Lorenzo Amati Italian National Institute for Astrophysics (INAF), Bologna

1. Measuring cosmological parameters with Gamma-Ray Bursts Lorenzo Amati Italian National Institute for Astrophysics (INAF), Bologna with main contributions by: M. Della Valle, F. Frontera, C. Guidorzi Nikko, Japan, 12-16 March 2012

2. Why looking for more cosmological probes ? • different distribution in redshift -> different sensitivity to different cosmological parameters

3. Guy et al. 2010 Astier et al. 2006 Recent results from SNLS (231 SNe Ia at 0.15 < z < 1.1, Guy et al. 2010) compared to those of Astier et al. 2006 (44 low redshift SNe along with the 71 SNe from the SNLS first year sample)

4. Each cosmological probe is characterized by possible systematics • e.g SN Ia: • 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)

5. Are GRB standard candles ? • all GRBs with measured redshift (~250, including a few short GRBs) lie at cosmological distances (z = 0.033 – 9.4) (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) Jakobsson, 2009 Amati, 2009

6. jet angles, derived from break time of optical afterglow light curve by assuming standard scenario, are of the order of few degrees • the collimation-corrected radiated energy spans the range ~5x1049 – 5x1052erg-> more clustered but still not standard Ghirlanda et al., 2004

7. GRB have huge luminosity, a redshift distribution extending far beyond SN Ia • high energy emission -> no extinction problems Ghirlanda et al, 2006

8. GRB have huge luminosity, a redshift distribution extending far beyond SN Ia • high energy emission -> no extinction problems • potentially powerful cosmological sources but need to investigate their properties to find ways to standardize them (if possible) Ghirlanda et al, 2006

9. 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 • measured spectrum + measured redshift -> intrinsic peak enery and radiated energy Ep,i = Epx (1 + z) 190 GRB Jakobsson (2009) Ep

10. ~260 GRBs with measured redshift, about 50% have measured spectra • both Ep, i and Eiso span several orders of magnitude and a distribution which can be described by a Gaussian plus a low – energy tail (“intrinsic” XRFs and sub-energetic events) 95 GRBs, sample of Amati, Frontera & Guidorzi, A&A (2009)

11. Amati et al. (A&A 2002): significant correlation between Ep,i and Eiso found based on a small sample of BeppoSAX GRBs with known redshift BeppoSAX GRBs

12. Ep,i – Eiso correlation for GRBs with known redshift confirmed and extended by measurements of ALL other GRB detectors with spectral capabilities 130 long GRBs as of Sept. 2011 BeppoSAX GRBs

13. Ep,i of Swift GRBs measured by Konus-WIND, Suzaku/WAM, Fermi/GBM and BAT (only when Ep inside or close to 15-150 keV and values provided by the Swift/BAT team (GCNs or Sakamoto et al. 2008). Swift GRBs

14. strong correlation but significant dispersion of the data around the best-fit power-law; the distribution of the residuals can be fit with a Gaussian with s(logEp,i) ~ 0.2 • the “extra-statistical scatter” of the data can be quantified by performing a fit whith a max likelihood method (D’Agostini 2005) which accounts for sample variance and the uncertainties on both X and Y quantities • with this method Amati et al. (2008, 2009) found an extrinsic scatter sint(logEp,i) ~0.18 and index and normalization t ~0.5 and ~100, respectively

15. “Standardizing” GRB with Ep,i-brightness correlations • 2004: evidence that by substituting Eiso with the collimation corrected energy Egthe logarithmic dispersion of the correlation decreases significantly and is low enough to allow its use to standardize GRB (Ghirlanda et al., Dai et al, and many)

16. BUT… • the Ep-Eg correlation is model dependent: slope depends on the assumptions on the circum-burst environment density profile (ISM or wind) • addition of a third observable introduces further uncertainties (difficulties in measuring t_break, chromatic breaks, model assumptions, subjective choice of the energy band in which compute T0.45, inhomogeneity on z of T0.45) and substantially reduces the number of GRB that can be used (e.g., #Ep,i – Eg~ ¼ #Ep,i – Eiso ) ISM WIND Nava et al.. , A&A, 2005: ISM (left) and WIND (right)

17. 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 ?

