Download Dark bursts - indico in2p3

Optically Dark
Gamma-Ray Bursts
Alina Volnova, Alexei Pozanenko
Space Research Institute (IKI)
Moscow, Russia
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 is a Dark GRB?
Greiner+ 2011
Dark bursts have bright X-ray radiation and faint optical radiation.
Possible nature of dark bursts:
high redshift
At 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)
AV (host)
 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:
different mechanism?

The mechanism of an optical and X-ray
afterglows may be different
(e.g., GRB 100614 и GRB 100615, D’Elia & Stratta
2011; GRB 090529, Xin+ 2012).
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.
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.
The source properties: X-ray plateau

Bright GRBs – 37% of events with X-ray plateau;

Dark GRBs – ~ 67% of events with X-ray plateau;
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 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 and find exact astrometric
position;

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;
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:
A V (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.
Comparison of the hosts:
SFR
Chen et al. 2012, the host galaxies of dark bursts show much higher value of starformation rate: at z = 1 – 2 <SFR> ~ 10 MO/yr, for z > 2 <SFR> ~ 60 MO/yr. (GRB
051008, Volnova+ 2014; GRB 060306, GRB 060814, Perley+ 2013)
Radio observations




In some cases dark GRBs may have a bright radio afterglow, which
is related to a dense circumburst medium (GRB 110709B, AVhost > 5.3m;
GRB 111215A, AVhost > 8.5m, Zauderer+ 2013). The combination of radio
and X-ray data allows to robustly determine the required extinction,
instead of simply assuming an optical to X-ray spectral index.
Millimeter observations of afterglows with JVLA, ALMA, PdB IRAM
etc. allow to determine subarcsecond positions of the GRB and may
help to find a robust association with its host galaxy.
Dark GRBs mostly prefer the host galaxies without radio
counterpart. It means that those galaxies do not have starformation
regions hidden by dense dust shells.
In few cases host galaxies of dark GRBs have a bright radio
counterpart, and SFRradio ~ 102 – 103 MSUN/yr >> SFRUV (1 host
galaxy from 15, e.g., GRB 051022, Perley+ 2013).
Dark GRB 051008

Only X-ray afterglow was discovered without
any OA starting 30 min after the trigger;

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
@ redshift zphot = 2.8 with MR = -21.0m, AV(host)
~ 0.49m, SFR = 60 – 70 MO/yr;

AV(LOS) > 2m, NH = 7.9 x 1022 cm-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 one of the first time, that the
GRB is found in a Lyman-break galaxy.
Volnova et al., 2014, http://arxiv.org/abs/1405.4139
Summary

20 – 35% of Swift long GRBs are optically dark.

~ 10 – 20% of all dark bursts are dark due to a high redshift (z ≥ 4).

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.

Dark GRBs have on average higher values of NH and AV (LoS) – ~40% of
dark bursts have AV (LoS) > 2m. 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).

Dark GRBs do not represent a distinct group among
long gamma-ray bursts.
Thank you for your attention!