83 research outputs found
3D global simulations of a cosmic-ray-driven dynamo in dwarf galaxies
Star-forming dwarf galaxies can be seen as the local proxies of the
high-redshift building blocks of more massive galaxies according to the current
paradigm of the hierarchical galaxy formation. They are low-mass objects, and
therefore their rotation speed is very low. Several galaxies are observed to
show quite strong magnetic fields. These cases of strong ordered magnetic
fields seem to correlate with a high, but not extremely high, star formation
rate. We investigate whether these magnetic fields could be generated by the
cosmic-ray-driven dynamo. The environment of a dwarf galaxy is unfavourable for
the large-scale dynamo action because of the very slow rotation that is
required to create the regular component of the magnetic field. We built a 3D
global model of a dwarf galaxy that consists of two gravitational components:
the stars and the dark-matter halo described by the purely phenomenological
profile proposed previously. We solved a system of magnetohydrodynamic (MHD)
equations that include an additional cosmic-ray component described by the
fluid approximation. We found that the cosmic-ray-driven dynamo can amplify the
magnetic field with an exponential growth rate. The -folding time is
correlated with the initial rotation speed. The final mean value of the
azimuthal flux for our models is of the order of few G and the system
reaches its equipartition level. The results indicate that the
cosmic-ray-driven dynamo is a process that can explain the magnetic fields in
dwarf galaxies.Comment: 6 pages, 4 figures, accepted for publication in A&
Cosmic-ray driven dynamo in the interstellar medium of irregular galaxies
Irregular galaxies are usually smaller and less massive than their spiral,
S0, and elliptical counterparts. Radio observations indicate that a magnetic
field is present in irregular galaxies whose value is similar to that in spiral
galaxies. However, the conditions in the interstellar medium of an irregular
galaxy are unfavorable for amplification of the magnetic field because of the
slow rotation and low shearing rate. We investigate the cosmic-ray driven
dynamo in the interstellar medium of an irregular galaxy. We study its
efficiency under the conditions of slow rotation and weak shear. The star
formation is also taken into account in our model and is parametrized by the
frequency of explosions and modulations of activity. The numerical model
includes a magnetohydrodynamical dynamo driven by cosmic rays that is injected
into the interstellar medium by randomly exploding supernovae. In the model, we
also include essential elements such as vertical gravity of the disk,
differential rotation approximated by the shearing box, and resistivity leading
to magnetic reconnection. We find that even slow galactic rotation with a low
shearing rate amplifies the magnetic field, and that rapid rotation with a low
value of the shear enhances the efficiency of the dynamo. Our simulations have
shown that a high amount of magnetic energy leaves the simulation box becoming
an efficient source of intergalactic magnetic fields.Comment: 9 pages, 6 figure
Cosmic-ray driven dynamo in galaxies
We present recent developments of global galactic-scale numerical models of
the Cosmic Ray (CR) driven dynamo, which was originally proposed by Parker
(1992). We conduct a series of direct CR+MHD numerical simulations of the
dynamics of the interstellar medium (ISM), composed of gas, magnetic fields and
CR components. We take into account CRs accelerated in randomly distributed
supernova (SN) remnants, and assume that SNe deposit small-scale, randomly
oriented, dipolar magnetic fields into the ISM. The amplification timescale of
the large-scale magnetic field resulting from the CR-driven dynamo is
comparable to the galactic rotation period. The process efficiently converts
small-scale magnetic fields of SN-remnants into galactic-scale magnetic fields.
The resulting magnetic field structure resembles the X-shaped magnetic fields
observed in edge-on galaxies.Comment: 6 pages, 4 figures, to appear in Proceedings of IAU Symp. 274,
Advances in Plasma Astrophysics, ed. A. Bonanno, E. de Gouveia dal Pino and
A. Kosoviche
A cosmic ray cocoon along the X-ray jet of M87?
