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Luminosity-time correlations for GRBs afterglows

Luminosity-time correlations for GRBs afterglows. Maria Giovanna Dainotti Department of Astronomy, Stanford University, Stanford, California In collaboration with First part of the talk M. Ostrowski (Crakow Observatory, Krakow, Poland),

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Luminosity-time correlations for GRBs afterglows

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  1. Luminosity-time correlationsfor GRBs afterglows Maria Giovanna Dainotti Department of Astronomy, Stanford University, Stanford, California In collaboration with First part of the talk M. Ostrowski (Crakow Observatory, Krakow, Poland), S. Capozziello , V. F. Cardone (Naples University, Naples, Italy) and R. Willingale (Leicester University, Leicester, UK) And Second part of the talk Vahe’ Petrosian and Jack Singal (Stanford University, Stanford, California) Nikko, Japan, 15-03-2012

  2. GRBs as possible cosmological tools? Nikko, Japan, 15-03-2012

  3. However • Notwithstanding the variety of their different peculiarities, some common • features may be identified looking at their light curves. • A crucial breakthrough : the Swift satellite • rapid follow-up of the afterglows in several wavelengths with better coverage than previous missions • a more complex behavior of the lightcurves, different from the broken power-law assumed in the past (Obrien et al. 2006,Sakamoto et al. 2007) • A significant step forward in determining common features in the afterglow • X-ray afterglow lightcurves of the full sample of Swift GRBs shows that they may be fitted by the same analytical expression (Willingale et al. 2007) • This provides the unprecedented opportunity to look for universal features that would allow us to recognize if GRBs are standard candles. Nikko, Japan, 15-03-2012

  4. Therefore, studies of correlations between GRB observables are the attempts pursued in this direction E_iso-E_peak (Lloyd & Petrosian 1999, Amatiet al. (2002,2009)) E_gamma,-E_peak (Ghirlanda et al. 2004, 2006) L-E_peak (Schaefer 2003,Yonekotu et al. 2004) L-V (Fenimore & Ramirez Ruiz 2000, Riechart et al. 2001, Norris et al. 2000,S03) Lpeak- peak_width (Willingale et al. 2010) Nikko, Japan, 15-03-2012

  5. Definition of some physical quantities Eisoisotropic energy integrated over T90 in the prompt emission. Epeak the peak of the energy in vu*Fvu spectrum. T90time interval between the 5% of the total countsand the 95%. The 90% of the emission is associated to the duration of the event. T45time T45 is the time spanned by the brightest 45 % of the total counts above the background. Tptime at the end of the prompt emission fitted within the Willingale et al. 2007 model. Tatime at the end of the plateau phase All the quantities presented in the talk are rescaled for the rest frame Nikko, Japan, 15-03-2012

  6. Afterglow LuminosityTime correlation- comparison between the well fitted by the Willingale model light curvesvs the irregular ones Dainotti et al. MNRAS, 391, L 79D (2008) Dainotti et al. ApJL, 722, L 215 (2010) Nikko, Japan, 15-03-2012

  7. Phenomenological model with SWIFT lightcurves Willingale et al. 2007 Nikko, Japan, 15-03-2012

  8. D’Agostini method (D’Agostini 2005 ) errors measurements on both x and y L*x(Ta) vs T*a distribution for the sample of 62 long afterglows Nikko, Japan, 15-03-2012

  9. Data and methodology • Sample : 77 afterglows, 66 long, 11 from IC class detected by Swift from January 2005 up to March 2009, namely all the GRBs with good coverage of data that obey to the Willingale model with firm redshift. • Redshifts : from the Greiner's web page http://www.mpe.mpg.de/jcg/grb.html. • Redshift range 0.08 <z < 8.2 • Spectrum for each GRB was computed during the plateau For some GRBs in the sample the error bars are so large that determination of the observables (Lx, Ta ) is not reliable. Therefore, we study effects of excluding such cases from the analysis (for details see Dainotti et al. 2011, ApJ 730, 135D ). To studythe low error subsamples we use the respective logarithmic errors bars to formally definethe error energy parameter Nikko, Japan, 15-03-2012

  10. Is the tight correlation due to bias selection effects? If we had had a selection effect we would have observed the red points only for the higher value of fluxes. The green triangles are XRFs, red points are the low error bar GRBs Nikko, Japan, 15-03-2012

  11. Long GRBs: • for the sample of 62 GRBs out of 66 long the ρLT= -0.76 and the fit line values are • b = −1.06 ± 0.28andlog a = 51.06 ± 1.02 • for the well fitted GRB afterglows (red points) ρLT= -0.93 and the fit is b =-1.05 ± 0.20 and log a=51.39 ± 0.90 "Short" IC GRBs in the sample • the fit line: b= −1.72 ± 0.22 and log a= 52.57±1.04 • a formally computed correlation coefficient for these 8 points ρLT=- 0.66. We have conservatively decided to drop short GRBs from the analysis, to deal with a physically more homogeneous sample of long GRBs (including also XRFs). Nikko, Japan, 15-03-2012

  12. Remarks on Lx-Ta and its interpretations • LT correlationis highly significant even including the large error points, and it monotonously converges to a limiting value-0.93 for σ(E)<0.09 • The subsample of 8 small error GRBs has redshifts reaching the value z=2.75, the GRB with z=8.26 disappears after decreasing σ(E) below 0.25. Models that predict the Lx-Ta anti-correlation: • energy injetion model from a spinning-down magnetar at the center of the fireballDall’ Osso et al. (2010), Xu & Huang (2011), Bernardini et al. (2011) • Accretion model onto the central engine as the long term powerhouse for the X-ray flux Cannizzo & Gerhels (2009), Cannizzo et al. 2010 • Prior emission model for the X-ray plateau Yamazaki (2009) • and the phenomenological model by Ghisellini et al. (2009). Nikko, Japan, 15-03-2012

