A Hidden Radio Halo in the Galaxy Cluster A 1682?

ABSTRACT Radio haloes are extended (∼Mpc), steep spectrum sources found in the central region of dynamically disturbed clusters of galaxies. Only a handful of radio haloes have been reported to reside in galaxy clusters with a mass $M_{500}\lesssim 5\times 10^{14}\, \mathrm{ M}_\odot$. In this paper, we present a LOw Frequency ARray (LOFAR) 144 MHz detection of a radio halo in the galaxy cluster Abell 990 with a mass of $M_{500}=(4.9\pm 0.3)\times 10^{14}\, \mathrm{ M}_\odot$. The halo has a projected size of ${\sim} 700\, {\rm kpc}$ and a flux density of $20.2\pm 2.2\, {\rm mJy}$ or a radio power of $1.2\pm 0.1\times 10^{24}\, {\rm W\, Hz}^{-1}$ at the cluster redshift (z = 0.144) that makes it one of the two haloes with the lowest radio power detected to date. Our analysis of the emission from the cluster with Chandra archival data using dynamical indicators shows that the cluster is not undergoing a major merger but is a slightly disturbed system with a mean temperature of $5\, {\rm keV}$. The low X-ray luminosity of $L_{\mathrm{ X}}=(3.66\pm 0.08)\times 10^{44}\, {\rm erg\, s}^{-1}$ in the 0.1–2.4 keV band implies that the cluster is one of the least luminous systems known to host a radio halo. Our detection of the radio halo in Abell 990 opens the possibility of detecting many more haloes in poorly explored less massive clusters with low-frequency telescopes such as LOFAR, Murchison Widefield Array (MWA, Phase II), and upgraded Giant Metrewave Radio Telescope (uGMRT).

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Reaching thermal noise at ultra-low radio frequencies

Astronomy and Astrophysics ◽

10.1051/0004-6361/202038663 ◽

2020 ◽

Vol 642 ◽

pp. A85 ◽

Cited By ~ 1

Author(s):

F. de Gasperin ◽

G. Brunetti ◽

M. Brüggen ◽

R. van Weeren ◽

W. L. Williams ◽

...

Keyword(s):

Thermal Noise ◽

Signal To Noise Ratio ◽

Low Frequency ◽

Galaxy Cluster ◽

Cosmic Ray ◽

Radio Halo ◽

Cosmic Ray Acceleration ◽

The Galaxy ◽

Background Turbulence ◽

Post Shock

Context. Ultra-low frequency observations (< 100 MHz) are particularly challenging because they are usually performed in a low signal-to-noise ratio regime due to the high sky temperature and because of ionospheric disturbances whose effects are inversely proportional to the observing frequency. Nonetheless, these observations are crucial for studying the emission from low-energy populations of cosmic rays. Aims. We aim to obtain the first thermal-noise limited (∼1.5 mJy beam−1) deep continuum radio map using the Low Frequency Array’s Low Band Antenna (LOFAR LBA) system. Our demonstration observation targeted the galaxy cluster RX J0603.3+4214 (known as the Toothbrush cluster). We used the resulting ultra-low frequency (39–78 MHz) image to study cosmic-ray acceleration and evolution in the post shock region considering the presence of a radio halo. Methods. We describe the data reduction we used to calibrate LOFAR LBA observations. The resulting image was combined with observations at higher frequencies (LOFAR 150 MHz and VLA 1500 MHz) to extract spectral information. Results. We obtained the first thermal-noise limited image from an observation carried out with the LOFAR LBA system using all Dutch stations at a central frequency of 58 MHz. With eight hours of data, we reached an rms noise of 1.3 mJy beam−1 at a resolution of 18″ × 11″. Conclusions. The procedure we developed is an important step towards routine high-fidelity imaging with the LOFAR LBA. The analysis of the radio spectra shows that the radio relic extends to distances of 800 kpc downstream from the shock front, larger than what is allowed by electron cooling time. Furthermore, the shock wave started accelerating electrons already at a projected distance of < 300 kpc from the crossing point of the two clusters. These results may be explained by electrons being re-accelerated downstream by background turbulence, possibly combined with projection effects with respect to the radio halo.

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A radio halo surrounding the Brightest Cluster Galaxy in RXCJ0232.2–4420: a mini-halo in transition?