Relativistic jets propagating through an ambient medium must produce some
observational effects along their side boundaries because of interactions
across the large velocity gradient. One possible effect of such an interaction
would be a sheared magnetic field structure at the jet boundaries, leading to a
characteristic radio polarization pattern. As proposed by Ostrowski, another
effect can come from the generation of a high energy cosmic ray component at
the boundary, producing dynamic effects on the medium surrounding the jet and
forming a cocoon dominated by cosmic rays with a decreased thermal gas
emissivity. We selected this process for our first attempt to look for the
effects of this type of interaction. We analyzed the Chandra X-ray data for the
radio galaxy M87 in order to verify if the expected regions of diminished
emissivity may be present near the spectacular X-ray jet in this source. The
detailed analysis of the data, merged from 42 separate observations, shows
signatures of lower emissivity surrounding the jet. In particular we detect an
intensity dip along the part of the jet, which would be approximately 150 pc x
2 kpc in size, if situated along the jet which is inclined toward us. Due to a
highly non-uniform X-ray background in the central region we are not able to
claim the discovery of a cosmic ray cocoon around the M87 jet: we only have
demonstrated that the data show morphological structures which could be
accounted for if a cosmic ray cocoon exists.Comment: 8 pages, 8 pictures accepted for publication in MNRA
The non-thermal superbubble in IC 10 : the generation of cosmic ray electrons caught in the act
Superbubbles are crucial for stellar feedback, with supposedly high (of the order of 10 per cent) thermalization rates. We combined multiband radio continuum observations from the Very Large Array (VLA) with Effelsberg data to study the non-thermal superbubble (NSB) in IC 10, a starburst dwarf irregular galaxy in the Local Group. Thermal emission was subtracted using a combination of Balmer Hα and VLA 32 GHz continuum maps. The bubble’s nonthermal spectrum between 1.5 and 8.8 GHz displays curvature and can be well fitted with a standard model of an ageing cosmic ray electron population. With a derived equipartition magnetic field strength of 44 ±8 μG, and measuring the radiation energy density from Spitzer MIPS maps as 5±1×10−11 erg cm−3, we determine, based on the spectral curvature, a spectral age of the bubble of 1.0 ± 0.3 Myr. Analysis of the LITTLE THINGS HI data cube shows an expanding HI hole with 100 pc diameter and a dynamical age of 3.8 ± 0.3 Myr, centred to within 16 pc on IC 10 X-1, a massive stellar mass black hole (M > 23 M⊙). The results are consistent with the expected evolution for a superbubble with a few massive stars, where a very energetic event like a Type Ic supernova/hypernova has taken place about 1 Myr ago. We discuss alternatives to this interpretationPeer reviewe
Cherenkov Telescope Array Data Management
Very High Energy gamma-ray astronomy with the Cherenkov Telescope Array (CTA)
is evolving towards the model of a public observatory. Handling, processing and
archiving the large amount of data generated by the CTA instruments and
delivering scientific products are some of the challenges in designing the CTA
Data Management. The participation of scientists from within CTA Consortium and
from the greater worldwide scientific community necessitates a sophisticated
scientific analysis system capable of providing unified and efficient user
access to data, software and computing resources. Data Management is designed
to respond to three main issues: (i) the treatment and flow of data from remote
telescopes; (ii) "big-data" archiving and processing; (iii) and open data
access. In this communication the overall technical design of the CTA Data
Management, current major developments and prototypes are presented.Comment: 8 pages, 2 figures, In Proceedings of the 34th International Cosmic
Ray Conference (ICRC2015), The Hague, The Netherlands. All CTA contributions
at arXiv:1508.0589
Magnetic fields in cosmic particle acceleration sources
We review here some magnetic phenomena in astrophysical particle accelerators
associated with collisionless shocks in supernova remnants, radio galaxies and
clusters of galaxies. A specific feature is that the accelerated particles can
play an important role in magnetic field evolution in the objects. We discuss a
number of CR-driven, magnetic field amplification processes that are likely to
operate when diffusive shock acceleration (DSA) becomes efficient and
nonlinear. The turbulent magnetic fields produced by these processes determine
the maximum energies of accelerated particles and result in specific features
in the observed photon radiation of the sources. Equally important, magnetic
field amplification by the CR currents and pressure anisotropies may affect the
shocked gas temperatures and compression, both in the shock precursor and in
the downstream flow, if the shock is an efficient CR accelerator. Strong
fluctuations of the magnetic field on scales above the radiation formation
