Issue |
A&A
Volume 517, July 2010
|
|
---|---|---|
Article Number | A61 | |
Number of page(s) | 9 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/200913966 | |
Published online | 06 August 2010 |
GRB 021004: Tomography of a
gamma-ray burst progenitor and its host galaxy![[*]](/icons/foot_motif.png)
A. J. Castro-Tirado1 - P. Møller2 - G. García-Segura3 - J. Gorosabel1 - E. Pérez1 - A. de Ugarte Postigo4 - E. Solano5 - D. Barrado17,5 - S. Klose6 - D. A. Kann6 - J. M. Castro Cerón7 - C. Kouveliotou8 - J. P. U. Fynbo9 - J. Hjorth9 - H. Pedersen9 - E. Pian2,10,11 - E. Rol12,13 - E. Palazzi14 - N. Masetti14 - N. R. Tanvir12 - P. M. Vreeswijk9 - M. I. Andersen9 - A. S. Fruchter15 - J. Greiner16 - R. A. M. J. Wijers13 - E. P. J. van den Heuvel13
1 - Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la
Astronomía s/n, 18.008 Granada, Spain
2 - European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748
Garching bei München, Germany
3 - Instituto de Astronomía, Universidad Nacional Autónoma de México,
Apdo. Postal 877, Ensenada 22800, Baja California, México
4 - INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807
Merate (LC), Italy
5 - Laboratorio de Astrofísica Estelar y Exoplanetas, Dpto.
Astrofísica, Centro de Astrobiología (CSIC/INTA), PO Box 78, 28691
Villanueva de la Cañada, Madrid, Spain
6 - Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778
Tautenburg, Germany
7 - European Space Agency (ESA), European Space Astronomy Centre
(ESAC), PO Box, Apdo. de correos 78, 28691 Villanueva de la Cañada,
Madrid, Spain
8 - NASA Marshall Space Flight Center, NSSTC, 320 Sparkman Drive,
Huntsville, Alabama 35805, USA
9 - Dark Cosmology Centre, Niels Bohr Institute, University of
Copenhagen, Juliane Maries Vej 30, 2100 København Ø, Denmark
10 - INAF - Osservatorio Astronomico di Trieste, via Tiepolo, 11, 34131
Trieste, Italy
11 - Scuola Normale Superiore di Pisa, Piazza dei Cavalieri 7, 56126
Pisa, Italy
12 - Department of Physics & Astronomy, University of
Leicester, University Road, Leicester LE1 7RH, UK
13 - Astronomical Institute ``Anton Pannekoek'', University of
Amsterdam, PO number 94249, 1090 GE, Amsterdam, The Netherlands
14 - INAF - IASF di Bologna, via Gobetti 101, 40129 Bologna, Italy
15 - Space Telescope Science Institute, 3700 San Martín Dr, Baltimore,
MD 21218-2463, USA
16 - Max-Planck-Institut für extraterrestrische Physik, 85748 Garching,
Germany
17 - Calar Alto Observatory, Centro Astronómico Hispano Alemán, C.
Jesús Durbán Remón, 2-2, 04004 Almería, Spain
Received 24 December 2009 / Accepted 26
March 2010
Abstract
Aims. We analyse the distribution of matter around
the progenitor star of gamma-ray burst GRB 021004 and the properties of
its host galaxy with high-resolution echelle and near-infrared
spectroscopy.
Methods. Observations were taken by the
8.2 m Very Large Telescope with the Ultraviolet and Visual
Echelle spectrograph (UVES) and the Infrared Spectrometer And Array
Camera (ISAAC) between 10 and 14 h after the onset of the
event.
Results. We report the first detection of emission
lines from a GRB host galaxy in the near-infrared, detecting H
and the [O III] doublet. These allow us to independently
measure the systemic redshift (
), which is not contaminated
by absorption as the Ly
line is, and infer the host galaxy properties. From the visual echelle
spectroscopy, we find several absorption-line groups spanning a range
of about 3000 km s-1 in
velocity relative to the redshift of the host galaxy. The absorption
profiles are very complex with both velocity-broadened components
extending over several 100 km s-1
and narrow lines with velocity widths of only
20 km s-1.
By analogy with QSO absorption line studies, the relative velocities,
widths, and degrees of ionization of the lines (``line-locking'',
``ionization-velocity correlation'') show that the progenitor had both
an extremely strong radiation field and several distinct mass-loss
phases (winds).
Conclusions. These results are consistent with GRB
progenitors being massive stars, such as luminous blue variables (LBVs)
or Wolf-Rayet stars, providing a detailed picture of the spatial and
velocity structure of the GRB progenitor star at the time of explosion.
The host galaxy is a prolific star-forming galaxy with a SFR
of 40
yr-1.
