196 research outputs found
Helium in natal HII regions: the origin of the X-ray absorption in gamma-ray burst afterglows
Soft X-ray absorption in excess of Galactic is observed in the afterglows of
most gamma-ray bursts (GRBs), but the correct solution to its origin has not
been arrived at after more than a decade of work, preventing its use as a
powerful diagnostic tool. We resolve this long-standing problem and find that
He in the GRB's host HII region is responsible for most of the absorption. We
show that the X-ray absorbing column density (N_Hx) is correlated with both the
neutral gas column density and with the optical afterglow extinction (Av). This
correlation explains the connection between dark bursts and bursts with high
N_Hx values. From these correlations we exclude an origin of the X-ray
absorption which is not related to the host galaxy, i.e. the intergalactic
medium or intervening absorbers are not responsible. We find that the
correlation with the dust column has a strong redshift evolution, whereas the
correlation with the neutral gas does not. From this we conclude that the
column density of the X-ray absorption is correlated with the total gas column
density in the host galaxy rather than the metal column density, in spite of
the fact that X-ray absorption is typically dominated by metals. The strong
redshift evolution of N_Hx/Av is thus a reflection of the cosmic metallicity
evolution of star-forming galaxies. We conclude that the absorption of X-rays
in GRB afterglows is caused by He in the HII region hosting the GRB. While dust
is destroyed and metals are stripped of all of their electrons by the GRB to
great distances, the abundance of He saturates the He-ionising UV continuum
much closer to the GRB, allowing it to remain in the neutral or singly-ionised
state. Helium X-ray absorption explains the correlation with total gas, the
lack of strong evolution with redshift as well as the absence of dust, metal or
hydrogen absorption features in the optical-UV spectra.Comment: 10 pages, 4 figures, submitted to Ap
In Memoriam: Professor Emeritus Richard O. Kummert
The Washington Law Review dedicates its October 2012 issue to Professor Richard O. Kummert who passed away last April at the age of seventy-nine. Professor Kummert served as the faculty advisor to the Washington Law Review for over four decades. The success of this publication owes, in many ways, to Professor Kummert\u27s steadfast guidance. The following memorial remarks come from his former students, colleagues, and friends. Many, but not all, of these remarks have been graciously adapted from speeches given at Professor Kummert\u27s memorial service, which was held at the University of Washington School of Law on April 29, 2012
Gas and dust properties in the afterglow spectra of GRB 050730
We present early WHT ISIS optical spectroscopy of the afterglow of gamma-ray
burst GRB 050730. The spectrum shows a DLA system with the highest measured
hydrogen column to date: N(HI) = 22.1 +/- 0.1 at the third-highest GRB redshift
z = 3.968. Our analysis of the Swift XRT X-ray observations of the early
afterglow show X-ray flares accompanied by decreasing X-ray absorption. From
both the optical and the X-ray spectra we constrain the dust and gas properties
of the host galaxy. We find the host to be a low metallicity galaxy, with low
dust content. Much of the X-ray absorbing gas is situated close to the GRB,
whilst the HI absorption causing the DLA is most likely located further out.Comment: 5 pages, 2 figures. Accepted for A&A Letter
Strategies for prompt searches for GRB afterglows: the discovery of the GRB 001011 optical/near-infrared counterpart using colour-colour selection
We report the discovery of the optical and near-infrared counterparts to GRB
001011. The GRB 001011 error box determined by Beppo-SAX was simultaneously
imaged in the near-infrared by the 3.58-m New Technology Telescope and in the
optical by the 1.54-m Danish Telescope ~8 hr after the gamma-ray event. Here we
implement the colour-colour discrimination technique proposed by Rhoads (2001)
and extend it using near-IR data as well. We present the results provided by an
automatic colour-colour discrimination pipe-line developed to discern the
different populations of objects present in the GRB 001011 error box. Our
software revealed three candidates based on single-epoch images. Second-epoch
observations carried out ~3.2 days after the burst revealed that the most
likely candidate had faded, thus identifying it with the counterpart to the
GRB. In deep R-band images obtained 7 months after the burst a faint
(R=25.38+/-0.25) elongated object, presumably the host galaxy of GRB 001011,
was detected at the position of the afterglow. The GRB 001011 afterglow is the
first discovered with the assistance of colour-colour diagram techniques. We
discuss the advantages of using this method and its application to error boxes
determined by future missions.Comment: Accepted for publication in Astronomy and Astrophysics, 13 pages, 16
figure
On the Afterglow of the X-ray Flash of 2003 July 23: Photometric evdence for an Off-Axis Gamma-Ray burst with an Associated Supernova
Peer reviewe
Comparison of estimated energy intake in children using a Web-based Dietary Assessment Software with accelerometer-estimated energy expenditure in children
Background: The OPUS (Optimal well-being, development and health for Danish children through a healthy New Nordic Diet) project carried out a school meal study to assess the impact of a New Nordic Diet (NND). The random controlled trial involved 834 children aged 8–11 in nine local authority schools in Denmark. Dietary assessment was carried out using a program known as WebDASC (Web-based Dietary Assessment Software for Children) to collect data from the children. Objective: To compare the energy intake (EI) of schoolchildren aged 8–11 estimated using the WebDASC system against the total energy expenditure (TEE) as derived from accelerometers worn by the children during the same period. A second objective was to evaluate the WebDASC's usability. Design: Eighty-one schoolchildren took part in what was the pilot study for the OPUS project, and they recorded their total diet using WebDASC and wore an accelerometer for two periods of seven consecutive days: at baseline, when they ate their usual packed lunches and at intervention when they were served the NND. EI was estimated using WebDASC, and TEE was calculated from accelerometer-derived activity energy expenditure, basal metabolic rate, and diet-induced thermogenesis. WebDASC's usability was assessed using a questionnaire. Parents could help their children record their diet and answer the questionnaire. Results: Evaluated against TEE as derived from the accelerometers worn at the same time, the WebDASC performed just as well as other traditional methods of collecting dietary data and proved both effective and acceptable with children aged 8–11, even with perhaps less familiar foods of the NND. Conclusions: WebDASC is a useful method that provided a reasonably accurate measure of EI at group level when compared to TEE derived from accelerometer-determined physical activity in children. WebDASC will benefit future research in this area
The optical/near-IR spectral energy distribution of the GRB 000210 host galaxy
We report on UBVRIZJsHKs-band photometry of the dark GRB 000210 host galaxy.