  13. Prompt – afterglow correlations Dainotti et al., MNRAS, 418,2202, 2011 A search for possible physical relations between the afterglow characteristic luminosity L*a ≡Lx(Ta) and the prompt emission quantities: 1.) the meanluminosity derived as <L*p>45=Eiso/T*45 2.)<L*p>90=Eiso/T*90 3.) <L*p>Tp=Eiso/T*p 4.) the isotropic energy Eiso Nikko, Japan, 15-03-2012

  14. L*a vs. <L*p>45for 62 long GRBs(the σ(E) ≤ 4 subsample). Nikko, Japan, 15-03-2012

  15. Correlation coefficients ρ for for the long GRB subsamples with the varying error parameter u (L*a, <L*p>45 ) - red (L*a, <L*p>90) - black (L*a, <L*p>Tp ) - green (L*a, Eiso ) - blue Nikko, Japan, 15-03-2012

  16. Conclusion I GRBs with well fitted afterglow light curves obey tight physical scalings, both in their afterglow properties and in the prompt-afterglow relations. We propose these GRBs as good candidates for the standard Gamma Ray Burst to be used both - in constructing the GRB physical models and • in cosmological applications • (Cardone, V.F., Capozziello, S. and Dainotti, M.G 2009, MNRAS, 400, 775C • Cardone, V.F., Dainotti, M.G., Capozziello, S., and Willingale, R.2010, MNRAS, 408, 1181C) Nikko, Japan, 15-03-2012

  17. Let’s go one step back • Are the tests performed so far enough? • Maybe not!!! • We need to answer the following question: • Is what we observe a truly representation of the events or there might be selection effect or biases? • Namely, the LT correlation is actually intrinsic to GRBs, or is only apparent and is induced by observational limitations? Nikko, Japan, 15-03-2012

  18. Update of the correlation with 100 GRBs ρ=-0.73 (improved from the 77 sample) a= 53.30b=-1.62±0.20 There is still compatibility in 1σ for the slope why is the correlation slope changing? Nikko, Japan, 15-03-2012

  19. Therefore, it is imperative to first determine the true correlations among the variables BEFORE • proceeding with any further application to cosmology • Or using the luminosity-time correlation as discriminant among theoretical models for the plateau emission Nikko, Japan, 15-03-2012

  20. Division in redshift bins of the sample of 77 GRBs(improved version of Dainotti et al. 2011, ApJ, 730, 135D ) Nikko, Japan, 15-03-2012

  21. Division in the same redshift bin of the updated sample 100 GRBs with firm redshft and plateau emission From a visual inspection it is hard to evaluate if there is redshift evolution or not. Therefore, we have applied the same test of Dainotti et al. 2011, ApJ, 730, 135D Namely we have checked that the slope of every redshift bin is consistent with every other, but it is not enough to answer definitely the question. Nikko, Japan, 15-03-2012

  22. But for a more profound and rigorous understanding we apply: The Efron & Petrosian method (EP) (ApJ, 399, 345,1992) • designed to obtain unbiased correlations, distributions, and evolution with redshift from a data set truncated due to observational biases. • corrects for instrumental threshold selection effect and redshift evolution • has been already successfully applied to GRBs (Lloyd,N., & Petrosian, V. ApJ, 1999) Nikko, Japan, 15-03-2012

  23. The technique • Investigation whether the variables of the distributions, L*Xand T*aare correlated with redshift or are statistically independent. • Namely, do we have luminosity and time evolution? • If yes, • how to remove the evolution? By defining new and independent variables! Nikko, Japan, 15-03-2012

  24. How the new variables are built? Nikko, Japan, 15-03-2012

  25. How to compute g(z) and f(z)? • The EP method deals with • data subsets that can be constructed to be independent of the truncation limit suffered by the entire sample. • This is done by creating 'associated sets', which include all objects that could have been observed given a certain limiting luminosity. • We have to determine the limiting luminosity for the sample Nikko, Japan, 15-03-2012

  26. Luminosity vs z Nikko, Japan, 15-03-2012

  27. A specialized version of the Kendall rank correlation coefficient, τ • a statistic tool used to measure the association between two measured quantities • takes into account the associated sets and not the whole sample • produces a single parameter whose value directly rejects or accepts the hypothesis of independence. • The values of kL and kT for which τ L,z= 0 and τ T,z= 0 are the ones that best fit the luminosity and time evolution respectively. Nikko, Japan, 15-03-2012

  28. τ L,z Nikko, Japan, 15-03-2012

  29. τ T,z Nikko, Japan, 15-03-2012

  30. The new approach: • Never applied in literature so far to find the true slope in two dimensions • We consider again the method of the associated set to find the slope of the uninvolved correlation: • The slope computed with this method is compatible with observational results in 1 σ1.6<b<2.5 Nikko, Japan, 15-03-2012

  31. Ta*= Ta’ + α LOG10(Lx’/Lo) Nikko, Japan, 15-03-2012

  32. Conclusion II • The correlation still exists!!!! • The Conclusion of part I are confirmed! • It can be useful as model discriminator Nikko, Japan, 15-03-2012

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