On the Challenges of Cosmic-Ray Proton Shock Acceleration in the Intracluster Medium

Wittor, Denis

doi:10.1016/j.newast.2020.101550

Cited by 12 publications

(10 citation statements)

References 101 publications

(159 reference statements)

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“…Since many years, the non-detection of the γ -ray signal from accelerated cosmic ray protons is posing a problem to the model (e.g. Wittor et al 2020Wittor et al , 2021. Still, the acceleration efficiencies for DSA from the thermal pool are too low to explain the observed high radio powers of most relics (e.g.…”

mentioning

confidence: 99%

Exploring the spectral properties of radio relics – I: integrated spectral index and Mach number

Wittor

Ettori

Vazza

et al. 2021

Monthly Notices of the Royal Astronomical Society

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Radio relics are the manifestation of electrons presumably being shock (re-)accelerated to high energies in the outskirts of galaxy clusters. However, estimates of the shocks’ strength yield different results when measured with radio or X-ray observations. In general, Mach numbers obtained from radio observations are larger than the corresponding X-ray measurements. In this work, we investigate this Mach number discrepancy. For this purpose, we used the cosmological code enzo to simulate a sample of galaxy clusters that host bright radio relics. For each relic, we computed the radio Mach number from the integrated radio spectrum and the X-ray Mach number from the X-ray surface brightness and temperature jumps. Our analysis suggests that the differences in the Mach number estimates follow from the way in which different observables are related to different parts of the underlying Mach number distribution: radio observations are more sensistive to the high Mach numbers present only in a small fraction of a shock’s surface, while X-ray measurements reflect the average of the Mach number distribution. Moreover, X-ray measurements are very sensitive to the relic’s orientation. If the same relic is observed from different sides, the measured X-ray Mach number varies significantly. On the other hand, the radio measurements are more robust, as they are unaffected by the relic’s orientation.

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mentioning

confidence: 99%

Exploring the spectral properties of radio relics – I: integrated spectral index and Mach number

Wittor

Ettori

Vazza

et al. 2021

Monthly Notices of the Royal Astronomical Society

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“…A small inclination of the merging axis with respect to the line of sight ( 10-20 • ) could explain why a shock wave and a spectral gradient have been not observed. In cosmological simulations, face-on relics show complex morphologies that consist of filaments, possibly polarized, similar to the diffuse source in A523 (e.g., Skillman et al 2013;Wittor et al 2019;Wittor 2021). In these simulations, the spectral indices vary across the relics' surfaces, but they lack the typical spectral steepening towards the cluster center.…”

Section: Discussionmentioning

confidence: 90%

Spectral study of the diffuse synchrotron source in the galaxy cluster Abell 523

Vacca,

Shimwell,

Perley

et al. 2022

Preprint

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The galaxy cluster Abell 523 (A523) hosts an extended diffuse synchrotron source historically classified as a radio halo. Its radio power at 1.4 GHz makes it one of the most significant outliers in the scaling relations between observables derived from multi-wavelength observations of galaxy clusters: it has a morphology that is different and offset from the thermal gas, and it has polarized emission at 1.4 GHz typically difficult to observe for this class of sources. A magnetic field fluctuating on large spatial scales (∼1 Mpc) can explain these peculiarities but the formation mechanism for this source is not yet completely clear. To investigate its formation mechanism, we present new observations obtained with the LOw Frequency ARray at 120-168 MHz and the Jansky Very Large Array at 1-2 GHz, which allow us to study the spectral index distribution of this source. According to our data the source is observed to be more extended at 144 MHz than previously inferred at 1.4 GHz, with a total size of about 1.8 Mpc and a flux density 𝑆 144 MHz = (1.52 ± 0.31) Jy. The spectral index distribution of the source is patchy with an average spectral index 𝛼 ∼ 1.2 between 144 MHz and 1.410 GHz, while an integrated spectral index 𝛼 ∼ 2.1 has been obtained between 1.410 GHz and 1.782 GHz. A previously unseen patch of steep spectrum emission is clearly detected at 144 MHz in the south of the cluster. Overall, our findings suggest that we are observing an overlapping of different structures, powered by the turbulence associated with the primary and a possible secondary merger.

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“…Since many years, the non-detection of the γ-ray signal from accelerated cosmic-ray protons is posing a problem to the model (e.g. Ryu et al 2019;Ha et al 2019;Wittor et al 2020;Wittor 2021). Still, the acceleration efficiencies for DSA from the thermal pool are too low to explain the observed high radio powers of most relics (e.g.…”

Section: Introductionmentioning

confidence: 99%

Exploring the spectral properties of radio relics I: Integrated spectral index and Mach number

Wittor,

Ettori,

Vazza

et al. 2021

Preprint

Self Cite

View full text Add to dashboard Cite

Radio relics are the manifestation of electrons presumably being shock (re-)accelerated to high energies in the outskirts of galaxy clusters. However, estimates of the shocks' strength yield different results when measured with radio or X-ray observations. In general, Mach numbers obtained from radio observations are larger than the corresponding X-ray measurements. In this work, we investigate this Mach number discrepancy. For this purpose, we used the cosmological code ENZO to simulate a sample of galaxy clusters that host bright radio relics. For each relic, we computed the radio Mach number from the integrated radio spectrum and the X-ray Mach number from the X-ray surface brightness and temperature jumps. Our analysis suggests that the differences in the Mach number estimates follow from the way in which different observables are related to different parts of the underlying Mach number distribution: radio observations are more sensistive to the high Mach numbers present only in a small fraction of a shock's surface, while X-ray measurements reflect the average of the Mach number distribution. Moreover, X-ray measurements are very sensitive to the relic's orientation. If the same relic is observed from different sides, the measured X-ray Mach number varies significantly. On the other hand, the radio measurements are more robust, as they are unaffected by the relic's orientation.

show abstract

On the Challenges of Cosmic-Ray Proton Shock Acceleration in the Intracluster Medium

Cited by 12 publications

References 101 publications

Exploring the spectral properties of radio relics – I: integrated spectral index and Mach number

Exploring the spectral properties of radio relics – I: integrated spectral index and Mach number

Spectral study of the diffuse synchrotron source in the galaxy cluster Abell 523

Exploring the spectral properties of radio relics I: Integrated spectral index and Mach number

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