332 research outputs found
Massive Stars as Major Factories of Galactic Cosmic Rays
The identification of major contributors to the locally observed fluxes of
Cosmic Rays (CRs) is a prime objective towards the resolution of the
long-standing enigma of CRs. We report on a compelling similarity of the energy
and radial distributions of multi-TeV CRs extracted from observations of very
high energy (VHE) -rays towards the Galactic Center (GC) and two
prominent clusters of young massive stars, Cyg~OB2 and Westerlund~1. This
resemblance we interpret as a hint that CRs responsible for the diffuse VHE
-ray emission from the GC are accelerated by the ultracompact stellar
clusters located in the heart of GC. The derived decrement of the CR
density with the distance from a star cluster is a distinct signature of
continuous, over a few million years, CR injection into the interstellar
medium. The lack of brightening of the -ray images toward the stellar
clusters excludes the leptonic origin of -radiation. The hard, type power-law energy spectra of parent protons continues up to
1 PeV. The efficiency of conversion of kinetic energy of stellar winds
to CRs can be as high as 10 percent implying that the young massive stars may
operate as proton PeVatrons with a dominant contribution to the flux of highest
energy galactic CRs.Comment: minor revisions have been applied to address the referees' comments,
conclusion unchange
Time-dependent modeling of TeV-detected, young pulsar wind nebulae
The increasing sensitivity of instruments at X-ray and TeV energies have
revealed a large number of nebulae associated with bright pulsars. Despite this
large data set, the observed pulsar wind nebulae (PWNe) do not show a uniform
behavior and the main parameters driving features like luminosity,
magnetization, and others are still not fully understood. To evaluate the
possible existence of common evolutive trends and to link the characteristics
of the nebula emission with those of the powering pulsar, we selected a sub-set
of 10 TeV detections which are likely ascribed to young PWNe and model the
spectral energy distribution with a time-dependent description of the nebulae's
electron population. In 9 of these cases, a detailed PWNe model, using
up-to-date multiwavelength information, is presented. The best-fit parameters
of these nebula are discussed, together with the pulsar characteristics. We
conclude that TeV PWNe are particle-dominated objects with large
multiplicities, in general far from magnetic equipartition, and that relatively
large photon field enhancements are required to explain the high level of
Comptonized photons observed. We do not find significant correlations between
the efficiencies of emission at different frequencies and the magnetization.
The injection parameters do not appear to be particularly correlated with the
pulsar properties either. We find that a normalized comparison of the SEDs
(e.g., with the corresponding spin-down flux) at the same age significantly
reduces the spectral distributions dispersion.Comment: 38 pages, 20 figures, 8 tables. Accepted for publication in the
Journal of High Energy Astrophysics (JHEAp
The effects of magnetic field, age, and intrinsic luminosity on Crab-like pulsar wind nebulae
We investigate the time-dependent behavior of Crab-like pulsar wind nebulae
(PWNe) generating a set of models using 4 different initial spin-down
luminosities (), 8 values
of magnetic fraction ( 0.001, 0.01, 0.03, 0.1, 0.5, 0.9, 0.99, and
0.999, i.e., from fully particle dominated to fully magnetically dominated
nebulae), and 3 distinctive ages: 940, 3000, and 9000 years. We find that the
self-synchrotron Compton (SSC) contribution is irrelevant for =0.1, 1,
and 10% of the Crab power, disregarding the age and the magnetic fraction. SSC
only becomes relevant for highly energetic ( of the Crab), particle
dominated nebulae at low ages (of less than a few kyr), located in a FIR
background with relatively low energy density. Since no pulsar other than Crab
is known to have these features, these results clarify why the Crab Nebula, and
only it, is SSC dominated. No young PWN would be detectable at TeV energies if
the pulsar's spin-down power is 0.1% Crab or lower. For 1% of the Crab
spin-down, only particle dominated nebulae can be detected by H.E.S.S.-like
telescopes when young enough (with details depending on the precise injection
and environmental parameters). Above 10% of the Crab's power, all PWNe are
detectable by H.E.S.S.-like telescopes if they are particle dominated, no
matter the age. The impact of the magnetic fraction on the final SED is varied
and important, generating order of magnitude variations in the luminosity
output for systems that are otherwise the same (equal , , injection,
and environment).Comment: Accepted for publication in MNRA
Is there room for highly magnetized pulsar wind nebulae among those non-detected at TeV?
