-
[show abstract]
[hide abstract]
ABSTRACT: Motivated by the anomalous dust-to-metal ratios (DTM) derived in the
literature for gamma-ray burst (GRB) damped Ly-alpha absorbers (DLAs), we
measure these ratios using the dust-depletion pattern observed in UV/optical
afterglow spectra, associated with the interstellar medium (ISM) at the GRB
host-galaxy redshifts. Our sample consists of 20 GRB absorbers and a comparison
sample of 72 QSO-DLAs, overall at redshift 1.2 < z < 4.0 and down to Z = 0.002
Zsol metallicities. The DTM in QSO- and GRB-DLAs increases both with
metallicity and metal column density, spanning ~10--110% the Galactic value and
pointing to a non-universal dust-to-metal ratio. The low values of DTM indicate
that low metallicity systems have lower dust fractions than typical spiral
galaxies and perhaps that the dust in these systems is produced inefficiently,
i.e. by formation in asymptotic giant branch (AGB) winds and by grain growth in
the low-metallicity regime, with little contribution from supernovae (SNe). On
the other hand, some GRB- and QSO-DLAs show high DTM values out to z ~ 4,
requiring rapid dust-production, such as in SN ejecta, but also in AGB winds
and via grain growth for the highest metallicity systems. GRB-DLAs overall
follow the DTM properties of QSO-DLAs, GRBs probing up to higher column and
volume densities. For comparison, the DTM that we derive for the SMC and LMC
are ~82--100% and ~98% of the Galactic value, respectively. The literature DTM
value of the low-metallicity galaxy I Zw 18 (< 37%) is consistent with the DTM
distribution that we find. The dust extinction AV increases steeply with the
column density of iron in dust, N(Fe)dust, calculated from relative metal
abundances, confirming that dust extinction is mostly occurring in the host
galaxy ISM. Most GRB-DLAs display log N(Fe)dust > 14.7, above which several
QSO-DLAs reveal H2, making GRB-DLAs promising candidates for molecular
research.
05/2013;
-
T. Krühler,
C. Ledoux,
J. P. U. Fynbo, P. M. Vreeswijk,
S. Schmidl,
D. Malesani,
L. Christensen,
A. De Cia,
J. Hjorth,
P. Jakobsson, [......],
R. Sánchez-Ramírez,
P. Schady,
S. Schulze,
V. Sudilovsky,
N. R. Tanvir,
G. Tagliaferri,
D. J. Watson,
K. Wiersema,
R. A. M. J. Wijers,
D. Xu
[show abstract]
[hide abstract]
ABSTRACT: [Abridged] Molecular hydrogen H_2 is a key requirement for star-formation,
frequently observed along sightlines in the Galaxy and to the Magellanic
Clouds, but notoriously hard to detect directly beyond z ~ 0. In the DLAs
associated with long gamma-ray bursts (GRBs), which are tightly linked to
vigorous star-formation, H_2 has remained largely elusive, and has been
unequivocally detected only towards GRB 080607 and possibly towards GRB 060206.
Here, we present the discovery of H_2-rich gas, including the presence of
vibrationally-excited H_2^* in the optical spectrum of the afterglow of GRB
120815A at z=2.36 obtained with X-shooter at the VLT. The galactic environment
of GRB 120815A is characterized by a strong DLA with log(N(H)/cm^-2) = 21.95
+/- 0.10, prominent H_2 absorption in the Lyman-Werner bands (log(N(H_2)/cm^-2)
= 20.53 +/- 0.04) and thus a molecular gas fraction log f(H_2)=-1.14 +/- 0.10.
The distance d between the absorbing neutral gas and GRB 120815A is constrained
via photo-excitation modeling of fine-structure and meta-stable transitions of
FeII and NiII to d = 0.5 +/- 0.1 kpc. The DLA metallicity ([Zn/H] = -1.15 +/-
0.11), visual extinction (A_V < 0.15 mag) and dust depletion ([Zn/Fe] = 1.01
+/- 0.09) are intermediate between the values of well-studied, H_2-deficient
GRB-DLAs observed at high spectral resolution, and the approximately solar
metallicity, highly-obscured and H_2-rich GRB 080607 sightline. With respect to
N(H), metallicity, as well as dust-extinction and depletion, GRB 120815A is
fairly representative of the average properties of GRB-DLAs. This demonstrates
that molecular hydrogen is present in at least a fraction of the more typical
GRB-DLAs, and H_2 and H_2^* are probably more wide-spread among GRB-selected
systems than the few examples of previous detections would suggest.
