arXiv subjects

Andra Stroe

Publications and source records attributed to Andra Stroe.

40 records · Page 3Linked to original sources

arXiv2014

Spectral age modelling of the `Sausage' cluster radio relic

CIZA J2242.8+5301 is a post-core passage, binary merging cluster that hosts a large, thin, arc-like radio relic, nicknamed the `Sausage', tracing a relatively strong shock front. We perform spatially-resolved spectral fitting to the available radio data for this radio relic, using a variety of spectral ageing models, with the aim of finding a consistent set of parameters for the shock and radio plasma. We determine an injection index of $0.77^{+0.03}_{-0.02}$ for the relic plasma, significantly steeper than was found before. Standard particle acceleration at the shock front implies a Mach number $M=2.90^{+0.10}_{-0.13}$, which now matches X-ray measurements. The shock advance speed is $v_\mathrm{shock}\approx2500$ km s$^{-1}$, which places the core passage of the two subclusters $0.6-0.8$ Gyr ago. We find a systematic spectral age increase from $0$ at the northern side of the relic up to $\sim60$ Myr at $\sim145$ kpc into the downstream area, assuming a $0.6$ nT magnetic field. Under the assumption of freely-ageing electrons after acceleration by the `Sausage' shock, the spectral ages are hard to reconcile with the shock speed derived from X-ray and radio observations. Re-acceleration or unusually efficient transport of particle in the downstream area and line-of-sight mixing could help explain the systematically low spectral ages.

astro-ph.GA↗

arXiv2014

Spectral and morphological signatures of cluster merger shocks: CIZA J2242.8+5301

The CIZA J2242.8+5301 cluster hosts an extraordinary, narrow, Mpc-wide, diffuse patch of radio emission tracing travelling shock waves, called a relic. We perform a multi-wavelength of observations of the cluster. We discover radio spectral steepening and increasing spectral curvature in the shock downstream area, indicative of emission produced by spectrally-aged electrons, following a JP ageing model. The curved integrated spectrum towards $16$ GHz challenges the favoured relic formation model. Further, we find a boosting the number density of H$α$ emitters by a factor of $10$, possibly caused by interactions of the galaxies with the shock wave.

astro-ph.GA↗

arXiv2014

The highest-frequency detection of a radio relic: 16-GHz AMI observations of the `Sausage' cluster

We observed the cluster CIZA J2242.8+5301 with the Arcminute Microkelvin Imager at $16$ GHz and present the first high radio-frequency detection of diffuse, non-thermal cluster emission. This cluster hosts a variety of bright, extended, steep-spectrum synchrotron-emitting radio sources, associated with the intra-cluster medium, called radio relics. Most notably, the northern, Mpc-wide, narrow relic provides strong evidence for diffusive shock acceleration in clusters. We detect a puzzling, flat-spectrum, diffuse extension of the southern relic, which is not visible in the lower radio-frequency maps. The northern radio relic is unequivocally detected and measures an integrated flux of $1.2\pm0.3$ mJy. While the low-frequency ($<2$ GHz) spectrum of the northern relic is well represented by a power-law, it clearly steepens towards $16$ GHz. This result is inconsistent with diffusive shock acceleration predictions of ageing plasma behind a uniform shock front. The steepening could be caused by an inhomogeneous medium with temperature/density gradients or by lower acceleration efficiencies of high energy electrons. Further modelling is necessary to explain the observed spectrum.

astro-ph.CO↗

arXiv2013

The role of cluster mergers and travelling shocks in shaping the H$α$ luminosity function at $\bf z\sim0.2$: `sausage' and `toothbrush' clusters

The most extreme cluster mergers can lead to massive cluster-wide travelling shock waves. The CIZA J2242.8+5301 ('sausage') and 1RXS J0603.3+4213 (`toothbrush') clusters ($z\sim0.2$) host enormous radio-emitting shocks with simple geometry. We investigate the role of mergers and shocks in shaping the H$α$ luminosity function, using custom-made narrow-band filters matching the cluster redshifts mounted on the INT. We surveyed $\sim0.28$ deg$^2$ for each cluster and found $181$ line emitters in the `sausage' (volume of $3.371\times10^3$ Mpc$^3$ for H$α$ at $z=0.1945$) and $141$ in the `toothbrush' ($4.546\times10^3$ Mpc$^3$ for H$α$ at $z=0.225$), out of which $49$ (`sausage') and $30$ (`toothbrush') are expected to be H$α$. We build luminosity functions for the field-of-view down to an average limiting star formation rate of $0.14$ M$_{\odot}$ yr$^{-1}$, find good agreement with field luminosity functions at $z=0.2$, but significant differences between the shapes of the luminosity functions for the two clusters. We discover extended, tens-of-kpc-wide H$α$ haloes in galaxies neighbouring relics, which were possibly disrupted by the passage of the shock wave. By comparing the `sausage' cluster with blank fields and other clusters, we also uncover an order of magnitude boost (at $9σ$ level) in the normalisation $ϕ^*$ of the luminosity function in the relic areas. Our results suggest that cluster mergers may play an important role in the evolution of cluster galaxies through shock-induced star formation.

astro-ph.CO↗