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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;
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[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;
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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
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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;
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S. Covino,
S. Campana,
M. L. Conciatore,
V. D'Elia,
E. Palazzi,
C. C. Thöne,
S. D. Vergani,
K. Wiersema,
M. Brusasca,
A. Cucchiara, [......],
P. Goldoni,
C. Henriksen,
K. D. Horne,
G. Malaspina,
E. J. A. Meurs,
E. Pian,
L. Stella,
G. Tagliaferri,
P. Ward,
F. M. Zerbi
[show abstract]
[hide abstract]
ABSTRACT: Context: Multiwavelength observations of gamma-ray burst prompt and afterglow emission are a key tool to disentangle the various possible emission processes and scenarios proposed to interpret the complex gamma-ray burst phenomenology. Aims: We collected a large dataset on GRB060908 in order to carry out a comprehensive analysis of the prompt emission as well as the early and late afterglow. Methods: Data from Swift-BAT, -XRT and -UVOT together with data from a number of different ground-based optical/NIR and millimeter telescopes allowed us to follow the afterglow evolution from about a minute from the high-energy event down to the host galaxy limit. We discuss the physical parameters required to model these emissions. Results: The prompt emission of GRB060908 was characterized by two main periods of activity, spaced by a few seconds of low intensity, with a tight correlation between activity and spectral hardness. Observations of the afterglow began less than one minute after the high-energy event, when it was already in a decaying phase, and it was characterized by a rather flat optical/NIR spectrum which can be interpreted as due to a hard energy-distribution of the emitting electrons. On the other hand, the X-ray spectrum of the afterglow could be fit by a rather soft electron distribution. Conclusions: GRB060908 is a good example of a gamma-ray burst with a rich multi-wavelength set of observations. The availability of this dataset, built thanks to the joint efforts of many different teams, allowed us to carry out stringent tests for various interpretative scenarios showing that a satisfactorily modeling of this event is challenging. In the future, similar efforts will enable us to obtain optical/NIR coverage comparable in quality and quantity to the X-ray data for more events, therefore opening new avenues to progress gamma-ray burst research. Comment: A&A, in press. 11 pages, 5 figures
07/2010;
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A. J. Levan,
J. P. Osborne,
N. R. Tanvir,
K. L. Page,
E. Rol,
B. Zhang,
M. R. Goad,
P. T. O'Brien,
R. S. Priddey,
D. Bersier,
D. N. Burrows, R. Chapman,
A. S. Fruchter,
P. Giommi,
N. Gehrels,
M. A. Hughes,
S. Pak,
C. Simpson,
G. Tagliaferri,
and E. Vardoulaki
[show abstract]
[hide abstract]
ABSTRACT: We present the discovery of XRF 050215B and its afterglow. The burst was detected by the Swift BAT during the check-out phase, and observations with the X-Ray Telescope began approximately 30 minutes after the burst. These observations found a faint, slowly fading X-ray afterglow near the center of the error box as reported by the BAT. Infrared data obtained at UKIRT after 10 hr also revealed a very faint K-band afterglow. The afterglow appears unusual since it is very faint, especially in the infrared, with K > 20 only 9 hr postburst. The X-ray and infrared light curves exhibit a slow, monotonic decay with α ~ 0.8 and no evidence for a steepening associated with the jet break to 10 days postburst. We discuss possible explanations for the faintness and slow decay in the context of present models for the production of X-ray flashes.
