621 research outputs found
Radio-X-ray Synergy to discover and Study Jetted Tidal Disruption Events
Observational consequences of tidal disruption of stars (TDEs) by
supermassive black holes (SMBHs) can enable us to discover quiescent SMBHs,
constrain their mass function, study formation and evolution of transient
accretion disks and jet formation. A couple of jetted TDEs have been recently
claimed in hard X-rays, challenging jet models, previously applied to
-ray bursts and active galactic nuclei. It is therefore of paramount
importance to increase the current sample. In this paper, we find that the best
strategy is not to use up-coming X-ray instruments alone, which will yield
between several (e-Rosita) and a couple of hundreds (Einstein Probe) events per
year below redshift one. We rather claim that a more efficient TDE hunter will
be the Square Kilometer Array (SKA) operating {\it in survey mode} at 1.4 GHz.
It may detect up to several hundreds of events per year below with
a peak rate of a few tens per year at . Therefore, even if the
jet production efficiency is {\it not } as assumed here, the predicted
rates should be large enough to allow for statistical studies. The
characteristic TDE decay of , however, is not seen in radio, whose
flux is quite featureless. {\it Identification} therefore requires localization
and prompt repointing by higher energy instruments. If radio candidates would
be repointed within a day by future X-ray observatories (e.g. Athena and
LOFT-like missions), it will be possible to detect up to X-ray
counterparts, almost up to redshift . The shortcome is that only for
redshift below the trigger times will be less than 10 days from
the explosion. In this regard the X-ray surveys are better suited to probe the
beginning of the flare, and are therefore complementary to SKA.Comment: Astrophysical Journal (revised version
Evolution of globular cluster systems in three galaxies of the Fornax cluster
We studied and compared the radial profiles of globular clusters and of the
stellar bulge component in three galaxies of the Fornax cluster observed with
the WFPC2 of the HST. A careful comparative analysis of these distributions
confirms that stars are more concentrated toward the galactic centres than
globular clusters, in agreement with what was observed in many other galaxies.
If the observed difference is the result of evolution of the globular cluster
system sstarting from initial profiles similar to those of the halo--bulge
stellar components, a relevant fraction of their mass (74%, 47%, 52% for NGC
1379, NGC 1399 and NGC 1404, respectively) is disappeared in the inner regions,
likely contributing to the nuclear field population, local dynamics and high
energy phenomena in the primeval life of the galaxy. An indication in favour of
the evolutionary interpretation of the difference between the globular cluster
system and stellar bulge radial profiles is given by the positive correlation
we found between the value of the mass lost from the GCS and the central
galactic black hole mass in the set of seven galaxies for which these data are
available.Comment: paper submitted to MNRAS; 8 pages, including 4 figures and 1 tabl
LOFT as a discovery machine for jetted Tidal Disruption Events
This is a White Paper in support of the mission concept of the Large
Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We
discuss the potential of LOFT for the study of jetted tidal disruption events.
For a summary, we refer to the paper.Comment: White Paper in Support of the Mission Concept of the Large
Observatory for X-ray Timin
Perspectives of blazar studies with future space missions
Since the AGILE and Fermi launch, the synergy between gamma-ray experiments and other space- and ground-based observatories has been the key to carry out multi-wavelength campaign aimed at understanding the physical mechanisms responsible for the observed gamma-ray emission in astrophysical sources. Blazars are the best examples of astrophysical sources where this strategy has been applied. The big efforts put in place for blazars to obtain coordinated observations with a broad coverage of the electromagnetic spectrum are providing new diagnostics of the physical processes at work in these sources, raising a lot of challenges for the theoretical interpretation. These could be partially solved through further observations with ground- and space-based facilities, therefore requiring new advances in technology and mission profile design. We will discuss how the lessons learned from current -ray observatories represent an important heritage for future missions expected to play a crucial role in the understanding of extreme phenomena in the high-energy domain
The assembly of massive galaxies from NIR observations of the Hubble Deep Field South
We use a deep K(AB)<25 galaxy sample in the Hubble Deep Field South to trace
the evolution of the cosmological stellar mass density from z~ 0.5 to z~3. We
find clear evidence for a decrease of the average stellar mass density at high
redshift, 2<z<3.2, that is 15^{+25}_{-5}% of the local value, two times higher
than what observed in the Hubble Deep Field North. To take into account for the
selection effects, we define a homogeneous subsample of galaxies with
10^{10}M_\odot \leq M_* \leq 10^{11}M_\odot: in this sample, the mass density
at z>2 is 20^{+20}_{-5} % of the local value. In the mass--limited subsample at
z>2, the fraction of passively fading galaxies is at most 25%, although they
can contribute up to about 40% of the stellar mass density. On the other hand,
star--forming galaxies at z>2 form stars with an average specific rate at least
~4 x10^{-10} yr, 3 times higher than the z<~1 value. This
implies that UV bright star--forming galaxies are substancial contributors to
the rise of the stellar mass density with cosmic time. Although these results
are globally consistent with --CDM scenarios, the present rendition of
semi analytic models fails to match the stellar mass density produced by more
massive galaxies present at z>2.Comment: Accepted for publication on ApJLetter
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