Key words: gamma-ray bust: general - techniques: spectroscopic - stars: Wolf-Rayet - galaxies: starburst - cosmology: observations
1 Introduction
The afterglows of long-duration gamma-ray bursts (GRBs), which are linked to the explosions of massive stars (see Woosley & Bloom 2006, for a review), are the most luminous optical sources in the universe for short periods of time (Kann et al. 2007; Bloom et al. 2009). Low-resolution optical spectroscopy only was initially usable for determining the redshift and placed them at cosmological distances (Metzger et al. 1997). Deeper insight came with the first medium-resolution spectrum, obtained with Keck ESI, of the afterglow of GRB 000926 (Castro et al. 2003). The first true high-resolution echelle spectra were obtained for GRB 020813 (Fiore et al. 2005), but they were of low signal-to-noise ratio (herafter S/N). The first echelle spectra of high S/N were finally obtained for GRB 021004, the focus of this work (see also Fiore et al. 2005).
Spectroscopy allows deep insight into the environments of GRBs (Prochaska et al. 2006). Some highlights include the possible detection of a Galactic superwind in the host galaxy of GRB 030329 (Thöne et al. 2007), and variable absorption lines that result from direct UV pumping by the luminous GRB afterglow (D'Elia et al. 2009; Vreeswijk et al. 2007; Dessauges-Zavadsky et al. 2006). These detections have become possible for more afterglows due to the rapid localization capabilities of the Swift satellite (Gehrels et al. 2004) in combination with the rapid-response mode (RRM) now available for the Ultraviolet-Visual Echelle Spectrograph (UVES) at the Very Large Telescope (VLT) (e.g., D'Elia et al. 2009; Vreeswijk et al. 2007). The typical wavelength region has been increased to now extend from the ultraviolet into the K band near-infrared (nIR) by the second-generation instrument X-Shooter at VLT (D'Odorico et al. 2006; see de Ugarte Postigo et al. 2010, for a first result).
GRB 021004 was detected at 12:06:14 universal time (UT) on
2002,
October 4 with the gamma-ray instrument FREGATE, the wide-field X-ray
Monitor (WXM) and the soft X-ray camera (SXC) aboard the High-Energy
Transient Explorer (HETE-2) (Shirasaki
et al. 2002). GRB 021004,
was a moderately bright, long-duration (
T90=100
s) event with
fluences of erg cm-2
(7-30 keV) and
erg cm-2
(30-400 keV) (Barraud
et al. 2002).
The GRB was rapidly localized in-flight and the position was
reported
in less than a minute, allowing rapid ground-based follow-up that
identified the fading optical afterglow of GRB 021004
(Fox et al. 2003).
This prompted follow-up observations at many
observatories, including extensive long-term coverage at X-rays (Sako &
Harrison 2002a,b),
radio
(Berger
et al. 2002; Pooley 2002b; Frail &
Berger 2002; Pooley
2002a),
millimeter (de Ugarte Postigo
et al. 2005), near-IR (Fynbo et al. 2005; de Ugarte Postigo
et al. 2005),
and optical wavelengths, both ground-based
(Pandey
et al. 2003; Holland et al. 2003; de Ugarte Postigo
et al. 2005; Bersier et al. 2003; Mirabal
et al. 2003; Uemura et al. 2003; Fox et al.
2003; Kawabata
et al. 2004) and
space-based (Fynbo et al. 2005).
The isotropic energy release during the prompt emission of
this GRB
was modest, with log (Kann et al. 2010), and the
dust extinction for this somehow reddish afterglow
(
)
was also low as is typical of
many well-observed afterglows (Kann
et al. 2006). Despite the low amount of energy
that was promptly released, this is among the most luminous afterglows
ever detected
(Kann et al. 2006),
even compared to a much larger Swift-era
sample (Kann et al. 2010).
The multiwavelength temporal evolution of the
GRB 021004 afterglow can be explained by multiple energy injections
(Björnsson
et al. 2004; de Ugarte Postigo et al. 2005),
although other scenarios cannot be discarded
(Lazzati et al. 2002).
GRB 021004 remains one of the most well-observed
afterglows ever.
Early-time low and medium resolution spectroscopic
observations
allowed a redshift of z=2.33 to be determined on
the basis of
Ly-
absorption and emission lines (Chornock
& Filippenko 2002). Several
absorption systems with outflow velocities of a few 1000 km s-1were
also reported (Savaglio
et al. 2002; Salamanca et al. 2002) and
studied in detail by
Møller et al. (2002b),
Wang et al. (2003), Matheson et al. (2003), Schaefer et al. (2003), Mirabal et al. (2003), Starling et al. (2005), and
Lazzati et al. (2006).
