527 research outputs found
Multi-wavelength analysis of the field of the dark burst GRB 031220
We have collected and analyzed data taken in different spectral bands (from
X-ray to optical and infrared) of the field of GRB031220 and we present results
of such multiband observations. Comparison between images taken at different
epochs in the same filters did not reveal any strong variable source in the
field of this burst. X-ray analysis shows that only two of the seven Chandra
sources have a significant flux decrease and seem to be the most likely
afterglow candidates. Both sources do not show the typical values of the R-K
colour but they appear to be redder. However, only one source has an X-ray
decay index (1.3 +/- 0.1) that is typical for observed afterglows. We assume
that this source is the best afterglow candidate and we estimate a redshift of
1.90 +/- 0.30. Photometric analysis and redshift estimation for this object
suggest that this GRB can be classified as a Dark Burst and that the
obscuration is the result of dust extinction in the circum burst medium or
inside the host galaxy.Comment: 7 pages, 5 figures, accepted for publication on A&
Searching for biosignatures by their rotational spectrum: global fit and methyl group internal rotation features of dimethylsulphoxide up to 116 GHz
The identification and quantification of molecules in interstellar space and atmospheres of planets in the solar systems and in exoplanets rely on spectroscopicmethods and laboratory work is essential to provide the community with the spectral features needed to analyse cosmological observations. Rotational spectroscopy in particular, with its intrinsic high resolution,
allows the unambiguous identification of biomolecular building blocks and biosignature gases which can be correlated with the origin of life or the identification of habitable planets.We report the extension of the measured rotational transition frequencies of dimethylsulphoxide and its 34S and 13C isotopologues in the millimetrewave range (59.6–78.4
GHz) by use of an absorption spectrometer based on the supersonic expansion technique. Hyperfine patterns related to the methyl group internal rotation were analysed in the microwave range region (6–18 GHz) with a Pulsed Jet Fourier Transform spectrometer at extremely high resolution (2 kHz) and reliable predictions up to 116 GHz are provided.
The focus on sulphur-bearing molecules is motivated by the fact that sulphur is largely involved in the intra- and
inter-molecular hydrogen bonds in proteins and although it is the 10th most abundant element in the known Universe, understanding its chemistry is still amatter of debate.Moreover, sulphur-bearingmolecules, in particular dimethylsulphoxide, have been indicated as possible biosignature gases to be monitored in the search of habitable exoplanets
There is a short gamma-ray burst prompt phase at the beginning of each long one
We compare the prompt intrinsic spectral properties of a sample of short
Gamma--ray Burst (GRB) with the first 0.3 seconds (rest frame) of long GRBs
observed by Fermi/GBM. We find that short GRBs and the first part of long GRBs
lie on the same E_p--E_iso correlation, that is parallel to the relation for
the time averaged spectra of long GRBs. Moreover, they are indistinguishable in
the E_p--L_iso plane. This suggests that the emission mechanism is the same for
short and for the beginning of long events, and both short and long GRBs are
very similar phenomena, occurring on different timescales. If the central
engine of a long GRB would stop after ~0.3 * (1+z) seconds the resulting event
would be spectrally indistinguishable from a short GRB.Comment: 14 pages, 6 figures, MNRAS accepte
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
A complete sample of bright Swift short Gamma-Ray Bursts
We present a carefully selected sample of short gamma-ray bursts (SGRBs)
observed by the Swift satellite up to June 2013. Inspired by the criteria we
used to build a similar sample of bright long GRBs (the BAT6 sample), we
selected SGRBs with favorable observing conditions for the redshift
determination on ground, ending up with a sample of 36 events, almost half of
which with a redshift measure. The redshift completeness increases up to about
70% (with an average redshift value of z = 0.85) by restricting to those events
that are bright in the 15-150 keV Swift Burst Alert Telescope energy band. Such
flux-limited sample minimizes any redshift-related selection effects, and can
provide a robust base for the study of the energetics, redshift distribution
and environment of the Swift bright population of SGRBs. For all the events of
the sample we derived the prompt and afterglow emission in both the observer
and (when possible) rest frame and tested the consistency with the correlations
valid for long GRBs. The redshift and intrinsic X-ray absorbing column density
distributions we obtain are consistent with the scenario of SGRBs originated by
the coalescence of compact objects in primordial binaries, with a possible
minor contribution (~10%-25%) of binaries formed by dynamical capture (or
experiencing large natal kicks). This sample is expected to significantly
increase with further years of Swift activity.Comment: 17 pages, 8 figures, 7 tables. Accepted for publication in MNRA
Evidence of a Two-Component Jet in the Afterglow of GRB 070419A
A two-component jet model is proposed to explain the unusual afterglow of GRB