04/2013;
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[show abstract]
[hide abstract]
ABSTRACT: We model the time-variable absorption of FeII, FeIII, SiII, CII and CrII
detected in UVES spectra of GRB 080310, with the afterglow radiation exciting
and ionizing the interstellar medium in the host galaxy at a redshift of
z=2.42743. To estimate the rest-frame afterglow brightness as a function of
time, we use a combination of the optical VRI photometry obtained by the
RAPTOR-T telescope array -- which are presented in this paper -- and Swift's
X-Ray Telescope observations. Excitation alone, which has been successfully
applied for a handful of other GRBs, fails to describe the observed
column-density evolution in the case of GRB 080310. Inclusion of ionization is
required to explain the column-density decrease of all observed FeII levels
(including the ground state 6D9/2) and increase of the FeIII 7S3 level. The
large population of ions in this latter level (up to 10% of all FeIII) can only
be explained through ionization of FeII, whereby a large fraction of the
ionized FeII ions -- we calculate 31% using the Flexible Atomic (FAC) and Cowan
codes -- initially populate the 7S3 level of FeIII rather than the ground
state. This channel for producing a significant FeIII 7S3 level population may
be relevant for other objects in which absorption lines from this level -- the
UV34 triplet -- are observed, such as BAL quasars and Eta Carinae. This
provides conclusive evidence for time-variable ionization in the circumburst
medium, which to date has not been convincingly detected. However, the best-fit
distance of the neutral absorbing cloud to the GRB is 200--400 pc, i.e. similar
to GRB-absorber distance estimates for GRBs without any evidence for
ionization. We find that the presence of time-varying ionization in GRB 080310
is likely due to a combination of the super-solar iron abundance ([Fe/H]=+0.2)
and the low HI column density (log N(HI)=18.7). [abridged]
09/2012;
-
[show abstract]
[hide abstract]
ABSTRACT: We analyse high-resolution near-UV and optical spectra of the afterglow of
GRB 080310, obtained with the Very Large Telescope Ultraviolet and Visual
Echelle Spectrograph (VLT/UVES), to investigate the circumburst environment and
the interstellar medium of the gamma-ray burst (GRB) host galaxy. The VLT
rapid-response mode (RRM) enabled the observations to start only 13 minutes
after the Swift trigger and a series of four exposures to be collected before
dawn. A low neutral-hydrogen column-density (log N (HI) = 18.7) is measured at
the host-galaxy redshift of z = 2.42743. At this redshift, we also detect a
large number of resonance ground-state absorption lines (e.g., CII, MgII, AlII,
SiII, CrII, CIV, SiIV), as well as time-varying absorption from the
fine-structure levels of FeII. Time-varying absorption from a highly excited
FeIII energy level (7S3), giving rise to the so-called UV34 line triplet, is
also detected, for the first time in a GRB afterglow. The CrII ground-state and
all observed FeII energy levels are found to depopulate with time, whilst the
FeIII 7S3 level is increasingly populated. This absorption-line variability is
clear evidence of ionization by the GRB, which is for the first time
conclusively observed in a GRB afterglow spectrum. We derive ionic column
densities at each epoch of observations by fitting absorption lines with a
four-component Voigt-profile model. We perform CLOUDY photo-ionization
modelling of the expected pre-burst ionic column densities, to estimate that,
before the onset of the burst, [C/H] = -1.3 \pm 0.2, [O/H] < -0.8, [Si/H] =
-1.2 \pm 0.2, [Cr/H] = +0.7 \pm 0.2, and [Fe/H] = +0.2 \pm 0.2 for the
integrated line profile, indicating strong overabundances of iron and chromium.
For one of the components, we observe even more extreme ratios of [Si/Fe] \leq
-1.47 and [C/Fe] \leq -1.74. [abridged]
07/2012;
-
S. Schulze,
J. P. U. Fynbo,
B. Milvang-Jensen,
A. Rossi,
P. Jakobsson,
C. Ledoux,
A. De Cia,
T. Kruehler,
A. Mehner,
G. Bjoernsson, [......],
D. A. Perley,
J. Hjorth,
A. J. Levan,
N. R. Tanvir,
S. Ellison,
P. Moller,
G. Worseck,
R. Chapman,
A. Dall'Aglio,
G. Letawe
[show abstract]
[hide abstract]
ABSTRACT: We present the first search for galaxy counterparts of intervening high-z
(2<z< 3.6) sub-DLAs and DLAs towards GRBs. Our final sample comprises of five
intervening sub-DLAs and DLAs in four GRB fields. To identify candidate galaxy
counterparts of the absorbers we use deep optical and near-infrared imaging,
and low-, mid- and high-resolution spectroscopy acquired with 6 to 10-m class
telescopes, the Hubble and the Spitzer space telescopes. Furthermore, we use
the spectroscopic information and spectral-energy-distribution fitting
techniques to study them in detail. Our main result is the detection and
spectroscopic confirmation of the galaxy counterpart of the intervening DLA at
z=3.096 in the field of GRB 070721B (z_GRB=3.6298) as proposed by other
authors. We also identify good candidates for the galaxy counterparts of the
two strong MgII absorbers at z=0.6915 and 1.4288 towards GRB 050820A
(z_GRB=2.615). The properties of the detected DLA galaxy are typical for
Lyman-break galaxies (LBGs) at similar redshifts; a young, highly starforming
galaxy that shows evidence for a galactic outflow. This supports the hypothesis
that a DLA can be the gaseous halo of an LBG. In addition, we report a redshift
coincidence of different objects associated with metal lines in the same field,
separated by 130-161 kpc. The high detection rate of three correlated
structures on a length scale as small as ~150 kpc in two pairs of lines of
sight is intriguing. The absorbers in each of these are most likely not part of
the same gravitationally bound structure. They more likely represent groups of
galaxies.
