Dark Gamma-Ray Bursts and their Host Galaxies
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Transcript of Dark Gamma-Ray Bursts and their Host Galaxies
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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)
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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.
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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.
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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)
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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
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Dark bursts have bright X-ray radiation and faint optical radiation.
Greiner+ 2011
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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)
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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)
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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?
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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)
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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)
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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.
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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.
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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.
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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.
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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)
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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)
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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.
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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).
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Thank you for your attention!Thank you for your attention!