070419A. Regarding the optical light curve, a wide "jet" with an opening angle
of > 30-40 degrees is assumed to produce the late shallow decay, while the
three early power-law segments must be caused by a narrow jet with an opening
angle of about 2-4 degrees. Additional energy injections to both components are
required. Late X-ray emission may come from either the wide jet or the narrow
one. If the latter is correct, the jets may run into an ISM environment and the
temporal index of the late energy injection q may be about 0.65.Comment: 3 pages, PDF only, accepted for publication in Science in China
Series
Multiwavelength chase of GRB 031220 afterglow
Several gamma ray bursts (GRBs) with X-ray afterglow do not
show any optical-IR afterglow. The nature of this class of events, the so-called Dark Bursts, is still not clear. The optical absorption could be due to the interstellar
dust or to the high redshift of the event. Or, more simply, the non-detection of the optical transient should be due to the delay in the observation or to the rapid energy
decaying of these events. High spatial resolution X-ray observations are the most promising tool to investigate on such kind of events. We have collected and analyzed X-ray data and images taken in different spectral bands (optical and infrared) for GRB 031220 and we present the results of the analysis of multiband observations on the field of this burst. Comparison between images taken at different epochs in the same filters did not reveal any strongly variable sources. Photometric analysis and photometric redshift estimation of all possible afterglow candidates suggest that
this GRB can be classifie d as a Dark Burst
Optical and X-ray Rest-frame Light Curves of the BAT6 sample
We present the rest-frame light curves in the optical and X-ray bands of an
unbiased and complete sample of Swift long Gamma-Ray Bursts (GRBs), namely the
BAT6 sample. The unbiased BAT6 sample (consisting of 58 events) has the highest
level of completeness in redshift ( 95%), allowing us to compute the
rest-frame X-ray and optical light curves for 55 and 47 objects, respectively.
We compute the X-ray and optical luminosities accounting for any possible
source of absorption (Galactic and intrinsic) that could affect the observed
fluxes in these two bands. We compare the behaviour observed in the X-ray and
in the optical bands to assess the relative contribution of the emission during
the prompt and afterglow phases. We unarguably demonstrate that the GRBs
rest-frame optical luminosity distribution is not bimodal, being rather
clustered around the mean value Log(L) = 29.9 0.8 when estimated at
a rest frame time of 12 hr. This is in contrast with what found in previous
works and confirms that the GRB population has an intrinsic unimodal luminosity
distribution. For more than 70% of the events the rest-frame light curves in
the X-ray and optical bands have a different evolution, indicating distinct
emitting regions and/or mechanisms. The X-ray light curves normalised to the
GRB isotropic energy (E), provide evidence for X-ray emission still
powered by the prompt emission until late times ( hours after the burst
event). On the other hand, the same test performed for the E-normalised optical light curves shows that the optical emission is a
better proxy of the afterglow emission from early to late times.Comment: Accepted for publication in A&A: 10 pages, 5 figures, 2 table
The Afterglow and Environment of the Short GRB111117A
We present multi-wavelength observations of the afterglow of the short
GRB111117A, and follow-up observations of its host galaxy. From rapid optical
and radio observations we place limits of r \gtrsim 25.5 mag at \deltat \approx
0.55 d and F_nu(5.8 GHz) < 18 \muJy at \deltat \approx 0.50 d, respectively.
However, using a Chandra observation at t~3.0 d we locate the absolute position
of the X-ray afterglow to an accuracy of 0.22" (1 sigma), a factor of about 6
times better than the Swift-XRT position. This allows us to robustly identify
the host galaxy and to locate the burst at a projected offset of 1.25 +/- 0.20"
from the host centroid. Using optical and near-IR observations of the host
galaxy we determine a photometric redshift of z=1.3 (+0.3,-0.2), one of the
highest for any short GRB, and leading to a projected physical offset for the
burst of 10.5 +/- 1.7 kpc, typical of previous short GRBs. At this redshift,
the isotropic gamma-ray energy is E_{gamma,iso} \approx 3\times10^51 erg
(rest-frame 23-2300 keV) with a peak energy of E_{pk} \approx 850-2300 keV
(rest-frame). In conjunction with the isotropic X-ray energy, GRB111117A
appears to follow our recently-reported E_x,iso-E_gamma,iso-E_pk universal
scaling. Using the X-ray data along with the optical and radio non-detections
we find that for a blastwave kinetic energy of E_{K,iso} \approx E_{gamma,iso},
the circumburst density is n_0 \sim 3x10^(-4)-1 cm^-3 (for a range of
epsilon_B=0.001-0.1). Similarly, from the non-detection of a break in the X-ray
light curve at t<3 d, we infer a minimum opening angle for the outflow of
theta_j> 3-10 degrees (depending on the circumburst density). We conclude that
Chandra observations of short GRBs are effective at determining precise
positions and robust host galaxy associations in the absence of optical and
radio detections.Comment: ApJ accepted versio
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