198 research outputs found
The effects of polydispersity and metastability on crystal growth kinetics
We investigate the effect of metastable gas-liquid (G-L) separation on
crystal growth in a system of either monodisperse or slightly size-polydisperse
square well particles, using a simulation setup that allows us to focus on the
growth of a single crystal. Our system parameters are such that, inside the
metastable G-L binodal, a macroscopic layer of the gas phase "coats" the
crystal as it grows, consistent with experiment and theoretical free energy
considerations. Crucially, the effect of this metastable G-L separation on the
crystal growth rate depends qualitatively on whether the system is
polydisperse. We measure reduced polydispersity and qualitatively different
local size ordering in the crystal relative to the fluid, proposing that the
required fractionation is dynamically facilitated by the gas layer. Our results
show that polydispersity and metastability, both ubiquitous in soft matter,
must be considered in tandem if their dynamical effects are to be understood.Comment: Published in Soft Matter. DOI: 10.1039/C3SM27627
Persistent effects of pre-Columbian plant domestication on Amazonian forest composition
The extent to which pre-Columbian societies altered Amazonian landscapes is hotly debated. We performed a basin-wide analysis of pre-Columbian impacts on Amazonian forests by overlaying known archaeological sites in Amazonia with the distributions and abundances of 85 woody species domesticated by pre-Columbian peoples. Domesticated species are five times more likely to be hyperdominant than non-domesticated species. Across the basin the relative abundance and richness of domesticated species increases in forests on and around archaeological sites. In southwestern and eastern Amazonia distance to archaeological sites strongly influences the relative abundance and richness of domesticated species. Our analyses indicate that modern tree communities in Amazonia are structured to an important extent by a long history of plant domestication by Amazonian peoples
Local hydrological conditions influence tree diversity and composition across the Amazon basin
Tree diversity and composition in Amazonia are known to be strongly determined by the water supplied by precipitation. Nevertheless, within the same climatic regime, water availability is modulated by local topography and soil characteristics (hereafter referred to as local hydrological conditions), varying from saturated and poorly drained to well-drained and potentially dry areas. While these conditions may be expected to influence species distribution, the impacts of local hydrological conditions on tree diversity and composition remain poorly understood at the whole Amazon basin scale. Using a dataset of 443 1-ha non-flooded forest plots distributed across the basin, we investigate how local hydrological conditions influence 1) tree alpha diversity, 2) the community-weighted wood density mean (CWM-wd) – a proxy for hydraulic resistance and 3) tree species composition. We find that the effect of local hydrological conditions on tree diversity depends on climate, being more evident in wetter forests, where diversity increases towards locations with well-drained soils. CWM-wd increased towards better drained soils in Southern and Western Amazonia. Tree species composition changed along local soil hydrological gradients in Central-Eastern, Western and Southern Amazonia, and those changes were correlated with changes in the mean wood density of plots. Our results suggest that local hydrological gradients filter species, influencing the diversity and composition of Amazonian forests. Overall, this study shows that the effect of local hydrological conditions is pervasive, extending over wide Amazonian regions, and reinforces the importance of accounting for local topography and hydrology to better understand the likely response and resilience of forests to increased frequency of extreme climate events and rising temperatures
Constraints on axion-like particles with the Perseus Galaxy Cluster with MAGIC
Axion-like particles (ALPs) are pseudo-Nambu-Goldstone bosons that emerge in
various theories beyond the standard model. These particles can interact with
high-energy photons in external magnetic fields, influencing the observed
gamma-ray spectrum. This study analyzes 41.3 hrs of observational data from the
Perseus Galaxy Cluster collected with the MAGIC telescopes. We focused on the
spectra the radio galaxy in the center of the cluster: NGC 1275. By modeling
the magnetic field surrounding this target, we searched for spectral
indications of ALP presence. Despite finding no statistical evidence of ALP
signatures, we were able to exclude ALP models in the sub-micro electronvolt
range. Our analysis improved upon previous work by calculating the full
likelihood and statistical coverage for all considered models across the
parameter space. Consequently, we achieved the most stringent limits to date
for ALP masses around 50 neV, with cross sections down to GeV.Comment: 25 pages, 10 figures, accepted for publication in Physics of the Dark
Univers
Multi-year characterisation of the broad-band emission from the intermittent extreme BL Lac 1ES 2344+514