07/2012;
-
[show abstract]
[hide abstract]
ABSTRACT: We present 10 new gamma-ray burst (GRB) redshifts and another five redshift
limits based on host galaxy spectroscopy obtained as part of a large program
conducted at the Very Large Telescope (VLT). The redshifts span the range 0.345
< z < 2.54. Three of our measurements revise incorrect values from the
literature. The homogeneous host sample researched here consists of 69 hosts
that originally had a redshift completeness of 55% (with 38 out of 69 hosts
having redshifts considered secure). Our project, including VLT/X-shooter
observations reported elsewhere, increases this fraction to 77% (53/69), making
the survey the most comprehensive in terms of redshift completeness of any
sample to the full Swift depth, analyzed to date. We present the cumulative
redshift distribution and derive a conservative, yet small, associated
uncertainty. We constrain the fraction of Swift GRBs at high redshift to a
maximum of 14% (5%) for z > 6 (z > 7). The mean redshift of the host sample is
assessed to be > 2.2, with the 10 new redshifts reducing it significantly.
Using this more complete sample, we confirm previous findings that the GRB rate
at high redshift (z > 3) appears to be in excess of predictions based on
assumptions that it should follow conventional determinations of the star
formation history of the universe, combined with an estimate of its likely
metallicity dependence. This suggests that either star formation at high
redshifts has been significantly underestimated, for example due to a dominant
contribution from faint, undetected galaxies, or that GRB production is
enhanced in the conditions of early star formation, beyond that usually
ascribed to lower metallicity.
05/2012;
-
P ~M Vreeswijk,
C Ledoux,
A Smette,
S ~L Ellison,
A ~O Jaunsen,
M ~I Andersen,
A ~S Fruchter,
J ~P ~U Fynbo,
J Hjorth,
A Kaufer,
P Møller,
P Petitjean,
S Savaglio,
R ~A ~M ~J Wijers
åp. 08/2011; 532:C3.
-
A. De Cia,
R. L. C. Starling,
K. Wiersema,
A. J. van der Horst, P. M. Vreeswijk,
G. Björnsson,
A. de Ugarte Postigo,
P. Jakobsson,
A. J. Levan,
E. Rol,
S. Schulze,
N. R. Tanvir
[show abstract]
[hide abstract]
ABSTRACT: GRB 070125 is among the most energetic bursts detected and the most
extensively observed so far. Nevertheless, unresolved issues are still open in
the literature on the physics of the afterglow and on the GRB environment. In
particular, GRB 070125 was claimed to have exploded in a galactic halo
environment, based on the uniqueness of the optical spectrum and the
non-detection of an underlying host galaxy. In this work we collect all
publicly available data and address these issues by modelling the NIR-to-X-ray
spectral energy distribution (SED) and studying the high signal-to-noise
VLT/FORS afterglow spectrum in comparison with a larger sample of GRB
absorbers. The SED reveals a synchrotron cooling break in the UV, low
equivalent hydrogen column density and little reddening caused by a LMC- or
SMC-type extinction curve. From the weak MgII absorption at z=1.5477 in the
spectrum, we derived logN(MgII)=12.96+0.13-0.18 and upper limits on the ionic
column density of several metals. These suggest that the GRB absorber is most
likely an LLS with a 0.03Z\odot<Z<1.3Z\odot metallicity. The comparison with
other GRB absorbers places GRB070125 at the low end of the absorption line
equivalent width distribution, confirming that weak spectral features and
spectral-line poor absorbers are not so uncommon in afterglow spectra.
Moreover, we show that the effect of photo-ionization on the gas surrounding
the GRB, combined with a low N(HI) along a short segment of the line of sight
within the host galaxy, can explain the lack of spectral features in GRB070125.
