283 research outputs found
Analysis of X-ray flares in GRBs
We present a detailed study of the spectral and temporal properties of the
X-ray flares emission of several GRBs. We select a sample of GRBs which X-ray
light curve exhibits large amplitude variations with several rebrightenings
superposed on the underlying three-segment broken powerlaw that is often seen
in Swift GRBs. We try to understand the origin of these fluctuations giving
some diagnostic in order to discriminate between refreshed shocks and late
internal shocks. For some bursts our time-resolved spectral analysis supports
the interpretation of a long-lived central engine, with rebrightenings
consistent with energy injection in refreshed shocks as slower shells generated
in the central engine prompt phase catch up with the afterglow shock at later
times.Comment: 9 pages, 3 figures. Invited talk at the Swift-Venice 2006 meeting to
be published by "Il Nuovo Cimento
Selection effects shaping the Gamma Ray Burst redshift distributions
Long Gamma Ray Bursts hold the promise of probing star-formation and metal
enrichment up to very high redshifts. The present GRB samples with redshift
determination are largely incomplete and therefore a careful analysis of
selection effects is mandatory before any conclusion can be drawn from the
observed GRB redshift distribution. We study and compare three well defined
samples of long GRBs detected by Swift, HETE2 and BeppoSAX. We find that Swift
GRBs are slighly fainter and harder than BeppoSAX and HETE2 GRBs, as expected
due to the higher energy range in which Swift GRBs are detected and localized,
compared to BeppoSAX and HETE2. Gas and dust obscuration plays a role in
shaping the GRB samples and the present samples of GRBs with redshift. We argue
that the majority of the bright Swift GRBs without redshift might actually be
z<~2 events therefore the present Swift GRB sample with redshift is biased
against low-z GRBs. On the other hand, the detection of bright UV rest-frame
afterglows from high-z GRBs, and even from those with large X-ray obscuration,
implies a dust amount lower than in nearby GRBs,and/or a different dust
composition. If this is the case, the Swift sample of GRBs with redshifts is
probably a fair sample of the real high-z GRB population. The absence of high-z
GRBs in the BeppoSAX and HETE2 samples of GRBs with redshifts is probably due
to the fact at the time of BeppoSAX and HETE2 follow-up faint afterglows of
high redshift GRBs will have weaken below the spectroscopic capabilities of
even 10m class telescopes. The redshift distribution of a subsample of Swift
GRBs with distributions of peak-fluxes, X-ray obscuration and optical magnitude
at a fixed observing time similar to those of the BeppoSAX and HETE2 samples,
is roughly consistent with BeppoSAX+HETE2 redshift distribution.Comment: 9 pages, back to A&A after referee repor
Neutrino Induced Upward Going Muons from a Gamma Ray Burst in a Neutrino Telescope of Km^2 Area
The number of neutrino induced upward going muons from a single Gamma Ray
Burst (GRB) expected to be detected by the proposed kilometer scale IceCube
detector at the South Pole location has been calculated. The effects of the
Lorentz factor, total energy of the GRB emitted in neutrinos and its distance
from the observer (red shift) on the number of neutrino events from the GRB
have been examined. The present investigation reveals that there is possibility
of exploring the early Universe with the proposed kilometer scale IceCube
neutrino telescope.Comment: 18pages, 5 figures. Physical Review D in pres
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&
BENA435, a new cell-permeant photoactivated green fluorescent DNA probe
N′-(2,8-Dimethoxy-12-methyl-dibenzo [c,h] [1,5] naphthyridin-6-yl)-N,N-dimethyl-propane-1,3-diamine (BENA435) is a new cell-membrane permeant DNA dye with absorption/emission maxima in complex with DNA at 435 and 484 nm. This new reagent is unrelated to known DNA dyes, and shows a distinct preference to bind double-stranded DNA over RNA. Hydrodynamic studies suggest that BENA435 intercalates between the opposite DNA strands. BENA435 fluoresces much stronger when bound to dA/dT rather than dG/dC homopolymers. We evaluated 14 related dibenzonaphthyridine derivatives and found BENA435 to be superior in its in vivo DNA-binding properties. Molecular modelling was used to develop a model of BENA435 intercalation between base pairs of a DNA helix. BENA435 fluorescence in the nuclei of cells increases upon illumination, suggesting photoactivation. BENA435 represents thus the first known cell-permeant photoactivated DNA-binding dye
A study of the prompt and afterglow emission of the Short GRB 061201
Our knowledge of the intrinsic properties of short duration Gamma-Ray Bursts
has relied, so far, only upon a few cases for which the estimate of the
distance and an extended, multiwavelength monitoring of the afterglow have been
obtained. We carried out multiwavelength observations of the short GRB 061201
aimed at estimating its distance and studying its properties. We performed a
spectral and timing analysis of the prompt and afterglow emission and discuss
the results in the context of the standard fireball model. A clear temporal
break was observed in the X-ray light curve about 40 minutes after the burst
trigger. We find that the spectral and timing behaviour of the X-ray afterglow
is consistent with a jet origin of the observed break, although the optical
data can not definitively confirm this and other scenarios are possible. No
