2,929 research outputs found
Qualitative classification of mulchers
ArticleMulchers are helpful in forest establishment and tending. Numerous mulchers are available, so buyers can become disoriented when choosing one. This paper was aimed at creating a classification of mulchers based on the evaluation of the most important parameters: weight, required engine performance, and mulching diameter. Through ANOVA, and regression and correlation analyses of our database, we created six machine classes, devised their upper limits, and assigned mulchers to the classes. Class K1 (weight up to 1,300 kilograms; performance up to 75 kilowatts; mulching diameter up to 22 centimetres) was the most popular one with 88 mulchers. It was followed by class K2 (1,800 kilograms; 100 kilowatts; 27 centimetres) with 61 mulchers, class K4 (3,200 kilograms; 175 kilowatts; 41 centimetres) with 44 mulchers, class K3 (2,300 kilograms; 125 kilowatts; 31 centimetres) with 34 mulchers, class K5 (4,100 kilograms; 225 kilowatts; 51 centimetres) with 18 mulchers, and class K6 (no upper limit) with nine mulchers
VLBI Observations of SN 2008D
We report on two epochs of very-long-baseline interferometry (VLBI)
observations of the Type Ib/c supernova SN 2008D, which was associated with the
X-ray outburst XRF 080109. At our first epoch, at t = 30 days after the
explosion, we observed at 22 and 8.4 GHz, and at our second, at t = 133 days,
at 8.4 and 5.0 GHz. The VLBI observations allow us to accurately measure the
source's size and position at each epoch, and thus constrain its expansion
velocity and proper motion. We find the source at best marginally resolved at
both epochs, allowing us to place a 3sigma upper limit of ~0.75c on the
expansion velocity of a circular source. For an elongated source, our
measurements are compatible with mildly relativistic expansion. However, our
3sigma upper limit on the proper motion is 4 micro-arcsec/day, corresponding to
an apparent velocity of <0.6c, and is consistent with a stationary flux
centroid. This limit rules out a relativistic jet such as an gamma-ray burst
jet away from the line of sight, which would be expected to show apparent
proper motion of >c. Taken together, our measurements argue against the
presence of any long-lived relativistic outflow in SN 2008D. On the other hand,
our measurements are consistent with the nonrelativistic expansion velocities
of <30,000 km/s and small proper motions (<500 km/s) seen in typical
supernovae.Comment: Accepted for publication in the Astrophysical Journal Letter
Challenging GRB models through the broadband dataset of GRB060908
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 figure
VLBI and Archival VLA and WSRT Observations of the GRB 030329 Radio Afterglow
We present VLBI and archival Karl G. Jansky Very Large Array (VLA) and
Westerbork Synthesis Radio Telescope (WSRT) observations of the radio afterglow
from the gamma-ray burst (GRB) of 2003 March 29 (GRB 030329) taken between 672
and 2032 days after the burst. The EVLA and WSRT data suggest a simple power
law decay in the flux at 5 GHz, with no clear signature of any rebrightening
from the counter jet. We report an unresolved source at day 2032 of size
mas, which we use in conjunction with the expansion rate of the
burst to argue for the presence of a uniform, ISM-like circumburst medium.
We develop a semi-analytic method to model gamma-ray burst afterglows, and
apply it to the 5 GHz light curve to perform burst calorimetry. A limit of mas yr is placed on the proper motion, supporting the standard
afterglow model for gamma-ray bursts.Comment: 24 pages, 5 figure
The electromagnetic model of Gamma Ray Bursts
I describe electromagnetic model of gamma ray bursts and contrast its main
properties and predictions with hydrodynamic fireball model and its
magnetohydrodynamical extension. The electromagnetic model assumes that
rotational energy of a relativistic, stellar-mass central source
(black-hole--accretion disk system or fast rotating neutron star) is converted
into magnetic energy through unipolar dynamo mechanism, propagated to large
distances in a form of relativistic, subsonic, Poynting flux-dominated wind and
is dissipated directly into emitting particles through current-driven
instabilities. Thus, there is no conversion back and forth between internal and
bulk energies as in the case of fireball model. Collimating effects of magnetic
hoop stresses lead to strongly non-spherical expansion and formation of jets.
Long and short GRBs may develop in a qualitatively similar way, except that in
case of long bursts ejecta expansion has a relatively short, non-relativistic,
strongly dissipative stage inside the star. Electromagnetic and fireball models
(as well as strongly and weakly magnetized fireballs) lead to different early
afterglow dynamics, before deceleration time. Finally, I discuss the models in
view of latest observational data in the Swift era.Comment: solicited contribution to Focus Issue of New Journal of Physics, 27
pages, 4 figure
Optical Behavior of GRB 061121 around its X-Ray Shallow Decay Phase
Aims. We report on a detailed study of the optical afterglow of GRB 061121
with our original time-series photometric data. In conjunction with X-ray
observations, we discuss the origin of its optical and X-ray afterglows.
