211 research outputs found
Radio signature of cosmological structure formation shocks
In the course of the formation of cosmological structures, large shock waves
are generated in the intra-cluster medium. In analogy to processes in supernova
remnants, these shock waves may generate a significant population of
relativistic electrons which, in turn, produce observable synchrotron emission.
The extended radio relics found at the periphery of several clusters and
possibly also a fraction of radio halo emission may have this origin. Here we
derive an analytic expression for (i) the total radio power in the downstream
region of a cosmological shock wave and (ii) the width of the radio-emitting
region. These expressions predict a spectral slope close to -1 for strong
shocks. Moderate shocks, such as those produced in mergers between clusters of
galaxies, lead to a somewhat steeper spectrum. Moreover, we predict an upper
limit for the radio power of cosmological shocks. Comparing our results to the
radio relics in Abell115, 2256, and 3667, we conclude that the magnetic field
in these relics is typically at a level of 0.1 mu G. Magnetic fields in the
intra-cluster medium are presumably generated by the shocks themselves, this
allows us to calculate the radio emission as a function of the cluster
temperature. The resulting emissions agree very well with the radio
power-temperature relation found for cluster halos. Finally, we show that
cosmic accretion shocks generate less radio emission than merger shock waves.
The latter may, however, be detected with upcoming radio telescopes.Comment: 28 pages, 8 figures, MNRAS accepte
Do radio relics challenge diffusive shock acceleration?
Radio relics in galaxy clusters are thought to be associated with powerful
shock waves that accelerate particles via diffusive shock acceleration (DSA).
Among the particles accelerated by DSA, relativistic protons should outnumber
electrons by a large factor. While the relativistic electrons emit synchrotron
emission detectable in the radio band, the protons interact with the thermal
gas to produce gamma-rays in hadronic interactions. Using simple models for the
propagation of shock waves through clusters, the distribution of thermal gas
and the efficiency of DSA, we find that the resulting hadronic -ray
emission lies very close or above the upper limits from the FERMI data on
nearby clusters. This suggests that the relative acceleration efficiency of
electrons and protons is at odds with predictions from DSA. The inclusion of
re-accelerated "fossil" particles does not seem to solve the problem. Our study
highlights a possible tension of the commonly assumed scenario for the
formation of radio relics and we discuss possible solutions to the problem.Comment: 7 pages, 3 figures. Updated version to match with the published
version in MNRA
On the Formation of Cluster Radio Relics
(abridged) We present detailed 3-dimensional magneto-hydrodynamical
simulations of the passage of a radio plasma cocoon filled with turbulent
magnetic fields through a shock wave. Taking into account synchrotron, inverse
Compton and adiabatic energy losses and gains we evolved the relativistic
electron population to produce synthetic polarisation radio maps. On contact
with the shock wave the radio cocoons are first compressed and finally torn
into filamentary structures, as is observed in several cluster radio relics. In
the synthetic radio maps the electric polarisation vectors are mostly
perpendicular to the filamentary radio structures. If the magnetic field inside
the cocoon is not too strong, the initially spherical radio cocoon is
transformed into a torus after the passage of the shock wave. Very recent,
high-resolution radio maps of cluster radio relics seem to exhibit such
toroidal geometries in some cases. This supports the hypothesis that cluster
radio relics are fossil radio cocoons that have been revived by a shock wave.