Jakobsson et al. (2005)
also report the detection of the host galaxy Ly
line
in narrow-band imaging.
Fiore et al. (2005)
constrained the ionization parameters
of the various absorption components detected above Ly
(at an
observer rest-frame of 4050 Å) in GRB 021004, interpreting them as
density
fluctuations, as Lazzati et al.
(2002) did. Within the context of larger samples, the UVES
spectra
of this GRB were also studied by Chen
et al. (2007) (the general lack of
wind signatures in high-resolution spectra of GRB afterglows), Prochaska et al. (2008)
(N V absorption lines toward GRB afterglows), Fox
et al. (2008) (high-ionisation line systems toward
GRB afterglows), Tejos
et al. (2009,2007), and Vergani
et al. (2009) (study of Mg II foreground absorption
systems).
The original expectation, as detailed high quality spectroscopy of GRB afterglows became possible, was that we would rapidly learn much about the GRB progenitors by studying the complex absorption line systems they were expected to exhibit due to ejection events leading up to the final collapse. This expectation has thus far not been met.
Despite the large number of long GRB afterglow spectra that have now been obtained, GRB 021004 continues to be one with the most complex set of intrinsic (0-3000 km s-1 ejection velocity) absorption systems. At the time, it also held the place as the lowest detected HI column density, and it still ranks between the lowest seven measured. Its complexity originally suggested that it represented a display of ejecta (Møller et al. 2002a; Mirabal et al. 2003; Fiore et al. 2005) but this interpretation was later disputed by Chen et al. (2007). A final interpretation of the complex systems has not yet been agreed on, and this object remains the most probable candidate to exhibit rarely detected signatures of events prior to the collapse. The rarity alone would warrant a more detailed discussion of the spectral features. In addition, we include UVES data below 4050 ang for a more detailed discussion of the absorption systems (Sect. 3.1) and near-IR VLT/ISAAC spectroscopy allowing a more accurate determination of host properties and redshift (Sect. 3.2). We discuss our results in the light of progenitor models for GRBs in Sect. 4, and summarize our work in Sect. 5.
For a Hubble constant of H0
= 72 km s-1 Mpc-1,
a matter
density ,
and a cosmological constant
,
the luminosity distance to the host is
Gpc,
and the look-back time is 10.42 Gyr. All errors are given at a
level of confidence for a parameter of interest unless stated
otherwise.
2 Observations and data reduction
2.1 Optical observations
Observations were conducted with 8.2-m Very Large Telescope Units 1 (VLT/UT1, Antu) and 2 (VLT/UT2, Kueyen) at the European Southern Observatory (ESO) in Cerro Paranal (Chile). We obtained optical spectroscopy of the GRB afterglow starting 0.6 days after the burst using the Ultraviolet-Visual Echelle Spectrograph (UVES) on UT2. Contemporaneous nIR observations were taken using the Infrared Spectrometer And Array Camera (ISAAC) on UT1 (see Table 1 for a log of the observations).
Table 1: Journal of the VLT GRB 021004 optical/nIR spectroscopic observations.
Table 2: Individual high-velocity metal-lines systems in GRB 021004.
The optical data reduction was performed using the UVES
context
running under MIDAS.
The UVES context is structured into reduction recipes allowing for
data reduction in a semi-automatic manner.
A master bias frame was used for the bias subtraction. The order
tracing was
performed taking advantage of a first guess calibration solution based
on the
UVES physical model. In all cases, except for the red spectrum centered
on
8600 Å, which is affected by fringing effects, the flat field
correction
was applied after the order extraction. A Th-Ar lamp was used for
wavelength
calibration. The spectra were extracted using an optimal
extraction method, which provides the object signal and the variance.
After the
extraction, the spectra were resampled to constant wavelength bins and
the
orders merged into a single spectrum. Finally, both the standard
heliocentric
and vacuum wavelength corrections were applied.
2.2 Near-infrared observations
For the nIR observations, we used the short wavelength mode in low
resolution, which yields a spectral coverage of 1.42-1.83 m and
1.84-2.56
m
in the H and K bands,
respectively. The pixel scale of the ESO-Hawaii detector is 0
146/pixel.
For each band, 50 individual exposures of 60 s each were
taken, shifting the target along the slit every 5 exposures in H
(3 in K) to be able to remove the sky background, a
standard technique in infrared astronomy. We also collected dark and
flat field images, which were used in the reduction process performed
with IRAF
. After combining the
groups of exposures and extracting the individual spectra, we
calibrated them using the air-glow OH emission lines present in the 2
dimensional images. This allows an accurate wavelength calibration with
dispersions of 4 and 8 Å/pix for H and K,
respectively. Finally, the individual spectra were median-combined into
a single one. In the case of the H band spectrum,
we used a G2 spectral type star to remove the telluric absorption
bands. The spectra were flux-calibrated by means of the JHK-band
observations that we obtained either side of each science spectral
observation.