Methods. We observed the optical afterglow of Swift burst GRB 061121 with the
Kanata 1.5-m telescope at Higashi-Hiroshima Observatory. Our observation covers
a period just after an X-ray plateau phase. We also performed deep imaging with
the Subaru telescope in 2010 in order to estimate the contamination of the host
galaxy. Results. In the light curve, we find that the optical afterglow also
exhibited a break as in the X-ray afterglow. However, our observation suggests
a possible hump structure or a flattening period before the optical break in
the light curve. There is no sign of such a hump in the X-ray light curve.
Conclusions. This implies that the emitting region of optical was distinct from
that of X-rays. The hump in the optical light curve was possibly caused by the
passage of the typical frequency of synchrotron emission from another forward
shock distinct from the early afterglow. The observed decay and spectral
indices are inconsistent with the standard synchrotron-shock model. Hence, the
observation requires a change in microphysical parameters in the shock region
or a prior activity of the central engine. Alternatively, the emission during
the shallow decay phase may be a composition of two forward shock emissions, as
indicated by the hump structure in the light curve.Comment: 8 pages, including 4 figures and 2 tables. Accepted to A&
Long gamma-ray bursts without visible supernovae: a case study of redshift estimators and alleged novel objects
It has been argued that the observational limits on a supernova (SN)
associated with GRB060614 convincingly exclude a SN akin to SN1998bw as its
originator, and provide evidence for a new class of long-duration GRBs. We
discuss this issue in the contexts of indirect 'redshift estimators' and of the
fireball and cannonball models of GRBs. The latter explains the unusual
properties of GRB060614: at its debated but favoured low redshift (0.125) they
are predicted, as opposed to exceptional, if the associated core-collapse SN is
of a recently discovered, very faint type. We take the occasion to discuss the
'association' between GRBs and SNe.Comment: 19 pages, 2 figures. To be published in Ap
Variable polarization in the optical afterglow of GRB 021004
We present polarimetric observations of the afterglow of gamma-ray burst
(GRB) 021004, obtained with the Nordic Optical Telescope (NOT) and the Very
Large Telescope (VLT) between 8 and 17 hours after the burst. Comparison among
the observations shows a 45 degree change in the position angle from 9 hours
after the burst to 16 hours after the burst, and comparison with published data
from later epochs even shows a 90 degree change between 9 and 89 hours after
the burst. The degree of linear polarization shows a marginal change, but is
also consistent with being constant in time. In the context of currently
available models for changes in the polarization of GRBs, a homogeneous jet
with an early break time of t_b ~ 1 day provides a good explanation of our
data. The break time is a factor 2 to 6 earlier than has been found from the
analysis of the optical light curve. The change in the position angle of the
polarization rules out a structured jet model for the GRB.Comment: 5 pages, 2 figures. Published in A&A letter
The short GRB070707 afterglow and its very faint host galaxy
We present the results from an ESO/VLT campaign aimed at studying the
afterglow properties of the short/hard gamma ray burst GRB 070707. Observations
were carried out at ten different epochs from ~0.5 to ~80 days after the event.
The optical flux decayed steeply with a power-law decay index greater than 3,
later levelling off at R~27.3 mag; this is likely the emission level of the
host galaxy, the faintest yet detected for a short GRB. Spectroscopic
observations did not reveal any line features/edges that could unambiguously
pinpoint the GRB redshift, but set a limit z < 3.6. In the range of allowed
redshifts, the host has a low luminosity, comparable to that of long-duration
GRBs. The existence of such faint host galaxies suggests caution when
associating short GRBs with bright, offset galaxies, where the true host might
just be too dim for detection. The steepness of the decay of the optical
afterglow of GRB 070707 challenges external shock models for the optical
afterglow of short/hard GRBs. We argue that this behaviour might results from
prolonged activity of the central engine or require alternative scenarios.Comment: 6 pages, 5 figures, accepted by A&
GRB Fireball Physics: Prompt and Early Emission
We review the fireball shock model of gamma-ray burst prompt and early
afterglow emission in light of rapid follow-up measurements made and enabled by
the multi-wavelength Swift satellite. These observations are leading to a
reappraisal and expansion of the previous standard view of the GRB and its
fireball. New information on the behavior of the burst and afterglow on minutes
to hour timescales has led, among other results, to the discovery and follow-up
of short GRB afterglows, the opening up of the z>6 redshift range, and the
first prompt multi-wavelength observations of a long GRB-supernova. We discuss
the salient observational results and some associated theoretical issues.Comment: 23 pages. Published in the New Journal of Physics Focus Issue, "Focus
on Gamma-Ray Bursts in the Swift Era" (Eds. D. H. Hartmann, C. D. Dermer & J.
Greiner). V2: Minor change
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