The Astrophysical Journal 12/2008; 648(2):1132. · 6.02 Impact Factor
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A. J. Levan,
P. Jakobsson,
C. Hurkett,
N. R. Tanvir,
J. Gorosabel,
P. Vreeswijk,
E. Rol, R. Chapman,
N. Gehrels,
P. T. O'Brien,
J. P. Osborne,
R. S. Priddey,
C. Kouveliotou,
R. Starling,
D. Vanden Berk,
K. Wiersema
[show abstract]
[hide abstract]
ABSTRACT: We investigate the origin of the GRB 060912A, which has observational properties that make its classification as either a long or short burst ambiguous. Short-duration gamma-ray bursts (SGRBs) are thought to have typically lower energies than long-duration bursts, can be found in galaxies with populations of all ages and are likely to originate from different progenitors to the long-duration bursts. However, it has become clear that duration alone is insufficient to make a distinction between the two populations in many cases, leading to a desire to find additional discriminators of burst type. GRB 060912A had a duration of 6 s and occurred only ∼10 arcsec from a bright, low-redshift (z= 0.0936) elliptical galaxy, suggesting that this may have been the host, which would favour it being a short burst. However, our deep optical imaging and spectroscopy of the location of GRB 060912A using the Very Large Telescope (VLT) shows that GRB 060912A more likely originates in a distant star-forming galaxy at z= 0.937, and is most likely a long burst. This demonstrates the risk in identifying bright, nearby galaxies as the hosts of given gamma-ray bursts (GRBs) without further supporting evidence. Further, it implies that, in the absence of secure identifications, ‘host’ type, or more broadly discriminators that rely on galaxy redshifts, may not be good indicators of the true nature of any given GRB.
Monthly Notices of the Royal Astronomical Society 07/2007; 378(4):1439 - 1446. · 4.90 Impact Factor
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A. J. Levan,
N. R. Tanvir,
P. Jakobsson, R. Chapman,
J. Hjorth,
R. S. Priddey,
J. P. U. Fynbo,
K. Hurley,
B. L. Jensen,
R. Johnson,
J. Gorosabel,
A. J. Castro-Tirado,
M. Jarvis,
D. Watson,
K. Wiersema
[show abstract]
[hide abstract]
ABSTRACT: We present deep optical and infrared observations of the short duration GRB 050906. Although no X-ray or optical/IR afterglow was discovered to deep limits, the error circle of the GRB (as derived from the Swift BAT) is unusual incontaining the relatively local starburst galaxy IC328. This makes GRB 050906 a candidate burst from a soft-gamma repeater, similar to the giant flare from SGR 1806-20. The probability of chance alignment of a given BAT position with such a galaxy is small (<1%), although the size of the error circle (2.6 arcminute radius) is such that a higher-z origin can't be ruled out. Indeed, the error circle also includes a moderately rich galaxy cluster at z=0.43, which is a plausible location for the burst given the apparent preference that short GRBs have for regions of high mass density. No residual optical or infrared emission has been observed, either in the form of an afterglow or later time emission from any associated supernova-like event. We discuss the constraints these limits place on the progenitor of GRB 050906 based on the expected optical signatures from both SGRs and merging compact object systems.
06/2007;
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[show abstract]
[hide abstract]
ABSTRACT: Recent progress on the nature of short duration Gamma Ray Bursts (GRBs) has shown that a fraction of them originate in the local universe. These systems may well be the result of giant flares from Soft Gamma Repeaters (SGRs) believed to be magnetars (neutron stars with extremely large magnetic fields >= 10^14 G). If these magnetars are formed via the core collapse of massive stars, then it would be expected that the bursts should originate from predominantly young stellar populations. However, correlating the positions of BATSE short bursts with structure in the local universe reveals a correlation with all galaxy types, including those with little or no ongoing star formation. This is a natural outcome if, in addition to magnetars forming via the core collapse of massive stars, they also form via Accretion Induced Collapse following the merger of two white dwarfs, one of which is magnetic. We investigate this possibility and find that the rate of magnetar production via WD-WD mergers in the Milky Way is comparable to the rate of production via core collapse. However, while the rate of magnetar production by core collapse is proportional to the star formation rate, the rate of production via WD-WD mergers (which have long lifetimes) is proportional to the stellar mass density, which is concentrated in early-type systems. Therefore magnetars produced via WD-WD mergers may produce SGR giant flares which can be identified with early type galaxies. We also comment on the possibility that this mechanism could produce a fraction of the observed short duration GRB population at low redshift.
