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

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

3. 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

4. Dark GRBs history
GRB : 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 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.

5. 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)

6. βOX (T0+ = 11h) = lg (FX/FO) / lg (νO/νX)
Jakobsson+ 2004
βOX (T0+ = 11h) = lg (FX/FO) / lg (νO/νX)
2 ≤ p ≤ =>
0.5 ≤ βOX ≤
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

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

8. 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 with z = 6.70 (Greiner+ 2009)

9. 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, 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 AV > 9m, GRB AV > 11m, Perley+ 2013).
AV (host)

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

11. 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)

12. 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)

13. 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.

14. Host galaxies of dark GRBs
In general, blue galaxies (В – R = ) with median brightness M ~ -20m (Fruchter+ 2006), but red dusty starburst galaxies are not excluded (GRB 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 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.

15. 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.

16. 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.

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

18. 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 , Volnova+ 2013, in prep.; GRB , GRB , Perley+ 2013)

19. Dark GRB
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 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 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.

20. 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).

21. Thank you for your attention!