Finally, the non-detection of an underlying galaxy is consistent with a faint
GRB host galaxy, well within the GRB host brightness distribution. Thus, the
possibility that GRB070125 is simply located in the outskirts of a gas-rich,
massive star-forming region inside its small and faint host galaxy seems more
likely than a gas-poor, halo environment origin.
07/2011;
-
A. De Cia,
P. Jakobsson,
G. Björnsson, P. M. Vreeswijk,
V. S. Dhillon,
T. R. Marsh,
R. Chapman,
J. P. U. Fynbo,
C. Ledoux,
S. P. Littlefair,
D. Malesani,
S. Schulze,
A. Smette,
T. Zafar,
E. H. Gudmundsson
[show abstract]
[hide abstract]
ABSTRACT: We present high time resolution (1.09 s) photometry of GRB 080210 obtained with ULTRASPEC mounted on the ESO/3.6-m telescope, starting 68.22 min after the burst and lasting for 26.45 min. The light curve is smooth on both short (down to 2.18 s) and long time scales, confirmed by a featureless power spectrum. On top of the fireball power-law decay, bumps and wiggles at different time scales can, in principle, be produced by density fluctuations in the circumburst medium, by substructures in the jet or by refreshed shocks. Comparing our constraints with variability limits derived from kinematic arguments, we exclude under-density fluctuations producing flux dips larger than 1 per cent with time scales Δt > 9.2 min (2 per cent on Δt > 2.3 min for many fluctuating regions). In addition, we study the VLT/FORS2 afterglow spectrum, the optical-to-X-ray spectral energy distribution (SED) and the time decay. The SED is best fitted with a broken power law with slopes βopt= 0.71 ± 0.01 and βX= 1.59 ± 0.07, in disagreement with the fireball model, suggesting a non-standard afterglow for GRB 080210. We find AV= 0.18 ± 0.03 mag optical extinction due to SMC-like dust and an excess X-ray absorption of log(NH/cm−2) = 21.58+0.18−0.26 assuming solar abundances. The spectral analysis reveals a damped Lyα absorber (logcm−2) = 21.90 ± 0.10) with a low metallicity ([X/H]=−1.21 ± 0.16), likely associated with the interstellar medium of the GRB host galaxy (z= 2.641).
Monthly Notices of the Royal Astronomical Society 04/2011; 412(4):2229 - 2240. · 4.90 Impact Factor
-
A. De Cia,
P. Jakobsson,
G. Björnsson, P. M. Vreeswijk,
V. S. Dhillon,
T. R. Marsh,
R. Chapman,
J. P. U. Fynbo,
C. Ledoux,
S. P. Littlefair,
D. Malesani,
S. Schulze,
A. Smette,
T. Zafar,
E. H. Gudmundsson
[show abstract]
[hide abstract]
ABSTRACT: We present high time resolution (1.09 s) photometry of GRB 080210 obtained
with ULTRASPEC mounted on the ESO/3.6-m telescope, starting 68.22 min after the
burst and lasting for 26.45 min. The light curve is smooth on both short (down
to 2.18 s) and long time scales, confirmed by a featureless power spectrum. On
top of the fireball power-law decay, bumps and wiggles at different time scales
can, in principle, be produced by density fluctuations in the circumburst
medium, substructures in the jet or by refreshed shocks. Comparing our
constraints with variability limits derived from kinematic arguments, we
exclude under-density fluctuations producing flux dips larger than 1 per cent
with time scales \Deltat > 9.2 min (2 per cent on \Deltat > 2.3 min for many
fluctuating regions). In addition, we study the afterglow VLT/FORS2 spectrum,
the optical-to-X-ray spectral energy distribution (SED) and the time decay. The
SED is best fit with a broken power law with slopes {\beta}opt = 0.71 \pm 0.01
and {\beta}X = 1.59 \pm 0.07, in disagreement with the fireball model,
suggesting a non-standard afterglow for GRB 080210. We find AV = 0.18 \pm 0.03
mag optical extinction due to SMC-like dust and an excess X-ray absorption of
log (NH/cm-2) = 21.58 +0.18 -0.26 assuming Solar abundances. The spectral
analysis reveals a damped Ly{\alpha} absorber (log (NH I /cm-2) = 21.90 \pm
0.10) with a low metallicity ([X/H] = -1.21 \pm 0.16), likely associated with
the interstellar medium of the GRB host galaxy (z = 2.641).
