Cosmic ray production in superbubbles Vieu, T; Gabici, S; Tatischeff, V ...
Monthly notices of the Royal Astronomical Society,
03/2022, Volume:
512, Issue:
1
Journal Article
Peer reviewed
Open access
ABSTRACT
We compute the production of cosmic rays (CRs) in the dynamical superbubble (SB) produced by a cluster of massive stars. Stellar winds, supernova remnants, and turbulence are found to ...accelerate particles so efficiently that the non-linear feedback of the particles must be taken into account in order to ensure that the energy balance is not violated. High-energy particles do not scatter efficiently on the turbulence and escape quickly after each supernova explosion, which makes both their intensity inside the bubble and injection in the interstellar medium intermittent. On the other hand, the stochastic acceleration of low-energy particles hardens the spectra at GeV energies. Because CRs damp the turbulence cascade, this hardening is less pronounced when non-linearities are taken into account. Nevertheless, spectra with hard components extending up to 1–10 GeV and normalized to an energy density of 1–100 eV cm−3 are found to be typical signatures of CRs produced in SBs. Efficient shock reacceleration within compact clusters is further shown to produce hard, slightly concave spectra, while the presence of a magnetized shell is shown to enhance the confinement of CRs in the bubble and therefore the collective plasma effects acting on them. We eventually estimate the overall contribution of SBs to the Galactic CR content and show typical gamma-ray spectra expected from hadronic interactions in SB shells. In both cases, a qualitative agreement with observations is obtained.
Context. Supernova remnants interacting with molecular and atomic clouds are interesting X-ray sources for studies of broadband nonthermal emission. X-ray line emission in these systems can be ...produced by different processes, such as low-energy cosmic rays (LECRs) interacting with the cloud and fast ejecta fragments moving in the cloud. Aims. This paper is aimed at studying the origin of the non-thermal X-ray emission of the southwestern limb of SN 1006 beyond the main shock to determine whether the emission is due to LECRs diffusing in the cloud or to ejecta knots moving into the cloud. Methods. We analyzed the X-ray emission of the southwestern limb of SN 1006, where the remnant interacts with an atomic cloud, using three different X-ray telescopes: NuSTAR , Chandra , and XMM-Newton . We also performed a combined spectro-imaging analysis of this region. Results. Our analysis of the nonthermal X-ray emission of the southwestern limb of SN 1006 interacting with an atomic cloud has led to the detection of an extended X-ray source in the atomic cloud, approximately 2 pc upstream of the shock front. The source is characterized by a hard continuum (described by a power law with photon index Γ ∼ 1.4) and by Ne, Si, and Fe emission lines. The observed flux suggests that the origin of the X-ray emission is not associated with LECRs interacting with the cloud. On the other hand, the spectral properties of the source, together with the detection of an IR counterpart visible with Spitzer -MIPS at 24 μm, are in good agreement with the general expectations for a fast ejecta fragment moving within the atomic cloud. Conclusions. We detected X-ray and IR emission from a possible ejecta fragment, with an approximate radius of 1 × 10 17 cm and approximate mass of 10 −3 M ⊙ at about 2 pc out of the shell of SN 1006, in the interaction region between the southwestern limb of the remnant and the atomic cloud.
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FMFMET, NUK, UL, UM, UPUK
Supernova remnants interacting with molecular/atomic clouds are interesting X-ray sources to study broadband nonthermal emission. X-ray line emission in these systems can be produced by different ...processes, e.g. low energy cosmic rays interacting with the cloud and fast ejecta fragments moving in the cloud. The paper aims at studying the origin of the non-thermal X-ray emission of the southwestern limb of SN 1006 beyond the main shock, in order to distinguish if the emission is due to low energy cosmic rays diffusing in the cloud or to ejecta knots moving into the cloud. We analyzed the X-ray emission of the southwestern limb of SN 1006, where the remnant interacts with an atomic cloud, with three different X-ray telescopes ({NuSTAR, Chandra and XMM-Newton) and performed a combined spectro-imaging analysis of this region. The analysis of the non thermal X-ray emission of the southwestern limb of SN 1006, interacting with an atomic cloud, has shown the detection of an extended X-ray source in the atomic cloud, approximately \(2\) pc upstream of the shock front. The source is characterized by a hard continuum (described by a power law with photon index \(\Gamma\sim1.4\)) and by Ne, Si and Fe emission lines. The observed flux suggests that the origin of the X-ray emission is not associated with low energy cosmic rays interacting with the cloud. On the other hand, the spectral properties of the source, together with the detection of an IR counterpart visible with \textit{Spitzer}-MIPS at 24 \(\mu\)m are in good agreement with expectations for a fast ejecta fragment moving within the atomic cloud. We detected X-ray and IR emission from a possible ejecta fragment, with radius approximately 1\(\times10^{17}\) cm, and mass approximately \(10^{-3}M_\odot\) at about 2 pc out of the shell of SN 1006, in the interaction region between the southwestern limb of the remnant and the atomic cloud.
