Abstracts of Interest
Selected by:
Ryan Burley
Abstract: 2403.16518
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Title:Bimodal orientation distribution and head-tail asymmetry of a sample of filamentary molecular clouds
View PDF HTML (experimental)Abstract:The morphology of molecular clouds is crucial for understanding their origin and evolution. In this work, we investigate the morphology of the filamentary molecular clouds (filaments for short) using a portion of the $^{12}\text{CO} (J=1-0)$ data from the Milky Way Imaging Scroll Painting (MWISP) project. The data cover an area spanning $104.75^\circ <l< 150.25^\circ , \vert b\vert < 5.25^\circ$ in Galactic coordinates, with $V_\text{LSR}$ ranging from $-95$ to 25 $\text{km s}^{-1}$. Our primary focus is on the orientation and morphological asymmetry of the filaments. To achieve this, we apply several criteria on the data to create a sample of filaments with well-defined straight shape, and we use elliptical fitting to obtain the orientation of each filament, with an estimated error of $\sim1.6^\circ$ for the orientation. We find that the filament orientation with respect to the Galactic plane exhibits a bimodal distribution, a double-Gaussian fitting of which has two centres located at $-38.1^\circ $ and $42.0^\circ $, with 1$\sigma$ of the two Gaussian functions being $35.4^\circ$ and $27.4^\circ$. We do not find significant correlation between the orientation and other parameters, including the Galactic coordinates, radial velocity, velocity width, and physical scale. A considerable fraction of filaments ($\gtrsim 40$ per cent) display head-tail asymmetry, which suggests that mass concentration tends to occur at one end of the filaments.
Abstract: 2403.16650
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Title:Probing Stellar Clusters from Gaia DR2 as Galactic PeVatrons: I -- Expected Gamma-ray and Neutrino Emission
View PDF HTML (experimental)Abstract:Young & massive stellar clusters (SCs) are a potential source of galactic cosmic rays up to very high energies as a result of two possible acceleration scenarios. Collective stellar winds from massive member stars form a wind-blown bubble with a termination shock (TS) at which particle acceleration to PeV energies may occur. Furthermore, shock acceleration may occur at SNRs expanding inside the bubble. By applying a model of CR acceleration at both the wind TS and SNR shocks to catalogues of known SCs derived from Gaia DR2, we identify the most promising targets to search for evidence of PeVatron activity. Predictions for the secondary fluxes of gamma-ray and neutrino emission are derived based on particle acceleration at the collective wind TS and the subsequent hadronic interactions with the surrounding medium. Predictions from our modelling under baseline and optimistic scenarios are compared to data, finding consistent results. We estimate the detection prospects for future gamma-ray and neutrino experiments. We find that degree-scale angular sizes of the wind-blown bubbles are typical, that may pose a challenge for experimental detection. A shortlist of the most promising candidates is provided, with an anticipated flux range. Of order 10 SCs may be detectable with future facilities, and 1-5 could be currently operating as PeVatrons. Of these, three gamma-ray detected SCs have data within our predicted range. Our model can consistently describe gamma-ray measurements of SC emission. Several further as-yet-undetected SCs offer promising targets for future observations, although the flux range allowed by our model can be large (> factor 10). The large angular size of the wind-blown bubble may lead to low surface brightness emission, worsening the problem of source confusion. Nevertheless, we discuss how further work will help to constrain SCs as PeVatron candidates. (abridged)
Abstract: 2403.16802
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Title:Unveiling extended gamma-ray emission around HESS J1813-178
View PDF HTML (experimental)Abstract:HESS J1813$-$178 is a very-high-energy $\gamma$-ray source spatially coincident with the young and energetic pulsar PSR J1813$-$1749 and thought to be associated with its pulsar wind nebula (PWN). Recently, evidence for extended high-energy emission in the vicinity of the pulsar has been revealed in the Fermi Large Area Telescope (LAT) data. This motivates revisiting the HESS J1813$-$178 region, taking advantage of improved analysis methods and an extended data set. Using data taken by the High Energy Stereoscopic System (H.E.S.S.) experiment and the Fermi-LAT, we aim to describe the $\gamma$-ray emission in the region with a consistent model, to provide insights into its origin. We performed a likelihood-based analysis on 32 hours of H.E.S.S. data and 12 years of Fermi-LAT data and fit a spectro-morphological model to the combined datasets. These results allowed us to develop a physical model for the origin of the observed $\gamma$-ray emission in the region. In addition to the compact very-high-energy $\gamma$-ray emission centered on the pulsar, we find a significant yet previously undetected component along the Galactic plane. With Fermi-LAT data, we confirm extended high-energy emission consistent with the position and elongation of the extended emission observed with H.E.S.S. These results establish a consistent description of the emission in the region from GeV energies to several tens of TeV. This study suggests that HESS J1813$-$178 is associated with a $\gamma$-ray PWN powered by PSR J1813$-$1749. A possible origin of the extended emission component is inverse Compton emission from electrons and positrons that have escaped the confines of the pulsar and form a halo around the PWN.