11/2010;
-
J. P. U. Fynbo,
P. Laursen,
C. Ledoux,
P. Møller,
A. K. Durgapal,
P. Goldoni,
B. Gullberg,
L. Kaper,
J. Maund,
P. Noterdaeme,
G. Östlin,
M. L. Strandet,
S. Toft, P. M. Vreeswijk,
T. Zafar
[show abstract]
[hide abstract]
ABSTRACT: We have initiated a survey using the newly commissioned X-shooter spectrograph to target candidate relatively metal-rich damped Lyα absorbers (DLAs). Our rationale is that high-metallicity DLAs due to the luminosity–metallicity relation likely will have the most luminous galaxy counterparts. In addition, the spectral coverage of X-shooter allows us to search for not only Lyα emission, but also rest-frame optical emission lines. We have chosen DLAs where the strongest rest-frame optical lines ([O ii], [O iii], Hβ and Hα) fall in the near-infrared atmospheric transmission bands. In this first paper resulting from the survey, we report on the discovery of the galaxy counterpart of the zabs= 2.354 DLA towards the z= 2.926 quasar Q 2222−0946. This DLA is amongst the most metal-rich z > 2 DLAs studied so far at comparable redshifts and there is evidence for substantial depletion of refractory elements on to dust grains. We measure metallicities from Zn ii, Si ii, Ni ii, Mn ii and Fe ii of −0.46 ± 0.07, − 0.51 ± 0.06, − 0.85 ± 0.06, − 1.23 ± 0.06 and −0.99 ± 0.06, respectively. The galaxy is detected in the Lyα, [O iii]λλ4959, 5007 and Hα emission lines at an impact parameter of about 0.8 arcsec (6 kpc at zabs= 2.354). Based on the Hα line, we infer a star formation rate of 10 M⊙ yr−1, which is a lower limit due to the possibility of slit loss. Compared to the recently determined Hα luminosity function for z= 2.2 galaxies, the DLA-galaxy counterpart has a luminosity of L∼ 0.1L*Hα. The emission-line ratios are 4.0 (Lyα/Hα) and 1.2 ([O iii]/Hα). In particular, the Lyα line shows clear evidence for resonant scattering effects, namely an asymmetric, redshifted (relative to the systemic redshift) component and a much weaker blueshifted component. The fact that the blueshifted component is relatively weak indicates the presence of a galactic wind.The properties of the galaxy counterpart of this DLA are consistent with the prediction that metal-rich DLAs are associated with the most luminous of the DLA-galaxy counterparts.
Monthly Notices of the Royal Astronomical Society 11/2010; 408(4):2128 - 2136. · 4.90 Impact Factor
-
V. D'Elia,
J. P. U. Fynbo,
S. Covino,
P. Goldoni,
P. Jakobsson,
F. Matteucci,
S. Piranomonte,
J. Sollerman,
C. C. Thoene,
S. D. Vergani, [......],
A. de Ugarte Postigo,
H. Flores,
J. Hjorth,
L. Kaper,
A. J. Levan,
D. Malesani,
B. Milvang-Jensen,
E. Pian,
G. Tagliaferri,
N. R. Tanvir
[show abstract]
[hide abstract]
ABSTRACT: The aim of this paper is to study the environment and intervening absorbers of the gamma-ray burst GRB 090926A through analysis of optical spectra of its afterglow. We analyze medium resolution spectroscopic observations (R=10 000, corresponding to 30 km/s, S/N=15 - 30 and wavelength range 3000-25000) of the optical afterglow of GRB 090926A, taken with X-shooter at the VLT ~ 22 hr after the GRB trigger. The spectrum shows that the ISM in the GRB host galaxy at z = 2.1071 is rich in absorption features, with two components contributing to the line profiles. In addition to the ground state lines, we detect C II, O I, Si II, Fe II and Ni II excited absorption features. No host galaxy emission lines, molecular absorption features nor diffuse interstellar bands are detected in the spectrum. The Hydrogen column density associated to GRB 090926A is log N_H/cm^{-2} = 21.60 +/- 0.07, and the metallicity of the host galaxy is in the range [X/H] =3.2X10^{-3}-1.2X10^{-2} with respect to the solar values, i.e., among the lowest values ever observed for a GRB host galaxy. A comparison with galactic chemical evolution models has suggested that the host of GRB090926A is likely to be a dwarf irregular galaxy. We put an upper limit to the Hydrogen molecular fraction of the host galaxy ISM, which is f < 7X10^{-7}. We derive information on the distance between the host absorbing gas and the site of the GRB explosion. The distance of component I is found to be 2.40 +/- 0.15 kpc, while component II is located far away from the GRB, possibly at ~ 5 kpc. These values are compatible with that found for other GRBs. Comment: 12 pages, 8 .ps figures, accepted for publication in A&A. Abstract reduced due to space constraints
07/2010;
-
J. P. U. Fynbo,
P. Laursen,
C. Ledoux,
A. K. Durgapal,
P. Møller,
P. Goldoni,
B. Gullberg,
L. Kaper,
J. Maund,
P. Noterdaeme,
G. Ostlin,
M. L. Strandet,
S. Toft, P. M. Vreeswijk,
T. Zafar
[show abstract]
[hide abstract]
ABSTRACT: We have initiated a survey using the newly commissioned X-shooter
spectrograph to target candidate relatively metal-rich damped Lyman-alpha
absorbers (DLAs). The spectral coverage of X-shooter allows us to search for
not only Lyman-alpha emission, but also rest-frame optical emission lines. We
have chosen DLAs where the strongest rest-frame optical lines ([OII], [OIII],
Hbeta and Halpha) fall in the NIR atmospheric transmission bands. In this first
paper resulting from the survey, we report on the discovery of the galaxy
counterpart of the z_abs = 2.354 DLA towards the z=2.926 quasar Q2222$-0946.
