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Title: The case for electron re-acceleration at galaxy cluster shocks

Abstract

On the largest scales, the Universe consists of voids and filaments making up the cosmic web. Galaxy clusters are located at the knots in this web, at the intersection of filaments. Clusters grow through accretion from these large-scale filaments and by mergers with other clusters and groups. In a growing number of galaxy clusters, elongated Mpc-sized radio sources have been found. Also known as radio relics, these regions of diffuse radio emission are thought to trace relativistic electrons in the intracluster plasma accelerated by low-Mach-number shocks generated by cluster–cluster merger events. A long-standing problem is how low-Mach-number shocks can accelerate electrons so efficiently to explain the observed radio relics. Here, we report the discovery of a direct connection between a radio relic and a radio galaxy in the merging galaxy cluster Abell 3411–3412 by combining radio, X-ray and optical observations. This discovery indicates that fossil relativistic electrons from active galactic nuclei are re-accelerated at cluster shocks. Lastly, it also implies that radio galaxies play an important role in governing the non-thermal component of the intracluster medium in merging clusters.

Authors:
ORCiD logo [1];  [1];  [2];  [3];  [4];  [5]; ORCiD logo [6];  [7];  [8];  [1];  [1]; ORCiD logo [9]; ORCiD logo [10];  [11];  [12];  [1]; ORCiD logo [13];  [14];  [15]
  1. Harvard-Smithsonian Center for Astrophysics, Cambridge, MA (United States)
  2. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
  3. Univ. of California, Davis, CA (United States)
  4. National Centre for Radio Astrophysics, Pune (India)
  5. Pusan National Univ., Busan (Korea, Republic of). Dept. of Earth Sciences
  6. Ulsan National Inst. of Science and Technology (UNIST), Ulsan (Korea). Dept. of Physics; Korea Astronomy and Space Science Inst., Daejeon (Korea)
  7. Hamburg Univ., Hamburg (Germany). Hamburger Sternwarte
  8. Stanford Univ., CA (United States). Kavli Inst. for Particle Astrophysics and Cosmology
  9. Univ. of Notre Dame, IN (United States). Dept. of Physics and JINA Center for the Evolution of the Elements
  10. Univ. de Sao Paulo, Sao Paulo (Brazil). Dept. de Astronomia - Instituto de Astronomia, Geofisica e Ciencias Atmosfericas
  11. Univ. of California, Davis, CA (United States); Univ. de Lisboa, Lisbon (Portugal). Instituto de Astrofisica e Ciencias do Espaco
  12. Yonsei Univ., Seoul (Korea). Dept. of Astronomy and Center for Galaxy Evolution Research
  13. Lancaster Univ. (United Kingdom). Dept. of Physics; Leiden Univ. (Netherlands). Leiden Observatory
  14. European Southern Observatory, Garching (Germany)
  15. Johns Hopkins Univ., Baltimore, MD (United States). Department of Physics and Astronomy
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE; National Aeronautic and Space Administration (NASA); National Research Foundation of Korea (NRF)
OSTI Identifier:
1430992
Report Number(s):
LLNL-JRNL-703259
Journal ID: ISSN 2397-3366
Grant/Contract Number:
AC52-07NA27344; NAS8-03060; 2016R1A5A1013277; 2014R1A1A2057940; HST-HF2-51345.001-A; NAS5-26555
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Nature Astronomy
Additional Journal Information:
Journal Volume: 1; Journal Issue: 1; Journal ID: ISSN 2397-3366
Publisher:
Springer
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS

