Dark Gamma-Ray Dark Gamma-Ray Bursts and their Bursts and their Host GalaxiesHost Galaxies

Dark Gamma-Ray Dark Gamma-Ray Bursts and their Bursts and their Host GalaxiesHost Galaxies

Volnova Alina (IKI RAS),

Pozanenko Alexei (IKI RAS)

Present GRB discoveringPresent GRB discoveringPresent GRB discoveringPresent 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.

Dark bursts problemDark bursts problemDark bursts problemDark 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.

What does the “dark burst” mean?What does the “dark burst” mean?What does the “dark burst” mean?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)

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

Possible nature of dark bursts:Possible nature of dark bursts:high redshifthigh redshiftPossible nature of dark bursts:Possible nature of dark bursts:high redshifthigh 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)

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).

Possible nature of dark bursts:Possible nature of dark bursts:absorptionabsorptionPossible nature of dark bursts:Possible nature of dark bursts:absorptionabsorption

AV (host)

AV (LoS)

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:Possible nature of dark bursts:different mechanism?different mechanism?Possible nature of dark bursts:Possible nature of dark bursts:different mechanism?different mechanism?

Comparison of dark and bright bursts.Comparison of dark and bright bursts. The source propertiesThe source properties: : prompt emissionprompt emissionComparison of dark and bright bursts.Comparison of dark and bright bursts. The source propertiesThe source properties: : prompt emissionprompt 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 propertiesThe source properties: : LLX X and observed fluxand observed fluxComparison of dark and bright bursts.Comparison of dark and bright bursts. The source propertiesThe source properties: : LLX X and observed fluxand 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.Comparison of dark and bright bursts. Surrounding medium: AV(LoS) & NH

Comparison of dark and bright bursts.Comparison of dark and bright bursts. Surrounding medium: AV(LoS) & NH

Covino+ 2013 Zheng+ 2009

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 GRBsHost 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

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)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)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: SFRComparison 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 051008Dark GRB 051008Dark GRB 051008Dark 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 2006-2012 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 = -21.0m, AV(host)

~ 0.49m, SFR = 180 – 200 MO/yr;

AV(LoS) > 6.3m, 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.

SummarySummary 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 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).

Thank you for your attention!Thank you for your attention!