This DLA is amongst the most metal-rich z>2 DLAs studied so far at comparable
redshifts and there is evidence for substantial depletion of refractory
elements onto dust grains. We measure metallicities from ZnII, SiII, NiII, MnII
and FeII of -0.46+/-0.07, -0.51+/-0.06, -0.85+/-0.06, -1.23+/-0.06, and
-0.99+/-0.06, respectively. The galaxy is detected in the Lyman-alpha, [OIII]
lambda4959,5007 Halpha emission lines at an impact parameter of about 0.8
arcsec (6 kpc at z_abs = 2.354). We infer a star-formation rate of 10 M_sun
yr^-1, which is a lower limit due to the possibility of slit-loss. Compared to
the recently determined Halpha luminosity function for z=2.2 galaxies the
DLA-galaxy counterpart has a luminosity of L~0.1L^*_Halpha. The emission-line
ratios are 4.0 (Lyalpha/Halpha) and 1.2 ([OIII]/Halpha). The Lyalpha line shows
clear evidence for resonant scattering effects, namely an asymmetric,
redshifted (relative to the systemic redshift) component and a much weaker
blueshifted component. The fact that the blueshifted component is relatively
weak indicates the presence of a galactic wind. The properties of the galaxy
counterpart of this DLA is consistent with the prediction that metal-rich DLAs
are associated with the most luminous of the DLA-galaxy counterparts.
02/2010;
-
J ~P ~U Fynbo,
P Jakobsson,
J ~X Prochaska,
D Malesani,
C Ledoux,
A Ugarte Postigo,
M Nardini, P ~M Vreeswijk,
K Wiersema,
J Hjorth, [......],
E Rol,
P Schady,
R Starling,
N Tanvir,
D ~J Watson,
D Xu,
T Augusteijn,
F Grundahl,
J Telting,
P -O Quirion
[show abstract]
[hide abstract]
ABSTRACT: In this paper, Figure 14 is incomplete due to an error during production. We here provide the missing sub-figures. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile, under programs 275.D-5022, 075.D-0270, 077.D-0661, 077.D-0805, 078.D-0416, 079.D-0429, 080.D-0526, 081.A-0135, 281.D-5002, and 081.A-0856. Also based on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. Some of the data obtained herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.
apjs. 02/2010; 186:485-496.
-
J. P. U. Fynbo,
P. Jakobsson,
J. X. Prochaska,
D. Malesani,
C. Ledoux,
A. de Ugarte Postigo,
M. Nardini, P. M. Vreeswijk,
K. Wiersema,
J. Hjorth, [......],
E. Rol,
P. Schady,
R. L. C. Starling,
N. R. Tanvir,
D. J. Watson,
D. Xu,
T. Augusteijn,
F. Grundahl,
J. Telting,
and P.-O. Quirion
[show abstract]
[hide abstract]
ABSTRACT: We present a sample of 77 optical afterglows (OAs) of Swift detected gamma-ray bursts (GRBs) for which spectroscopic follow-up observations have been secured. Our first objective is to measure the redshifts of the bursts. For the majority (90%) of the afterglows, the redshifts have been determined from the spectra. We provide line lists and equivalent widths (EWs) for all detected lines redward of Lyα covered by the spectra. In addition to the GRB absorption systems, these lists include line strengths for a total of 33 intervening absorption systems. We discuss to what extent the current sample of Swift bursts with OA spectroscopy is a biased subsample of all Swift detected GRBs. For that purpose we define an X-ray-selected statistical sample of Swift bursts with optimal conditions for ground-based follow-up from the period 2005 March to 2008 September; 146 bursts fulfill our sample criteria. We derive the redshift distribution for the statistical (X-ray selected) sample and conclude that less than 18% of Swift bursts can be at z > 7. We compare the high-energy properties (e.g., γ-ray (15-350 keV) fluence and duration, X-ray flux, and excess absorption) for three subsamples of bursts in the statistical sample: (1) bursts with redshifts measured from OA spectroscopy; (2) bursts with detected optical and/or near-IR afterglow, but no afterglow-based redshift; and (3) bursts with no detection of the OA. The bursts in group (1) have slightly higher γ-ray fluences and higher X-ray fluxes and significantly less excess X-ray absorption than bursts in the other two groups. In addition, the fractions of dark bursts, defined as bursts with an optical to X-ray slope βOX < 0.5, is 14% in group (1), 38% in group (2), and >39% in group (3). For the full sample, the dark burst fraction is constrained to be in the range 25%-42%. From this we conclude that the sample of GRBs with OA spectroscopy is not representative for all Swift bursts, most likely due to a bias against the most dusty sight lines. This should be taken into account when determining, e.g., the redshift or metallicity distribution of GRBs and when using GRBs as a probe of star formation. Finally, we characterize GRB absorption systems as a class and compare them to QSO absorption systems, in particular the damped Lyα absorbers (DLAs). On average GRB absorbers are characterized by significantly stronger EWs for H I as well as for both low and high ionization metal lines than what is seen in intervening QSO absorbers. However, the distribution of line strengths is very broad and several GRB absorbers have lines with EWs well within the range spanned by QSO-DLAs. Based on the 33 z > 2 bursts in the sample, we place a 95% confidence upper limit of 7.5% on the mean escape fraction of ionizing photons from star-forming galaxies.