Aims. The BL Lac 1ES 2344+514 is known for temporary extreme properties characterised by a shift of the synchrotron spectral energy distribution (SED) peak energy νsynch;p above 1 keV. While those extreme states have only been observed during high flux levels thus far, additional multi-year observing campaigns are required to achieve a coherent picture. Here, we report the longest investigation of the source from radio to very high energy (VHE) performed so far, focussing on a systematic characterisation of the intermittent extreme states. Methods.We organised a monitoring campaign covering a 3-year period from 2019 to 2021.Morethan ten instruments participated in the observations in order to cover the emission from radio to VHE. In particular, sensitive X-ray measurements by XMM-Newton, NuSTAR, and AstroSat took place simultaneously with multi-hour MAGIC observations, providing an unprecedented constraint of the two SED components for this blazar. Results. While our results confirm that 1ES 2344+514 typically exhibits νsynch;p > 1 keV during elevated flux periods, we also find periods where the extreme state coincides with low flux activity. A strong spectral variability thus happens in the quiescent state, and is likely caused by an increase in the electron acceleration efficiency without a change in the electron injection luminosity. On the other hand, we also report a strong X-ray flare (among the brightest for 1ES 2344+514) without a significant shift of νsynch;p. During this particular flare, the X-ray spectrum is among the softest of the campaign. It unveils complexity in the spectral evolution, where the common harder-when-brighter trend observed in BL Lacs is violated. By combining Swift-XRT and Swift-UVOT measurements during a low and hard X-ray state, we find an excess of the UV flux with respect to an extrapolation of the X-ray spectrum to lower energies. This UV excess implies that at least two regions significantly contribute to the infrared/optical/ultraviolet/X-ray emission. Using the simultaneous MAGIC, XMM-Newton, NuSTAR, and AstroSat observations, we argue that a region possibly associated with the 10 GHz radio core may explain such an excess. Finally, we investigate a VHE flare, showing an absence of simultaneous variability in the 0.3-2 keV band. Using time-dependent leptonic modelling, we show that this behaviour, in contradiction to single-zone scenarios, can instead be explained by a two-component model
Multi-year characterisation of the broad-band emission from the intermittent extreme BL Lac 1ES~2344+514
The BL Lac 1ES 2344+514 is known for temporary extreme properties (e.g., a
shift of the synchrotron SED peak energy above 1keV). While
those extreme states were so far observed only during high flux levels,
additional multi-year observing campaigns are required to achieve a coherent
picture. Here, we report the longest investigation of the source from radio to
VHE performed so far, focusing on a systematic characterisation of the
intermittent extreme states. While our results confirm that 1ES 2344+514
typically exhibits 1keV during elevated flux periods, we also
find periods where the extreme state coincides with low flux activity. A strong
spectral variability thus happens in the quiescent state, and is likely caused
by an increase of the electron acceleration efficiency without a change in the
electron injection luminosity. We also report a strong X-ray flare (among the
brightest for 1ES 2344+514) without a significant shift of .
During this particular flare, the X-ray spectrum is among the softest of the
campaign. It unveils complexity in the spectral evolution, where the common
harder-when-brighter trend observed in BL Lacs is violated. During a low and
hard X-ray state, we find an excess of the UV flux with respect to an
extrapolation of the X-ray spectrum to lower energies. This UV excess implies
that at least two regions contribute significantly to the
infrared/optical/ultraviolet/X-ray emission. Using the simultaneous MAGIC,
XMM-Newton, NuSTAR, and AstroSat observations, we argue that a region possibly
associated with the 10 GHz radio core may explain such an excess. Finally, we
investigate a VHE flare, showing an absence of simultaneous variability in the
0.3-2keV band. Using a time-dependent leptonic modelling, we show that this
behaviour, in contradiction to single-zone scenarios, can instead be explained
by a two-component model.Comment: Accepted for publication in Astronomy & Astrophysic
MAGIC observations provide compelling evidence of hadronic multi-TeV emission from the putative PeVatron SNR G106.3+2.7