Abstract: 2403.17335
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Title:Cosmic-ray Acceleration in Core-Collapse Supernova Remnants with the Wind Termination Shock
View PDF HTML (experimental)Abstract:We investigate the attainable maximum energy of particles accelerated in the core-collapse supernova remnant (SNR) shock propagating in the free wind region with the Parker-spiral magnetic field, current sheet, and the wind termination shock (WTS) by using test particle simulations. This work focuses on Wolf-Rayet stars as progenitors. The magnetic field amplification in the free wind region (shock upstream region) is not considered in this work. Test particle simulations show that particles escaped from the core-collapse SNR reach and move along the WTS, and eventually return to the SNR shock from the poles or equator of the WTS. The particle attainable energy can be boosted by this cyclic motion between the SNR shock and WTS and can be larger than the particle energy that is limited by escape from the SNR shock. The particle energy limited by the cyclic motion between the SNR shock and WTS is about $10-100~{\rm TeV}$. Thus, the core-collapse SNR without upstream magnetic field amplification can be the origin of the break around $10~{\rm TeV}$ of the energy spectrum of observed cosmic ray protons and helium.
Abstract: 2403.18012
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Title:A closer scrutiny of cosmic ray proton energy losses
View PDF HTML (experimental)Abstract:The percent-level precision attained by modern cosmic ray (CR) observations motivates reaching a comparable or better control of theoretical uncertainties. Here we focus on energy-loss processes affecting low-energy CR protons ($\sim 0.1-5$ GeV), where the experimental errors are small and collisional effects play a comparatively larger role with respect to collisionless transport ones. We study three aspects of the problem: i) We quantitatively assess the role of the nuclear elastic cross-section, for the first time, providing analytical formulae for the stopping power and inelasticity. ii) We discuss the error arising from treating both elastic and pion production inelastic interactions as continuous energy loss processes, as opposed to catastrophic ones. The former is the approximation used in virtually all modern numerical calculations. iii) We consider sub-leading effects such as relativistic corrections, radiative and medium processes in ionization energy-losses. Our analysis reveals that neglecting i) leads to errors close to 1%, notably around and below 1 GeV; neglecting ii) leads to errors reaching about 3% within the considered energy range; iii) contributes to a minor effect, gauged at the level of 0.1%. Consequently, while iii) can currently be neglected, ii) warrants consideration, and we also recommend incorporating i) into computations. We conclude with some perspectives on further steps to be taken towards a high-precision goal of theoretical CR predictions regarding the treatment of energy-losses.