We make a time-dependent characterization of pulsar wind nebulae (PWNe)
surrounding some of the highest spin-down pulsars that have not yet been
detected at TeV. Our aim is assessing their possible level of magnetization. We
analyze the nebulae driven by J2022+3842 in G76.9+1.0, J0540-6919 in N158A (the
Crab twin), J1400--6325 in G310.6--1.6, and J1124--5916 in G292.0+0.18, none of
which have been found at TeV energies. For comparison we refer to published
models of G54.1+0.3, the Crab nebula, and develop a model for N157B in the
Large Magellanic Cloud (LMC). We conclude that further observations of N158A
could lead to its detection at VHE. According to our model, a FIR energy
density of 5 eV cm could already lead to a detection in H.E.S.S.
(assuming no other IC target field) within 50 hours of exposure and just the
CMB inverse Compton contribution would produce VHE photons at the CTA
sensitivity. We also propose models for G76.9+1.0, G310.6--1.6 and G292.0+1.8
which suggest their TeV detection in a moderate exposure for the latter two
with the current generation of Cherenkov telescopes. We analyze the possibility
that these PWNe are highly magnetized, where the low number of particles
explains the residual detection in X-rays and their lack of detection at TeV
energies.Comment: Accepted for publication in MNRA
Determination of the Night Sky Background around the Crab pulsar using its optical pulsation
The poor angular resolution of imaging gamma-ray telescopes is offset by the
large reflector areas of next generation telescopes such as MAGIC (17~m
diameter), which makes the study of optical emission associated with some
gamma-ray sources feasible. Furthermore, the extremely fast time response of
photomultipliers (PMs) makes them ideal detectors for fast (subsecond) optical
transients and periodic sources like pulsars. The optical pulse of the Crab
pulsar was detected with the HEGRA CT1 central pixel using a modified PM,
similar to the future MAGIC camera PMs. The purpose of these periodic
observations was to determine the light of the night sky (LONS) for the
galactic anticenter Crab region.Our results are between 2.5 and 3 times larger
than the previously measured LONS (outside the galactic plane), as expected
since the Crab pulsar is in the galactic plane, which implies a slightly higher
energy threshold for Crab observations, if the higher value of CT1 measured
LONS rate for galactic sources is used.Comment: 19 pages, 6 figures, accepted by Astroparticle Physic
CTA and cosmic-ray diffusion in molecular clouds
Molecular clouds act as primary targets for cosmic-ray interactions and are
expected to shine in gamma-rays as a by-product of these interactions. Indeed
several detected gamma-ray sources both in HE and VHE gamma-rays (HE: 100 MeV <
E 100 GeV) have been directly or indirectly associated with
molecular clouds. Information on the local diffusion coefficient and the local
cosmic-ray population can be deduced from the observed gamma-ray signals. In
this work we concentrate on the capability of the forthcoming Cherenkov
Telescope Array Observatory (CTA) to provide such measurements. We investigate
the expected emission from clouds hosting an accelerator, exploring the
parameter space for different modes of acceleration, age of the source, cloud
density profile, and cosmic ray diffusion coefficient. We present some of the
most interesting cases for CTA regarding this science topic. The simulated
gamma-ray fluxes depend strongly on the input parameters. In some cases, from
CTA data it will be possible to constrain both the properties of the
accelerator and the propagation mode of cosmic rays in the cloud.Comment: In Proceedings of the 2012 Heidelberg Symposium on High Energy
Gamma-Ray Astronomy. All CTA contributions at arXiv:1211.184
High energy gamma-ray emission powered by a young protostar: the case of S255 NIRS 3
Evidence of efficient acceleration of cosmic rays in massive young stellar
objects has been recently reported. Among these massive protostars, S255 NIRS 3
for which extreme flaring events associated with radio jets have been detected,
is one of the best objects to test this hypothesis. We search for gamma-ray
emission associated with this object in Fermi-LAT data and inspect the gas
content in different molecular lines using the MWISP survey. A GeV source
dubbed 4FGL J0613.1+1749c lies on top of the MYSO region, where two filamentary
~10 pc CO structures extend along the same direction of the sub-parsec radio
jets. We investigate the spectrum, morphology, and light curve of the gamma-ray
source and compare it with the theoretical emission expected from hadronic and
leptonic populations accelerated in the radio jets. We argue that the gamma-ray
source could be powered by particles accelerated in the S255 NIRS 3 jets,
radiating via Bremsstrahlung or proton-proton interaction, and with a
synchrotron component shinning in radio from primary or secondary electrons in
the case of a leptonic or hadronic population.Comment: accepted for publication in Monthly Notices of the Royal Astronomical
Societ
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