Citation Formats

van Weeren, Reinout J., Andrade-Santos, Felipe, Dawson, William A., Golovich, Nathan, Lal, Dharam V., Kang, Hyesung, Ryu, Dongsu, Bruggen, Marcus, Ogrean, Georgiana A., Forman, William R., Jones, Christine, Placco, Vinicius M., Santucci, Rafael M., Wittman, David, Jee, M. James, Kraft, Ralph P., Sobral, David, Stroe, Andra, and Fogarty, Kevin. The case for electron re-acceleration at galaxy cluster shocks. United States: N. p., 2017. Web. doi:10.1038/s41550-016-0005.
van Weeren, Reinout J., Andrade-Santos, Felipe, Dawson, William A., Golovich, Nathan, Lal, Dharam V., Kang, Hyesung, Ryu, Dongsu, Bruggen, Marcus, Ogrean, Georgiana A., Forman, William R., Jones, Christine, Placco, Vinicius M., Santucci, Rafael M., Wittman, David, Jee, M. James, Kraft, Ralph P., Sobral, David, Stroe, Andra, & Fogarty, Kevin. The case for electron re-acceleration at galaxy cluster shocks. United States. doi:10.1038/s41550-016-0005.
van Weeren, Reinout J., Andrade-Santos, Felipe, Dawson, William A., Golovich, Nathan, Lal, Dharam V., Kang, Hyesung, Ryu, Dongsu, Bruggen, Marcus, Ogrean, Georgiana A., Forman, William R., Jones, Christine, Placco, Vinicius M., Santucci, Rafael M., Wittman, David, Jee, M. James, Kraft, Ralph P., Sobral, David, Stroe, Andra, and Fogarty, Kevin. Wed . "The case for electron re-acceleration at galaxy cluster shocks". United States. doi:10.1038/s41550-016-0005. https://www.osti.gov/servlets/purl/1430992.
@article{osti_1430992,
title = {The case for electron re-acceleration at galaxy cluster shocks},
author = {van Weeren, Reinout J. and Andrade-Santos, Felipe and Dawson, William A. and Golovich, Nathan and Lal, Dharam V. and Kang, Hyesung and Ryu, Dongsu and Bruggen, Marcus and Ogrean, Georgiana A. and Forman, William R. and Jones, Christine and Placco, Vinicius M. and Santucci, Rafael M. and Wittman, David and Jee, M. James and Kraft, Ralph P. and Sobral, David and Stroe, Andra and Fogarty, Kevin},
abstractNote = {On the largest scales, the Universe consists of voids and filaments making up the cosmic web. Galaxy clusters are located at the knots in this web, at the intersection of filaments. Clusters grow through accretion from these large-scale filaments and by mergers with other clusters and groups. In a growing number of galaxy clusters, elongated Mpc-sized radio sources have been found. Also known as radio relics, these regions of diffuse radio emission are thought to trace relativistic electrons in the intracluster plasma accelerated by low-Mach-number shocks generated by cluster–cluster merger events. A long-standing problem is how low-Mach-number shocks can accelerate electrons so efficiently to explain the observed radio relics. Here, we report the discovery of a direct connection between a radio relic and a radio galaxy in the merging galaxy cluster Abell 3411–3412 by combining radio, X-ray and optical observations. This discovery indicates that fossil relativistic electrons from active galactic nuclei are re-accelerated at cluster shocks. Lastly, it also implies that radio galaxies play an important role in governing the non-thermal component of the intracluster medium in merging clusters.},
doi = {10.1038/s41550-016-0005},
journal = {Nature Astronomy},
number = 1,
volume = 1,
place = {United States},
year = {Wed Jan 04 00:00:00 EST 2017},
month = {Wed Jan 04 00:00:00 EST 2017}
}