Table 3: Line Identifications in the GRB 021004 OA spectra.
![]() |
Figure 1:
Overall view of GRB 021004 optical afterglow spectrum. These VLT/UVES
data were obtained |
Open with DEXTER |
3 Results
3.1 The blueshifted absorption-line systems
The VLT/UVES data set (Fig. 1) exhibits a
large number of absorption lines, as well as Ly
in emission
(see Table 3
for a list, and also Jakobsson
et al. 2005; Starling et al. 2005; Møller
et al. 2002b, on the Ly
emission line).
Some of the absorption lines are due to two foreground systems
at redshifts z=1.3820 and z=1.6020
(Mirabal
et al. 2003; Møller et al. 2002b; see Tejos et al.
2009,2007;
and Vergani et al. 2009,
for additional discussions about the foreground absorbers), but the
remaining lines are from
multiple systems found in the redshift range
.
In Fig. 2,
we show the three wide absorption-line complexes
in the redshift range .
The absorption profiles are plotted
in velocity space and marked C, D, and E
following the naming convention used by Møller
et al. (2002a).
The higher resolution of the UVES spectrum allowed us to identify
four narrow line systems embedded inside the velocity structure of the
broad systems C and E. We name those systems C1, C2, E1, and E2.
In the following, we discuss all of those systems in more detail in
order of increasing velocity
relative to the host redshift (z=2.3304, see
Sect. 3.2).
![]() |
Figure 2:
In this figure, we have converted the observed wavelengths into
velocities relative to the host redshift (z=2.3304)
for each ionic species significantly detected. Ly |
Open with DEXTER |
![]() |
Figure 3: A zoomed image of the high-resolution VLT/UVES spectrum at the location of the C1 and E1 narrow-line absorption systems. The data represent the coadded spectra of the different ions (N V + Si II + Al II + Al III) in velocity space. Thus, C1 has two subcomponents at 2803 km s-1 and 2837 km s-1 (low ionization, Si II + Al II, top) and 2810 km s-1 and 2860 km s-1 (high ionization, Si III + C IV, bottom). The ionization-velocity correlation also seen in QSOs (Møller et al. 1994) and WR nebulae (Smith et al. 1984) is noticeable. E1 divides equally into two subsystems at 114 km s-1 and 150 km s-1. |
Open with DEXTER |
E complex: The Ly
absorption width translates into velocities
in the range 40-470 km s-1,
while in C IV (where we observe the
narrowest trough), the range is 40-390 km s-1.
The difference
in the high-velocity cut-offs
(390 versus 470 km s-1) is
most easily understood
as a column density effect, i.e., the column density of the
high velocity end is too low for C IV to be detectable. Alternatively,
it could be caused by lower metallicity in the higher
velocity gas of system E, or a different ionization.
The simplest interpretation of the E complex is that it is caused by
gas,
spanning velocities from 40-470 km s-1
and densities decreasing from
the slowest to the fastest moving part.
Similar profiles have been observed in QSO broad absorption line (BAL)
systems, where velocities up to
have been found. BAL
systems are interpreted as velocity broadening due to ejection and/or
radiative acceleration. These BAL-like absorption troughs are not
meaningfully fitted by standard (Voigt) profiles that assume a
Gaussian velocity field.
Embedded in the E complex are two narrow-line
systems at 134 km s-1 (E1) and
262 km s-1 (E2). The strongest
component, E1,
is divided equally into two subcomponents at 114 km s-1
and
150 km s-1 (see Fig. 3)
and exhibits absorption from a large number of both high and low ionic
species
(N V, C IV, Fe II, Al II, Al III,
Si II, Si III, Si IV).
The metal line absorption of E1 has similarities to so-called damped
Ly
(DLA) systems, which in a few cases have been
identified with high redshift, star-forming galaxies (Møller et al. 2002b) but
it has too low HI column density along this sightline to be considered
a DLA.
Nevertheless, E1 is the system most likely to be identified with the
interstellar medium (ISM) of the host
galaxy of the GRB, whereby the velocity offset between the
host and the ISM (
134 km s-1)
represents the velocity
of the local absorbing cloud within the host. High and low ionic
species are also seen in the second system (E2), but at lower
significance.
The two rightmost dotted vertical lines in Fig. 2 mark the systems E1
and E2.
