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Lorenzo Amati INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Bologna

Gamma-Ray Bursts (GRB) as cosmological probes. Lorenzo Amati INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Bologna. Gamma-Ray Bursts: brightest cosmol. sources. most of the flux detected from 10-20 keV up to 1-2 MeV

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Lorenzo Amati INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Bologna

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  1. Gamma-Ray Bursts (GRB) as cosmological probes Lorenzo Amati INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Bologna

  2. Gamma-Ray Bursts: brightest cosmol. sources • 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, host galaxies

  5. optical spectroscopy of afterglow and/or host galaxy –> first measurements of GRB redshift • redshifts higher than 0.1 and up to >6 -> GRB are cosmological • their isotropic equivalent radiated energy is huge (up to 1054 erg in a few tens of s !) • std. scenarios for long and short

  6. Are GRB standard candles ? • all GRBs with measured redshift (~130, 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. “Standardizing” GRB with spectrum-energy correlations log(Ep,i )= 2.52 , s = 0.43 • 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 • Further analysis with updated samples confirmed the correlation and extended it to the weakest and softest events (X-Ray Flashes, XRF) • Significant extra-Poissonian scatter of the data around the best fit power-law: sext [log(Ep,i)] ~ 0.17 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 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

  13. Methods (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

  14. 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

  15. 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

  16. 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

  17. 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.)

  18. “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

  19. Using the simple Ep,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 Simple PL fit 70 GRB Amati et al. 2008

  20. 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

  21. 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, BAO, 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

  22. 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

  23. END OF THE TALK

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