Journal Article:
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  • An extreme case of electron shock drift acceleration (SDA) in low Mach number collisionless shocks is investigated as a plausible mechanism for the initial acceleration of relativistic electrons in large-scale shocks in galaxy clusters, where the upstream plasma temperature is of the order of 10 keV and the degree of magnetization is not too small. One-dimensional electromagnetic full particle simulations reveal that, even when a shock is rather moderate, a part of the thermal incoming electrons are accelerated and reflected through relativistic SDA and form a local non-thermal population just upstream of the shock. The accelerated electrons can self-generate localmore » coherent waves and further be back-scattered toward the shock by those waves. This may be a scenario for the first stage of the electron shock acceleration occurring at the large-scale shocks in galaxy clusters, such as CIZA J2242.8+5301, which have well-defined radio relics.« less
  • Radio relics are diffuse radio sources observed in galaxy clusters, probably produced by shock acceleration during cluster-cluster mergers. Their large size, of the order of 1 Mpc, indicates that the emitting electrons need to be (re)accelerated locally. The usually invoked diffusive shock acceleration models have been challenged by recent observations and theory. We report the discovery of complex radio emission in the Galaxy cluster PLCKG287.0+32.9, which hosts two relics, a radio halo, and several radio filamentary emission. Optical observations suggest that the cluster is elongated, likely along an intergalactic filament, and displays a significant amount of substructure. The peculiar featuresmore » of this radio relic are that (1) it appears to be connected to the lobes of a radio galaxy and (2) the radio spectrum steepens on either side of the radio relic. We discuss the origins of these features in the context of particle re-acceleration.« less
  • Electrons can be efficiently energized at interplanetary shocks and planetary bow shocks. The acceleration and reflection process is extremely sensitive to the angle {theta}{sub Bn} between the upstream magnetic field and the shock normal, and is most prominent at {theta} {sub Bn}{approximately} 90{degree}. The mechanism has been investigated by theoretical and simulation means, and can be interpreted as a fast Fermi process or as gradient drift acceleration. Previous work has been carried out for plane shocks only, and is expanded here to take into account the global curvature of a shock. Simple estimates suggest that this curvature may have amore » strong limiting effect on the acceleration to high energies, i.e., above several keV in case of the Earth's bow shock. The authors perform two-dimensional test particle calculations to address this question, and evaluate the reflected electron flux as a function of {theta} {sub Bn} at the shock surface. The shock profile is derived from hybrid code simulations, and modified to include the first order effects of a global curvature in the vicinity of {theta} {sub Bn}=90{degrees}. At low energies, the calculated fluxes exhibit a cut-off and a maximum, which can give rise to observed bump-on-tail distributions in the electron foreshock. Results at high energy show that while individual electrons gain less energy in a curved shock, concerning the flux this fact is largely offset by two-dimensional focusing effects. Electrons that drift into the shock over a wide area converge and stream out within a narrow spatial area, thus greatly enhancing the flux of reflected electrons. A {kappa} distribution of superthermal solar wind electrons (of index {kappa}=6) is capable of producing the observed large fluxes of reflected electrons at the Earth's bow shock up to energies of 10 to 15 keV, even when the global shock curvature is accounted for.« less
  • Electron acceleration to non-thermal energies is known to occur in low Mach number (M{sub s} ≲ 5) shocks in galaxy clusters and solar flares, but the electron acceleration mechanism remains poorly understood. Using two-dimensional (2D) particle-in-cell (PIC) plasma simulations, we showed in Paper I that electrons are efficiently accelerated in low Mach number (M{sub s} = 3) quasi-perpendicular shocks via a Fermi-like process. The electrons bounce between the upstream region and the shock front, with each reflection at the shock resulting in energy gain via shock drift acceleration. The upstream scattering is provided by oblique magnetic waves that are self-generatedmore » by the electrons escaping ahead of the shock. In the present work, we employ additional 2D PIC simulations to address the nature of the upstream oblique waves. We find that the waves are generated by the shock-reflected electrons via the firehose instability, which is driven by an anisotropy in the electron velocity distribution. We systematically explore how the efficiency of wave generation and of electron acceleration depend on the magnetic field obliquity, the flow magnetization (or equivalently, the plasma beta), and the upstream electron temperature. We find that the mechanism works for shocks with high plasma beta (≳ 20) at nearly all magnetic field obliquities, and for electron temperatures in the range relevant for galaxy clusters. Our findings offer a natural solution to the conflict between the bright radio synchrotron emission observed from the outskirts of galaxy clusters and the low electron acceleration efficiency usually expected in low Mach number shocks.« less
  • Electron acceleration to non-thermal energies in low Mach number (M{sub s} ≲ 5) shocks is revealed by radio and X-ray observations of galaxy clusters and solar flares, but the electron acceleration mechanism remains poorly understood. Diffusive shock acceleration, also known as first-order Fermi acceleration, cannot be directly invoked to explain the acceleration of electrons. Rather, an additional mechanism is required to pre-accelerate the electrons from thermal to supra-thermal energies, so they can then participate in the Fermi process. In this work, we use two- and three-dimensional particle-in-cell plasma simulations to study electron acceleration in low Mach number shocks. We focusmore » on the particle energy spectra and the acceleration mechanism in a reference run with M{sub s} = 3 and a quasi-perpendicular pre-shock magnetic field. We find that about 15% of the electrons can be efficiently accelerated, forming a non-thermal power-law tail in the energy spectrum with a slope of p ≅ 2.4. Initially, thermal electrons are energized at the shock front via shock drift acceleration (SDA). The accelerated electrons are then reflected back upstream where their interaction with the incoming flow generates magnetic waves. In turn, the waves scatter the electrons propagating upstream back toward the shock for further energization via SDA. In summary, the self-generated waves allow for repeated cycles of SDA, similarly to a sustained Fermi-like process. This mechanism offers a natural solution to the conflict between the bright radio synchrotron emission observed from the outskirts of galaxy clusters and the low electron acceleration efficiency usually expected in low Mach number shocks.« less