Characterizing the Uncertainty in Cluster Magnetic Fields Derived from Rotation Measures

Johnson, Andrew D.; Rudnick, Lawrence; Jones, T. W.; Mendygral, Peter; Dolag, K.

doi:10.3847/1538-4357/ab5d30

Cited by 22 publications

(18 citation statements)

References 53 publications

(71 reference statements)

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“…where B 0 = 4.7 µG and η B = 0.5. We note, however, that uncertainties on these parameters are relatively large and the parameters are degenerate (see also the recent work by Johnson et al 2020). They are constrained in the range ( B 0 = 3.9 µG; η B = 0.4) to ( B 0 = 5.4 µG; η B = 0.7) (see Bonafede et al 2010, for more details).…”

Section: Magnetic Fieldmentioning

confidence: 86%

γ-ray detection toward the Coma cluster withFermi-LAT: Implications for the cosmic ray content in the hadronic scenario

Adam

Goksu

Brown

et al. 2021

A&A

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The presence of relativistic electrons within the diffuse gas phase of galaxy clusters is now well established, thanks to deep radio observations obtained over the last decade, but their detailed origin remains unclear. Cosmic ray protons are also expected to accumulate during the formation of clusters. They may explain part of the radio signal and would lead to γ-ray emission through hadronic interactions within the thermal gas. Recently, the detection of γ-ray emission has been reported toward the Coma cluster with Fermi-LAT. Assuming that this γ-ray emission arises essentially from pion decay produced in proton-proton collisions within the intracluster medium (ICM), we aim at exploring the implication of this signal on the cosmic ray content of the Coma cluster and comparing it to observations at other wavelengths. We use the MINOT software to build a physical model of the Coma cluster, which includes the thermal target gas, the magnetic field strength, and the cosmic rays, to compute the corresponding expected γ-ray signal. We apply this model to the Fermi-LAT data using a binned likelihood approach, together with constraints from X-ray and Sunyaev-Zel’dovich observations. We also consider contamination from compact sources and the impact of various systematic effects on the results. We confirm that a significant γ-ray signal is observed within the characteristic radius θ500 of the Coma cluster, with a test statistic TS ≃ 27 for our baseline model. The presence of a possible point source (4FGL J1256.9+2736) may account for most of the observed signal. However, this source could also correspond to the peak of the diffuse emission of the cluster itself as it is strongly degenerate with the expected ICM emission, and extended models match the data better. Given the Fermi-LAT angular resolution and the faintness of the signal, it is not possible to strongly constrain the shape of the cosmic ray proton spatial distribution when assuming an ICM origin of the signal, but preference is found in a relatively flat distribution elongated toward the southwest, which, based on data at other wavelengths, matches the spatial distribution of the other cluster components well. Assuming that the whole γ-ray signal is associated with hadronic interactions in the ICM, we constrain the cosmic ray to thermal energy ratio within R500 to XCRp = 1.79−0.30+1.11% and the slope of the energy spectrum of cosmic rays to α = 2.80−0.13+0.67 (XCRp = 1.06−0.22+0.96% and α = 2.58−0.09+1.12 when including both the cluster and 4FGL J1256.9+2736 in our model). Finally, we compute the synchrotron emission associated with the secondary electrons produced in hadronic interactions assuming steady state. This emission is about four times lower than the overall observed radio signal (six times lower when including 4FGL J1256.9+2736), so that primary cosmic ray electrons or reacceleration of secondary electrons is necessary to explain the total emission. We constrain the amplitude of the primary to secondary electrons, or the required boost from reacceleration with respect to the steady state hadronic case, depending on the scenario, as a function of radius. Our results confirm that γ-ray emission is detected in the direction of the Coma cluster. Assuming that the emission is due to hadronic interactions in the intracluster gas, they provide the first quantitative measurement of the cosmic ray proton content in a galaxy cluster and its implication for the cosmic ray electron populations.

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Section: Magnetic Fieldmentioning

confidence: 86%

γ-ray detection toward the Coma cluster withFermi-LAT: Implications for the cosmic ray content in the hadronic scenario

Adam

Goksu

Brown

et al. 2021

A&A

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“…The reasons for a non-detection of synchrotron emission from the bridge region at the level of A399-401, for example, may lie in the absence of any one of the four points mentioned at the beginning of this section. If we assume that the relation between thermal gas density and magnetic field strength found in galaxy clusters is also valid in the intercluster region, we can estimate the magnetic field strength using the observed scaling B ∝ n 0.5 e (Johnson et al 2020). In cluster centres with electron number densities of n e ∼ 0.1 cm −3 , typical field values of 1-10 µG are observed.…”