Design of DC–DC converter for harvesting maximum power from the multiple piezoelectric energy harvesters is a challenging task. In this work, a method to obtain maximum power from the multiple ...piezoelectric energy harvesters for supercapacitor charging is proposed. The method involves acquiring energy from each harvester by time-multiplexed operation of the multi-input buck–boost converter. The maximum power from each harvester is extracted by operating the converter to match the impedance of each harvester to the load impedance. The impedance matching is done by operating the converter with optimal duty cycle. The proposed method is experimentally evaluated, and the charging rate of supercapacitor is found to be higher while charging by the proposed method as compared to charging directly through the rectifier. The proposed method involves a single converter circuit for extracting energy from multiple piezoelectric energy harvesters, so that the component utilisation and its associated losses are very much reduced.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The low power energy harvesters need efficient single-stage direct ac–dc conversion evading diode bridge rectifier. An active rectifier circuit is proposed for piezoelectric energy harvester working ...on the principle of the buck–boost converter. The active rectifier circuit provides dual output with a reduced number of components. The analysis of the active rectifier is carried out, and expression for the optimum duty cycle is derived for maximum power extraction. The active rectifier configuration is extended for connecting multiple piezoelectric energy harvesters, and maximum power extraction is achieved through time multiplexed switching of energy harvesters. Proposed active rectifier topology is validated through simulation and experimentation. The results demonstrate that the harvested power is improved by the factor of 1.4 and 3.2 for single input and multiple input configurations, respectively, as compared to the power harvested using dual output rectifier. The charging time of the supercapacitor is reduced by 17 min while charging through the single input configuration and 15 min while charging through the multiple input configuration of the proposed active rectifier circuit.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The \(\mathrm{H}_2\) ionisation rate in the Central Molecular Zone, located in the Galactic Centre region, is estimated to be \(\zeta\sim2\times10^{-14}~\mathrm{s}^{-1}\), based on observations of ...H\(_3^+\) lines. This value is 2-3 orders of magnitude larger than that measured anywhere else in the Galaxy. A high cosmic-ray density has been invoked to explain the unusually high ionisation rate. However, this excess is not seen in the \(\gamma\)-ray emission from this region, which is produced by high-energy cosmic rays. Therefore, an excess is expected only in the low-energy cosmic-ray spectrum. Here, we derive constraints on this hypothetical low-energy component in the cosmic-ray spectra and we question its plausibility. To do so, we solve numerically the cosmic-ray transport equation in the Central Molecular Zone, considering spatial diffusion, advection in the Galactic wind, reacceleration in the ambient turbulence, and energy losses due to interactions with matter and radiation in the interstellar medium. We derive stationary solutions under the assumption that cosmic rays are continuously injected by a source located in the Galactic Centre. The high-energy component in the cosmic-ray spectrum is then fitted to available \(\gamma\)-ray and radio data, and a steep low-energy component is added to the cosmic-ray spectrum to explain the large ionisation rates. We find that injection spectra of \(p^{-7}\) for protons below \(p_{enh,p}c\simeq780~\mathrm{MeV}\) and \(p^{-5.2}\) for electrons below \(p_{enh,e}c=1.5~\mathrm{GeV}\) are needed to reach the observed ionisation rates. This corresponds to an extremely large cosmic-ray power of the order \(\sim10^{40-41}~\mathrm{erg}\,\mathrm{s}^{-1}\) injected at the Galactic Centre. We conclude that cosmic rays alone can not explain the high ionisation rates in the Galactic Centre region.
We compute the production of cosmic rays in the dynamical superbubble produced by a cluster of massive stars. Stellar winds, supernova remnants and turbulence are found to accelerate particles so ...efficiently that the nonlinear feedback of the particles must be taken into account in order to ensure that the energy balance is not violated. High energy particles do not scatter efficiently on the turbulence and escape quickly after each supernova explosion, which makes both their intensity inside the bubble and injection in the interstellar medium intermittent. On the other hand, the stochastic acceleration of low energy particles hardens the spectra at GeV energies. Because cosmic rays damp the turbulence cascade, this hardening is less pronounced when nonlinearities are taken into account. Nevertheless, spectra with hard components extending up to 1 to 10 GeV and normalised to an energy density of 1 to 100 eV cm\(^{-3}\) are found to be typical signatures of cosmic rays produced in superbubbles. Efficient shock reacceleration within compact clusters is further shown to produce hard, slightly concave spectra, while the presence of a magnetised shell is shown to enhance the confinement of cosmic rays in the bubble and therefore the collective plasma effects acting on them. We eventually estimate the overall contribution of superbubbles to the galactic cosmic ray content and show typical gamma-ray spectra expected from hadronic interactions in superbuble shells. In both cases, a qualitative agreement with observations is obtained.
