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X-ray absorption and Dark Bursts population of BAT6 sample

X-ray absorption and Dark Bursts population of BAT6 sample. Andrea Melandri INAF-Osservatorio Astronomico di Brera. S. Campana (OAB) S. Covino (OAB) P. D’Avanzo (OAB) D. Fugazza (OAB). G. Ghirlanda (OAB) G. Ghisellini (OAB) L. Nava (APC) R. Salvaterra (IASF-MI).

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X-ray absorption and Dark Bursts population of BAT6 sample

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  1. X-ray absorption and Dark Bursts population of BAT6 sample Andrea Melandri INAF-Osservatorio Astronomico di Brera S. Campana (OAB) S. Covino (OAB) P. D’Avanzo (OAB) D. Fugazza (OAB) G. Ghirlanda (OAB) G. Ghisellini (OAB) L. Nava (APC) R. Salvaterra (IASF-MI) B. Sbarufatti (OAB) G. Tagliaferri (OAB) S.D. Vergani (OAB)

  2. BAT6 : a complete sample Swift/BAT [180/540] Jakobsson [132/248] Swift/BAT6 [55/58] BAT6: Jakobsson criteria + bright in the BAT band 6 times worst sensitivity with respect to BAT Salvaterra et al. 2012, ApJ

  3. complete sample: overview • highlights of some results obtained so far from our complete sample: • GRB luminosity function (Salvaterra+ 2012) • need of evolution? • prompt emission spectral correlations (D’Avanzo+ 2012) • complete vs incomplete samples • redshift evolution? • Correlations of γand X-rays rest frame properties (Nava+ 2012) • relative contribution of prompt and afterglow • radiative efficiency • distribution of the X-ray absorptions (Campana+ 2012) • complete vs incomplete samples • dark bursts (Melandri+ 2012) • redshift and luminosity distributions • nature Paolo D’Avanzo’s talk on Wed

  4. X-ray absorption

  5. the distribution of NH shows two under-populated regions incomplete sample under-populated <log(NH)>=21.9±0.1 Campana et al. 2010

  6. the region of low-z and high-NH is now populated (bias in the non-complete sample: difficult to get z of highly absorbed low-z GRB) complete sample <log(NH)>=21.7±0.5 consistent with incomplete sample results intervening systems Campana et al. 2012, MNRAS

  7. dark bursts

  8. dark bursts in the complete sample two operative definitions: 1. Jakobsson+ (2004) βox<0.5 at 11h (obs.) ~30% (<36%) 2. van der Horst+ (2009) βox<βx-0.5 at 11h ~20% (<26%) possible explanation: 1. dust 2. optically faint/X bright 3. high-z Melandri et al. 2012, MNRAS

  9. X-ray busted at early times: flares? plateau phase? Optical busted? darkness evolution early dark & late bright always bright early: 50% Jak (27% vdH) late: 30% Jak (20% vdH) always dark Melandri et al. 2012, MNRAS

  10. redshift and luminosity distributions KS~0.35 KS~0.06-0.12 KS~0.1 Melandri et al. 2011, MNRAS

  11. dark bursts show a higher NH with respect to no-dark (KS<4x10-5) X-ray absorption ☐: vdH dark Campana et al. 2012, MNRAS

  12. dark bursts show a higher NH with respect to no-dark (KS<4x10-5) X-ray absorption possible explanation: 1. dust 2. optically faint/X bright 3. high-z ☐: vdH dark Campana et al. 2012, MNRAS

  13. Conclusions • This complete sample (90%) of bright long Swift GRBs allows for the first time to study the GRB population in an unbiased way • distribution of the X-ray absorptions • the low-z/high-NH region is populated • the high-z/low-NH region is still under-populated: intervening systems? • dark bursts (20%-30%): • redshift and luminosity distributions are similar to no-dark GRBs • darkness evolution can help to select “real” DB • higher NH suggesting metal-rich (dust-rich) local environment

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