D Complex:
The D complex consists of a single, wide BAL-like component seen in Ly,
C IV, and
Si III. Both the C IV (1548 [0pt]Å) and the Ly
troughs
provide a consistent relative velocity range from
585 to 1005 km s-1. We find
residual flux at the bottom of both the Ly
and C IV
(1548 [0pt]Å)
troughs indicating that the gas is optically thin. We also see
significant variations in the optical thickness as a
function of relative velocity; in particular, one may note
the two sharp ``dips'' marking shells of higher column density at
the leading and the trailing edges. Those dips are seen in Ly
,
in both C IV lines and in Si III (marked by dotted lines in
Fig. 2). The Si III trough is optically thick between those
two edges, but then appears to have an optically thin extension towards
lower ejection velocities. However, this trough overlaps with Ly
of system C, and the extension is found to belong to system C (see
below).
It is remarkable that the D and E systems are separated in velocity by exactly the amount required to shift the C IV 1550 Å line complex right on top of the C IV 1548 Å one in the spectrum (Savaglio et al. 2002; Møller et al. 2002b). This shift is known as absorption-absorption ``line-locking'' and is not uncommon in QSO spectra where radiative acceleration is important (e.g., Srianand et al. 2002). The line-locking between the D and E systems strongly suggests that a process similar to that seen in intrinsic QSO absorption systems is indeed involved here, i.e., radiative acceleration.
C Complex:
As discussed above, the wide Ly
trough of system C
is partly caused by Si III absorption from system D, but also contains
Ly
absorption of a wide component spanning the velocity range
2730-3250 km s-1, because we
detect C II, C IV, Si II, Si III, Al II, and N V absorption in this
velocity range (Fig. 2).
The finely tuned velocity offset causing a precise overlap between
the Ly
(C complex) and Si III (D complex) lines could
be due to chance coincidence
(although with small probability, see below). Given the evidence of
line-locking between the two C IV lines between systems D and E,
the overlap is most
likely the result of radiative acceleration leading to line-locking
of two expanding shells corresponding to the C and D complexes.
Embedded in C, we identify two narrow systems at
3050 km s-1 (C2)
and at 2810 km s-1
(C1). C2 is a high ionisation system
that exhibits strong N V and C IV absorption, but no
absorption from singly
ionized ions. C1 is a low ionization system with moderate C IV
absorption
and clear detections of the singly ionized species Al II and
Si II,
which usually identifies a cloud optically thick at the Lyman limit
(see Table 3).
It is most likely that C2, although farthest away from the host
redshift in velocity
space, corresponds to the part of the wind that is nearest to the
progenitor
site in physical space. This ionization-velocity
correlation is also observed in QSOs: the high ionization
lines are often seen at the highest relative velocities for the
absorption systems in QSO spectra
(e.g., Møller et al. 1994).
As for E1, C1 consists of two subcomponents, detected at
2803 km s-1 and
2837 km s-1 (low ionization)
and at 2810 km s-1 and 2860 km
s-1 (high ionization). We again observe the
ionization-velocity
correlation seen already between C1 and C2 (Fig. 2).
In a similar way to Fiore
et al. (2005), we do consider C1 as part of the
ejected systems simply on the basis of the low probability that is
extrinsic, although we cannot exclude its association with the
neighbouring galaxy detected by HST (Chen et al. 2007). In that
case, it should be
realized that the comparatively low H I column density reported by
Fynbo et al. (2005)
would be the sum of the H I column of the two galaxies, such that they
both have even lower H I columns.
3.2 The near-IR emission lines
![]() |
Figure 4:
Bottom panels: a zoomed image of the GRB
021004 nIR afterglow spectrum. These VLT/ISAAC data were obtained |
Open with DEXTER |
Assuming a redshift of the host of z=2.3351
(Møller et al. 2002b,
based on
Ly
emission), we searched the ISAAC spectra for redshifted lines
of H I (H
,
H
)
and [O III]
(4959 Å/5007 Å). There are tentative detections at
low
significance of H
and the [O III] lines.
To confirm the reality of the lines, we used the method that we
introduced in Jensen et al.
(2001).
Our method briefly involves first rebinning the spectra into redshifts
for the rest wavelength of each expected line, in our case the lines
of H
and [O III] (4959 Å/5007 Å). The three
``redshift spectra'' were then coadded (with weights 2, 1, and 1
because
of the slightly higher S/N of the tentative H
detection). The
resulting combined line is clearly detected, as shown in Fig. 4.
To this combined line, we fitted a Gaussian (overplotted as a
blue dotted line)
and measured the systemic redshift to be ,
which
is significantly different from the values derived from the Ly
emission
line (which may be absorbed or modified by resonant
scattering) or from the absorption systems (which are affected by the
local velocity of the absorbing cloud in the host galaxy). The width
of the Gaussian was found to be exactly the spectral resolution, so the
lines are unresolved and we can place an upper limit on the intrinsic
line width of 600 km s-1 FWHM.