The Astrophysical Journal Supplement Series 11/2009; 185(2):526. · 13.46 Impact Factor
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J. P. U. Fynbo,
P. Jakobsson,
J. X. Prochaska,
D. Malesani,
C. Ledoux,
A. de Ugarte Postigo,
M Nardini, P. M. Vreeswijk,
K. Wiersema,
J. Hjorth, [......],
E. Rol,
P. Schady,
R. Starling,
N. Tanvir,
D. J. Watson,
D Xu,
T. Augusteijn,
F. Grundahl,
J. Telting,
P. -O. Quirion
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ABSTRACT: (Abridged). We present a sample of 77 optical afterglows (OAs) of Swift detected GRBs for which spectroscopic follow-up observations have been secured. We provide linelists and equivalent widths for all detected lines redward of Ly-alpha. We discuss to what extent the current sample of Swift bursts with OA spectroscopy is a biased subsample of all Swift detected GRBs. For that purpose we define an X-ray selected sample of Swift bursts with optimal conditions for ground-based follow up from the period March 2005 to September 2008; 146 bursts fulfill our sample criteria. We derive the redshift distribution for this sample and conclude that less than 19% of Swift bursts are at z>7. We compare the high energy properties for three sub-samples of bursts in the sample: i) bursts with redshifts measured from OA spectroscopy, ii) bursts with detected OA, but no OA-based redshift, and iii) bursts with no detection of the OA. The bursts in group i) have significantly less excess X-ray absorption than bursts in the other two groups. In addition, the fraction of dark bursts is 14% in group i), 38% in group ii) and > 39% in group iii). From this we conclude that the sample of GRBs with OA spectroscopy is not representative for all Swift bursts, most likely due to a bias against the most dusty sight-lines. Finally, we characterize GRB absorption systems as a class and compare them to QSO absorption systems, in particular DLAs. On average GRB absorbers are characterized by significantly stronger EWs for HI as well as for both low and high ionization metal lines than what is seen in intervening QSO absorbers. Based on the z>2 bursts in the sample we place a 95% confidence upper limit of 7.5% on the mean escape fraction of ionizing photons from star-forming galaxies. Comment: 175 pages, 14 figures. Accepted for publication in ApJS. A full resolution version of the paper can be found here: http://www.astro.ku.dk/~jfynbo/grb_lowres.pdf
07/2009;
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ABSTRACT: We aim to understand the nature of the absorbing neutral gas in the galaxies hosting high-redshift long-duration GRBs and to determine their physical conditions. We report the detection of a significant number of previously unidentified allowed transition lines of Fe+, involving the fine structure of the ground term and that of other excited levels, from the zabs=3.969, log N(H0)=22.10 DLA system located in the host galaxy of GRB 050730. The time-dependent evolution of the observed Fe+ energy-level populations is modelled by assuming the excitation mechanism is fluorescence following excitation by ultraviolet photons. This UV pumping model successfully reproduces the observations, yielding a burst/cloud distance (defined to the near-side of the cloud) of d=440\pm 30 pc and a linear cloud size of l=520{+240}{-190} pc. We discuss these results in the context of no detections of H2 and CI lines in a sample of seven z>1.8 GRB host galaxies observed with VLT/UVES. We show that the lack of H2 can be explained by the low metallicities, [X/H]<-1, low depletion factors and, at most, moderate particle densities of the systems. This points to a picture where GRB-DLAs typically exhibiting very high H0 column densities are diffuse metal-poor atomic clouds with high kinetic temperatures, Tkin>~1000 K, and large physical extents, l>~100 pc. The properties of GRB-DLAs observed at high spectral resolution towards bright GRB afterglows differ markedly from the high metal and dust contents of GRB-DLAs observed at lower resolution. This difference likely results from the effect of a bias, against systems of high metallicity and/or close to the GRB, due to dust obscuration in the magnitude-limited GRB afterglow samples observed with high-resolution spectrographs. Comment: 16 pages, 8 figures, A&A in press
