High-resolution international LOFAR observations of 4C 43.15: spectral ages and injection indices in a high-z radio galaxy

Galaxie

Sub-arcsecond imaging with the International LOFAR Telescope I. Foundational calibration strategy and pipeline

The International LOFAR Telescope is an interferometer with stations spread across Europe. With baselines of up to ~2000 km, LOFAR has the unique capability of achieving sub-arcsecond resolution at frequencies below 200 MHz. However, it is technically and logistically challenging to process LOFAR data at this resolution. To date only a handful of publications have exploited this capability. Here we present a calibration strategy that builds on previous high-resolution work with LOFAR. It is implemented in a pipeline using mostly dedicated LOFAR software tools and the same processing framework as the LOFAR Two-metre Sky Survey (LoTSS). We give an overview of the calibration strategy and discuss the special challenges inherent to enacting high-resolution imaging with LOFAR, and describe the pipeline, which is publicly available, in detail. We demonstrate the calibration strategy by using the pipeline on P205+55, a typical LoTSS pointing with an 8 h observation and 13 international stations. We perform in-field delay calibration, solution referencing to other calibrators in the field, self-calibration of these calibrators, and imaging of example directions of interest in the field. We find that for this specific field and these ionospheric conditions, dispersive delay solutions can be transferred between calibrators up to ~1.5° away, while phase solution transferral works well over ~1°. We also demonstrate a check of the astrometry and flux density scale with the in-field delay calibrator source. Imaging in 17 directions, we find the restoring beam is typically ~0.3′′ ×0.2′′ although this varies slightly over the entire 5 deg2 field of view. We find we can achieve ~80–300 μJy bm−1 image rms noise, which is dependent on the distance from the phase centre; typical values are ~90 μJy bm−1 for the 8 h observation with 48 MHz of bandwidth. Seventy percent of processed sources are detected, and from this we estimate that we should be able to image roughly 900 sources per LoTSS pointing. This equates to ~ 3 million sources in the northern sky, which LoTSS will entirely cover in the next several years. Future optimisation of the calibration strategy for efficient post-processing of LoTSS at high resolution makes this estimate a lower limit

Sub-arcsecond imaging with the International LOFAR Telescope. I.: Foundational calibration strategy and pipeline

InstrumentationGalaxie

The Majorana project

Building a 0νβ β experiment with the ability to probe neutrino mass in the inverted hierarchy region requires the combination of a large detector mass sensitive to 0νβ β, on the order of 1-tonne, and unprecedented background levels, on the order of or less than 1 count per year in the 0νβ β signal region. The Majorana Collaboration proposes a design based on using high-purity enriched 76Ge crystals deployed in ultra- low background electroformed Cu cryostats and using modern analysis techniques that should be capable of reaching the required sensitivity while also being scalable to a 1- tonne size. To demonstrate feasibility, the collaboration plans to construct a prototype system, the Majorana Demonstrator, consisting of 30 kg of 86% enriched 76Ge detectors and 30 kg of natural or isotope-76-depleted Ge detectors. We plan to deploy and evaluate two different Ge detector technologies, one based on a p-type configuration and the other on n-type

High-resolution international LOFAR observations of 4C 43.15. Spectral ages and injection indices in a high-z radio galaxy

It has long been known that radio sources with the steepest spectra are preferentially associated with the most distant galaxies, the α − z relation, but the reason for this relation is an open question. The spatial distribution of spectra in high-z radio sources can be used to study this relation, and low-frequency observations are particularly important in understanding the particle acceleration and injection mechanisms. However, the small angular sizes of high-z sources together with the inherently low resolution of low-frequency radio telescopes until now has prevented high angular resolution low-frequency observations of distant objects. Here we present subarcsecond observations of a z = 2.4 radio galaxy at frequencies between 121 and 166 MHz. We measure the spatial distribution of spectra, and discuss the implications for models of the α−z relation. We targeted 4C 43.15 with the High Band Antennas of the International LOFAR Telescope with a range of baselines up to 1300 km. At the central frequency of 143 MHz we achieve an angular resolution of ~0.3″. By complementing our data with archival Very Large Array data we study the spectral index distribution across 4C 43.15 between 55 MHz and 8.4 GHz at resolutions of 0.4″ and 0.9″. With a magnetic field strength of B = 5.2 nT and fitted injection indices of αinjnorth = −0.8 and αinjsouth = −0.6, fitting a Tribble spectral ageing model results in a spectral age of τspec = 1.1 ± 0.1 Myr. We conclude that our data on 4C 43.15 indicates that inverse Compton losses could become comparable to or exceed synchrotron losses at higher redshifts and that inverse Compton losses could be a viable explanation for the α−z relation. Statistical studies of these objects will become possible in the future when wide-area subarcsecond surveys start