Using a redshift of 2.3304 and the width of the resolution
profile,
we then fitted Gaussian profiles to the data (the continuum level and
the amplitude of the Gaussian being the only free parameters) using
min-square
deviation. For the two [O III] lines, we also imposed a 1:3
flux ratio (Osterbrock 1989),
we found the following fluxes: flux = erg cm-2 s-1([O
III] 4959 Å),
erg cm-2 s-1([O
III] 5007 Å), and
erg cm-2
s-1 (H
). The H
flux is consistent with zero.
4 Discussion
Following the suggestion that there is often a link between
long-duration
GRBs and core-collapse supernovae (Stanek et al. 2003; Hjorth
et al. 2003) as proposed by Woosley
(1993)
we know that GRBs represent the end stages of massive-star evolution.
As we now describe, the results presented here provide
independent evidence of a massive star (40
)
progenitor
and additional insight into the immediate surroundings and prior
evolution of the star leading up to the catastrophic event.
4.1 A LBV / Wolf-Rayet progenitor
The probability that the three absorption systems C1, C2, and D lie in
front of the GRB host galaxy as a chance coincidence is very small and
can be computed using the number density of intervening C IV absorbers
at 1.8<z<3.5 of
per unit redshift (Sargent
et al. 1998).
We find that the probability of finding three of them within the
redshift range
spanned by C1, C2, and D is
.
Fiore et al. (2005)
used a similar argument but
added the column densities to obtain an even lower probablility.
Like Fiore et al. (2005),
we conclude that the absorbers must be intrinsic
to the host galaxy. The large blueshift then makes it unlikely that
the physical location can be anywhere else than close to the
GRB progenitor.
Furthermore, the probability that the C IV 1550 Å
line for
complex E falls on top
of the C IV 1548 Å line for complex D and that the Ly
line
for system C falls on top of the Si III line for system D by chance is
small,
of order of a
few percent for each, although difficult to quantify due to the complex
profiles.
Therefore, it makes sense to seek a physical explanation of the complex
structure of the C, D, and E systems.
The multiple absorption-line systems found in the spectrum of
GRB 021004 are naturally explained by multiple shell
structures
formed by the stellar winds of a massive progenitor star (Schaefer et al. 2003).
These line-driven winds are expected for very massive stars
(García Segura
et al. 1996; Castor et al. 1975) that
end their lives as
Wolf-Rayet stars (WR) (de Koter
et al. 1997), after passing through an unstable
luminous blue variable (LBV) phase (Langer
et al. 1999). Stellar evolution
calculations
for massive stars predict a number of different phases in which the
wind
properties associated with those phases, i.e., the velocities and
mass-loss rates, become strongly time-varying (Langer et al. 1999; Maeder 1983).
The minimum
initial mass for a star to become a LBV is (Langer
et al. 1994; Maeder & Meynet 2000).
During the course of their evolution, starting as O-class stars
(O
LBV
WR
SN),
stars have distinct winds
in which their velocities are proportional to the escape velocities.
Thus, for each transition where a fast wind (i.e., when the stellar
radius
was small)
follows a slow one (larger stellar radius), a shell of swept-up
material
is formed. Within this simple scheme, computations of the circumstellar
medium around a 60
star at the zero-age main sequence
(García Segura et al. 1996)
may produce the number of distinct compressed shells in the
kinematic range seen in Fig. 2.
![]() |
Figure 5: A sketch showing the absorption-line complexes C, D, and E along the line of sight to GRB 021004, not drawn to scale. Velocities are given in km s-1. Complex C, with the highest relative velocity, is most likely the closest to the progenitor site in space and is probably formed during the WR phase. The high-ionization sub-component C2 is the closest one to the progenitor whereas the colder sub-component C1 is further out and we cannot exclude that it could even be related to the neighbouring galaxy detected by HST (Chen et al. 2007). Complex E has the lowest relative velocity, and the broad part of the E complex is most likely caused by the oldest wind formed when the progenitor evolved from being an O star into the LBV phase or in the early LBV phase. The narrow E1 component has the properties expected for the ISM of the host galaxy. We believe that the D complex is caused by winds formed during the unstable LBV phase. |
Open with DEXTER |
During the LBV phase, when hydrogen shell burning occurs,
the stellar models are affected by several instabilities that cannot
yet be modelled self-consistently and thus the unstable LBV
phase cannot be fully resolved (Langer
et al. 1994). However, several LBV observations show
that
a number of different swept-up shells can be formed. In particular, the
D
complex discussed above is akin to the Homunculus nebula around Car,
which contains
and expands at a speed in the range
700-1000 km s-1 (Humphreys & Davidson 1994).
As the formation of each of these
swept-up shells occurs when the LBV reaches the Eddington limit
(Langer et al. 1999),
massive stars are able to form several compressed shells before
ending their lives.
