Abstract:We investigate ionization and heating of gas in the dense, shielded clumps/cores of molecular clouds bathed by an influx of energetic, charged cosmic rays (CRs). These molecular clouds have complex structures, with substantial variation in their physical properties over a wide range of length scales. The propagation and distribution of CRs is thus regulated accordingly, in particular, by the magnetic fields threaded through the clouds and into the dense regions within. We have found that a specific heating rat… Show more
“…In this section, we probe 6.4 keV line emissions from different cloud regions, toward understanding LECR populations within Sgr B2 in the context of both ambient LECRs in the CMZ and LECR production in the cloud. The problem of CR transport into and within MCs is model dependent and has been the subject of several theoretical works with different predictions for the geometry of energy deposition by LECRs, all of which necessarily make simplifying assumptions about the gas distribution and magnetic field structure, for either a generic cloud or Sgr B2 specifically (Dogiel et al 2015;Morlino & Gabici 2015;Gabici 2013;Owen et al 2021).…”
Located ∼ 100 pc from the dynamic center of the Milky Way, the molecular cloud Sagittarius B2 (Sgr B2) is the most massive such object in the Galactic Center region. In X-rays, Sgr B2 shows a prominent neutral Fe Kα line at 6.4 keV and continuum emission beyond 10 keV, indicating highenergy, non-thermal processes in the cloud. The Sgr B2 complex is an X-ray reflection nebula whose total emissions have continued to decrease since the year 2001 as it reprocesses what are likely one or more past energetic outbursts from the supermassive black hole Sagittarius A*. The X-ray reflection model explains the observed time-variability of the Fe Kα and hard X-ray emissions, and it provides a window into the luminous evolutionary history of our nearest supermassive black hole. In light of evidence of elevated cosmic particle populations in the Galactic Center, recent interest has also focused on X-rays from Sgr B2 as a probe of low-energy (sub-GeV) cosmic particles. In contrast to the timevarying X-ray reflection, in this case we can assume that the X-ray flux contribution from interactions of low-energy cosmic particles is constant in time, such that upper limits on low-energy cosmic particle populations may be obtained using the lowest flux levels observed from the cloud. Here, we present the most recent and correspondingly dimmest NuSTAR and XMM-Newton observations of Sgr B2, from 2018. These reveal small-scale variations within lower density portions of the Sgr B2 complex, including brightening features, yet still enable the best upper limits on X-rays from low-energy cosmic particles in Sgr B2. We also present Fe Kα fluxes from cloud regions of different densities, facilitating comparison with models of ambient low-energy cosmic particle interactions throughout the cloud.
“…In this section, we probe 6.4 keV line emissions from different cloud regions, toward understanding LECR populations within Sgr B2 in the context of both ambient LECRs in the CMZ and LECR production in the cloud. The problem of CR transport into and within MCs is model dependent and has been the subject of several theoretical works with different predictions for the geometry of energy deposition by LECRs, all of which necessarily make simplifying assumptions about the gas distribution and magnetic field structure, for either a generic cloud or Sgr B2 specifically (Dogiel et al 2015;Morlino & Gabici 2015;Gabici 2013;Owen et al 2021).…”
Located ∼ 100 pc from the dynamic center of the Milky Way, the molecular cloud Sagittarius B2 (Sgr B2) is the most massive such object in the Galactic Center region. In X-rays, Sgr B2 shows a prominent neutral Fe Kα line at 6.4 keV and continuum emission beyond 10 keV, indicating highenergy, non-thermal processes in the cloud. The Sgr B2 complex is an X-ray reflection nebula whose total emissions have continued to decrease since the year 2001 as it reprocesses what are likely one or more past energetic outbursts from the supermassive black hole Sagittarius A*. The X-ray reflection model explains the observed time-variability of the Fe Kα and hard X-ray emissions, and it provides a window into the luminous evolutionary history of our nearest supermassive black hole. In light of evidence of elevated cosmic particle populations in the Galactic Center, recent interest has also focused on X-rays from Sgr B2 as a probe of low-energy (sub-GeV) cosmic particles. In contrast to the timevarying X-ray reflection, in this case we can assume that the X-ray flux contribution from interactions of low-energy cosmic particles is constant in time, such that upper limits on low-energy cosmic particle populations may be obtained using the lowest flux levels observed from the cloud. Here, we present the most recent and correspondingly dimmest NuSTAR and XMM-Newton observations of Sgr B2, from 2018. These reveal small-scale variations within lower density portions of the Sgr B2 complex, including brightening features, yet still enable the best upper limits on X-rays from low-energy cosmic particles in Sgr B2. We also present Fe Kα fluxes from cloud regions of different densities, facilitating comparison with models of ambient low-energy cosmic particle interactions throughout the cloud.