Context. Certain types of supernova remnants (SNRs) in our Galaxy are assumed to be PeVatrons, capable of accelerating cosmic rays (CRs) to ∼ PeV energies. However, conclusive observational evidence for this has not yet been found. The SNR G106.3+2.7, detected at 1- 100 TeV energies by different γ-ray facilities, is one of the most promising PeVatron candidates. This SNR has a cometary shape, which can be divided into a head and a tail region with different physical conditions. However, in which region the 100 TeV emission is produced has not yet been identified because of the limited position accuracy and/or angular resolution of existing observational data. Additionally, it remains unclear as to whether the origin of the γ-ray emission is leptonic or hadronic. Aims. With the better angular resolution provided by new MAGIC data compared to earlier γ-ray datasets, we aim to reveal the acceleration site of PeV particles and the emission mechanism by resolving the SNR G106.3+2.7 with 0.1 resolution at TeV energies. Methods. We observed the SNR G106.3+2.7 using the MAGIC telescopes for 121.7 h in total - after quality cuts - between May 2017 and August 2019. The analysis energy threshold is ∼0.2 TeV, and the angular resolution is 0.07-0.1. We examined the γ-ray spectra of different parts of the emission, whilst benefitting from the unprecedented statistics and angular resolution at these energies provided by our new data. We also used measurements at other wavelengths such as radio, X-rays, GeV γ-rays, and 10 TeV γ-rays to model the emission mechanism precisely. Results. We detect extended γ-ray emission spatially coincident with the radio continuum emission at the head and tail of SNR G106.3+2.7. The fact that we detect a significant γ-ray emission with energies above 6.0 TeV from only the tail region suggests that the emissions above 10 TeV detected with air shower experiments (Milagro, HAWC, Tibet ASγ and LHAASO) are emitted only from the SNR tail. Under this assumption, the multi-wavelength spectrum of the head region can be explained with either hadronic or leptonic models, while the leptonic model for the tail region is in contradiction with the emission above 10 TeV and X-rays. In contrast, the hadronic model could reproduce the observed spectrum at the tail by assuming a proton spectrum with a cutoff energy of ∼1 PeV for that region. Such high-energy emission in this middle-aged SNR (4-10 kyr) can be explained by considering a scenario where protons escaping from the SNR in the past interact with surrounding dense gases at present. Conclusions. The γ-ray emission region detected with the MAGIC telescopes in the SNR G106.3+2.7 is extended and spatially coincident with the radio continuum morphology. The multi-wavelength spectrum of the emission from the tail region suggests proton acceleration up to ∼PeV, while the emission mechanism of the head region could either be hadronic or leptonic
MAGIC detection of GRB 201216C at z = 1.1
Gamma-ray bursts (GRBs) are explosive transient events occurring at cosmological distances, releasing a large amount of energy as electromagnetic radiation over several energy bands. We report the detection of the long GRB 201216C by the MAGIC telescopes. The source is located at z = 1.1 and thus it is the farthest one detected at very high energies. The emission above 70 GeV of GRB 201216C is modelled together with multiwavelength data within a synchrotron and synchrotron self-Compton (SSC) scenario. We find that SSC can explain the broad-band data well from the optical to the very-high-energy band. For the late-time radio data, a different component is needed to account for the observed emission. Differently from previous GRBs detected in the very-high-energy range, the model for GRB 201216C strongly favours a wind-like medium. The model parameters have values similar to those found in past studies of the afterglows of GRBs detected up to GeV energies
MAGIC observations provide compelling evidence of the hadronic multi-TeV emission from the putative PeVatron SNR G106.3+2.7
The SNR G106.3+2.7, detected at 1--100 TeV energies by different -ray
facilities, is one of the most promising PeVatron candidates. This SNR has a
cometary shape which can be divided into a head and a tail region with
different physical conditions. However, it is not identified in which region
the 100 TeV emission is produced due to the limited position accuracy and/or
angular resolution of existing observational data. Additionally, it remains
unclear whether the origin of the -ray emission is leptonic or
hadronic. With the better angular resolution provided by these new MAGIC data
compared to earlier -ray datasets, we aim to reveal the acceleration
site of PeV particles and the emission mechanism by resolving the SNR
G106.3+2.7 with 0.1 resolution at TeV energies. We detected extended
-ray emission spatially coincident with the radio continuum emission at
the head and tail of SNR G106.3+2.7. The fact that we detected a significant
-ray emission with energies above 6.0 TeV from the tail region only
suggests that the emissions above 10 TeV, detected with air shower experiments
(Milagro, HAWC, Tibet AS and LHAASO), are emitted only from the SNR
tail. Under this assumption, the multi-wavelength spectrum of the head region
can be explained with either hadronic or leptonic models, while the leptonic
model for the tail region is in contradiction with the emission above 10 TeV
and X-rays. In contrast, the hadronic model could reproduce the observed
spectrum at the tail by assuming a proton spectrum with a cutoff energy of
PeV for the tail region. Such a high energy emission in this
middle-aged SNR (4--10 kyr) can be explained by considering the scenario that
protons escaping from the SNR in the past interact with surrounding dense gases
at present.Comment: 13 pages, 7 figures, Accepted for publication in A&
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