Fitting a grid of spectral templates to its Spectral Energy Distribution (SED),
we derived a photometric redshift (z=0.842\+0.0540.042) which is in excellent
agreement with the spectroscopic one (z=0.8463+/-0.0002; Piro et al. 2002). The
best fit to the SED is obtained with a blue starburst template with an age of
0.181\+0.0370.026 Gyr. We discuss the implications of the inferred low value of
Av and the age of the dominant stellar population for the non detection of the
GRB 000210 optical afterglow.Comment: 4 pages, 1 figure, contribution to the Rome 2002 GRB worksho
The GRB-SN connection: GRB030329 and XRF030723
The attempt to secure conclusive, spectroscopic evidence for the GRB/SN
connection has been a central theme in most GRB observing time proposals since
the discovery of the very unusual GRB980425 associated with the peculiar type
Ib/c SN1998bw. GRB030329 provided this evidence to everybody's satisfaction. In
this contribution we show the results of a spectroscopic campaign of the
supernova associated with GRB030329 carried out at ESOs Very Large Telescope.
We also present preliminary results from a photometric and spectroscopic
campaign targeting the X-ray Flash of July 23.Comment: 6 pages, 3 figures, to appear in the Santa Fe GRB Conference
Proceedings, 200
The red optical afterglow of GRB 030725
We present a photometric study of the optical counterpart of the
long-duration Gamma Ray Burst (GRB) 030725, which triggered the HETE FREGATE
and WXM instruments on July 25th, 2003, and lasted more than 160s. An optical
counterpart was identified at the Bronberg Observatory in South Africa about 7
hours after the burst occurred. The optical afterglow (OA) was observed between
4 and 15 days after the burst with the 1.54m Danish telescope at La Silla in
the V, Rc, and Ic bands. We fit a broken power law to the data and determine a
break time in the light curve between 16 hours and 4.7 days after the first
detection of the burst. The decay slope is alpha1 = -0.59 +0.59/-0.44 before
and alpha2 = -1.43 +/- 0.06 after the break. A bump may be present in the light
curve, only significant at the 2-sigma level, 13.9 days after the main burst.
The spectral slope of the OA, measured 12 days after the burst, is -2.9 +/- 0.6
, i.e. it falls in the extreme red end of the distribution of previous OA
spectral slopes. Observations of the field 8 months after the burst with the
EMMI instrument on the NTT telescope (La Silla) resulted in an upper limit of
Rc=24.7 mag for the host galaxy of GRB 030725. The OA of GRB 030725 was
discovered at a private, non-professional observatory and we point out that
with the current suite of gamma ray satellites, an effort to organize future
contributions of amateur observers may provide substantial help in GRB light
curve follow up efforts.Comment: Accepted for publication in A&A, 6 pages, 2 figure
A multi-colour study of the dark GRB 000210 host galaxy and its environment
We present UBVRIZJsHKs broad band photometry of the host galaxy of the dark
gamma-ray burst (GRB) of February 10, 2000. These observations represent the
most exhaustive photometry given to date of any GRB host galaxy. A grid of
spectral templates have been fitted to the Spectral Energy Distribution (SED)
of the host. The derived photometric redshift is z=0.842^+0.054_-0.042, which
is in excellent agreement with the spectroscopic redshift (z=0.8463+/-0.0002)
proposed by Piro et al. (2002) based on a single emission line. Furthermore, we
have determined the photometric redshift of all the galaxies in an area of
6'x6' around the host galaxy, in order to check for their overdensity in the
environment of the host. We find that the GRB 000210 host galaxy is a
subluminous galaxy (L ~ 0.5+/-0.2 L*), with no companions above our detection
threshold of 0.18+/-0.06 L*. Based on the restframe ultraviolet flux a star
formation rate of 2.1+/-0.2 Solar Masses per year is estimated. The best fit to
the SED is obtained for a starburst template with an age of 0.181^+0.037_-0.026
Gyr and a very low extinction (Av~0). We discuss the implications of the
inferred low value of Av and the age of the dominant stellar population for the
non detection of the GRB 000210 optical afterglow.Comment: 10 pages with 4 encapsulated PostScript figures included. Accepted
for publication in Astronomy & Astrophysic
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