423 research outputs found
The circumburst density profile around GRB progenitors: a statistical study
According to our present understanding, long GRBs originate from the collapse
of massive stars while short bursts are due to the coalescence of compact
stellar objects. Since the afterglow evolution is determined by the circumburst
density profile, n(r), traversed by the fireball, it can be used to distinguish
between a so-called ISM profile, n(r) = const., and a free stellar wind, . Our goal is to derive the most probable circumburst density
profile for a large number of Swift-detected bursts using well-sampled
afterglow light curves in the optical and X-ray bands. We combined all publicly
available optical and Swift/X-ray afterglow data from June 2005 to September
2009 to find the best-sampled late-time afterglow light curves. After applying
several selection criteria, our final sample consists of 27 bursts, including
one short burst. The afterglow evolution was then studied within the framework
of the fireball model. We find that the majority (18) of the 27 afterglow light
curves are compatible with a constant density medium (ISM case). Only 6 of the
27 afterglows show evidence for a wind profile at late times. In particular, we
set upper limits on the wind termination-shock radius, , for GRB fireballs
which are propagating into an ISM profile and lower limits on for those
which were found to propagate through a wind medium. Observational evidence for
ISM profiles dominates in GRB afterglow studies, implying that most GRB
progenitors might have relatively small wind termination-shock radii. A smaller
group of progenitors, however, seems to be characterised by notably more
extended wind regions.Comment: A&A, accepted (Oct 26, 2010); 20 pages in journal format; 6 pages
main text, 13 pages Appendix, 1 page references, 6 tables and 2 figures;
included comments by the referee and language editor; removed grey colouring
of the table
GROND coverage of the main peak of Gamma-Ray Burst 130925A
Prompt or early optical emission in gamma-ray bursts is notoriously difficult
to measure, and observations of the dozen cases show a large variety of
properties. Yet, such early emission promises to help us achieve a better
understanding of the GRB emission process(es).
We performed dedicated observations of the ultra-long duration (T90 about
7000 s) GRB 130925A in the optical/near-infrared with the 7-channel "Gamma-Ray
Burst Optical and Near-infrared Detector" (GROND) at the 2.2m MPG/ESO
telescope. We detect an optical/NIR flare with an amplitude of nearly 2 mag
which is delayed with respect to the keV--MeV prompt emission by about 300--400
s. The decay time of this flare is shorter than the duration of the flare (500
s) or its delay.
While we cannot offer a straightforward explanation, we discuss the
implications of the flare properties and suggest ways toward understanding it.Comment: 9 pages, 9 figures, accepted for publ. in A&
A complete sample of bright Swift Gamma-Ray Bursts: X-ray afterglow luminosity and its correlation with the prompt emission
We investigate wheter there is any correlation between the X-ray afterglow
luminosity and the prompt emission properties of a carefully selected
sub-sample of bright Swift long Gamma-Ray Bursts (GRBs) nearly complete in
redshift (~90%). Being free of selection effects (except flux limit), this
sample provides the possibility to compare the rest frame physical properties
of GRB prompt and afterglow emission in an unbiased way. The afterglow X-ray
luminosities are computed at four different rest frame times (5 min, 1 hr, 11
hr and 24 hr after trigger) and compared with the prompt emission isotropic
energy E_iso, the isotropic peak luminosity L_iso and the rest frame peak
energy E_peak. We find that the rest frame afterglow X-ray luminosity do
correlate with these prompt emission quantities, but the significance of each
correlation decreases over time. This result is in agreement with the idea that
the GRB X-ray light curve can be described as the result of a combination of
different components whose relative contribution and weight change with time,
with the prompt and afterglow emission dominating at early and late time,
respectively. In particular, we found evidence that the plateau and the shallow
decay phase often observed in GRB X-ray light curves are powered by activity
from the central engine. The existence of the L_X-E_iso correlation at late
times (t_rf > 11 hr) suggests a similar radiative efficiency among different
bursts with on average about 6% of the total kinetic energy powering the prompt
emission.Comment: 8 pages, 4 figures, 3 tables. Accepted for publication in MNRA
Optical Classification of Gamma-Ray Bursts in the Swift Era
We propose a new method for the classification of optically dark gamma-ray
bursts (GRBs), based on the X-ray and optical-to-X-ray spectral indices of GRB
afterglows, and utilizing the spectral capabilities of Swift. This method
depends less on model assumptions than previous methods, and can be used as a
quick diagnostic tool to identify optically sub-luminous bursts. With this
method we can also find GRBs that are extremely bright at optical wavelengths.
We show that the previously suggested correlation between the optical darkness
and the X-ray/gamma-ray brightness is merely an observational selection effect.Comment: 6 pages, 3 figures; accepted for publication in Ap
The X-ray absorbing column density of a complete sample of bright Swift Gamma-Ray Bursts
A complete sample of bright Swift Gamma-ray Bursts (GRBs) has been recently
selected by Salvaterra et al. (2011). The sample has a high level of
completeness in redshift (91%). We derive here the intrinsic absorbing X-ray
column densities of these GRBs making use of the Swift X-ray Telescope data.
