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Dark Gamma-Ray Bursts and their Host Galaxies

Dark Gamma-Ray Bursts and their Host Galaxies. Volnova Alina ( IKI RAS ) , Pozanenko Alexei ( IKI RAS ). Present GRB discovering. Swift ( launched on 20.11.2004) 3 telescopes : BAT ( 15 – 150 keV ), XRT ( 0.3 – 10 keV), UVOT ( 6 bands in the range 1800 - 6000 A).

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Dark Gamma-Ray Bursts and their Host Galaxies

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  1. Dark Gamma-Ray Bursts and their Host Galaxies Volnova Alina(IKI RAS), Pozanenko Alexei(IKI RAS)

  2. Present GRB discovering Swift (launchedon 20.11.2004) • 3 telescopes: BAT (15 – 150 keV), XRT(0.3 – 10 keV), UVOT (6 bands in the range 1800 - 6000 A). • Fast localization of the event in gamma (t ~ seconds, accuracy ~ 3'), X-ray (t ~ a few tens of seconds, accuracy < 8'')and in optics(t ~ a few tens of seconds to a few tens of minutes, accuracy< 1''). • Quick distribution of the information via GCN (co-ordination with ground-based observatory). GBM/Fermi (launched on 11.06.2008) • Operational range10 keV – 40 MeV. Main feature is a possibility of a registration of very high energy photons from GRBs. • Localization accuracy (statistical + systematical) 3 – 15 degrees.

  3. Dark bursts problem • GRB 970228: the discovering of the first optical afterglow (OA) (van Paradijs+ 1997)raised a question: are ALL GRBs accompanied by an OA? • Further observations showed that the discovery of an OA occurs only in 20-30% of cases(ex., Fynbo+ 2001 and Lazzati+ 2002); • With the beginning of the operation of Swift and many ground-based telescopes with fast reaction the number of dark bursts became among 20% (Cenko+ 2009, Greiner+ 2011) and~25-35% (Melandri+ 2012)of the total LGRBs number.

  4. What does the “dark burst” mean? If we assume the fireball model, where F ~ ν-β,than β depends onpand νc: (Sari, Piran, Narayan 1998)

  5. Jakobsson+ 2004 • βOX (T0+ = 11h) = lg (FX/FO) / lg (νO/νX) • 2 ≤ p ≤ 2.5 => • 0.5 ≤ βOX≤ 1.25 • νc > 1018 Hzνc< 1014 Hz • than dark GRBs haveβOX < 0,5 • van der Horst+ 2009 • assuming that both X-ray and optical components are produced by synchrotron radiation optical spectral indexβОshould be equal to βXor toβX – 0.5 and • βX – 0.5 < βОX < βX • for dark GRBsβОX < βX – 0.5

  6. Greiner+ 2011 Dark bursts have bright X-ray radiation and faint optical radiation.

  7. Possible nature of dark bursts:high redshift For z ≥ 4 optical radiation is effectively absorbed in Lyα-forest (~ 10-20% of the total number of dark bursts, Zheng+ 2009, Greiner+ 2011). E.g. GRB 080913 with z = 6.70 (Greiner+ 2009)

  8. AV (LoS) AV (host) Possible nature of dark bursts:absorption • the absorption in the medium of the host galaxy (bulk absorption). ~ 25% of dark GRBs haveAV > 0.8m, which @z ~ 2 givesAV > 3 (Perley+ 2009, Greiner+ 2011) • The absorption in the ISM on the line-of-sight to the burst source (e.g., GRB 051022 AV > 9m, GRB 070521 AV > 11m, Perley+ 2013).

  9. Possible nature of dark bursts:different mechanism? • The mechanism of an optical and X-ray afterglows may be different (e.g., Zhang+ 2006; GRB 100614 и GRB 100615, D’Elia & Stratta 2011; GRB 090529, Xin+ 2012).

