on both WM and WL but with this method GRB are no more an indipendent cosmological probe

  21. “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 achromatic break) • Recent evidence that the dispersion of the Lp-Ep-T0.45 correlation is significantly higher than thought before Campana et al. 2007 Rossi et al. 2008

  22. Conclusions • 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, in order to avoid circularity problem • need solid physical interpretation • identification and understanding of possible sub-classes of GRB not following correlations

  23. GRB as cosmological beacons • 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

  24. END OF THE TALK

  25. BACK UP SLIDES

  26. physics of prompt emission (e.g. Zhang & Meszaros 2002) • 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 or • under different assumptions and to be combined with and in scenarios in whch for comptonized thermal emission from the photosphere dominates (e.g. Rees & Meszaros 2005)

  27. jet geometry and structure • XRF-GRB unification models • viewing angle effects • identification and nature of different classes of GRBs (as I will discuss next) Uniform/variable jet PL-structured /universal jet Uniform/variable jet PL-structured /universal jet

  28. Need of understending the physics behind spectral energy correlations • 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)

  29. Ep,i-Eg correlation need to clarify and understand the problem lack of chromatic breaks and possible outliers • 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 061007, Mundell et al. Schady et al. GRB 050416a

  30. need to reduce systematics on Ep with broad energy bandspectroscopy • 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.)

  31. need detectors 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 + GRBD (5cm thick NaI, 850cm2 geom.area) WFM: 2mm thick CZT detector, eff. Area~500 cm2@100 keV, FOV~1.4sr GRB with 15-150 keV fluence 10-6 erg cm-2 and Ep=300 keV. Left: WFM only, Right: WFM + GRBD

  32. and 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

  33. need to increase number and reduce selection effects of redshift estimates • 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)

  34. Next experiments: what is needed ? • to test the correlations, to better constrain their slope, normalization and dispersion, to understand the physics behind them and study the behaviour of different sub-classes of GRB, and to calibrate them for cosmological use, need to: • 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 HETE-2 (2-400 keV) and Konus (from 15 keV to several MeV) for simultaneously detected events

  35. presently, Ep,i values are mainly provided by HETE-2 (but only up to 400 keV -> mostly CPL fits) or Konus/Wind (> 15 keV, fits with Band function); in some cases, useful spectral information also from BAT, Suzaku and INTEGRAL • need of new GRB detectors capable to extend down to a few keV (or even below) and up to at least ~1 MeV: e.g., a combination of a sensitive low energy detector (CZT, silicon microstrips or SDC diodes) and moderate area scintillator high energy detector (NaI, CsI, BGO) 2008(-2011 ?): GLAST (AGILE) + Swift: • accurate Ep and z estimate (plus full X-ray coverage and possibly study of GeV emission) for simultaneously detected events • however, even by assuming that Swift will follow-up ALL GLAST GRB, no more than 30 GRB with Ep and z in 3 years

  36. 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 • unique mission with these capabilities in the 2010-2015 (at least) time frame • SVOM will allow a step forward in the test, understanding and use (GRB physics, cosmology) of spectral – energy correlations

  37. Full calibration and exploitation of spectral-energy correlation for cosmology 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 the estimates of cosmological parameters

  38. THE END

  39. 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 • naturally explained collimated emission • energy budget up to 1051 - 1052 erg • short duration GRBs (< 5 s) • clean circum-burst environment • GRBs in the outer regions of the host galaxy

  40. before EDGE main contributions expected from GLAST+Swift and SVOM • EDGE will ~triplicate the GRB in the sample and substantially increase the accuracy in cosmological parameters • EDGE will allow cosm. param. estimates based on an homogeneous sample • the very high number of GRB known z and Ep that will be reached with EDGE will allow calibration of the correlations (i.e. find Ep for a sufficent number of GRB at same z within 10% and thus solve circularity problem) • EDGE GRB, SNAP SN Ia, other probes (e.g. clusters): complementarity and sinergy (different z ranges), cross-checks, reduction of systematics • EDGE will also allow a robust test of the correlations themselves (selection effects, outliers, etc.)

  41. What is needed ? • to test the correlations, to better constrain their slope, normalization and dispersion, to understand the physics behind them and study the behaviour of different sub-classes of GRB, and to calibrate them for cosmological use, need to: • 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 HETE-2 (2-400 keV) and Konus (from 15 keV to several MeV) for simultaneously detected events

  42. Spectral-energy correlations: physics, peculiar GRB and possible outliers, tests, debates

  43. Origin of spectral-energy correlations • 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)

  44. physics of prompt emission (e.g. Zhang & Meszaros 2002) • 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 or • under different assumptions and to be combined with and in scenarios in whch for comptonized thermal emission from the photosphere dominates (e.g. Rees & Meszaros 2005)

  45. jet geometry and structure • XRF-GRB unification models • viewing angle effects • identification and nature of different classes of GRBs (as I will discuss next) Uniform/variable jet PL-structured /universal jet Uniform/variable jet PL-structured /universal jet

  46. are sub-energetic GRBs really outliers ? • 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 • the most common explanations for the (apparent ?) sub-energetic nature of GRB980425 and GRB031203 and their violation of the Ep-Eiso and Ep-Eg correlations assume that they are NORMAL events seen very off-axis (e.g. Yamazaki et al. 2003, Ramirez-Ruiz et al. 2005

  47. 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 • also XRF 020903 is very weak and soft (sub-energetic GRB prompt emission) and is ocnsistent with the Ep-Eiso correlation Amati et al., 2007

  48. 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

  49. Testing the Ep,i – Eiso correlation • claims that a high fraction of BATSE events (without z) are inconsistent with the correlation (e.g. Nakar & Piran 2004, Band & Preece 2005 • Ghirlanda et al. (2005), Bosnjak et al. (2005), Pizzichini et al. (2005): BATSE GRB with unknown redshift are consistentwith the correlation • different conclusions but general consensus: Swift will allow reduction of selection effects both in GRB detection and in z estimates and will allow us to stringently test the correlation

  50. 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 • 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

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