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

Outline Darkness of Gamma-ray burst Physical origin of darkness Properties of Dark and Normal GRB/hosts Examples of Dark burst Conclusions

Current main GRB detectors Swift: good localization (1-8"), quick positional data distributed via GCN, a possibility to search optical GRB components Fermi: wider energy range, all-sky field of view, a possibility of a registration of very high energy photons from GRBs

Dark GRBs history 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.

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)

βOX (T0+ = 11h) = lg (FX/FO) / lg (νO/νX) Jakobsson+ 2004 βOX (T0+ = 11h) = 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 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

Dark bursts have bright X-ray radiation and faint optical radiation. 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 (LoS) the absorption in the medium of the host galaxy (bulk absorption). ~ 25% of dark GRBs have AV > 0.8m, which @ z ~ 2 gives AV > 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). AV (host)

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).

Comparison of dark and bright bursts Comparison of dark and bright bursts. The 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 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(LoS) & NH Zheng+ 2009 Covino+ 2013 37% of dark bursts have AV(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.

Host galaxies of dark GRBs In general, blue galaxies (В – R = 0.3-0.7) with median brightness M ~ -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.

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)> ~ 1m, and optically bright bursts prefer more transparent galaxies.

AV (host) vs. AV (LoS) 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 star-formation 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 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 = 24.1m); 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 zphot = 2.8 with MR = -22.5m, AV(host) ~ 0.3m, SFR = 60 – 70 MO/yr; AV(LoS) > 4m, NH = 7.9 x 1022 см-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. It’s a first time, that the GRB is found in a Lyman-break galaxy.

Summary 10 – 25% of Swift GRBs are optically dark. The sources of dark GRBs do not show the difference in distribution of main parameters (Eiso, Epeak, z), but dark bursts have on average a higher X-ray luminosity. Dark GRBs have on average higher values of NH and AV (LoS) – ~40% of dark bursts have AV (LoS) > 2m. 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).

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