“…Within the Milky Way, cosmic-ray transport studies and observations have indicated an average diffusion coefficient between D 0 = 10 28 -3 × 10 28 cm 2 s −1 (Evoli et al 2020). However, regarding the dense gas, studies have shown a spread over several orders of magnitude, from D 0 = 10 27 -10 30 cm 2 s −1 (Dogiel et al 2015;Owen et al 2021). As such, it is even more paramount to understand what is constraining the CR transport within molecular gas, and how the local environment changes the diffusion coefficient.…”
Low-energy cosmic rays, in particular protons with energies below 1 GeV, are significant drivers of the thermochemistry of molecular clouds. However, these cosmic rays are also greatly impacted by energy losses and magnetic field transport effects in molecular gas. Explaining cosmic ray ionization rates of 10 −16 s −1 or greater in dense gas requires either a high external cosmic ray flux, or local sources of MeV-GeV cosmic ray protons. We present a new local source of low-energy cosmic rays in molecular clouds: first order Fermi-acceleration of protons in regions undergoing turbulent reconnection in molecular clouds. We show from energetic-based arguments there is sufficient energy within the magneto-hydrodynamic turbulent cascade to produce ionization rates compatible with inferred ionization rates in molecular clouds. As turbulent reconnection is a volume-filling process, the proposed mechanism can produce a near-homogeneous distribution of low-energy cosmic rays within molecular clouds.
“…The choice of spectral model is discussed in detail in [6], however its exact form (if reasonable) does not substantially affect the results of this work. The diffusive term ∇ • [𝐷 (𝐸, 𝒔)∇𝑛] is governed by the coefficient 𝐷 (𝐸, 𝒔) which depends on the gyro-scattering radius (or frequency) of the CRs of energy 𝐸 in their local magnetic field, as well as turbulence and magnetohydrodynamical (MHD) perturbations along local magnetic field vectors.…”
Section: Cr Propagation In MC Complexesmentioning
confidence: 99%
“…Here 𝜆 1 = 𝜆 td (|𝜔 L |/𝜔 p,0 ) is the CR resonant scattering length scale parallel to the background magnetic field line. The turbulent decay length scale is 𝜆 td ≈ 𝑣 A 𝜏 td , (where we use 𝜏 td = 2 Myrfor discussion, see [6]). The CR gyro-frequency is 𝜔 L , with a sign convention set by the charge (in units of proton charge).…”
Section: Empirical Diffusion Parametermentioning
confidence: 99%
“…5]. In this work, we consider an empirical propagation and heating model of CRs in MC environments, which was first introduced in [6]. This accounts for CR cooling and interaction mechanisms relevant to CR species (protons and electrons) in a self-consistent manner.…”
Molecular clouds are complex magnetized structures, with variations over a broad range of length scales. Ionization in dense, shielded clumps and cores of molecular clouds is thought to be caused by charged cosmic rays (CRs). These CRs can also contribute to heating the gas deep within molecular clouds, and their effect can be substantial in environments where CRs are abundant. CRs propagate predominantly by diffusion in media with disordered magnetic fields. The complex magnetic structures in molecular clouds therefore determine the propagation and spatial distribution of CRs within them, and hence regulate their local ionization and heating patterns.Optical and near-infrared (NIR) polarization of starlight through molecular clouds is often used to trace magnetic fields. The coefficients of CR diffusion in magnetized molecular cloud complexes can be inferred from the observed fluctuations in these optical/NIR starlight polarisations. Here, we present calculations of the expected CR heating patterns in the star-forming filaments of IC 5146, determined from optical/NIR observations. Our calculations show that local conditions give rise to substantial variation in CR propagation. This affects the local CR heating power. Such effects are expected to be severe in star-forming galaxies rich in CRs. The molecular clouds in these galaxies could evolve differently to those in galaxies where CRs are less abundant.
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