18. Amati et al. (2008): let’s make a step backward and focus on the Ep,i – Eiso correlation Eiso<->Liso 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

19. Amati et al. (2008): let’s make a step backward and focus on the Ep,i – Eiso correlation Eiso<->Liso 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

20. does the extrinsic scatter of the Ep,i-Eiso correlation vary with the cosmological parameters used to compute Eiso ? Dl = Dl (z , H0 , WM , WL ,…) 70 GRB Amati et al. 2008

21. a fraction of the extrinsic scatter of the Ep,i-Eiso correlation is indeed due to the cosmological parameters used to compute Eiso • Evidence, independent on SN Ia or other cosmological probes, that, if we are in a flat LCDM universe , WM is lower than 1 Simple PL fit Amati et al. 2008 Amati et al. 2008

22. By using a maximum likelihood method the extrinsic scatter can be parametrized and quantified (e.g., Reichart 2001, D’Agostini 2005) • WM can be constrained to 0.04-0.43 (68%) and 0.02-0.71 (90%) for a flat LCDM universe (WM = 1 excluded at 99.9% c.l.) • significant constraints on both WM and WL expected from sample enrichment 70 (real) GRBs 70 (real) + 150 (sim.) GRBs Amati et al. 2008

23. analysis of the most updated sample of 137 GRBs shows significant improvements w/r to the sample of 70 GRBs of Amati et al. (2008) • this evidence supports the reliability and perspectives of the use of the Ep,i – Eiso correlation for the estimate of cosmological parameters 114 GRBs 137 GRBs 70 GRBs

25. GRB

26. the simulatenous operation of Swift, Fermi/GBM, Konus-WIND is allowing an increase of the useful sample (z + Ep) at a rate of 15-20 GRB/year, providing an increasing accuracy in the estimate of cosmological parameters • future GRB experiments (e.g., SVOM) and more investigations (physics, methods, calibration) will improve the significance and reliability of the results and allow to go beyond SN Ia cosmology (e.g. investigation of dark energy) 300 GRB 300 GRB Adapted from Amati+ 12 and Ghirlanda+ 2007

27. Conclusions and perspectives • Given theirhuge radiated energies and redshift distribution extending from ~ 0.1 up to > 9, GRBs are potentially a very powerful cosmological probe, complementary to other probes (e.g., SN Ia, clusters, BAO) • TheEp,i – Eiso correlationis one of the most robust (no firm evidence of significant selection / instrumental effects) and intriguing properties of GRBs and a promising tool for cosmological parameters • Analysis in the last years (>2008) provide already evidence, independent on , e.g., SN Ia, that if we live in a flat LCDM universe,Wm is < 1 at >99.9% c.l.(c2minimizes at Wm ~ 0.25, consistent with “standard” cosmology) • the simulatenous operation of Swift, Fermi/GBM, Konus-WIND is allowing anincrease of the useful sample (z + Ep) at a rate of 15-20 GRB/year, providing an increasing accuracy in the estimate of cosmological parameters • future GRB experiments (e.g., SVOM) and more investigations (physics, methods, calibration)will allow to go beyond SN Ia cosm. (e.g.,dark energy EOS)

28. Backup slides

29. Reliability of the Ep,i – Eiso correlation • different GRB detectors are characterized by different detection and spectroscopy sensitivity as a function of GRB intensity and spectrum • this may introduce relevant selection effects / biases in the observed Ep,i – Eiso and other correlations: hot debate within GRB community Band 2008 Ghirlanda et al. 2008

30. ? OK

31. Amati, Frontera & Guidorzi (2009): the normalization of the correlation varies only marginally using GRBs measured by individual instruments with different sensitivities and energy bands Amati , Frontera & Guidorzi 2009

32. Up to date (Sept. 2011) Ep,i – Eiso plane: 131 long GRBs, 4 XRFs, 13 short GRBs

33. method: unknown redshift -> convert the Ep,i – Eiso correlation into an Ep,obs – Fluence correlation Intrinsic (cosm. Rest-frame) plane Observer’s plane 3 s 2 s 2 s GRBs WITHOUT redshift (thousands) GRBs WITH redshift (130)

34. Amati, Dichiara et al. (2011, in prep.): consider fluences and spectra from the Goldstein et al. (2010) BATSE complete spectral catalog (on line data) • considered long (777) and short (89) GRBs with fit with the Band-law and uncertainties on Ep and fluence < 40% LONG SHORT LONG • most long GRBs are potentially consistent with the Ep.i – Eiso correlation, most short GRBs are not

35. ALL long GRBs with 20% uncertainty on Ep and fluence (525) are potentially consistent with the correlation LONG, 20% unc. LONG, 40% unc.

