436 research outputs found
Luminosity function and jet structure of Gamma Ray Bursts
The structure of Gamma Ray Burst (GRB) jets impacts on their prompt and
afterglow emission properties. The jet of GRBs could be uniform, with constant
energy per unit solid angle within the jet aperture, or it could instead be
structured, namely with energy and velocity that depend on the angular distance
from the axis of the jet. We try to get some insight about the still unknown
structure of GRBs by studying their luminosity function. We show that low
(1e46-1e48 erg/s) and high (i.e. with L > 1e50 erg/s) luminosity GRBs can be
described by a unique luminosity function, which is also consistent with
current lower limits in the intermediate luminosity range (1e48-1e50} erg/s).
We derive analytical expressions for the luminosity function of GRBs in uniform
and structured jet models and compare them with the data. Uniform jets can
reproduce the entire luminosity function with reasonable values of the free
parameters. A structured jet can also fit adequately the current data, provided
that the energy within the jet is relatively strongly structured, i.e. E propto
theta^{-k} with k > 4. The classical E propto theta^{-2} structured jet model
is excluded by the current data.Comment: 11 pages, 2 tables, 7 figures, submitted to MNRA
Limits to tDCS effects in language:failures to modulate word production in healthy participants with frontal or temporal tDCS
Transcranial direct current stimulation (tDCS) is a method of non-invasive brain stimulation widely used to modulate cognitive functions. Recent studies, however, suggests that effects are unreliable, small and often non-significant at least when stimulation is applied in a single session to healthy individuals. We examined the effects of frontal and temporal lobe anodal tDCS on naming and reading tasks and considered possible interactions with linguistic activation and selection mechanisms as well possible interactions with item difficulty and participant individual variability. Across four separate experiments (N, Exp 1A = 18; 1B = 20; 1C = 18; 2 = 17), we failed to find any difference between real and sham stimulation. Moreover, we found no evidence of significant effects limited to particular conditions (i.e., those requiring suppression of semantic interference), to a subset of participants or to longer RTs. Our findings sound a cautionary note on using tDCS as a means to modulate cognitive performance. Consistent effects of tDCS may be difficult to demonstrate in healthy participants in reading and naming tasks, and be limited to cases of pathological neurophysiology and/or to the use of learning paradigms
Bulk Lorentz factors of Gamma-Ray Bursts
Knowledge of the bulk Lorentz factor of GRBs allows us to
compute their comoving frame properties shedding light on their physics. Upon
collisions with the circumburst matter, the fireball of a GRB starts to
decelerate, producing a peak or a break (depending on the circumburst density
profile) in the light curve of the afterglow. Considering all bursts with known
redshift and with an early coverage of their emission, we find 67 GRBs with a
peak in their optical or GeV light curves at a time . For another
106 GRBs we set an upper limit . We show that
is due to the dynamics of the fireball deceleration and not to the passage of a
characteristic frequency of the synchrotron spectrum across the optical band.
Considering the of 66 long GRBs and the 85 most constraining upper
limits, using censored data analysis methods, we reconstruct the most likely
distribution of . All are larger than the time when the prompt emission peaks, and are much larger than the time when the fireball becomes transparent. The reconstructed distribution of
has median value 300 (150) for a uniform (wind) circumburst
density profile. In the comoving frame, long GRBs have typical isotropic
energy, luminosity, and peak energy erg, erg s ,
and keV in the homogeneous (wind) case. We
confirm that the significant correlations between and the rest frame
isotropic energy (), luminosity () and peak energy
() are not due to selection effects. Assuming a typical opening
angle of 5 degrees, we derive the distribution of the jet baryon loading which
is centered around a few .Comment: 19 pages, 11 figures, 6 tables. Accepted for publication on Astronomy
& Astrophysic
Unveiling the population of orphan Gamma Ray Bursts
Gamma Ray Bursts are detectable in the gamma-ray band if their jets are
oriented towards the observer. However, for each GRB with a typical theta_jet,
there should be ~2/theta_jet^2 bursts whose emission cone is oriented elsewhere
in space. These off-axis bursts can be eventually detected when, due to the
deceleration of their relativistic jets, the beaming angle becomes comparable
to the viewing angle. Orphan Afterglows (OA) should outnumber the current
population of bursts detected in the gamma-ray band even if they have not been
conclusively observed so far at any frequency. We compute the expected flux of
the population of orphan afterglows in the mm, optical and X-ray bands through
a population synthesis code of GRBs and the standard afterglow emission model.