Section: Upper Limit On the Radio Bridgementioning

confidence: 97%

Radio observations of the merging galaxy cluster system Abell 3391-Abell 3395

Brüggen

Reiprich

Bulbul

et al. 2021

A&A

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The pre-merging system of galaxy clusters Abell 3391-Abell 3395 located at a mean redshift of 0.053 has been observed at 1 GHz in an ASKAP/EMU Early Science observation as well as in X-rays with eROSITA. The projected separation of the X-ray peaks of the two clusters is ∼50 or ∼ 3.1 Mpc. Here we present an inventory of interesting radio sources in this field around this cluster merger. While the eROSITA observations provide clear indications of a bridge of thermal gas between the clusters, neither ASKAP nor MWA observations show any diffuse radio emission coinciding with the X-ray bridge. We derive an upper limit on the radio emissivity in the bridge region of J 1 GHz < 1.2 × 10 −44 W Hz −1 m −3. A non-detection of diffuse radio emission in the X-ray bridge between these two clusters has implications for particle-acceleration mechanisms in cosmological large-scale structure. We also report extended or otherwise noteworthy radio sources in the 30 deg 2 field around Abell 3391-Abell 3395. We identified 20 Giant Radio Galaxies, plus 7 candidates, with linear projected sizes greater than 1 Mpc. The sky density of field radio galaxies with largest linear sizes of > 0.7 Mpc is ≈ 1.7 deg −2 , three times higher than previously reported. We find no evidence for a cosmological evolution of the population of Giant Radio Galaxies. Moreover, we find seven candidates for cluster radio relics and radio halos.

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“…Using φ peak,res,char ∼ σ φpeak,res,cluster ≈ 17 rad m -2 , we get M Thermal ≈ 2.4 × 10 11 1 BμG M , which for comparison, is approximately three times the mass of the X-ray-emitting hot ICM (but see Johnson et al 2020 for discussion of inherent uncertainties in such measurements). If there are typically N magnetic field reversals along the LOS to sources in our sample, M Thermal will be larger by a factor of ∼N 1/2 .…”

Section: Mass and Phase Of The Ionised Cluster Gasmentioning

confidence: 98%

“…This can distribute μG-level magnetic fields throughout cluster volumes (e.g. Xu et al 2009), inducing significant variability in the Faraday rotation signal of clusters with otherwise similar properties (Xu et al 2011), and degeneracies in the interpretation of such data (Johnson et al 2020). Nevertheless, following Anderson et al (2018), we can crudely estimate the baryonic gas mass generating the Faraday depth enhancement as follows.…”

Section: Mass and Phase Of The Ionised Cluster Gasmentioning

confidence: 99%

Early Science from POSSUM: Shocks, turbulence, and a massive new reservoir of ionised gas in the Fornax cluster

Anderson

Heald

Eilek

et al. 2021

Publ. Astron. Soc. Aust.

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We present the first Faraday rotation measure (RM) grid study of an individual low-mass cluster—the Fornax cluster—which is presently undergoing a series of mergers. Exploiting commissioning data for the POlarisation Sky Survey of the Universe’s Magnetism (POSSUM) covering a ${\sim}34$ square degree sky area using the Australian Square Kilometre Array Pathfinder (ASKAP), we achieve an RM grid density of ${\sim}25$ RMs per square degree from a 280-MHz band centred at 887 MHz, which is similar to expectations for forthcoming GHz-frequency ${\sim}3\pi$ -steradian sky surveys. These data allow us to probe the extended magnetoionic structure of the cluster and its surroundings in unprecedented detail. We find that the scatter in the Faraday RM of confirmed background sources is increased by $16.8\pm2.4$ rad m−2 within 1 $^\circ$ (360 kpc) projected distance to the cluster centre, which is 2–4 times larger than the spatial extent of the presently detectable X-ray-emitting intracluster medium (ICM). The mass of the Faraday-active plasma is larger than that of the X-ray-emitting ICM and exists in a density regime that broadly matches expectations for moderately dense components of the Warm-Hot Intergalactic Medium. We argue that forthcoming RM grids from both targeted and survey observations may be a singular probe of cosmic plasma in this regime. The morphology of the global Faraday depth enhancement is not uniform and isotropic but rather exhibits the classic morphology of an astrophysical bow shock on the southwest side of the main Fornax cluster, and an extended, swept-back wake on the northeastern side. Our favoured explanation for these phenomena is an ongoing merger between the main cluster and a subcluster to the southwest. The shock’s Mach angle and stand-off distance lead to a self-consistent transonic merger speed with Mach 1.06. The region hosting the Faraday depth enhancement also appears to show a decrement in both total and polarised radio emission compared to the broader field. We evaluate cosmic variance and free-free absorption by a pervasive cold dense gas surrounding NGC 1399 as possible causes but find both explanations unsatisfactory, warranting further observations. Generally, our study illustrates the scientific returns that can be expected from all-sky grids of discrete sources generated by forthcoming all-sky radio surveys.

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Characterizing the Uncertainty in Cluster Magnetic Fields Derived from Rotation Measures

Cited by 22 publications

References 53 publications

γ-ray detection toward the Coma cluster withFermi-LAT: Implications for the cosmic ray content in the hadronic scenario

γ-ray detection toward the Coma cluster withFermi-LAT: Implications for the cosmic ray content in the hadronic scenario

Radio observations of the merging galaxy cluster system Abell 3391-Abell 3395

Early Science from POSSUM: Shocks, turbulence, and a massive new reservoir of ionised gas in the Fornax cluster

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