Monthly Notices of the Royal Astronomical Society Letters ◽

10.1093/mnrasl/slz061 ◽

2019 ◽

Vol 486 (1) ◽

pp. L80-L84 ◽

Cited By ~ 3

Author(s):

Ruta Kale ◽

Krishna M Shende ◽

Viral Parekh

Keyword(s):

Radio Source ◽

Surface Brightness ◽

Galaxy Cluster ◽

Cosmic Ray ◽

Radio Sources ◽

Cluster Galaxy ◽

X Ray ◽

Radio Halo ◽

The Galaxy ◽

Radio Power

ABSTRACT Diffuse radio sources associated with the intra-cluster medium are direct probes of the cosmic ray electrons and magnetic fields. We report the discovery of a diffuse radio source in the galaxy cluster RXCJ0232.2–4420 (SPT-CL J0232–4421, z = 0.2836) using 606 MHz observations with the Giant Metrewave Radio Telescope. The diffuse radio source surrounds the Brightest Cluster Galaxy in the cluster-like typical radio mini-haloes. However the total extent of it is 550 × 800 kpc2, which is larger than mini-haloes and similar to that of radio haloes. The BCG itself is also a radio source with a marginally resolved core at 7 arcsec (30 kpc) resolution. We measure the 606 MHz flux density of the RH to be 52 ± 5 mJy. Assuming a spectral index of 1.3, the 1.4 GHz radio power is 4.5 × 1024 W Hz−1. The dynamical state of the cluster has been inferred to be 'relaxed’ and also as 'complex’, depending on the classification methods based on the morphology of the X-ray surface brightness. This system thus seems to be in the transition phase from a mini-halo to a radio halo.

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Diffuse radio emission in the galaxy cluster SPT-CL J2031−4037: a steep-spectrum intermediate radio halo?

Monthly Notices of the Royal Astronomical Society Letters ◽

10.1093/mnrasl/slaa002 ◽

2020 ◽

Vol 493 (1) ◽

pp. L28-L32 ◽

Cited By ~ 1

Author(s):

Ramij Raja ◽

Majidul Rahaman ◽

Abhirup Datta ◽

Jack O Burns ◽

H T Intema ◽

...

Keyword(s):

Radio Emission ◽

Spectral Index ◽

Low Frequency ◽

Galaxy Cluster ◽

Transition System ◽

Cluster Galaxy ◽

Radio Halo ◽

Spectrum Radio ◽

Upper Limits ◽

The Galaxy

ABSTRACT The advent of sensitive low-frequency radio observations has revealed a number of diffuse radio objects with peculiar properties that are challenging our understanding of the physics of the intracluster medium. Here, we report the discovery of a steep-spectrum radio halo surrounding the central brightest cluster galaxy (BCG) in the galaxy cluster SPT-CL J2031−4037. This cluster is morphologically disturbed yet has a weak cool core, an example of a cool-core/non-cool-core transition system, which harbours a radio halo ∼0.7 Mpc in size. The halo emission detected at 1.7 GHz is less extended compared to that in the 325 MHz observation, and the spectral index of the part of the halo visible at the 325 MHz to 1.7 GHz frequencies was found to be −1.35 ± 0.07. Also, P1.4 GHz was found to be 0.77 × 1024 W Hz−1, which falls in the region where radio mini-haloes, halo upper limits and ultra-steep-spectrum (USS) haloes are found in the P1.4 GHz–LX plane. Additionally, simulations presented in the paper provide support for the scenario of the steep spectrum. The diffuse radio emission found in this cluster may be a steep-spectrum ‘intermediate’ or ‘hybrid’ radio halo that is transitioning into a mini-halo.

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Ultra-steep spectrum emission in the merging galaxy cluster Abell 1914

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833992 ◽

2019 ◽

Vol 622 ◽

pp. A22 ◽

Cited By ~ 6

Author(s):

S. Mandal ◽

H. T. Intema ◽

T. W. Shimwell ◽

R. J. van Weeren ◽

A. Botteon ◽

...

Keyword(s):

Low Frequency ◽

Galaxy Cluster ◽

Diffuse Emission ◽

Radio Observations ◽

Very Large Array ◽

Radio Halo ◽

The Galaxy ◽

Distinct Source ◽

Relativistic Particles ◽

Massive Galaxy

A number of radio observations have revealed the presence of large synchrotron-emitting sources associated with the intra-cluster medium. There is strong observational evidence that the emitting particles have been (re-)accelerated by shocks and turbulence generated during merger events. The particles that are accelerated are thought to have higher initial energies than those in the thermal pool but the origin of such mildly relativistic particles remains uncertain and needs to be further investigated. The galaxy cluster Abell 1914 is a massive galaxy cluster in which X-ray observations show clear evidence of merging activity. We carried out radio observations of this cluster with the LOw Frequency ARay (LOFAR) at 150 MHz and the Giant Metrewave Radio Telescope (GMRT) at 610 MHz. We also analysed Very Large Array (VLA) 1.4 GHz data, archival GMRT 325 MHz data, CFHT weak lensing data and Chandra observations. Our analysis shows that the ultra-steep spectrum source (4C38.39; α ≲ −2), previously thought to be part of a radio halo, is a distinct source with properties that are consistent with revived fossil plasma sources. Finally, we detect some diffuse emission to the west of the source 4C38.39 that could belong to a radio halo.

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XMM-Newton EPIC observation of the galaxy cluster A 3667

Astronomy and Astrophysics ◽

10.1051/0004-6361:20035812 ◽

2004 ◽

Vol 426 (1) ◽

pp. 1-9 ◽

Cited By ~ 33

Author(s):

U. G. Briel ◽

A. Finoguenov ◽

J. P. Henry

Keyword(s):

Galaxy Cluster ◽

The Galaxy ◽

Cluster A

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A joint XMM-NuSTAR observation of the galaxy cluster Abell 523: Constraints on inverse Compton emission

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834644 ◽

2019 ◽

Vol 628 ◽

pp. A83 ◽

Cited By ~ 3

Author(s):

F. Cova ◽

F. Gastaldello ◽

D. R. Wik ◽

W. Boschin ◽

A. Botteon ◽

...

