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Short GRBs: spectra & redshifts

Short GRBs: spectra & redshifts. G. Ghirlanda Osservatorio Astronomico Brera giancarlo.ghirlanda@brera.inaf.it. in collaboration with:.

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Short GRBs: spectra & redshifts

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  1. Short GRBs: spectra & redshifts G. Ghirlanda Osservatorio Astronomico Brera giancarlo.ghirlanda@brera.inaf.it in collaboration with: Ghisellini G. (OAB), Celotti A. (Sissa), Firmani C.(OAB), Lazzati D. (JILA), Guzzo L. (OAB), Magliocchetti M. (OAT), Mereghetti S. (IASF), Nardini M. (OAB), Nava L. (OAB), Tavecchio F. (OAB) Venezia – Swift 2006

  2. Q1: how do short GRB spectra look like? Short GRB spectra (“harder”) VS Long GRB spectra (“softer”) Q2: which is the distance scale to short GRBs? Spectroscopically measured redshifts VS Redshift inferred from statistical analysis

  3. Long and short • LONG GRBs: • Spectral shape complex (smoothly joined powerlaws…) & spectral diversity [e.g. Band et al. 1993] • Spectral evolution on ms timescales [e.g. Ford et al. 1995, Preece et al. 2000] • … AIMS: • Characterize short GRB spectra • Compare with long <2 sec softer harder Hurley et al. 1992; Kouveliotou et al. 1993; Tavani 1998; Cline et al.1999; Yi-ping Qui et al. 2001 …

  4. <HR32(short)>~6 <HR32(long)>~3.8 F(100-300keV)>3x10-7 erg/cm2 Spectral analysis: E~0.03-1.8MeV t~0.2 sec Model: CPL (Band, BKNPL) 28/36 with enough S/N Ph/s/cm2 Ghirlanda, Ghisellini, Celotti 2004

  5. Short vs Long: average spectra long long short short Log Epeak a Short <α>~-0.58+-0.10 Long <α>~-1.05+-0.14 K-S  P=0.04% Long bursts: Preece et al. 2000; GG, Celotti, Ghisellini 2002

  6. Short vs First 2 s of Long long long short short a Log Epeak

  7. 81/157 with enough S/N Ph/s/cm2

  8. Swift Hete2 81 28 a 81 28 Log Epeak [keV] Ghirlanda et al. 2005

  9. Short/Hard – Long/Soft paradigm (based on the HR) revisited: Short GRBs are harder because their low energy spectral component (α) is harder while their peak energy Epeak is similar wrt Long GRBs long short Short GRB spectra are similar to the first 2 sec of long GRB short A larger fraction (wrt to long) of short GRBs violate the OTSSM limit and a larger fraction of short bursts are consistent with Th-BB (or modified BBPL). long Post-Batse short GRBs (few) populate the soft tail of the spectral param distributions (Ep,α) GG et al. 2006

  10. Swift-BAT Batse Integral Hete-2 Derive the instrumental sensitivity as a function of the spectral hardness of the GRB … where the sensitivity is expressed as the minimum flux that a burst (with certain assumed spectral properties) should have to satisfy the simplest trigger condition: Sburst> n Sbkg Band et al. 2001, 2006

  11. Instrument sensitivity versus spectral properties

  12. Instrumental sensitivity (TRIGGER ALG. ONLY) + Short Batse GRBs

  13. Redshift distribution:spectroscopic measures 73 Long 29 Swift 15 Sax 17 Hete … 12 other Still few Short GRBs with measured redshifts to infer their N(z), … N(L), … 050925 050709 050509B 050724 051221 050813 040924 GG 2006 Swift 5/7 with redshift Short GRB

  14. Anisotropic distribution of short GRBs Short Long Large scale structure z<0.5 Magliocchetti et al. 2004

  15. Short vs Early type (<Sbc) Short Long RC3 galaxy (2300) z<0.007 Short correlate with local galaxies and better with local early type gal. ~10-20% of Batse Short GRBs originate within 100 Mpc Tanvir et al. 2005

  16. 050509B Bloom et al. 2006 050813 050709 Gladders et al. 2005 790613 (old!) Covino et al. 2005 050724 Berger et al. 2005 Gal-Yam et al. 2005 Short GRB hosts

  17. … let’s try with clusters … (previous evidences of cluster-GRB correlation e.g. Kolatt & Piran 1996, Struble & Rood 1997 and discussion by Hurley et al. 1999; Gorosabel et al 1997) 497 short GRBs 1540 long GRBs 448 REFLEX(Boeringher et al. 2004) 484 NORAS (391) (Boeringher et al. 2000) Z<0.459 • Short GRB-Cluster positive correlation signal • No corr signal with long • Short do not trace exactely clusters • No preference for early types Ghirlanda, Magliochetti, Ghisellini, Guzzo, 2006, MNRAS

  18. L*~1.5-2 X 10^44 erg/s Stronger correlation signal with sub-luminous cl. Stronger correlation signal with lower redshift cuts

  19. Statistical analysis of large BATSE GRB sample (autocorrelation, local galaxy-grb cross correlation, cluster-grb cross correlation) Z<0.1 Note that if Short GRBs are at z<0.1 Eiso~10^48 erg Short GRBs: distance scale Q2: which is the distance scale to short GRBs? Direct (spectroscopic) redshift measures of ~ 5(swift)+2(Hete) short bursts Z~0.1-1.0 • Possible solutions to be searched in any (or a combination of) • swift preference to detect larger z bursts (as also seen for the long GRB population, e.g. Berger et al. 2005) • Possible contamination of the S-grb pop by extra-gal SGR (Hurley et al. 2005, but Nakar et al. 2005, Lazzati et al. 2005) • complex N(L) (e.g. Guetta & Piran 2005, Nakar et al. 2005, Gal-Yam et al. 2005)

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