36. the Ep,i– Liso correlation holds also within a good fraction of GRBs (Liang et al.2004, Firmani et al. 2008, Frontera et al. 2009, Ghirlanda et al. 2009): robust evidence for a physical origin and clues to explanation Frontera et al. 2012 (ApJ,subm.) Liang et al., ApJ, 2004

37. selection effects in the process leading to the redshift estimate are also likely to play a relevant role (e.g., Coward 2008) • Swift: reduction of selection effects in redshift -> Swift GRBs expected to provide a robust test of the Ep,i – Eiso correlation

38. Ep,i of Swift GRBs measured by Konus-WIND, Suzaku/WAM, Fermi/GBM and BAT (only when Ep inside or close to 15-150 keV and values provided by the Swift/BAT team (GCNs or Sakamoto et al. 2008):Swift GRBs are consistent with the Ep,i – Eiso correlation Red points = Swift GRBs Slope ~ 0.5 s (logEp,i) ~ 0.2 Gaussian distribution of data scatter

39. definite evidence that short GRBs DO NOT follow the Ep.i – Eiso correlation: a tool to distinguish between short and long events and to get clues on their different nature (e.g., Amati 2006, Piranomonte et al. 2008, Ghirlanda et al. 2009)

40. the only long GRB outlier to the correlation is the peculiar GRB980425 (very low redshift z = 0.0085, sub-energetic, inconsistent with most other GRB properties) GRB 980425

41. 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

42. Method (e.g., Ghirlanda et al, Firmani et al., Dai et al., Zhang et al.): Ep,i = Ep,obsx (1 + z) , tb,i = tb / /1 + z) Dl = Dl (z , H0 , WM , WL ,…) • fit the correlation and construct an Hubble diagram for each set of cosmological parameters -> derive c.l. contours based on chi-square

43. Calibrating the Ep,i – Eiso correlation with SN Ia • several authors (e.g., Kodama et al., 2008; Liang et al., 2008, Li et al. 2008, Tsutsui et al. 2009, Capozziello & Izzo 2010) calibrated the correlation at z < 1.7 by using the luminosity distance – redshift relation derived from SN Ia • The aim is to extend the SN Ia Hubble diagram up to redshift where the luminosity distance is more sensitive to dark energy properties and evolution • but with this method GRB are no more an independent cosmological probe

44. the quest for high-z GRB • for both cosmological parameters and SFR evolution studies it is of fundamental importance to increase the detection rate of high-z GRBs • Swift recently changed the BAT trigger threshold to this purpouse • the detection rate can be increased by lowering the low energy bound of the GRB detector trigger energy band adapted from Salvaterra et al. 2008

45. 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) • in last years, Ep estimates for some Swift GRBs from Konus (from 15 keV to several MeV) and, to minor extent, RHESSI and SUZAKU NARROW BAND BROAD BAND

46. present and near future: main contribution expected from joint Fermi + Swift measurements • Up to 2009: ~290 Fermi/GBM GRBs, Ep estimates for ~90%, ~35 simultaneously detected by Swift (~13%), 13 with Ep and z estimates (~10% of Swift sample) • 2008 pre-Fermi : 61 Swift detections, 5 BAT Ep (8%), 15 BAT + KONUS + SUZAKU Ep estimates (25%), 20 redshift (33%), 11 with Ep and z estimates (~15% of Swift sample) • Fermi provides a dramatic increase in Ep estimates (as expected), but a only small fraction of Fermi GRBs is detected / localized by Swift (~15%) -> low number of Fermi GRBs with Ep and z (~5%). • It is difficult to improve this number: BAT FOV much narrower than Fermi/GBM; similar orbits, each satellite limited by Earth occultation but at different times, … ) • Summary: 15-20 GRB/year in the Ep,i – Eiso plane

47. In the > 2014 time frame a significant step forward expected from SVOM: • spectral study of prompt emission in 5-5000 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, high-z GRB • substantial increase of the number of GRB with known z and Ep -> test of correlations and calibration for their cosmological use