Discovery of a Double Radio Relic in ZwCl1447.2+2619: A Rare Testbed for Shock-acceleration Models with a Peculiar Surface-brightness Ratio

We report a discovery of a double radio relic in the cluster merger ZwCl1447.2+2619 (z = 0.376) with uGMRT observations at 420 MHz and 700 MHz. The linear sizes of the northern and southern relics are ∼0.3 Mpc and ∼1.2 Mpc, respectively, which is consistent with the theoretical expectation that a larger relic is produced in the less massive subcluster side. However, ZwCl1447.2+2619 is unlike other known double radio relic systems, where the larger relics are much more luminous by several factors. In this merger, the higher surface brightness of the smaller northern relic makes its total radio luminosity comparable to that of the much larger southern relic. The surface brightness ratio ∼0.1 between the two radio relics differs significantly from the relation observed in other double radio relic systems. From our radio spectral analysis, we find that both relics signify similar weak shocks with Mach numbers of 2.9 ± 0.8 and 2.0 ± 0.7 for the northern and southern relics, respectively. Moreover, the northern relic is connected to a discrete radio source with an optical counterpart, which indicates the possible presence of cosmic-ray injection and reacceleration. Therefore, we propose that this atypical surface brightness ratio can be explained with the particle acceleration efficiency precipitously dropping in the weak shock regime and/or with reacceleration of fossil cosmic rays. Our multi-wavelength analysis and numerical simulation suggest that ZwCl1447.2+2619 is a postmerger, which has experienced a near head-on collision ∼0.7 Gyr ago.

DZHIRGA EARTHQUAKE on April 27, 2014, Mw=4.9, КР=13.3, I0=7 (Northern Baikal Region)

We consider the April 27, 2014, Mw=4.9 Dzhirga earthquake, occurred within the north-eastern end of the Barguzin Depression. The event is the second one by energy level in the study area for the period of instrumental observations. The earthquake was followed by an aftershock sequence of 75 weak shocks recorded till the end of the year. The focal mechanism was determined from surface wave records from 13 digital broadband seismic stations of the IRIS networks. The obtained solution demonstrates the normal fault kinematics with both nodal planes striking north-east that corresponds to the regional fault pattern. The focal depth is estimated equal to 19 km. The maximum observed intensity was 5 points on the MSK-64 scale at the epicentral distance of 13 km. The Dzhirga earthquake analysis made it possible to fill in the lack of seismological information about the study area. The data obtained can be used for refinement of seismic hazard assessment of the north of the Barguzin Basin.

Propagation of weak shocks in cool-core galaxy clusters in two-temperature magnetohydrodynamics with anisotropic thermal conduction

ABSTRACT We investigate how different magnetohydrodynamic models of propagation of a weak (Mach number ∼1.2) shock in the core of a galaxy cluster affect its observational appearance, using the Perseus cluster as our fiducial model. In particular, we study how thermal conduction, both isotropic and anisotropic, and ion–electron temperature equilibration modify a weak shock. Strong thermal conduction is expected to produce an electron temperature precursor. Less prominent pressure and density precursors are formed as well. A longer equilibration time largely reduces the density precursor but does not change the electron temperature precursor much. When thermal conduction becomes anisotropic, the intracluster magnetic field imprints its characteristic spatial scale on the distortions of the shock induced by heat fluxes.

SENSIBLE EARTHQUAKES 2013 in MOLDOVA: August 11 with КР=11.8, October 6 with КР=14.1, October 15 with КР=12.3 (Romania–Moldova)

