824 research outputs found
Evidence for anisotropy in the distribution of short-lived gamma-ray bursts
Measurements of the two-point angular correlation function w(\theta) for 407
short gamma-ray bursts collected in the Current BATSE Catalogue reveal a ~2
\sigma deviation from isotropy on angular scales \theta ~ 2-4 degrees. Such an
anisotropy is not observed in the distribution of long gamma-ray bursts and
hints to the presence of repeated bursts for up to ~13% of the sources under
exam. However, the available data cannot exclude the signal as due to the
presence of large-scale structure. Under this assumption, the amplitude of the
observed w(\theta) is compatible with those derived for different populations
of galaxies up to redshifts ~0.5, result that suggests short gamma-ray bursts
to be relatively local sources.Comment: 5 pages, 4 figures, submitted to MNRA
Time Resolved GRB Spectroscopy
We present the main results of a study of time-resolved spectra of 43 intense
GRBs detected by BATSE. We considered the 4-parameter Band model and the
Optically Thin Synchrotron Shock model (OTSSM). We find that the large majority
of time-resolved spectra of GRBs are in remarkable agreement with the OTSSM.
However, about 15 % of initial GRB pulses show an apparent low-energy photon
suppression. This phenomenon indicates that complex radiative conditions
modifying optically thin emission may occur during the initial phases of some
GRBs.Comment: 5 pages, 3 figures, Paper presented at the 5th Huntsville Symposium,
Huntsville (Alabama) Oct. 199
On the correlation of Short Gamma--Ray Bursts and Clusters of galaxies
We cross correlate Gamma--Ray Bursts and X--Ray selected clusters of galaxies
at z\leq0.45. We find a positive 2\sigma signal for the angular
cross--correlation function w_{bc}(\theta) on scales \theta\leq 3 deg between
short GRBs and clusters. Conversely, no correlation is found between clusters
and the population of long GRBs. The comparison with the cluster
autocorrelation function shows that short GRBs do not trace the cluster
distribution as not all short GRBs are found in clusters. A higher signal in
w_{bc}(\theta) is found if we only consider the cluster population up to z=0.1.
By comparing the short burst autocorrelation function with model predictions we
then constrain short bursts to mostly originate within \sim 270 Mpc (i.e. z\leq
0.06). Our analysis also reveals that short GRBs are better correlated with
``normal'' galaxies. The double compact object merger model for short GRBs
would associate them preferentially to early--type galaxies but the present
statistics do not allow us to exclude that at least a fraction of these events
might also take place in late--type galaxies, in agreement with recent
evidences.Comment: 5 pages, 5 figures, accepted for publication in MNRA
General properties of overlap probability distributions in disordered spin systems. Toward Parisi ultrametricity
For a very general class of probability distributions in disordered Ising
spin systems, in the thermodynamical limit, we prove the following property for
overlaps among real replicas. Consider the overlaps among s replicas. Add one
replica s+1. Then, the overlap q(a,s+1) between one of the first s replicas,
let us say a, and the added s+1 is either independent of the former ones, or it
is identical to one of the overlaps q(a,b), with b running among the first s
replicas, excluding a. Each of these cases has equal probability 1/s.Comment: LaTeX2e, 11 pages. Submitted to Journal of Physics A: Mathematical
and General. Also available at
http://rerumnatura.zool.su.se/stefano/ms/ghigu.p
Evidence of two spectral breaks in the prompt emission of gamma ray bursts
The long-lasting tension between the observed spectra of gamma ray bursts
(GRBs) and the predicted synchrotron emission spectrum might be solved if
electrons do not completely cool. Evidence for incomplete cooling was recently
found in Swift GRBs with prompt observations down to 0.1 keV and in one bright
Fermi burst, GRB 160625B. Here we systematically search for evidence of
incomplete cooling in the spectra of the ten brightest short and long GRBs
observed by Fermi. We find that in 8/10 long GRBs there is compelling evidence
of a low energy break (below the peak energy) and good agreement with the
photon indices of the synchrotron spectrum (respectively -2/3 and -3/2 below
the break and between the break and the peak energy). Interestingly, none of
the ten short GRBs analysed shows a break but the low energy spectral slope is
consistent with -2/3. In a standard scenario, these results imply a very low
magnetic field in the emission region (B' ~ 10 G in the comoving frame), at odd
with expectations.Comment: 14 pages, 15 figures, in press, accepted for publication in A&
The evolution of signal form: Effects of learned versus inherited recognition
Organisms can learn by individual experience to recognize relevant stimuli
in the environment or they can genetically inherit this ability from their
parents. Here, we ask how these two modes of acquisition affect signal evolution, focusing in particular on the exaggeration and cost of signals. We argue first, that faster learning by individual receivers cannot be a driving force for the evolution of exaggerated and costly signals unless signal senders are related or the same receiver and sender meet repeatedly. We argue instead that biases in receivers’ recognition mechanisms can promote the evolution of costly exaggeration in signals. We provide support for this hypothesis by simulating coevolution between senders and receivers, using artificial neural networks as a model of receivers’ recognition mechanisms. We analyse the joint effects of receiver biases, signal cost and mode of acquisition, investigating the circumstances under which learned recognition gives rise to more exaggerated signals than inherited recognition. We conclude the paper by discussing the relevance of our results to a number of biological scenarios
Long Gamma-Ray Bursts as standard candles
As soon as it was realized that long GRBs lie at cosmological distances,
attempts have been made to use them as cosmological probes. Besides their use
as lighthouses, a task that presents mainly the technological challenge of a
rapid deep high resolution follow-up, researchers attempted to find the Holy
Grail: a way to create a standard candle from GRB observables. We discuss here
the attempts and the discovery of the Ghirlanda correlation, to date the best
method to standardize the GRB candle. Together with discussing the promises of
this method, we will underline the open issues, the required calibrations and
how to understand them and keep them under control. Even though GRB cosmology
is a field in its infancy, ongoing work and studies will clarify soon if and
how GRBs will be able to keep up to the promises.Comment: To appear in the proceedings of the 16th Annual October Astrophysics
Conference in Maryland "Gamma Ray Bursts in the Swift Era", eds. S. Holt, N.
Gehrels & J. Nouse
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