We note that, since the LBV stellar wind is asymmetric and
often leads to a
bipolar or at least asymmetric nebula (Gruendl
et al. 2000), the GRB has to be beamed
in the same direction as the wind for us to see the shells in
absorption.
Similar, but less spectacular, complex absorption has been detected in
GRB 030226 (Klose et al. 2004;
but see Shin et al. 2006)
and GRB 050505
(Berger et al. 2006);
this effect is not seen, however, in several other
cases with similar quality data (Chen
et al. 2007). It is possible, therefore, that GRBs
for which no such complex velocity structure is seen have different
relative
orientations of the wind and GRB jet.
Highly ionized gas (
K) of e.g., C IV, N
V or Si IV, similar to
that found in the environment of GRB 021004,
has already been reported in a number of galactic ring nebulae
surrounding
Wolf-Rayet stars (Boronson
et al. 1997). To form these gas
shells, forward shocks moving at
km s-1,
or reverse shocks
at
-103 km s-1
are required.
The most natural explanation of the C complex is that it was formed
during the WR phase, with C1 and C2 lying in an expanding free wind,
before
reaching the shocked stellar wind. The
ionization-velocity correlation seen in the systems C1 and C2 has also
been detected in WR nebulae, but at lower velocities
(
100-150 km s-1;
Smith et al. 1984).
In addition to the detailed picture of the spatial and
velocity structure
of the GRB progenitor star at the time of explosion (Fig. 5), our
observations have also provided insight into the physical mechanism
producing the
progenitor shell structure. The C1 and C2 profiles are not
P-Cygni, but almost symmetric profiles, thus ruling out their being
very close to
the burst (within 0.2 pc)
and accelerated by the GRB ionizing
flux to the observed velocities, as has been proposed elsewhere
(Mirabal
et al. 2003; Schaefer et al. 2003). The
detected line-locking also favours radiative
acceleration by the progenitor star.
4.2 A starburst host galaxy
An extremely blue host galaxy was detected in late-time imaging (Jakobsson et al. 2005; Fynbo et al. 2005; de Ugarte Postigo et al. 2005). To derive the star-formation rate (SFR), we studied the emission lines seen in the combined optical-nIR spectrum.
The emission line parameters were measured by applying a
Gaussian fit to the
emission line and a flat fit to the continuum. If all the Hemission is
attributed to star formation in the host galaxy, we can
compute the SFR to be SFR (
yr-1) =
(erg/s)
(Kennicutt et al. 1994).
In our assumed cosmology,
the measured H
intensity transforms into a H
luminosity
erg s-1.
This implies a SFR (without any corrections) of
yr-1,
which is much higher than the present-day rate
in our Galaxy. From the total measured Ly
intensity
(Castro-Tirado et al. 2002),
we derive a Ly
luminosity
erg s-1,
which implies a
yr-1
in agreement with
Fynbo et al. (2005)
and Jakobsson et al. (2005).
The Ly
/H
ratio of 2 implies a low dust content in the GRB host galaxy, as also
inferred by de Ugarte Postigo
et al. (2005) and Kann
et al. (2006) from the afterglow SED; there is also
no excess absorption beyond the (low) Galactic value detected in Chandra
X-ray observations (Sako &
Harrison 2002a). This is a high value for the SFR (see also Castro-Tirado et al. 2007),
although the
S/N of the nIR spectrum implies a large
uncertainty. The derived SFR implies that most of the ongoing star
formation is unobscured, which is also in agreement with the SCUBA
results at 850
m
(Tanvir et al. 2004).
The Ly
photons produced in the ionized nebula experience a
continuous resonant scattering in the presence of HI. In a dust-free
nebula, this mechanism effectively traps the Ly
photons
producing a typical P Cygni line profile in both emission and
absorption,
depending on the details of the density and velocity field. However, if
even a little dust is present, most Ly
photons will disappear
by heating dust grains, because radiation absorption by dust has a
maximum cross-section close to the wavelength of Ly
.
Thus, the
observed complex Ly
profile is the result of both mechanisms,
resonance scattering which determines the profile shape, and dust
absorption,
which decreases yet further the intensity of the emission line. The
Ly
P Cygni profile can be understood in terms of these
mechanisms acting during the evolution of the swept-up HI supershell
produced by a massive starburst in the host galaxy of GRB 021004. It
corresponds to stage 4 in the evolutionary scheme developed by
Tenorio-Tagle et al. (1999)
and Más-Hesse et al. (2003),
a stage compatible with GRB 021004 being the evolution endpoint of a WR
star.
5 Conclusions
We have identified several absorption line groups spanning a range of
about 3000
km s-1 in velocity relative to the redshift of
the host galaxy.
