539 research outputs found
Prompt observations of Gamma-Ray Bursts with Swift.
This thesis uses early-time and simultaneous data from all three instruments on-board Swift to explore how the conditions of long Gamma-Ray Bursts (GRBs) and their environment affect their observed prompt and afterglow properties. I firstly analyse two long GRBs with properties that distinguish them from the more standard class of long GRB XRF 050406 and GRB 061007. The X-Ray Flash XRF 050406 is a class of GRB with softer prompt emission spectra than is typically observed. At the time, Swift UVOT observations of XRF 050406 were the earliest to be taken of an XRF optical counterpart, and the temporal and spectral Swift multi-wavelength data indicate that the bursts' softness is due to a geometrical effect where the GRB is observed off-axis. GRB 061007 is the brightest GRB to be detected by Swift and is accompanied by an exceptionally luminous afterglow that had a V-band magnitude < 11.1 at 80 s after the prompt emission. Although several properties of GRB 061007 axe comparable to that of more standard GRBs, the brightness and the similarity in the decay rate of the X-ray, UV/optical and 7-ray emission from 100 s after the trigger require either an excessively large kinetic energy or highly collimated outflow. To study GRB local environments, I analyse the X-ray and UV/optical spectral energy distributions of seven GRBs, and determine the column density and dust extinction in the GRB local environment. Using the SMC, LMC and Milky Way extinction curves to model the host galaxy dust, I find the SMC model to provide the best fit to the majority of the sample, indicating that the local environments of long GRBs are characteristic of irregular, low metallicity galaxies. I investigate the factors that contribute to the extinction and absorption in GRB afterglows, and the implications for the host galaxy properties
A Surprising Lack of LGRB Metallicity Evolution with Redshift
Recent additions to the population of Long-duration Gamma Ray Burst (LGRB)
host galaxies with measured metallicities and host masses allow us to
investigate how the distributions of both these properties change with
redshift. We form a sample out to z of 2.5 which we show does not have strong
redshift dependent populations biases in mass and metallicity measurements.
Using this sample, we find a surprising lack of evolution in the LGRB
metallicity distribution across different redshifts and in particular the
fraction of LGRB hosts with relatively high-metallicity, that is those with
12+log(O/H) > 8.4, remains essentially constant out to z = 2.5. This result is
at odds with the evolution in the mass metallicity relation of typical
galaxies, which become progressively more metal poor with increasing redshift.
By converting the measured LGRB host masses and redshifts to expected
metallicities using redshift appropriate mass-metallicity relations, we further
find that the increase in LGRB host galaxy mass distribution with redshift seen
in the Perley et al. (2016) SHOALS sample is consistent with that needed to
preserve a non-evolving LGRB metallicity distribution. However, the estimated
LGRB host metallicity distribution is at least a quarter dex higher at all
redshifts than the measured metallicity distribution. This corresponds to about
a factor of two in raw metallicity and resolves much of the difference between
the LGRB host metallicity cutoffs determined by Graham & Fruchter (2017) and
Perley et al. (2016). As LGRB hosts do not follow the general mass metallicity
relations, there is no substitute for actually measuring their metallicities.Comment: 20 pages, 7 figures, 10 table
The long gamma-ray burst rate and the correlation with host galaxy properties
To answer questions on the start and duration of the epoch of reionisation,
periods of galaxy mergers and properties of other cosmological encounters, the
cosmic star formation history (CSFH), is of fundamental importance. Using the
association of long gamma-ray bursts (LGRBs) with the death of massive stars
and their ultra-luminous nature, the CSFH can be probed to higher redshifts
than current conventional methods. Unfortunately, no consensus has been reached
on the manner in which the LGRB rate (LGRBR) traces the CSFH, leaving many of
the questions mentioned mostly unexplored by this method. Observations by the
GRB NIR detector (GROND) over the past 4 years have, for the first time,
acquired highly complete LGRB samples. Driven by these completeness levels and
new evidence of LGRBs also occurring in more massive and metal rich galaxies
than previously thought, the possible biases of the LGRBR-CSFH connection are
investigated over a large range of galaxy properties. The CSFH is modelled
using empirical fits to the galaxy mass function and galaxy star formation
rates. Biasing the CSFH by metallicity cuts, mass range boundaries, and other
unknown redshift dependencies, a LGRBR is generated and compared to the highly
complete GROND sample. It is found that there is no strong preference for a
metallicity cut or fixed galaxy mass boundaries and that there are no unknown
redshift effects, in contrast to previous work which suggest values of
Z/Z_sun~0.1-0.3. From the best-fit models, we predict that ~1.2% of the LGRB
burst sample exists above z=6. The linear relationship between the LGRBR and
the CSFH suggested by our results implies that redshift biases present in
previous LGRB samples significantly affect the inferred dependencies of LGRBs
on their host galaxy properties. Such biases can lead to, e.g., an
interpretation of metallicity limitations and evolving LGRB luminosity
functions.Comment: 15 pages, 14 figures, 7 tables, accepted for publication in Astronomy
& Astrophysic
The environment of the SN-less GRB 111005A at z = 0.0133
The collapsar model has proved highly successful in explaining the properties
of long gamma-ray bursts (GRBs), with the most direct confirmation being the
detection of a supernova (SN) coincident with the majority of nearby long GRBs.
