A shock at the radio relic position in Abell 115 Botteon, A; Gastaldello, F; Brunetti, G ...
Monthly notices of the Royal Astronomical Society. Letters,
07/2016, Volume:
460, Issue:
1
Journal Article
Peer reviewed
Open access
We analysed a deep Chandra observation (334 ks) of the galaxy cluster Abell 115 and detected a shock cospatial with the radio relic. The X-ray surface brightness profile across the shock region ...presents a discontinuity, corresponding to a density compression factor
$\mathcal {C}=2.0\pm 0.1$
, leading to a Mach number
$\mathcal {M}=1.7\pm 0.1$
(
$\mathcal {M}=1.4{\rm -}2$
including systematics). Temperatures measured in the upstream and downstream regions are consistent with what expected for such a shock:
$T_{\rm u}=4.3^{+1.0}_{-0.6}\:\rm {keV}$
and
$T_{\rm d}=7.9^{+1.4}_{-1.1}\:\rm {keV}$
, respectively, implying a Mach number
$\mathcal {M}=1.8^{+0.5}_{-0.4}$
. So far, only few other shocks discovered in galaxy clusters are consistently detected from both density and temperature jumps. The spatial coincidence between this discontinuity and the radio relic edge strongly supports the view that shocks play a crucial role in powering these synchrotron sources. We suggest that the relic is originated by shock re-acceleration of relativistic electrons rather than acceleration from the thermal pool. The position and curvature of the shock and the associated relic are consistent with an off-axis merger with unequal mass ratio where the shock is expected to bend around the core of the less massive cluster.
Multifrequency Very Long Baseline Array (VLBA) observations were performed to study the radio morphology and synchrotron spectra of four high-frequency peaking radio sources. They are resolved into ...several compact components and the radio emission is dominated by hotspots/lobes. The core region is detected unambiguously in J1335+5844 and J1735+5049. The spectra of the main source components peak above 3 GHz. Assuming that the spectral peak is produced by synchrotron self-absorption, we estimate the magnetic field directly from observable quantities: in half of the components it agrees with the equipartition field, while in the others the difference exceeds an order of magnitude. By comparing the physical properties of the targets with those of larger objects, we found that the luminosity increases with linear size for sources smaller than a few kpc, while it decreases for larger objects. The asymmetric sources J1335+5844 and J1735+5049 suggest that the ambient medium is inhomogeneous and is able to influence the evolution of the radio emission even during its first stages. The core luminosity increases with linear size for sources up to a few kpc, while it seems constant for larger sources, suggesting an evolution independent of source total luminosity.
Context. Giant radio halos (RH) are diffuse Mpc-scale synchrotron sources detected in some massive and merging galaxy clusters. An unbiased study of the statistical properties of RHs is crucial to ...constraining their origin and evolution. Aims. We investigate the occurrence of RHs and its dependence on the cluster mass in a Sunyaev-Zel'dovich (SZ)-selected sample of galaxy clusters, which is nearly a mass-selected sample. Moreover, we analyse the connection between RHs and merging clusters. Methods. We selected from the Planck SZ catalogue clusters with M > or = 6 x 10 super(14)Mmiddot in circle at z = 0.08-0.33 and we searched for the presence of RHs using the NVSS for z < 0.2 and the GMRT RH survey (GRHS) and its extension (EGRHS) for 0.2 < z < 0.33. We used archival Chandra X-ray data to derive information on the dynamical status of the clusters. Results. We confirm that RH clusters are merging systems while the majority of clusters without RH are relaxed, thus supporting the idea that mergers play a fundamental role in the generation of RHs. We find evidence for an increase in the fraction of clusters with RHs with the cluster mass and this is in line with expectations derived on the basis of the turbulence re-acceleration scenario. Finally, we discuss the effect of the incompleteness of our sample on this result.
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The first blazar observed at z > 6 Belladitta, S.; Moretti, A.; Caccianiga, A. ...