07/2009;
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Á. Elíasdóttir,
J. P. U. Fynbo,
J. Hjorth,
C. Ledoux,
D. J. Watson,
A. C. Andersen,
D. Malesani, P. M. Vreeswijk,
J. X. Prochaska,
J. Sollerman,
and A. O. Jaunsen
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ABSTRACT: We report the clear detection of the 2175 Å dust absorption feature in the optical afterglow spectrum of the gamma-ray burst (GRB) GRB 070802 at a redshift of z = 2.45. This is the highest redshift for a detected 2175 Å dust bump to date, and it is the first clear detection of the 2175 Å bump in a GRB host galaxy, while several tens of optical afterglow spectra without the bump have been recorded in the past decade. The derived extinction curve gives AV = 0.8-1.5 depending on the assumed intrinsic slope. Of the three local extinction laws, a Large Magellanic Cloud (LMC) type extinction gives the best fit to the extinction curve of the host of GRB 070802. Besides the 2175 Å bump we find that the spectrum of GRB 070802 is characterized by unusually strong low-ionization metal lines and possibly a high metallicity for a GRB sightline ([Si/H] = –0.46 ± 0.38, [Zn/H] = –0.50 ± 0.68). In particular, the spectrum of GRB 070802 is unique for a GRB spectrum in that it shows clear C I absorption features, leading us to propose a correlation between the presence of the bump and C I. The gas-to-dust ratio for the host galaxy is found to be significantly lower than that of other GRB hosts with N(H I)/AV = (2.4 ± 1.0) × 1021 cm–2 mag–1, which lies between typical Milky Way and LMC values. Our results are in agreement with the tentative conclusion reached by Gordon et al. that the shape of the extinction curve, in particular the presence of the bump, is affected by the UV flux density in the environment of the dust.
The Astrophysical Journal 05/2009; 697(2):1725. · 6.02 Impact Factor
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D. Malesani,
J. P. U. Fynbo,
J. Hjorth,
G. Leloudas,
J. Sollerman,
M. D. Stritzinger, P. M. Vreeswijk,
D. J. Watson,
J. Gorosabel,
M. J. Michałowski,
C. C. Thöne
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ABSTRACT: SN 2008D was discovered following up a bright X‐ray transient that exploded in the nearby spiral galaxy NGC 2770 at redshift z = 0.007. We present early spectra (taken 1.75 days after the X‐ray transient) which first allowed the identification of this object as a supernova. The early light curve shows a blue, fading excess, due to the envelope cooling after the shock breakout. At later stages, the spectra developed He lines, making the supernova of type Ib. The properties of SN 2008D are different from those of SNe associated with gamma‐ray bursts.
AIP Conference Proceedings. 05/2009; 1111(1):627-628.
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J. Greiner,
T. Krühler,
J. P. U. Fynbo,
A. Rossi,
R. Schwarz,
S. Klose,
S. Savaglio,
N. R. Tanvir,
S. McBreen,
T. Totani, [......],
J. Hjorth,
E. Jehin,
A. J. Levan,
E. W. Liang,
D. Malesani,
T.-S. Pyo,
S. Schulze,
G. Szokoly,
K. Terada,
and K. Wiersema
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ABSTRACT: We report on the detection by Swift of GRB 080913, and subsequent optical/near-infrared follow-up observations by GROND, which led to the discovery of its optical/NIR afterglow and the recognition of its high-z nature via the detection of a spectral break between the i' and z' bands. Spectroscopy obtained at the ESO-VLT revealed a continuum extending down to λ = 9400 Å, and zero flux for 7500 Å<λ < 9400 Å, which we interpret as the onset of a Gunn-Peterson trough at z = 6.695± 0.025 (95.5% confidence level), making GRB 080913 the highest-redshift gamma-ray burst (GRB) to date, and more distant than the highest-redshift QSO. We note that many redshift indicators that are based on promptly available burst or afterglow properties have failed for GRB 080913. We report on our follow-up campaign and compare the properties of GRB 080913 with bursts at lower redshift. In particular, since the afterglow of this burst is fainter than typical for GRBs, we show that 2 m class telescopes can identify most high-redshift GRBs.
The Astrophysical Journal 03/2009; 693(2):1610. · 6.02 Impact Factor