11/2006;
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Páll Jakobsson,
Johan P. U. Fynbo,
C. Ledoux,
P. Vreeswijk,
D. A. Kann,
Jens Hjorth,
R. S. Priddey,
N. R. Tanvir,
D. Reichart,
Jorge Gorosabel, [......],
D. Watson,
J. Sollerman,
A. S. Fruchter,
A. de Ugarte Postigo,
K. Wiersema,
G. Bjornsson, R. Chapman,
C. C. Thone,
K. Pedersen,
Brian L. Jensen
[show abstract]
[hide abstract]
ABSTRACT: Context. Before the launch of the Swift satellite, the majority of the gamma-ray burst (GRB) afterglows for which Lya was redshifted into the observable spectrum showed evidence for a damped Lya absorber. This small sample indicated that GRBs explode either in galaxies, or regions within them, having high neutral hydrogen column densities. Aims. To increase the spectroscopic sample of GRBs with z > 2 and hence establish the N(H I) distribution along GRB lines-of-sight. Methods. We have obtained six z > 2 GRB afterglow spectra and fitted the Lya absorption line in each case to determine N(H I). This has been complemented with 12 other Swift N(H I) values from the literature. Results. We show that the peak of the GRB N(H I) distribution is qualitatively consistent with a model where GRBs originate in Galactic-like molecular clouds. However, a systematic difference, in particular an excess of low column-density systems compared to the predictions, indicates that selection effects and conditions within the cloud (e.g. strong ionization) influence the observed N(H I) range. We also report the discovery of Lya emission from the GRB 060714 host, corresponding to a star-formation rate of approximately 0.8 M-circle dot yr(-1). Finally, we present accurate redshifts of the six bursts: z = 3.240 +/- 0.001 (GRB 050319), z = 2.198 +/- 0.002 (GRB 050922C), z = 3.221 +/- 0.001 (GRB 060526), z = 3.425 +/- 0.002 (GRB 060707), z = 2.711 +/- 0.001 (GRB 060714) and z = 3.686 +/- 0.002 (GRB 060906).
10/2006;
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C. P. Hurkett,
J. P. Osborne,
K. L. Page,
E. Rol,
M. R. Goad,
P. T. O'Brien,
A. Beardmore,
O. Godet,
D. N. Burrows,
N. R. Tanvir, [......],
B. Zhang,
D. Malesani,
J. E. Hill,
J. A. Kennea, R. Chapman,
V. La Parola,
M. Perri,
P. Romano,
R. Smith,
N. Gehrels
[show abstract]
[hide abstract]
ABSTRACT: We report the discovery and subsequent multiwavelength afterglow behaviour of the high-redshift (z= 4.27) Gamma Ray Burst (GRB) 050505. This burst is the third most-distant burst, measured by spectroscopic redshift, discovered after GRB 000131 (z= 4.50) and GRB 050904 (z= 6.29). GRB 050505 is a long GRB with a multipeaked γ-ray light curve, with a duration of T90= 63 ± 2 s and an inferred isotropic release in γ-rays of ∼ 4.44 × 1053 erg in the 1–104 keV rest-frame energy range. The Swift X-Ray Telescope followed the afterglow for 14 d, detecting two breaks in the light curve at 7.4 +1.5−1.5 and 58.0 +9.9−15.4 ks after the burst trigger. The power-law decay slopes before, between and after these breaks were 0.25+0.16−0.17, 1.17+0.08−0.09 and 1.97+0.27−0.28, respectively. The light curve can also be fitted with a ‘smoothly broken’ power-law model with a break observed at ∼T+ 18.5 ks, with decay slopes of ∼0.4 and ∼1.8, before and after the break, respectively. The X-ray afterglow shows no spectral variation over the course of the Swift observations, being well fitted with a single power law of photon index ∼1.90. This behaviour is expected for the cessation of the continued energization of the interstellar medium shock, followed by a break caused by a jet, either uniform or structured. Neither break is consistent with a cooling break. The spectral energy distribution, indeed, shows the cooling frequency to be below the X-ray but above the optical frequencies. The optical–X-ray spectrum also shows that there is significant X-ray absorption in excess of that due to our Galaxy but very little optical–ultraviolet extinction, with E(B−V) ≈ 0.10 for a Small Magellanic Cloud like extinction curve.
