1,440 research outputs found
The Gamma Ray Burst Luminosity Function in the Light of the Swift 2-year Data
We compute the luminosity function (LF) and the formation rate of long gamma
ray bursts (GRBs) by fitting the observed differential peak flux distribution
obtained by the BATSE satellite in three different scenarios: i) GRBs follow
the cosmic star formation and their LF is constant in time; ii) GRBs follow the
cosmic star formation but the LF varies with redshift; iii) GRBs form
preferentially in low-metallicity environments. We find that the differential
peak flux number counts obtained by BATSE and by Swift can be reproduced using
the same LF and GRB formation rate, indicating that the two satellites are
observing the same GRB population. We then check the resulting redshift
distributions in the light of Swift 2-year data, focusing in particular on the
relatively large sample of GRBs detected at z>2.5. We show that models in which
GRBs trace the cosmic star formation and are described by a constant LF are
ruled out by the number of high-z Swift detections. This conclusion does not
depend on the redshift distribution of bursts that lack of optical
identification, nor on the existence of a decline in star formation rate at
z>2, nor on the adopted faint-end of the GRB LF. Swift observations can be
explained by assuming that the LF varies with redshift and/or that GRB
formation is limited to low-metallicity environments.Comment: 7 pages, 3 figures, ApJ Letter in pres
On the detection of very high redshift Gamma Ray Bursts with Swift
We compute the probability to detect long Gamma Ray Bursts (GRBs) at z>5 with
Swift, assuming that GRBs form preferentially in low-metallicity environments.
The model fits well both the observed BATSE and Swift GRB differential peak
flux distribution and is consistent with the number of z>2.5 detections in the
2-year Swift data. We find that the probability to observe a burst at z>5
becomes larger than 10% for photon fluxes P<1 ph s^{-1} cm^{-2}, consistent
with the number of confirmed detections. The corresponding fraction of z>5
bursts in the Swift catalog is ~10%-30% depending on the adopted metallicity
threshold for GRB formation. We propose to use the computed probability as a
tool to identify high redshift GRBs. By jointly considering promptly-available
information provided by Swift and model results, we can select reliable z>5
candidates in a few hours from the BAT detection. We test the procedure against
last year Swift data: only three bursts match all our requirements, two being
confirmed at z>5. Other three possible candidates are picked up by slightly
relaxing the adopted criteria. No low-z interloper is found among the six
candidates.Comment: 5 pages, 2 figures, MNRAS in pres
High-Redshift Superclustering of QSO Absorption Line Systems on 100 Mpc Scales
We have analyzed the clustering of C IV absorption line systems in an
extensive new catalog of heavy element QSO absorbers. The catalog permits
exploration of clustering over a large range in both scale (from about 1 to
over 300 Mpc) and redshift (z from 1.2 to 4.5). We find significant evidence
(5.0 sigma) that C IV absorbers are clustered on comoving scales of 100 Mpc and
less --- similar to the size of voids and walls found in galaxy redshift
surveys of the local universe --- with a mean correlation function over these scales. We find, on these scales, that the mean
correlation function at low (z=1.7), medium (z=2.4), and high redshift (z=3.0)
is , , and , respectively.
Thus, the superclustering is present even at high redshift; furthermore, it
does not appear that the superclustering scale, in comoving coordinates, has
changed significantly since then. We find 7 QSOs with rich groups of absorbers
(potential superclusters) that account for a significant portion of the
clustering signal, with 2 at redshift . We find that the
superclustering is just as evident if we take instead of 0.5;
however, the inferred scale of clustering is then 240 Mpc , which is larger
than the largest scales of clustering known at present. This discrepancy may be
indicative of a larger value of , and hence . The evolution of
the correlation function on 50 Mpc scales is consistent with that expected in
cosmologies with density parameter ranging from 0.1 to 1.
Finally, we find no evidence for clustering on scales greater than 100 Mpc
() or 240 Mpc ().Comment: 16 LaTeX pages with 3 encapsulated Postscript figures included, uses
AASTeX (v. 4.0) available at ftp://ftp.aas.org/pubs/ , to appear in The
Astrophysical Journal Letter
The luminosity function of cluster galaxies. II. Data reduction procedures applied to the cluster Abell 496
We initiated a large project aimed to estimate the Luminosity Function of
galaxies in clusters and to evaluate its relation to cluster morphology. With
this paper we deem necessary to outline the general procedures of the data
reduction and details of the data analysis. The cluster sample includes the
brightest southern ROSAT all-sky survey clusters with z < 0.1. These have been
observed in three colours g, r, i, and mapped up to a few core radii using a
mosaic of CCD frames. E/S0 galaxies in the cluster core are singled out both by
morphology (for the brightest galaxies), and by colour. The details of the data
reduction procedure are illustrated via the analysis of the cluster Abell 496,
which has been used as a pilot cluster for the whole program. The related
photometric catalogue consists of 2355 objects. The limiting magnitudes (the
reference Surface Brightness is given in parenthesis) in the various colours
are respectively g(25.5) = 24.14, r(25.5) = 24.46, i(25.0) = 23.75$. These
correspond to the limiting absolute magnitudes -12.28, -11.96 and -12.67
(H_0=50 km/sec/Mpc).Comment: 17 pages, 19 ps figures, aa.cl
The X-ray light curve of Gamma-ray bursts: clues to the central engine
We present the analysis of a large sample of gamma-ray burst (GRB) X-ray
light curves in the rest frame to characterise their intrinsic properties in
the context of different theoretical scenarios. We determine the morphology,
time scales, and energetics of 64 long GRBs observed by \emph{Swift}/XRT
\emph{without} flaring activity. We furthermore provide a one-to-one comparison
to the properties of GRBs \emph{with} X-ray flares. We find that the steep
decay morphology and its connection with X-ray flares favour a scenario in
which a central engine origin. We show that this scenario can also account for
the shallow decay phase, provided that the GRB progenitor star has a
self-similar structure with a constant envelope-to-core mass ratio . However, difficulties arise for very long duration
( s) shallow phases. Alternatively, a spinning-down magnetar
whose emitted power refreshes the forward shock can quantitatively account for
the shallow decay properties. In particular we demonstrate that this model can
account for the plateau luminosity vs. end time anticorrelation.Comment: 12 pages, 8 figures, accepted for publication in A&
A detector of gravitational waves based on coupled microwave cavities
Since 1978 superconducting coupled cavities have been proposed as sensitive
detector of gravitational waves. The interaction of the gravitational wave with
the cavity walls, and the resulting motion, induces the transition of some
electromagnetic energy from an initially excited cavity mode to an empty one.
The energy transfer is maximum when the frequency of the wave is equal to the
frequency difference of the two cavity modes. In this paper the basic
principles of the detector are discussed. The interaction of a gravitational
wave with the cavity walls is studied in the proper reference frame of the
detector, and the coupling between two electromagnetic normal modes induced by
the wall motion is analyzed in detail. Noise sources are also considered; in
particular the noise coming from the brownian motion of the cavity walls is
analyzed. Some ideas for the developement of a realistic detector of
gravitational waves are discussed; the outline of a possible detector design
and its expected sensitivity are also shown.Comment: 29 pages, 12 eps figures. Typeset by REVTe
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