length in the shock vicinity result in intermittent structures observable in
synchrotron emission images. Resonant and non-resonant CR streaming
instabilities in the shock precursor can generate mesoscale magnetic fields
with scale-sizes comparable to supernova remnants and even superbubbles. This
opens the possibility that magnetic fields in the earliest galaxies were
produced by the first generation Population III supernova remnants and by
clustered supernovae in star forming regions.Comment: 30 pages, Space Science Review
Sensitivity of the Cherenkov Telescope Array to TeV photon emission from the Large Magellanic Cloud
A deep survey of the Large Magellanic Cloud at ∼0.1-100 TeV photon energies with the Cherenkov Telescope Array is planned. We assess the detection prospects based on a model for the emission of the galaxy, comprising the four known TeV emitters, mock populations of sources, and interstellar emission on galactic scales. We also assess the detectability of 30 Doradus and SN 1987A, and the constraints that can be derived on the nature of dark matter. The survey will allow for fine spectral studies of N 157B, N 132D, LMC P3, and 30 Doradus C, and half a dozen other sources should be revealed, mainly pulsar-powered objects. The remnant from SN 1987A could be detected if it produces cosmic-ray nuclei with a flat power-law spectrum at high energies, or with a steeper index 2.3-2.4 pending a flux increase by a factor of >3-4 over ∼2015-2035. Large-scale interstellar emission remains mostly out of reach of the survey if its >10 GeV spectrum has a soft photon index ∼2.7, but degree-scale 0.1-10 TeV pion-decay emission could be detected if the cosmic-ray spectrum hardens above >100 GeV. The 30 Doradus star-forming region is detectable if acceleration efficiency is on the order of 1−10 per cent of the mechanical luminosity and diffusion is suppressed by two orders of magnitude within <100 pc. Finally, the survey could probe the canonical velocity-averaged cross-section for self-annihilation of weakly interacting massive particles for cuspy Navarro-Frenk-White profiles
Sensitivity of the Cherenkov Telescope Array to spectral signatures of hadronic PeVatrons with application to Galactic Supernova Remnants
The local Cosmic Ray (CR) energy spectrum exhibits a spectral softening at
energies around 3~PeV. Sources which are capable of accelerating hadrons to
such energies are called hadronic PeVatrons. However, hadronic PeVatrons have
not yet been firmly identified within the Galaxy. Several source classes,
including Galactic Supernova Remnants (SNRs), have been proposed as PeVatron
candidates. The potential to search for hadronic PeVatrons with the Cherenkov
Telescope Array (CTA) is assessed. The focus is on the usage of very high
energy -ray spectral signatures for the identification of PeVatrons.
Assuming that SNRs can accelerate CRs up to knee energies, the number of
Galactic SNRs which can be identified as PeVatrons with CTA is estimated within
a model for the evolution of SNRs. Additionally, the potential of a follow-up
observation strategy under moonlight conditions for PeVatron searches is
investigated. Statistical methods for the identification of PeVatrons are
introduced, and realistic Monte--Carlo simulations of the response of the CTA
observatory to the emission spectra from hadronic PeVatrons are performed.
Based on simulations of a simplified model for the evolution for SNRs, the
detection of a -ray signal from in average 9 Galactic PeVatron SNRs is
expected to result from the scan of the Galactic plane with CTA after 10 hours
of exposure. CTA is also shown to have excellent potential to confirm these
sources as PeVatrons in deep observations with hours of
exposure per source.Comment: 34 pages, 16 figures, Accepted for publication in Astroparticle
Physic
Sensitivity of the Cherenkov Telescope Array for probing cosmology and fundamental physics with gamma-ray propagation
The Cherenkov Telescope Array (CTA), the new-generation ground-based observatory for γ astronomy, provides unique capabilities to address significant open questions in astrophysics, cosmology, and fundamental physics. We study some of the salient areas of γ cosmology that can be explored as part of the Key Science Projects of CTA, through simulated observations of active galactic nuclei (AGN) and of their relativistic jets. Observations of AGN with CTA will enable a measurement of γ absorption on the extragalactic background light with a statistical uncertainty below 15% up to a redshift z=2 and to constrain or detect γ halos up to intergalactic-magnetic-field strengths of at least 0.3 pG . Extragalactic observations with CTA also show promising potential to probe physics beyond the Standard Model. The best limits on Lorentz invariance violation from γ astronomy will be improved by a factor of at least two to three. CTA will also probe the parameter space in which axion-like particles could constitute a significant fraction, if not all, of dark matter. We conclude on the synergies between CTA and other upcoming facilities that will foster the growth of γ cosmology.</p
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