Monthly Notices of the Royal Astronomical Society 05/2006; 368(3):1101 - 1109. · 4.90 Impact Factor
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J B Haislip,
M C Nysewander,
D E Reichart,
A Levan,
N Tanvir,
S B Cenko,
D B Fox,
P A Price,
A J Castro-Tirado,
J Gorosabel, [......],
M J Jarvis,
M Moles,
E Alfaro,
K M Ivarsen,
N D Kumar,
C E Mack,
C M Zdarowicz,
N Gehrels,
S Barthelmy,
D N Burrows
[show abstract]
[hide abstract]
ABSTRACT: Gamma-ray bursts (GRBs) and their afterglows are the most brilliant transient events in the Universe. Both the bursts themselves and their afterglows have been predicted to be visible out to redshifts of z approximately 20, and therefore to be powerful probes of the early Universe. The burst GRB 000131, at z = 4.50, was hitherto the most distant such event identified. Here we report the discovery of the bright near-infrared afterglow of GRB 050904 (ref. 4). From our measurements of the near-infrared afterglow, and our failure to detect the optical afterglow, we determine the photometric redshift of the burst to be z = 6.39 - 0.12 + 0.11 (refs 5-7). Subsequently, it was measured spectroscopically to be z = 6.29 +/- 0.01, in agreement with our photometric estimate. These results demonstrate that GRBs can be used to trace the star formation, metallicity, and reionization histories of the early Universe.
Nature 04/2006; 440(7081):181-3. · 36.28 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: Gamma-ray bursts (GRBs) divide into two classes: 'long', which typically have initial durations of T90 > 2 s, and 'short', with durations of T90 < 2 s (where T90 is the time to detect 90% of the observed fluence). Long bursts, which on average have softer gamma-ray spectra, are known to be associated with stellar core-collapse events-in some cases simultaneously producing powerful type Ic supernovae. In contrast, the origin of short bursts has remained mysterious until recently. A subsecond intense 'spike' of gamma-rays during a giant flare from the Galactic soft gamma-ray repeater, SGR 1806-20, reopened an old debate over whether some short GRBs could be similar events seen in galaxies out to approximately 70 Mpc (refs 6-10; redshift z approximately 0.016). Shortly after that, localizations of a few short GRBs (with optical afterglows detected in two cases) have shown an apparent association with a variety of host galaxies at moderate redshifts. Here we report a correlation between the locations of previously observed short bursts and the positions of galaxies in the local Universe, indicating that between 10 and 25 per cent of short GRBs originate at low redshifts (z < 0.025).
Nature 12/2005; 438(7070):991-3. · 36.28 Impact Factor
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J. Haislip,
M. Nysewander,
D. Reichart,
A. Levan,
N. Tanvir,
S. Cenko,
D. Fox,
P. Price,
A. Castro-Tirado,
J. Gorosabel, [......],
A. Gal-Yam,
M. Kasliwal,
R. Hudec,
S. Vitek,
P. Kubanek,
J. Crain,
A. Foster,
M. Bayliss,
J. Clemens,
J. Bartelme
[show abstract]
[hide abstract]
ABSTRACT: In 2000, Lamb and Reichart predicted that gamma-ray bursts (GRBs) and their afterglows occur in sufficient numbers and at sufficient brightnesses at very high redshifts (z > 5) to eventually replace quasars as the preferred probe of element formation and reionization in the early universe and to be used to characterize the star-formation history of the early universe, perhaps back to when the first stars formed. Here we report the discovery of the afterglow of GRB 050904 and the identification of GRB 050904 as the first very high redshift GRB. We measure its redshift to be 6.39(+0.11,-0.12), which is consistent with the reported spectroscopic redshift (6.29 +/- 0.01). Furthermore, just redward of Ly-alpha the flux is suppressed by a factor of three on the first night, but returns to expected levels by the fourth night. We propose that this is due to absorption by molecular hydrogen that was excited to rovibrational states by the GRB's prompt emission, but was then overtaken by the jet. Now that very high redshift GRBs have been shown to exist, and at least in this case the afterglow was very bright, observing programs that are designed to capitalize on this science will likely drive a new era of study of the early universe, using GRBs as probes. Comment: Submitted to Nature, 11 pages, 3 figures, LaTeX
09/2005;
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