The absorption profiles are very complex with both velocity-broadened
components extending over several 100 km s-1
and narrow lines
with velocity widths of only 20 km s-1.
By analogy with
QSO absorption line studies, the relative velocities, widths, and
degrees of ionization of the lines (``line-locking'',
``ionization-velocity correlation'') show that the progenitor had
both an extremely strong radiation field and several distinct mass-loss
phases (winds). These results are consistent with GRB progenitors
being massive stars, such as LBVs or Wolf-Rayet stars, and provides
additional insight into the nature of these progenitors and their
immediate environments.
The host galaxy is a prolifically star-forming galaxy at a
systemic redshift z=2.3304, which has a SFR
of 40
yr-1
as also found by Fynbo et al.
(2005) and de Ugarte Postigo
et al. (2005), reinforcing the potential
association of some GRB with starburst galaxies (Gorosabel et al. 2005; Christensen
et al. 2004, and
references there in).
The Swift mission with a predicted lifetime of ten years (Gehrels et al. 2004) will certainly provide us the opportunity to carry out high-resolution spectroscopy for dozens of future GRBs, and to ascertain physical/chemical properties common to all GRB outflows.
AcknowledgementsWe are grateful to M. Cerviño, M. Más-Hesse, R. González, G. Tenorio-Tagle and S. Vergani for fruitful discussions as well as the anonymous referee for the useful suggestions. This research has also been partially supported by the Spanish Ministry programmes AYA2004-01515, AYA 2007-06377, AYA 2009-14000-C03-01 and ESP 2002-04124-C03-01 (including FEDER funds). The Dark Cosmology Centre is funded by the DNRF. Some of the authors acknowledge benefits from collaboration within the EU FP5 Research Training Network ``Gamma-Ray Bursts: An Enigma and a Tool''.
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Footnotes
- ... galaxy
- Based on observations taken with the ESO's 8.2 m Very Large Telescope in Chile.
- ...
) - See http://www.aoc.nrao.edu/ dfrail/grb021004.dat for the complete VLA data set.
- ... MIDAS
- MIDAS (Munich Imaging Data Analysis System) is developed and maintained by the European Southern Observatory (ESO). http://www.eso.org/projects/esomidas/midas-distrib.html
- ... IRAF
- IRAF is the Image Reduction and Analysis Facility, a general purpose software system for the reduction and analysis of astronomical data. IRAF is written and supported by the IRAF programming group at the National Optical Astronomy Observatories (NOAO) in Tucson, Arizona, USA.
All Tables
Table 1: Journal of the VLT GRB 021004 optical/nIR spectroscopic observations.
Table 2: Individual high-velocity metal-lines systems in GRB 021004.
Table 3: Line Identifications in the GRB 021004 OA spectra.
All Figures
![]() |
Figure 1:
Overall view of GRB 021004 optical afterglow spectrum. These VLT/UVES
data were obtained |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
In this figure, we have converted the observed wavelengths into
velocities relative to the host redshift (z=2.3304)
for each ionic species significantly detected. Ly |
Open with DEXTER | |
In the text |
![]() |
Figure 3: A zoomed image of the high-resolution VLT/UVES spectrum at the location of the C1 and E1 narrow-line absorption systems. The data represent the coadded spectra of the different ions (N V + Si II + Al II + Al III) in velocity space. Thus, C1 has two subcomponents at 2803 km s-1 and 2837 km s-1 (low ionization, Si II + Al II, top) and 2810 km s-1 and 2860 km s-1 (high ionization, Si III + C IV, bottom). The ionization-velocity correlation also seen in QSOs (Møller et al. 1994) and WR nebulae (Smith et al. 1984) is noticeable. E1 divides equally into two subsystems at 114 km s-1 and 150 km s-1. |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Bottom panels: a zoomed image of the GRB
021004 nIR afterglow spectrum. These VLT/ISAAC data were obtained |
Open with DEXTER | |
In the text |
![]() |
Figure 5: A sketch showing the absorption-line complexes C, D, and E along the line of sight to GRB 021004, not drawn to scale. Velocities are given in km s-1. Complex C, with the highest relative velocity, is most likely the closest to the progenitor site in space and is probably formed during the WR phase. The high-ionization sub-component C2 is the closest one to the progenitor whereas the colder sub-component C1 is further out and we cannot exclude that it could even be related to the neighbouring galaxy detected by HST (Chen et al. 2007). Complex E has the lowest relative velocity, and the broad part of the E complex is most likely caused by the oldest wind formed when the progenitor evolved from being an O star into the LBV phase or in the early LBV phase. The narrow E1 component has the properties expected for the ISM of the host galaxy. We believe that the D complex is caused by winds formed during the unstable LBV phase. |
Open with DEXTER | |
In the text |
Copyright ESO 2010
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