  10. Comparison of dark and bright bursts. The source properties: prompt emission The distributions ofEiso ,Epeak and Liso do not differ significantly between optically dark and bright GRBs in case of the homogeneous selection (Melandri+ 2012)

  11. Comparison of dark and bright bursts. The source properties: LX and observed flux Dark GRBs have in general higher X-ray luminosity, higher observed X-ray flux and lower observed optical flux in case of the homogeneous selection (Melandri+ 2012)

  12. Comparison of dark and bright bursts.Surrounding medium: AV(LoS) &NH Zheng+ 2009 Covino+ 2013 37% of dark bursts haveAV(LoS)> 2 mag. (only 5-10% of optically bright bursts have AV(LoS)> 2 mag).NH of dark bursts is higher than that of optically bright burst approximately by an order.

  13. Host galaxies of dark GRBs • In general, blue galaxies(В – R = 0.3-0.7) with median brightnessM ~ -20m (Fruchter+ 2006), but red dusty starburst galaxies are not excluded (GRB 070521 Perley+ 2009); • In many cases the observations of the burst host galaxy is the only way to determine the distance to its source; • Currently, host galaxies have been found and studied at redshifts as high as 4.7 (GRB 100219A Thöne+ 2012). • The study of the host galaxies of dark GRBs helps to determine the nature of these events; • But when the OA is absent the observer may find more than one galaxy in the X-ray localization circle.

  14. Comparison of the hosts: color indexR - Ks Perley+ 2013, the host galaxies of dark GRBs appear red in comparison with those of optically bright bursts.

  15. Comparison of the hosts: AV (host) Perley+ 2013, the host galaxies of dark bursts have on average higher extinction<AV(host)> ~ 1m, and optically bright bursts prefer more transparent galaxies.

  16. AV (host) vs. AV (LoS) Dark GRBs occur more often in dusty galaxies with rather inhomogeneous distribution of absorbing medium (Perley+ 2013)

  17. Comparison of the hosts: SFR Chen et al. 2012, the host galaxies of dark bursts show much higher value of star-formation rate: forz = 1 – 2 <SFR> ~ 10 MO/yr, forz > 2 <SFR> ~ 60 MO/yr. (GRB 051008, Volnova+ 2013, in prep.; GRB 060306, GRB 060814, Perley+ 2013)

  18. Dark GRB 051008 • Only X-ray afterglow was discovered starting 30 min after the trigger, βOX < 0.02; • The host galaxy was discovered by Shajn telescope in Crimea (R = 23.9m); • The observations of the host galaxy were performed in 2006-2012inUBgVRIiZK’bands (+UVOT/Swift data) with the telescopes: Shajn (CrAO), АZТ-22(Maidanak), NOT (La Palma), Keck I, Gemini N (Mauna Kea); • The host is a Lyman-break starburst galaxy @ redshft zphot = 2.8 with MR = -21.0m, AV(host) ~ 0.49m, SFR = 180 – 200 MO/yr; • AV(LoS) > 6.3m, NH = 7.9 x 1022см-1, Eiso = 1.1 x 1054 erg, Eγ = 4.6 – 6.8 x 1050 erg, θjet ~ 2°; • The most probable nature of the burst darkness is a significant absorption in a dense medium surrounding the source of the burst.

  19. Summary • About 10 – 25% of GRBs are optically dark. • The sources of dark bursts do not show the difference in distribution of prompt properties (Eiso, Epeak, Liso), but dark bursts have on average higher X-ray luminosity and observed X-ray flux in contrast with lower optical observed flux. • Dark bursts perform on average higher values ofNH andAV (LoS) – ~40% of dark bursts haveAV (LoS) > 2m. • Host galaxies of dark GRBs have redder color indexes, higher SFR and bulk absorption in the host, ~ 30% of dark GRBsare located in the galaxies with more inhomogeneous distribution of absorbing medium. • In most cases the GRB is dark due to a significant absorption of the optical radiation in the medium of the host galaxy (bulk or local).

  20. Thank you for your attention!

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