Recent low-frequency radio observations at 140 MHz discovered a 3 Mpc-longbridge of diffuse emission connecting the galaxy clusters Abell 0399 and Abell0401. We present follow-up observations at 60 MHz to constrain the spectralindex of the bridge, which so far has only been detected at 140 and 144 MHz. Weanalysed deep (~18 hours) LOw Frequency ARray (LOFAR) Low Band Antenna (LBA)data at 60 MHz to detect the bridge at very low frequencies. We then conducteda multi-frequency study with LOFAR HBA data at 144 MHz and uGMRT data at 400MHz. Assuming second-order Fermi mechanisms for the re-acceleration ofrelativistic electrons driven by turbulence in the radio bridge regions, wecompare the observed radio spectrum with theoretical synchrotron models. Thebridge is detected in the 75'' resolution LOFAR image at 60 MHz and itsemission fully connects the region between the two galaxy clusters. Between 60MHz and 144 MHz we found an integrated spectral index value of -1.44 +\- 0.16for the bridge emission. For the first time, we produced spectral index andrelated uncertainties maps for a radio bridge. We produce a radio spectrum,which show significant steepening between 144 and 400 MHz. This detection atlow frequencies provides important information on the models of particleacceleration and magnetic field structure on very extended scales. The spectralindex gives important clues to the origin of inter-cluster diffuse emission.The steepening of the spectrum above 144 MHz can be explained in a turbulentre-acceleration framework, assuming that the acceleration timescales are longerthan ~200 Myr.
Fast radio bursts (FRBs) are energetic, millisecond-duration radio pulsesobserved at extragalactic distances and whose origins are still a subject ofheated debate. A fraction of the FRB population have shown repeating bursts,however it's still unclear whether these represent a distinct class of sources.We investigated the bursting behaviour of FRB 20220912A, one of the most activerepeating FRBs known thus far. In particular, we focused on its burst energydistribution, linked to the source energetics, and its emission spectrum, withthe latter directly related to the underlying emission mechanism. We monitoredFRB 20220912A at $408$ MHz with the Northern Cross radio telescope and at $1.4$GHz using the $32$-m Medicina Grueff radio telescope. Additionally, weconducted $1.2$ GHz observations taken with the upgraded Giant Meter Wave RadioTelescope (uGMRT) searching for a persistent radio source coincident with FRB20220912A, which included high energy observations in the $0.3-10$ keV,$0.4-100$ MeV and $0.03-30$ GeV energy range. We report $16$ new bursts fromFRB 20220912A at $408$ MHz during the period between October 16$^{\rm th}$ 2022and December 31$^{\rm st}$ 2023. Their cumulative spectral energy distributionfollows a power law with slope $\alpha_E = -1.3 \pm 0.2$ and we measured arepetition rate of $0.19 \pm 0.03$ hr$^{-1}$ for bursts having a fluence of$\mathcal{F} \geq 17$ Jy ms. Furthermore, we report no detections at 1.4 GHzfor $\mathcal{F} \geq 20$ Jy ms. These non-detections imply an upper limit of$\beta < -2.3$, with $\beta$ being the $408$ MHz $-$ $1.4$ GHz spectral indexof FRB 20220912A. This is inconsistent with positive $\beta$ values found forthe only two known cases in which an FRB has been detected in separate spectralbands. We find that FRB 20220912A shows a decline of four orders of magnitudein its bursting activity at $1.4$ GHz over a .. (abridged)
Superclusters are the largest-scale environments where a number of galaxyclusters interact with each other through minor/major mergers and grow viaaccretion along cosmic filaments. We focus on the A3528 complex in the core ofthe Shapley Supercluster. This system includes three clusters, A3528 (composeditself by two sub-clusters, namely A3528N and A3528S), A3532 and A3530, andpresents a mildly active dynamical state. We study how minor mergers affect theevolution of radio galaxies and whether they are able to re-acceleraterelativistic electrons in the ICM. We used observations from the uGMRT (Band 3,4 and 5) and MeerKAT (L-band) telescopes to obtain images and spectral indexmaps over a wide frequency band and spatial resolutions. We compare these datawith those from the SRG/eROSITA X-ray telescope. We detect faint diffuse radioemission associated with the radio galaxies. The BCGs in A3528S and A3532 showfilaments of diffuse radio emission which extend for $\sim200-400$ kpc out ofthe radio galaxy. The spectral index of these filaments is extremely steep andalmost constant ($\alpha\sim -2, -2.5$). Contrary to the radio tails in A3528N,the spectral properties of these radio filaments are not consistent withstandard models of plasma ageing. We also detect roundish diffuse radioemission around the BCG in A3528S which could be classified as a radiomini-halo. The radio tail in this cluster appears longer that in earlierdetections, being $\sim300$ kpc long at all frequencies. We linked the presenceof extended radio emission in the form of filaments and threads in the A3528complex with the effect of minor mergers. This is reinforced by the increasingX-ray fluctuations in correspondence with the radio extended emission inA3528S. Despite the less energy involved, our findings support the hypothesisthat these events can re-energise plasma originating from radio galaxies.
The inner disk of the young star PDS 70 may be a site of rocky planetformation, with two giant planets detected further out. Solids in the innerdisk may inform us about the origin of this inner disk water and nature of thedust in the rocky planet-forming regions. We aim to constrain the chemicalcomposition, lattice structure, and grain sizes of small silicate grains in theinner disk of PDS 70, observed both in JWST/MIRI MRS and Spitzer IRS. We use adust fitting model, called DuCK, based on a two-layer disk model. We useGaussian Random Field and Distribution of Hollow Spheres models to obtain twosets of dust opacities. The third set of opacities is obtained from aerosolspectroscopy. We use stoichiometric amorphous silicates, forsterite, andenstatite in our analysis. We also used iron-rich and magnesium-rich amorphoussilicate and fayalite dust species to study the iron content. The GaussianRandom Field opacity agrees well with the observed spectrum. In both MIRI andSpitzer spectra, amorphous silicates are the dominant dust species. Crystallinesilicates are dominated by iron-poor olivine. We do not find strong evidencefor enstatite. Moreover, the MIRI spectrum indicates larger grain sizes thanthe Spitzer spectrum, indicating a time-variable small grain reservoir. Theinner PDS 70 disk is dominated by a variable reservoir of optically thin warmamorphous silicates. We suggest that the small grains detected in the inner PDS70 disk are likely transported inward from the outer disk as a result offiltration and fragmentation at the ice line. In addition, the variationbetween MIRI and Spitzer data can be explained by the grain growth over 15years and a dynamical inner disk where opacity changes occur resulting from thehighly variable hot innermost dust reservoir.