Abstract: 2403.18484
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Title:Stellar wind bubbles of OB stars as Galactic cosmic-ray re-accelerators
View PDF HTML (experimental)Abstract:Cosmic rays are highly energetic messengers propagating in magnetized plasma, which are, possibly but not exclusively, accelerated at astrophysical shocks. Amongst the variety of astrophysical objects presenting shocks, the huge circumstellar stellar wind bubbles forming around very massive stars, are potential non thermal emitters. We present the 1D magnetohydrodynamical simulation of the evolving magnetized surroundings of a single, OB type main sequence 60 Mo star, which is post processed to calculate the re-acceleration of preexisting non-thermal particles of the Galactic cosmic ray background. It is found that the forward shock of such circumstellar bubble can, during the early phase (1 Myr) of its expansion, act as a substantial reaccelerator of pre existing interstellar cosmic rays. This results in an increasing excess emission flux by a factor of 5, the hadronic component producing gamma-rays by pion0 decay being more important than those by synchrotron and inverse Compton radiation mechanisms. We propose that this effect is at work in the circumstellar environments of massive stars in general and we conjecture that other nebulae such as the stellar wind bow shocks of runaway massive stars also act as Galactic cosmic-ray re-accelerators. Particularly, this study supports the interpretation of the enhanced hadronic emission flux measured from the surroundings of kappa Ori as originating from the acceleration of pre-existing particles at the forward shock of its wind bubble.
Abstract: 2403.18516
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Title:Multi-View Deep Learning for Imaging Atmospheric Cherenkov Telescopes
View PDF HTML (experimental)Abstract:This research note concerns the application of deep-learning-based multi-view-imaging techniques to data from the H.E.S.S. Imaging Atmospheric Cherenkov Telescope array. We find that the earlier the fusion of layer information from different views takes place in the neural network, the better our model performs with this data. Our analysis shows that the point in the network where the information from the different views is combined is far more important for the model performance than the method used to combine the information.
Abstract: 2403.18689
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Title:Supernova Archaeology with X-Ray Binary Winds -- The Case of GRO J1655-40
View PDF HTML (experimental)Abstract:Supernovae are responsible for the elemental enrichment of the galaxy and some are postulated to leave behind a black hole. In a stellar binary system the supernova pollutes its companion, and the black hole can accrete back its own debris and emit X-rays. In this sequence of events, which is only poorly understood, winds are ejected, and observed through X-ray absorption lines. Measuring abundances of elements in the wind can lead to inferences about the historical explosion and possibly identify the long-gone progenitor of the compact object. Here, we re-analyze the uniquely rich X-ray spectrum of the 2005 outburst of GRO J1655-40. We reconstruct the absorption measure distribution (AMD) of the wind, and find that it increases sharply with ionization from H-like O up to H-like Ca, and then flattens out. The AMD is then used to measure relative abundances of 18 different elements. The present abundances are in partial agreement with a previous work with discrepancies mostly for low-Z elements. The overabundance of odd-Z elements hints at a high-metallicity, high-mass ($\simeq25\,M_\odot$) progenitor. Interestingly, the abundances are different from those measured in the companion atmosphere, indicating that the wind entrains lingering ambient supernova debris. This can be expected since the current total stellar mass of the binary ($<10\,M_\odot$) is much less than the progenitor mass.
Abstract: 2403.15794
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Title:Modeling Circumstellar Gas Emission around a White Dwarf Using Cloudy
View PDF HTML (experimental)Abstract:The chemical composition of an extrasolar planet is fundamental to its formation, evolution and habitability. In this study, we explore a new way to measure the chemical composition of the building blocks of extrasolar planets, by measuring the gas composition of the disrupted planetesimals around white dwarf stars. As a first attempt, we used the photo-ionization code Cloudy to model the circumstellar gas emission around a white dwarf Gaia J0611$-$6931 under some simplified assumptions. We found most of the emission lines are saturated and the line ratios approaching the ratios of thermal emission; therefore only lower limits to the number density can be derived. Silicon is the best constrained element in the circumstellar gas and we derived a lower limit of 10$^{10.3}$ cm$^{-3}$. In addition, we placed a lower limit on the total amount of gas to be 1.8 $\times$ 10$^{19}$ g. Further study is needed to better constrain the parameters of the gas disk and connect it to other white dwarfs with circumstellar gas absorption.
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