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

2. 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). 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 range 10 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 on p and ν c : (Sari, Piran, Narayan 1998)

5.  Jakobsson+ 2004 β OX (T0+ = 11 h ) = lg (F X /F O ) / lg (ν O /ν X ) 2 ≤ p ≤ 2.5 => 0.5 ≤ β OX ≤ 1.25 ν c > 10 18 Hz ν c < 10 14 Hz than dark GRBs have β OX < 0,5  van der Horst+ 2009 assuming that both X-ray and optical components are produced by synchrotron radiation optical spectral index β О should be equal to β X or to β X – 0.5 and β X – 0.5 < β ОX < β X for dark GRBs β ОX < β X – 0.5

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

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

9. 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 ). Possible nature of dark bursts: different mechanism?

10. Comparison of dark and bright bursts. The source properties: prompt emission The distributions of E iso, E peak and L iso 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: L X 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. Comparison of dark and bright bursts. Surrounding medium: A V (LoS) & N H Covino+ 2013 Zheng+ 2009 37% of dark bursts have A V (LoS) > 2 mag. (only 5-10% of optically bright bursts have A V (LoS) > 2 mag). N H 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 brightness M ~ -20 m ( 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 index R - K s Perley+ 2013, the host galaxies of dark GRBs appear red in comparison with those of optically bright bursts.

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

16. A V (host) vs. A V (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: for z = 1 – 2 ~ 10 M O /yr, for z > 2 ~ 60 M O /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, β OX < 0.02;  The host galaxy was discovered by Shajn telescope in Crimea (R = 23.9 m );  The observations of the host galaxy were performed in 2006-2012 in UBgVRIiZK’ 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 z phot = 2.8 with M R = -21.0 m, A V(host) ~ 0.49 m, SFR = 180 – 200 M O /yr;  A V(LoS) > 6.3 m, N H = 7.9 x 10 22 см -1, E iso = 1.1 x 10 54 erg, E γ = 4.6 – 6.8 x 10 50 erg, θ jet ~ 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 (E iso, E peak, L iso ), 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 of N H and A V (LoS) – ~40% of dark bursts have A V (LoS) > 2 m. Host galaxies of dark GRBs have redder color indexes, higher SFR and bulk absorption in the host, ~ 30% of dark GRBs are 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!