We estimate the detection rate of OA by on-going and forthcoming surveys. The
average duration of OA as transients above a given limiting flux is derived and
described with analytical expressions: in general OA should appear as daily
transients in optical surveys and as monthly/yearly transients in the mm/radio
band. We find that ~ 2 OA yr^-1 could already be detected by Gaia and up to 20
OA yr^-1 could be observed by the ZTF survey. A larger number of 50 OA yr^-1
should be detected by LSST in the optical band. For the X-ray band, ~ 26 OA
yr^-1 could be detected by the eROSITA. For the large population of OA
detectable by LSST, the X-ray and optical follow up of the light curve (for the
brightest cases) and/or the extensive follow up of their emission in the mm and
radio band could be the key to disentangle their GRB nature from other
extragalactic transients of comparable flux density.Comment: 9 pages, 4 figures, 2 tables. Accepted for publication by Astronomy
and Astrophysic
A comprehensive stepped planing hull systematic series: Part 1 - Resistance test
This work addresses the experimental study of a new systematic series of stepped planing hulls. Indeed, the interest in the stepped planing hulls is constantly growing, both in the industrial/commercial and academic fields. Designers and boat builders have been orienting toward the multi-stepped hulls solution to ensure good dynamic stability, reliable seakeeping and operability at high speeds. However, there is a lack of a compre hensive stepped hull systematic series with various step configurations including a forward V-shaped step, as typically used on modern boats. For the abovementioned reasons, a systematic series of eight different models of stepped hulls have been developed and tested. The towing tank tests have been carried out at the naval basin of the Universita ` degli Studi di Napoli âFederico IIâ Dipartimento di Ingegneria Industriale (DII) in calm water at different speeds (Frâ = 1.077â6.774) and for three different static trim conditions. All models are built with a transparent bottom to visualize the wetted surface and the eventual development of vortices generated behind the step. The eight models are defined by modifying three significant design parameters for stepped hulls (i.e. the number of steps, longitudinal step position, and step height)
Short GRBs at the dawn of the gravitational wave era
We derive the luminosity function and redshift distribution of short Gamma
Ray Bursts (SGRBs) using (i) all the available observer-frame constraints (i.e.
peak flux, fluence, peak energy and duration distributions) of the large
population of Fermi SGRBs and (ii) the rest-frame properties of a complete
sample of Swift SGRBs. We show that a steep with a>2.0
is excluded if the full set of constraints is considered. We implement a Monte
Carlo Markov Chain method to derive the and functions
assuming intrinsic Ep-Liso and Ep-Eiso correlations or independent
distributions of intrinsic peak energy, luminosity and duration. To make our
results independent from assumptions on the progenitor (NS-NS binary mergers or
other channels) and from uncertainties on the star formation history, we assume
a parametric form for the redshift distribution of SGRBs. We find that a
relatively flat luminosity function with slope ~0.5 below a characteristic
break luminosity ~3 erg/s and a redshift distribution of SGRBs
peaking at z~1.5-2 satisfy all our constraints. These results hold also if no
Ep-Liso and Ep-Eiso correlations are assumed. We estimate that, within ~200 Mpc
(i.e. the design aLIGO range for the detection of GW produced by NS-NS merger
events), 0.007-0.03 SGRBs yr should be detectable as gamma-ray events.
Assuming current estimates of NS-NS merger rates and that all NS-NS mergers
lead to a SGRB event, we derive a conservative estimate of the average opening
angle of SGRBs: ~3-6 deg. Our luminosity function implies an
average luminosity L~1.5 erg/s, nearly two orders of magnitude
higher than previous findings, which greatly enhances the chance of observing
SGRB "orphan" afterglows. Efforts should go in the direction of finding and
identifying such orphan afterglows as counterparts of GW events.Comment: 13 pages, 5 figures, 2 tables. Accepted for publication in Astronomy
& Astrophysics. Figure 5 and angle ranges corrected in revised versio
FFAs and QT intervals in obese women with visceral adiposity: Effects of sustained weight loss over 1 year
We evaluated 66 obese patients grouped by waist-to-hip ratio (WHR) into group A (WHR > 0.85, n = 30) and group B (WHR †0.85, n = 36), before and after 1 yr of diet-induced weight loss compared with 25 nonobese women. Before diet, the longest values of QT intervals and the highest levels of FFA and catecholamines were in group A (P < 0.01). In obese women (both groups), the corrected QT (QTc); interval correlated with plasma FFA (P < 0.01) and catecholamine (P < 0.02) concentrations. After 1 yr of diet, at the same levels of body weight reduction, the decrement of the QTc interval (P < 0.02), FFA (P < 0.01) and catecholamine (P < 0.02) levels were significantly greater in-group A than group B. In multivariate analysis, the decline of the QTc interval after weight loss was associated with changes in plasma FFA independently of changes in WHR and plasma catecholamines. Our data suggest that the QTc interval is tightly correlated with plasma FFA levels; shortening of cardiac repolarization times in the course of long-lasting weight reduction may reduce the risk of ventricular electrical instability, especially in women with abdominal adiposity
The high-redshift gamma-ray burst GRB140515A
High-redshift gamma-ray bursts have several advantages for the study of the
distant universe, providing unique information about the structure and
properties of the galaxies in which they exploded. Spectroscopic identification
with large ground-based telescopes has improved our knowledge of the class of
such distant events. We present the multi-wavelength analysis of the high-
Swift gamma-ray burst GRB140515A (). The best estimate of the
neutral hydrogen fraction of the intergalactic medium (IGM) towards the burst
is . The spectral absorption lines detected for this event
are the weakest lines ever observed in gamma-ray burst afterglows, suggesting
that GRB140515A exploded in a very low density environment. Its circum-burst
medium is characterised by an average extinction (A) that
seems to be typical of events. The observed multi-band light curves
are explained either with a very flat injected spectrum () or with a
multi-component emission (). In the second case a long-lasting central
engine activity is needed in order to explain the late time X-ray emission. The
possible origin of GRB140515A from a Pop III (or from a Pop II stars with local
environment enriched by Pop III) massive star is unlikely.Comment: 10 pages, 8 figures, 5 tables, submitted to Astronomy & Astrophysic
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