816 research outputs found
High redshift X-ray galaxy clusters. II. The L_X-T relationship revisited
In this paper we re-visit the observational relation between X-ray luminosity
and temperature for high-z galaxy clusters and compare it with the local L_X-T
and with theoretical models. To these ends we use a sample of 17 clusters
extracted from the Chandra archive supplemented with additional clusters from
the literature, either observed by Chandra or XMM-Newton, to form a final
sample of 39 high redshift (0.25 < z < 1.3) objects. Different statistical
approaches are adopted to analyze the L_X-T relation. The slope of the L_X-T
relation of high redshift clusters is steeper than expected from the
self-similar model predictions and steeper, even though still compatible within
the errors, than the local L_X-T slope. The distant cluster L_X-T relation
shows a significant evolution with respect to the local Universe: high-z
clusters are more luminous than the local ones by a factor ~2 at any given
temperature. The evolution with redshift of the L_X-T relation cannot be
described by a single power law nor by the evolution predicted by the
self-similar model. We find a strong evolution, similar or stronger than the
self-similar model, from z = 0 to z <0.3 followed by a much weaker, if any,
evolution at higher redshift. The weaker evolution is compatible with
non-gravitational models of structure formation. According to us a
statistically significant sample of nearby clusters (z < 0.25) should be
observed with the current available X-ray telescopes to completely exclude
observational effects due to different generation detectors and to understand
this novel result.Comment: 14 pages, 10 postscript figures. Accepted for publication in
Astronomy & Astrophysics. Corrected typo
High redshift X-ray galaxy clusters. I. The impact of point sources on the cluster properties
The current generation of X-ray observatories like Chandra allows studies
with very fine spatial details. It is now possible to resolve X-ray point
sources projected into the cluster diffuse emission and exclude them from the
analysis to estimate the ``correct'' X-ray observables. In order to verify the
incidence of point sources on the cluster thermal emission and to evaluate the
impact of their non-thermal emission on the determination of cluster
properties, we used a sample of 18 high-z (0.25 < z < 1.01) clusters from the
Chandra archive. We performed a detailed analysis of the cluster properties and
compared the changes observed in the X-ray observables, like temperature and
luminosity or their inter-relation, when one keeps the point sources in the
analysis. The point sources projected into the cluster extended emission affect
the estimates of cluster temperature or luminosity considerably (up to 13% and
17% respectively). These percentages become even larger for clusters with z >
0.7 where temperature and luminosity increase up to 24% and 22%, respectively.
Thus the point sources should be removed to correctly estimate the cluster
properties. However the inclusion of the point sources does not impact
significantly the slope and normalization of the L-T relationship since for
each cluster the correction to be applied to T and L produces a moderate shift
in the L-T plane almost parallel to the best-fit of the ``correct'' L-T
relation.Comment: 16 pages, 18 postscript figures. Accepted for publication in
Astronomy & Astrophysic
GW170817: implications for the local kilonova rate and for surveys from ground-based facilities
We compute the local rate of events similar to GRB 170817A, which has been
recently found to be associated with a kilonova (KN) outburst. Our analysis
finds an observed rate of such events of R
Gpcyr. After comparing at their face values this density of sGRB
outbursts with the much higher density of Binary Neutron Star (BNS) mergers of
1540 Gpcyr, estimated by LIGO-Virgo
collaboration, one can conclude, admittedly with large uncertainty that either
only a minor fraction of BNS mergers produces sGRB/KN events or the sGRBs
associated with BNS mergers are beamed and observable under viewing angles as
large as . Finally we provide preliminary estimates
of the number of sGRB/KN events detected by future surveys carried out with
present/future ground-based/space facilities, such as LSST, VST, ZTF, SKA and
THESEUS.Comment: MNRAS accepted, 6 pages, 1 figur
The Radio Luminosity Function of the NEP Distant Cluster Radio Galaxies
A complete sample of 18 X-ray selected clusters of galaxies belonging to the
ROSAT North Ecliptic Pole (NEP) survey has been observed with the Very Large
Array at 1.4 GHz. These are the most distant clusters in the X-ray survey with
redshift in the range 0.3 < z < 0.8.Seventy-nine radio sources are detected
within half an Abell radius with an observed peak brightness >=0.17 mJy/beam,
except for three sources, belonging to the same cluster, which have a higher
peak brightness limit of 0.26 mJy/beam. The NEP field source counts are in good
agreement with the source counts of a comparison survey, the VLA-VIRMOS deep
field survey, indicating that the NEP sample is statistically complete.
Thirty-two out of the 79 sources are within 0.2 Abell radii, twenty-two of them
are considered cluster members based on spectroscopic redshifts or their
optical magnitude and morphological classification. The cluster radio galaxies
are used to construct the Radio Luminosity Function (RLF) of distant X-ray
selected clusters. A comparison with two nearby cluster RLFs shows that the NEP
RLF lies above the local ones, has a steeper slope at low radio powers (<=
10^(24) W/Hz) and shows no evidence for a break at about 6 X 10^(24) W/Hz which
is observed in the nearby cluster RLFs. We discuss briefly the origin and
possible explanations of the differences observed in the radio properties of
nearby and distant clusters of galaxies. The main result of this study is that
the RLF of the distant X-ray clusters is very different from that of the local
rich Abell clusters.Comment: 32 pages, 14 figures, Latex file with use of bib.tex. To appear in
Astronomy and Astrophysics, Main Journal. To appear in Astronomy and
Astrophysics Main Journal. To appear in Astronomy & Astrophysics, Main
Journa
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
Electromagnetic follow-up of gravitational wave sources and the case of GW150914
Simultaneous observations of an electromagnetic counterpart from gravitational wave (GW) sources is a powerful tool to gain a complete understanding of the astrophysical event, as well as to support GW data analysis. This proceeding summarizes the expected electromagnetic counterpart of GW sources detectable by the advanced LIGO and Virgo, the follow-up strategies to detect them with groundand space-based observatories, together with an overview of the follow-up campaign of the first GW signal, GW150914, detected with the Advanced LIGO detectors by the LIGO/Virgo Collaboration team
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