underlying host galaxy down to R~26 mag was found after fading of the optical
afterglow. Thus, no secure redshift could be measured for this burst. The
nearest galaxy is at z=0.111 and shows evidence of star formation activity. We
discuss the association of GRB 061201 with this galaxy and with the ACO S 995
galaxy cluster, from which the source is at an angular distance of 17'' and
8.5', respectively. We also test the association with a possible undetected,
positionally consistent galaxy at z~1. In all these cases, in the jet
interpretation, we find a jet opening angle of 1-2 degrees.Comment: 10 pages, 7 figures, accepted for publication in A&
The exceptionally extended flaring activity in the X-ray afterglow of GRB 050730 observed with Swift and XMM-Newton
We present the results of a detailed spectral and temporal analysis of Swift
and XMM-Newton observations of the high redshift (z=3.969) GRB 050730. The
X-ray afterglow of GRB 050730 was found to decline with time with superimposed
intense flaring activity that extended over more than two orders of magnitude
in time. Seven distinct re-brightening events starting from 236 s up to 41.2 ks
after the burst were observed. The underlying decay of the afterglow was well
described by a double broken power-law model with breaks at t_1= 237 +/- 20 s
and t_2 = 10.1 (-2.2) (+4.6) ks. The temporal decay slopes before, between and
after these breaks were alpha_1 = 2.1 +/- 0.3, alpha_2 = 0.44 (-0.08) (+0.14)
and alpha_3 = 2.40 (+0.07) (-0.09), respectively. The spectrum of the X-ray
afterglow was well described by a photoelectrically absorbed power-law with an
absorbing column density N_H=(1.28 +/- 0.26) 10^22 cm^-2 in the host galaxy.
Strong X-ray spectral evolution during the flaring activity was present. In the
majority of the flares (6/7) the ratio Delta_t/t_p between the duration of the
event and the time when the flare peaks was nearly constant and about 0.6-0.7.
We showed that the observed spectral and temporal properties of the first three
flares are consistent with being due both to high-latitude emission, as
expected if the flares were produced by late internal shocks, or to refreshed
shocks, i.e. late time energy injections into the main afterglow shock by slow
moving shells ejected from the central engine during the prompt phase. The
event fully satisfies the E_p-E_iso Amati relation while is not consistent with
the E_p-E_jet Ghirlanda relation.Comment: 12 pages, 5 figures, accepted for publication in A&
decays in SUSY models without R-parity
Being strictly forbidden in the standard model, experimental detection of the
lepton flavor violating decays and would constitute an unmistakable indication of new physics. We
study these decays in supersymmetric models without R-parity and without lepton
number. In order to derive order of magnitude predictions for the branching
ratios, we assume a horizontal U(1) symmetry with horizontal charges chosen to
explain the magnitude of fermion masses and quark mixing angles. We find that
the branching ratios for decays with a pair in the final state are
not particularly suppressed with respect to the lepton flavor conserving
channels. In general in these models {\rm B}[b\to\mu^+\mu^-(X)]\lsim {\rm
B}[b(\bar b)\to\tau^+\mu^-(X)] \lsim {\rm B}[b\to\tau^+\tau^-(X)]. While in
some cases the rates for final states can be up to one order of
magnitude larger than the lepton flavor violating channel, due to better
efficiencies for muon detection and to the absence of standard model
contributions, decays into final states appear to be better suited to
reveal this kind of new physics.Comment: 15 pages, LaTeX, 3 ps-figures (uses epsfig.sty) Minor typos
corrected, one normalization factor added to Eq. (3.11). To be published on
Phys. Rev.
Heavy --> Light semileptonic decays of pseudoscalar mesons from lattice QCD
We have computed the form factors for B --> pi and D --> K(pi) semileptonic
decays on the lattice by using full non-perturbative O(a) improvement, in the
quenched approximation. Our results are expressed in terms of few parameters
which describe the q^2-dependence and normalization of the form factors.Comment: 28 pages (LaTeX2e), 11 PostScript figures (version to appear in
Nucl.Phys.B
From AMANDA to IceCube
The first string of the neoteric high energy neutrino telescope IceCube
successfully began operating in January 2005. It is anticipated that upon
completion the new detector will vastly increase the sensitivity and extend the
reach of AMANDA to higher energies. A discussion of the IceCube's discovery
potential for extra-terrestrial neutrinos, together with the prospects of new
physics derived from the ongoing AMANDA research will be the focus of this
paper. Preliminary results of the first antarctic high energy neutrino
telescope AMANDA searching in the muon neutrino channel for localized and
diffuse excess of extra-terrestrial neutrinos will be reviewed using data
collected between 2000 and 2003. Neutrino flux limits obtained with the
all-flavor dedicated UHE and cascade analyses will be described. A first
neutrino spectrum above one TeV in agreement with atmospheric neutrino flux
expectations and no extra-terrestrial contribution will be presented, followed
by a discussion of a limit for neutralino CDM candidates annihilating in the
center of the Sun.Comment: 15 pages, 8 figures Invited talk contribution at 5th International
Conference on Non-accelerator New Physics (NANP 05), Dubna, Russia, 20-25 Jun
200
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