For a late-stage relic the ratio of its global diameter to the filament
diameter should correlate with the shock strength. Finally, we argue that the
total radio polarisation of radio relic should be well correlated with the
3-dimensional orientation of the shock wave that produced the relic.Comment: accepted by MNRAS, 10 pages, 13 figures, some modifications due to
comments of a refere
Thermal and non-thermal traces of AGN feedback: results from cosmological AMR simulations
We investigate the observable effects of feedback from Active Galactic Nuclei
(AGN) on non-thermal components of the intracluster medium (ICM). We have
modelled feedback from AGN in cosmological simulations with the adaptive mesh
refinement code ENZO, investigating three types of feedback that are sometimes
called quasar, jet and radio mode. Using a small set of galaxy clusters
simulated at high resolution, we model the injection and evolution of Cosmic
Rays, as well as their effects on the thermal plasma. By comparing, both, the
profiles of thermal gas to observed profiles from the ACCEPT sample, and the
secondary gamma-ray emission to the available upper limits from FERMI, we
discuss how the combined analysis of these two observables can constrain the
energetics and mechanisms of feedback models in clusters. Those modes of AGN
feedback that provide a good match to X-ray observations, yield a gamma-ray
luminosity resulting from secondary cosmic rays that is about below the
available upper limits from FERMI. Moreover, we investigate the injection of
turbulent motions into the ICM from AGN, and the detectability of these motions
via the analysis of line broadening of the Fe XXIII line. In the near future,
deeper observations/upper-limits of non-thermal emissions from galaxy clusters
will yield stringent constraints on the energetics and modes of AGN feedback,
even at early cosmic epochs.Comment: 24 pages, 20 figures. MNRAS accepted. A version of the paper with
higher quality figures can be found at this url:
http://www.ira.inaf.it/~vazza/papers/feedback_vazza.pd
The structure and fate of white dwarf merger remnants
We present a large parameter study where we investigate the structure of
white dwarf (WD) merger remnants after the dynamical phase. A wide range of WD
masses and compositions are explored and we also probe the effect of different
initial conditions. We investigated the degree of mixing between the WDs, the
conditions for detonations as well as the amount of gas ejected. We find that
systems with lower mass ratios have more total angular momentum and as a result
more mass is flung out in a tidal tail. Nuclear burning can affect the amount
of mass ejected. Many WD binaries that contain a helium-rich WD achieve the
conditions to trigger a detonation. In contrast, for carbon-oxygen transferring
systems only the most massive mergers with a total mass above ~2.1 solar masses
detonate. Even systems with lower mass may detonate long after the merger if
the remnant remains above the Chandrasekhar mass and carbon is ignited at the
centre. Finally, our findings are discussed in the context of several possible
observed astrophysical events and stellar systems, such as hot subdwarfs, R
Coronae Borealis stars, single massive white dwarfs, supernovae of type Ia and
other transient events. A large database containing 225 white dwarf merger
remnants is made available via a dedicated web page.Comment: 23 pages, 15 figures, submitted to MNRAS. A database containing 225
WD merger remnants is available for download at
http://www.hs.uni-hamburg.de/DE/Ins/Per/Dan/wdwd_remnants.htm
Shock heating by FR I radio sources in galaxy clusters
Feedback by active galactic nuclei (AGN) is frequently invoked to explain the
cut-off of the galaxy luminosity function at the bright end and the absence of
cooling flows in galaxy clusters. Meanwhile, there are recent observations of
shock fronts around radio-loud AGN. Using realistic 3D simulations of jets in a
galaxy cluster, we address the question what fraction of the energy of active
galactic nuclei is dissipated in shocks. We find that weak shocks that
encompass the AGN have Mach numbers of 1.1-1.2 and dissipate at least 2% of the
mechanical luminosity of the AGN. In a realistic cluster medium, even a
continuous jet can lead to multiple shock structures, which may lead to an
overestimate of the AGN duty cycles inferred from the spatial distribution of
waves.Comment: accepted by MNRAS Letter
Trade-offs in a 1 Tbps MIMO Communication System Between an Airship and an Array of Ground Receive Antennas
As demand for higher data-rate wireless communications increases, so will the interest in multiple-input and multiple-output (MIMO) systems. In a single transmitter, single receiver communication system, there is a fundamental limit to the data-rate capacity of the system proportional to the systems bandwidth. Since increasing the bandwidth is expensive and limited, another option is increasing the system\u27s capacity by adding multiple antennas at the transmitter and receiver to create a MIMO communication system. With a T transmitter, R receiver MIMO communication system, TR channels are created which allow extremely high data-rates. MIMO systems are attractive because they are extremely robust as they are able to operate when encountering channels with severe attenuation also known as deep fades. MIMO systems are known for their ability to achieve extremely high data-rates created by the multiple channels while improving bit error rate (BER) through diversity
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