Astronomy & astrophysics,
03/2020, Volume:
635
Journal Article
Peer reviewed
Open access
We present the discovery of PSO J030947.49+271757.31, the radio brightest (23.7 mJy at 1.4 GHz) active galactic nucleus (AGN) at
z
> 6.0. It was selected by cross-matching the NRAO VLA Sky Survey ...and the Panoramic Survey Telescope and Rapid Response System PS1 databases and its high-
z
nature was confirmed by a dedicated spectroscopic observation at the Large Binocular Telescope. A pointed
Neil Gehrels Swift
Observatory XRT observation allowed us to measure a flux of ∼3.4 × 10
−14
erg s
−1
cm
−2
in the 0.5–10 keV energy band, which also makes this object the X-ray brightest AGN ever observed at
z
> 6.0. Its flat radio spectrum (
α
νr
< 0.5), very high radio loudness (
R
> 10
3
), and strong X-ray emission, compared to the optical, support the hypothesis of the blazar nature of this source. Assuming that this is the only blazar at this redshift in the surveyed area of sky, we derive a space density of blazars at
z
∼ 6 and with
M
1450 Å
< −25.1 of 5.5
+11.2
−4.6
× 10
−3
Gpc
−3
. From this number, and assuming a reasonable value of the bulk velocity of the jet (Γ = 10), we can also infer a space density of the entire radio-loud AGN population at
z
∼ 6 with the same optical/UV absolute magnitude of 1.10
+2.53
−0.91
Gpc
−3
. Larger samples of blazars will be necessary to better constrain these estimates.
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We present Director’s Discretionary Time multi-frequency observations obtained with the
Jansky
Very Large Array and the Very Long Baseline Array (VLBA) of the blazar PSO J030947.49+271757.31 ...(hereafter PSO J0309+27) at
z
= 6.10 ± 0.03. The milliarcsecond angular resolution of our VLBA observations at 1.5, 5, and 8.4 GHz unveils a bright one-sided jet extended for ∼500 parsecs in projection. This high-
z
radio-loud active galactic nucleus is resolved into multiple compact sub-components that are embedded in a more diffuse and faint radio emission that enshrouds them in a continuous jet structure. We directly derive limits on some physical parameters from observable quantities such as viewing angle and Lorentz and Doppler factors. If PSO J0309+27 is a genuine blazar, as suggested by its X-ray properties, then we find that its bulk Lorentz factor must be relatively low (lower than 5). This value would be in favour of a scenario currently proposed to reconcile the paucity of high-
z
blazars with current predictions. Nevertheless, we cannot exclude that PSO J0309+27 is seen under a larger viewing angle, which would imply that the X-ray emission must be enhanced, for example, by inverse Compton scattering with the cosmic microwave background. More stringent constraints on the bulk Lorentz factor in PSO J0309+27 and on these factors in the other high-
z
blazars are necessary to test whether their properties are intrinsically different from those of the low-
z
blazar population.
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Aims. The aim of the present work is to constrain the Coma cluster magnetic field strength, its radial profile and power spectrum by comparing Faraday rotation measure (RM) images with numerical ...simulations of the magnetic field. Methods. We have analyzed polarization data for seven radio sources in the Coma cluster field observed with the Very Large Array at 3.6, 6 and 20 cm, and derived Faraday rotation measures with kiloparsec scale resolution. Random three dimensional magnetic field models have been simulated for various values of the central intensity B0 and radial power-law slope η, where η indicates how the field scales with respect to the gas density profile. Results. We derive the central magnetic field strength, and radial profile values that best reproduce the RM observations. We find that the magnetic field power spectrum is well represented by a Kolmogorov power spectrum with minimum scale ~2 kpc and maximum scale ~34 kpc. The central magnetic field strength and radial slope are constrained to be in the range (B0 = 3.9 μG; η = 0.4) and (B0 = 5.4 μG; η = 0.7) within 1σ. The best agreement between observations and simulations is achieved for B0 = 4.7 μG; η = 0.5. Values of B0 > 7 μG and <3 μG as well as η < 0.2 and η > 1.0 are incompatible with RM data at 99% confidence level.