This distribution has a mean value of log(NH/cm-2)=21.7+-0.5. This value is
consistent with the distribution of the column densities derived from the total
sample of GRBs with redshift. We find a mild increase of the intrinsic column
density with redshift. This can be interpreted as due to the contribution of
intervening systems along the line of sight. Making use of the spectral index
connecting optical and X-ray fluxes at 11 hr (beta_OX), we investigate the
relation of the intrinsic column density and the GRB `darkness'. We find that
there is a very tight correlation between dark GRBs and high X-ray column
densities. This clearly indicates that the dark GRBs are formed in a metal-rich
environment where dust must be present.Comment: MNRAS, 6 pages, 3 figures, 1 tabl
GRB 021004: Tomography of a gamma-ray burst progenitor and its host galaxy
We analyse the distribution of matter around the progenitor star of gamma-ray
burst GRB 021004 as well as the properties of its host galaxy with
high-resolution echelle as well as near-infrared spectroscopy. Observations
were taken by the 8.2m Very Large Telescope with the Ultraviolet and Visual
Echelle spectrograph (UVES) and the Infrared Spectrometer And Array Camera
(ISAAC) between 10 and 14 hours after the onset of the event. We report the
first detection of emission lines from a GRB host galaxy in the near-infrared,
detecting H-alpha and the [O III] doublet. These allow an independent
measurement of the systemic redshift (z = 2.3304 +/- 0.0005) which is not
contaminated by absorption as the Ly-alpha line is, and the deduction of
properties of the host galaxy. From the visual echelle spectroscopy, we find
several absorption line groups spanning a range of about 3,000 km/s 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
and narrow lines with velocity widths of only 20 km/s. 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 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 about 40 solar masses per year.Comment: 11 pages, 5 figures. Accepted for publication in Astronomy and
Astrophysics
On the nature of the extremely fast optical rebrightening of the afterglow of GRB 081029
Context. After the launch of the Swift satellite, the Gamma-Ray Burst (GRB)
optical light-curve smoothness paradigm has been questioned thanks to the
faster and better sampled optical follow-up, which has unveiled a very complex
behaviour. This complexity is triggering the interest of the whole GRB
community. The GROND multi-channel imager is used to study optical and
near-infrared (NIR) afterglows of GRBs with unprecedented optical and
near-infrared temporal and spectral resolution. The GRB 081029 has a very
prominent optical rebrightening event and is an outstanding example of the
application of the multi-channel imager to GRB afterglows. Aims. Here we
exploit the rich GROND multi-colour follow-up of GRB 081029 combined with XRT
observations to study the nature of late-time rebrightenings that appear in the
optical-NIR light-curves of some GRB afterglows. Methods. We analyse the
optical and NIR observations obtained with the seven-channel Gamma-Ray burst
Optical and Near-infrared Detector (GROND) at the 2.2 m MPI/ESO telescope and
the X-ray data obtained with the XRT telescope on board the Swift observatory.
The multi-wavelength temporal and spectral evolution is discussed in the
framework of different physical models. Results. The extremely steep optical
and NIR rebrightening observed in GRB 081029 cannot be explained in the
framework of the standard forward shock afterglow model. The absence of a
contemporaneous X-ray rebrightening and the evidence of a strong spectral
evolution in the optical-NIR bands during the rise suggest two separate
components that dominate in the early and late-time lightcurves, respectively.
The steepness of the optical rise cannot be explained even in the framework of
the alternative scenarios proposed in the literature unless a late-time
activity of the central engine is assumed.Comment: 9 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
Assessing quality of life in psychosocial and mental health disorders in children:a comprehensive overview and appraisal of generic health related quality of life measures
Background: Mental health problems often arise in childhood and adolescence and can have detrimental effects on people's quality of life (QoL). Therefore, it is of great importance for clinicians, policymakers and researchers to adequately measure QoL in children. With this review, we aim to provide an overview of existing generic measures of QoL suitable for economic evaluations in children with mental health problems. Methods: First, we undertook a meta-review of QoL instruments in which we identified all relevant instruments. Next, we performed a systematic review of the psychometric properties of the identified instruments. Lastly, the results were summarized in a decision tree. Results: This review provides an overview of these 22 generic instruments available to measure QoL in children with psychosocial and or mental health problems and their psychometric properties. A systematic search into the psychometric quality of these instruments found 195 suitable papers, of which 30 assessed psychometric quality in child and adolescent mental health. Conclusions: We found that none of the instruments was perfect for use in economic evaluation of child and adolescent mental health care as all instruments had disadvantages, ranging from lack of psychometric research, no proxy version, not being suitable for young children, no age-specific value set for children under 18, to insufficient focus on relevant domains (e.g. social and emotional domains)
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