Dark GammaRay Bursts and their Host Galaxies Volnova

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) n 3 telescopes: BAT (15 – 150 ke. V), XRT (0. 3 – 10 ke. V), UVOT (6 bands in the range 1800 - 6000 A). n 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''). n Quick distribution of the information via GCN (co-ordination with ground-based observatory). GBM/Fermi (launched on 11. 06. 2008) n Operational range 10 ke. V – 40 Me. V. Main feature is a possibility of a registration of very high energy photons from GRBs. n Localization accuracy (statistical + systematical) 3 – 15 degrees.

Dark bursts problem n GRB 970228: the discovering of the first optical afterglow (OA) (van Paradijs+ 1997) raised a question: are ALL GRBs accompanied by an OA? n Further observations showed that the discovery of an OA occurs only in 20 -30% of cases (ex. , Fynbo+ 2001 and Lazzati+ 2002); n 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.

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

q Jakobsson+ 2004 βOX (T 0+ = 11 h) = lg (FX/FO) / lg (νO/νX) 2 ≤ p ≤ 2. 5 => 0. 5 ≤ βOX ≤ 1. 25 νc > 1018 Hz νc < 1014 Hz than dark GRBs have βOX < 0, 5 q van der Horst+ 2009 assuming that both X-ray and optical components are produced by synchrotron radiation optical spectral index βО should be equal to βX or to βX – 0. 5 and βX – 0. 5 < βОX < βX for dark GRBs βОX < βX – 0. 5

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

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)

Possible nature of dark bursts: absorption AV (Lo. S) ¨ the absorption in the medium of the host galaxy (bulk absorption). ~ 25% of dark GRBs have AV > 0. 8 m, which @ z ~ 2 gives AV > 3 (Perley+ 2009, Greiner+ 2011) AV (host) ¨ The absorption in the ISM on the line-of-sight to the burst source (e. g. , GRB 051022 AV > 9 m, GRB 070521 AV > 11 m, Perley+ 2013).

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

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

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)

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

Host galaxies of dark GRBs n n n 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 100219 A 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.

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

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

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

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

Dark GRB 051008 q Only X-ray afterglow was discovered starting 30 min after the trigger, βOX < 0. 02; q The host galaxy was discovered by Shajn telescope in Crimea (R = 23. 9 m); q The observations of the host galaxy were performed in 2006 -2012 in UBg. VRIi. ZK’ bands (+ UVOT/Swift data) with the telescopes: Shajn (Cr. AO), АZТ-22 (Maidanak), NOT (La Palma), Keck I, Gemini N (Mauna Kea); q The host is a Lyman-break starburst galaxy @ redshft zphot = 2. 8 with MR = -21. 0 m, AV(host) ~ 0. 49 m, SFR = 180 – 200 MO/yr; q AV(Lo. S) > 6. 3 m, NH = 7. 9 x 1022 см-1, Eiso = 1. 1 x 1054 erg, Eγ = 4. 6 – 6. 8 x 1050 erg, θjet ~ 2°; q The most probable nature of the burst darkness is a significant absorption in a dense medium surrounding the source of the burst.

Summary n About 10 – 25% of GRBs are optically dark. n 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. n Dark bursts perform on average higher values of NH and AV (Lo. S) – ~40% of dark bursts have AV (Lo. S) > 2 m. n 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. n 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).

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