The article presents instrumental and macroseismic data on earthquakes of the Carpathian region in 2013, felt on the territory of the Republic of Moldova. Three earthquakes, with an intermediate focal depth, occurred in the Vrancea zone: August 11 with h=93 km, КР=11.8; October 6 with h=140 km, КР=14.1; Octo-ber 15 with h=144 km, КР=12.3. Also in 2013, a swarm of small earthquakes was recorded, near Galati (Ro-mania) and Giurgiulesti (Moldova). The swarm arose September 23, its shocks were recorded for two months. More than 370 seismic events happened, of which three had КР  11 (September 29, 30, and Octo-ber 4 with КР=11.2, 10.9, 10.6). The intensity of shaking reached 5. For earthquakes, September 29, 30 and October 4 regional solutions for the focal mechanism from the Mostryukov and Petrov catalog (GO "Borok" IPE RAS) were received. The event of August 11, 2013, with КР=11.8, occurred at a depth of 93 km on the northeastern margin of the focal zone of Vrancea, felt at the epicenter with intensity I=4. In Chisinau, only some people felt weak shocks (I=2). The two largest earthquakes of 2013 occurred on October 6 at 01h 37m (КР=14.2, mb=5.3, I0=6) and on October 15 at 19h 33m (КР=12.3, mb=4.8), in the central part of the focal zone Vrancea. Between these events, three more earthquakes were noted with ML=3–3.5. However, the most extensive macroseismic manifestations arose from the main shock – October 6, which caused shaking in the territory of four states – in Romania, in the north of Bulgaria, in the south-west of Ukraine, and in Moldova. The reason for such a significant spread of tremors lies in a large focal depth of 140 km. But for the same reason, the intensity of the macroseismic impact did not exceed I=5 in Romania and I=4–5 in Moldova and Ukraine. As a result, a map of the intensity distribution of the October 6 earthquake is given, built on the basis of data processing in 104 locations. Various solutions for the focal mechanisms of intermediate earthquakes in 2013 are presented. The consistent manifestation of a swarm of crustal earthquakes and a number of earthquakes of intermediate depth confirms the unity of tectonic processes in the Carpathian region. The accumulated seismic stresses were preliminarily realized in the form of crustal shocks, then powerful discharge occurred in the Vrancea region, which interrupted a series of events associated with Sfintul-Gheorghe and Peceneaga–Camena faults.

A radio ridge connecting two galaxy clusters in a filament of the cosmic web

Galaxy clusters are the most massive gravitationally bound structures in the Universe. They grow by accreting smaller structures in a merging process that produces shocks and turbulence in the intracluster gas. We observed a ridge of radio emission connecting the merging galaxy clusters Abell 0399 and Abell 0401 with the Low-Frequency Array (LOFAR) telescope network at 140 megahertz. This emission requires a population of relativistic electrons and a magnetic field located in a filament between the two galaxy clusters. We performed simulations to show that a volume-filling distribution of weak shocks may reaccelerate a preexisting population of relativistic particles, producing emission at radio wavelengths that illuminates the magnetic ridge.

Cavities, shocks and a cold front around 3C 320

We present results obtained from the analysis of a total of 110 ks of Chandra observations of the Fanaroff–Riley class II (FR II) radio galaxy 3C 320, located at the centre of a cluster of galaxies at redshift z = 0.342. A pair of X-ray cavities has been detected at an average distance of ∼38 kpc along the east and west directions, with cavity energy, age and total power equal to ∼7.7 × 1059 erg, ∼7 × 107 yr and ∼3.5 × 1044 erg s−1, respectively. The cooling luminosity within the cooling radius of ∼100 kpc was found to be Lcool ∼ 8.5 × 1043 erg s−1. Comparison of these two estimates (total cavity power and cooling luminosity) implies that the cavity power is sufficiently high to balance radiative loss. A pair of weak shocks has also been detected at distances of ∼47 and ∼76 kpc surrounding the radio bubbles. Using the observed density jumps of ∼1.8 and ∼2.1 at the shock locations along the east and west directions, we estimate the Mach numbers ($\mathcal {M}$) to be ∼1.6 and ∼1.8, respectively. A sharp surface-brightness edge was also detected at a relatively larger radius (∼80 kpc) along the south direction. The density jump at this surface-brightness edge was estimated to be ∼1.6 and it is probably due to the presence of a cold front in this cluster. The far-infrared luminosity yielded a star formation rate of 51 M⊙ yr−1, which is a quarter of the cooling rate ($\dot{M}$ ∼ 192 M⊙ yr−1).