Keyword(s):

Magnetic Field ◽

Field Strength ◽

Magnetic Field Strength ◽

Galaxy Cluster ◽

X Ray ◽

Radio Halo ◽

Upper Limit ◽

Inverse Compton ◽

The Galaxy ◽

The Magnetic Field

Aims. We present the results of a joint XMM-Newton and NuSTAR observation (200 ks) of the galaxy cluster Abell 523 at z = 0.104. The peculiar morphology of the cluster radio halo and its outlier position in the radio power P(1.4 GHz) – X-ray luminosity plane make it an ideal candidate for the study of radio and X-ray correlations and for the search of inverse Compton (IC) emission. Methods. We constructed bi-dimensional maps for the main thermodynamic quantities (i.e., temperature, pressure and entropy) derived from the XMM observations to describe the physical and dynamical state of the cluster’s intracluster medium (ICM) in detail. We performed a point-to-point comparison in terms of surface brightness between the X-ray and radio emissions to quantify their morphological discrepancies. Making use of NuSTAR’s unprecedented hard X-ray focusing capability, we looked for IC emission both globally and locally after properly modeling the purely thermal component with a multi-temperature description. Results. The thermodynamic maps obtained from the XMM observation suggest the presence of a secondary merging process that could be responsible for the peculiar radio halo morphology. This hypothesis is supported by the comparison between the X-ray and radio surface brightnesses, which shows a broad intrinsic scatter and a series of outliers from the best-fit relation, corresponding to those regions that could be influenced by a secondary merger. The global NuSTAR spectrum can be explained by purely thermal gas emission, and there is no convincing evidence that an IC component is needed. The 3σ upper limit on the IC flux in the 20−80 keV band is in the [2.2−4.0] × 10−13 erg s−1 cm−2 range, implying a lower limit on the magnetic field strength in the B > [0.23 − 0.31] μG range. Locally, we looked for IC emission in the central region of the cluster radio halo finding a 3σ upper limit on the 20−80 keV nonthermal flux of 3.17 × 10−14 erg s−1 cm−2, corresponding to a lower limit on the magnetic field strength of B ≳ 0.81 μG.

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Diffuse galaxy cluster emission at 168 MHz within the Murchison Widefield Array Epoch of Reionization 0-h field

Publications of the Astronomical Society of Australia ◽

10.1017/pasa.2021.7 ◽

2021 ◽

Vol 38 ◽

Author(s):

S. W. Duchesne ◽

M. Johnston-Hollitt ◽

A. R. Offringa ◽

G. W. Pratt ◽

Q. Zheng ◽

...

Keyword(s):

Galaxy Clusters ◽

Galaxy Cluster ◽

Scaling Relations ◽

Cluster Emission ◽

X Ray ◽

Diffuse Sources ◽

Radio Halo ◽

The Galaxy ◽

Murchison Widefield Array

Abstract We detect and characterise extended, diffuse radio emission from galaxy clusters at 168 MHz within the Epoch of Reionization 0-h field: a $45^{\circ} \times 45^{\circ}$ region of the southern sky centred on R. A. ${}= 0^{\circ}$ , decl. ${}=-27^{\circ}$ . We detect 29 sources of interest; a newly detected halo in Abell 0141; a newly detected relic in Abell 2751; 4 new halo candidates and a further 4 new relic candidates; and a new phoenix candidate in Abell 2556. Additionally, we find nine clusters with unclassifiable, diffuse steep-spectrum emission as well as a candidate double relic system associated with RXC J2351.0-1934. We present measured source properties such as their integrated flux densities, spectral indices ( $\alpha$ , where $S_\nu \propto \nu^\alpha$ ), and sizes where possible. We find several of the diffuse sources to have ultra-steep spectra including the halo in Abell 0141, if confirmed, showing $\alpha \leq -2.1 \pm 0.1$ with the present data making it one of the steepest-spectrum haloes known. Finally, we compare our sample of haloes with previously detected haloes and revisit established scaling relations of the radio halo power ( $P_{1.4}$ ) with the cluster X-ray luminosity ( $L_{\textrm{X}}$ ) and mass ( $M_{500}$ ). We find that the newly detected haloes and candidate haloes are consistent with the $P_{1.4}$ – $L_{\textrm{X}}$ and $P_{1.4}$ – $M_{500}$ relations and see an increase in scatter in the previously found relations with increasing sample size likely caused by inhomogeneous determination of $P_{1.4}$ across the full halo sample. We show that the MWA is capable of detecting haloes and relics within most of the galaxy clusters within the Planck catalogue of Sunyaev–Zel’dovich sources depending on exact halo or relic properties.

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