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ABSTRACT
We present results on global very long baseline interferometry (VLBI) observations at 327 MHz of 18 compact steep-spectrum (CSS) and GHz-peaked spectrum (GPS) radio sources from the 3C and ...the Peacock & Wall catalogues. About 80 per cent of the sources have a ‘double/triple’ structure. The radio emission at 327 MHz is dominated by steep-spectrum extended structures, while compact regions become predominant at higher frequencies. As a consequence, we could unambiguously detect the core region only in three sources, likely due to self-absorption affecting its emission at this low frequency. Despite their low surface brightness, lobes store the majority of the source energy budget, whose correct estimate is a key ingredient in tackling the radio source evolution. Low-frequency VLBI observations able to disentangle the lobe emission from that of other regions are therefore the best way to infer the energetics of these objects. Dynamical ages estimated from energy budget arguments provide values between 2 × 103 and 5 × 104 yr, in agreement with the radiative ages estimated from the fit of the integrated synchrotron spectrum, further supporting the youth of these objects. A discrepancy between radiative and dynamical ages is observed in a few sources where the integrated spectrum is dominated by hotspots. In this case the radiative age likely represents the time spent by the particles in these regions, rather than the source age.
Abstract
Radio haloes and radio relics are diffuse synchrotron sources that extend over Mpc-scales and are found in a number of merger galaxy clusters. They are believed to form as a consequence of ...the energy that is dissipated by turbulence and shocks in the intracluster medium (ICM). However, the precise physical processes that generate these steep synchrotron spectrum sources are still poorly constrained. We present a new LOFAR observation of the double galaxy cluster Abell 1758. This system is composed of A1758N, a massive cluster hosting a known giant radio halo, and A1758S, which is a less massive cluster whose diffuse radio emission is confirmed here for the first time. Our observations have revealed a radio halo and a candidate radio relic in A1758S, and a suggestion of emission along the bridge connecting the two systems which deserves confirmation. We combined the LOFAR data with archival VLA and GMRT observations to constrain the spectral properties of the diffuse emission. We also analysed a deep archival Chandra observation and used this to provide evidence that A1758N and A1758S are in a pre-merger phase. The ICM temperature across the bridge that connects the two systems shows a jump which might indicate the presence of a transversal shock generated in the initial stage of the merger.
Context. Radio relics are extended synchrotron sources thought to be produced by shocks in the outskirts of merging galaxy clusters. The cluster Abell 2256 hosts one of the most intriguing examples ...in this class of sources. It has been found that this radio relic has a rather flat integrated spectrum at low frequencies that would imply an injection spectral index for the electrons that is inconsistent with the flattest allowed by the test particle diffusive shock acceleration (DSA). Aims. We aim at testing the origins of the radio relic in Abell 2256. Methods. We performed new high-frequency observations at 2273, 2640, and 4850 MHz. Combining these new observations with images available in the literature, we constrain the radio-integrated spectrum of the radio relic in Abell 2256 over the widest sampled frequency range collected so far for this class of objects (63−10 450 MHz). Moreover, we used X-ray observations of the cluster to check the temperature structure in the regions around the radio relic. Results. We find that the relic keeps an unusually flat behavior up to high frequencies. Although the relic integrated spectrum between 63 and 10 450 MHz is not inconsistent with a single power law with α6310 450 = 0.92 ± 0.02, we find hints of a steepening at frequencies >1400 MHz. The two frequency ranges 63−1369 MHz and 1369−10 450 MHz are, indeed, best represented by two different power laws, with α631369 = 0.85 ± 0.01 and α136910 450 = 1.00 ± 0.02. This broken power law would require special conditions to be explained in terms of test-particle DSA, e.g., non-stationarity of the spectrum, which would make the relic in A2256 a rather young system, and/or non-stationarity of the shock. On the other hand, the single power law would make of this relic the one with the flattest integrated spectrum known so far, even flatter than what is allowed in the test-particle approach to DSA. We find a rather low temperature ratio of T2/T1 ~ 1.7 across the G region of the radio relic and no temperature jump across the H region. However, in both regions projection effects might have affected the measurements, thereby reducing the contrast.
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