Within this model, a long GRB is produced by the core-collapse of a metal-poor,
rapidly rotating, massive star. The detection of some long GRBs in metal-rich
environments, and more fundamentally the three examples of long GRBs (GRB
060505, GRB 060614 and GRB 111005A) with no coincident SN detection down to
very deep limits is in strong contention with theoretical expectations. In this
paper we present MUSE observations of the host galaxy of GRB 111005A, which is
the most recent and compelling example yet of a SN-less, long GRB. At
z=0.01326, GRB 111005A is the third closest GRB ever detected, and second
closest long duration GRB, enabling the nearby environment to be studied at a
resolution of 270 pc. From the analysis of the MUSE data cube, we find GRB
111005A to have occurred within a metal-rich environment with little signs of
ongoing star formation. Spectral analysis at the position of the GRB indicates
the presence of an old stellar population (tau > 10 Myr), which limits the mass
of the GRB progenitor to M_ZAMS<15 Msolar, in direct conflict with the
collapsar model. Our deep limits on the presence of any SN emission combined
with the environmental conditions at the position of GRB 111005A necessitate
the exploration of a novel long GRB formation mechanism that is unrelated to
massive stars.Comment: Now accepted by A&A. Manuscript replaced to match accepted version.
Some additional discussion added, and velocity map of the host galaxy now
include
Dust-to-metal ratios in damped Lyman-alpha absorbers: Fresh clues to the origins of dust and optical extinction towards gamma-ray bursts
Motivated by the anomalous dust-to-metal ratios derived in the literature for
gamma-ray burst (GRB) damped Lyman-alpha absorbers (DLAs), we measure these
ratios using the dust-depletion pattern observed in UV/optical afterglow
spectra associated with the ISM at the GRB host-galaxy redshifts. Our sample
consists of 20 GRB absorbers and a comparison sample of 72 QSO-DLAs with
redshift 1.2 < z < 4.0 and down to Z = 0.002 Z_Sol metallicities. The
dust-to-metal ratio in QSO- and GRB-DLAs increases both with metallicity and
metal column density, spanning ~10--110% of the Galactic value and pointing to
a non universal dust-to-metal ratio. The low values of dust-to-metal ratio
suggest 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 grain growth in the low-metallicity regime with
negligible contribution from supernovae (SNe) and asymptotic giant branch (AGB)
stars. On the other hand, some GRB- and QSO-DLAs show high dust-to-metal ratio
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 dust-to-metal-ratio properties of QSO-DLAs, GRBs
probing up to larger column and volume densities. For comparison, the
dust-to-metal ratio that we derive for the SMC and LMC are ~82--100% and ~98%
of the Galactic value, respectively. The literature dust-to-metal ratio of the
low-metallicity galaxy I Zw 18 (< 37%) is consistent with the 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
(abridged).Comment: 14 pages, 9 figures. A&A, in pres
GRB 081029: Understanding Multiple Afterglow Components
We present an analysis of the unusual optical light curve of the gamma-ray
burst GRB~081029, which occurred at a redshift of z = 3.8479$. We combine X-ray
and optical observations from the Swift X-Ray Telescope and the Swift
UltraViolet/Optical Telescope with optical and infrared data obtained using the
REM and ROTSE telescopes to construct a detailed data set extending from 86 s
to approximately 100,000 s after the BAT trigger. Our data also cover a wide
energy range, from 10 keV to 0.77 eV (1.24 Angstrom to 16,000 Angstrom). The
X-ray afterglow shows a shallow initial decay followed by a rapid decay
starting at about 18,000s. The optical and infrared afterglow, however, shows
an uncharacteristic rise at about 5000 s that does not correspond to any
feature in the X-ray light curve. Our data are not consistent with synchrotron
radiation from a single-component jet interacting with an external medium. We
do, however, find that the observed light curve can be explained using
multi-component model for the jet.Comment: 4 pages, 3 figures, to appear in the AIP Conference Proceedings for
the Gamma-Ray Burst 2010 Conference, Annapolis, MD, USA, November 201
High-redshift blazars through nustar eyes
The most powerful sources among the blazar family are MeV blazars. Often
detected at , they usually display high X- and \gm-ray luminosities,
larger-than-average jet powers and black hole masses .
In the present work we perform a multiwavelength study of three high redshift
blazars: 3FGL J0325.5+2223 (), 3FGL J0449.0+1121 (), and 3FGL
J0453.22808 (), analysing quasi simultaneous data from GROND,
\swift-UVOT and XRT, \nustar, and \fermi-LAT. Our main focus is on the hard
X-ray band recently unveiled by \nustar~(379 keV) where these objects show a
hard spectrum which enables us to constrain the inverse Compton peak and the
jet power. We found that all three targets resemble the most powerful blazars,
with the synchrotron peak located in the sub-millimeter range and the inverse
Compton peak in the MeV range, and therefore belong to the MeV blazar class.
Using a simple one zone leptonic emission model to reproduce the spectral
energy distributions, we conclude that a simple combination of synchrotron and
accretion disk emission reproduces the infrared-optical spectra while the X-ray
to \gm-ray part is well reproduced by the inverse Compton scattering of low
energy photons supplied by the broad line region. The black hole masses for
each of the three sources are calculated to be . The three studied sources have jet power at the level of, or
beyond, the accretion luminosity.Comment: 4 figures, 3 tables, accepted for publication in Ap
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