LOFAR observations of the XMM-LSS field

Abstract

We present observations of the XMM Large-Scale Structure (XMM-LSS) field observed with the LOw Frequency ARray (LOFAR) at 120-168 MHz. Centred at a J2000 declination of −4.5 • , this is a challenging field to observe with LOFAR because of its low elevation with respect to the array. The low elevation of this field reduces the effective collecting area of the telescope, thereby reducing sensitivity. This low elevation also causes the primary beam to be elongated in the north-south direction, which can introduce side lobes in the synthesised beam in this direction. However the XMM-LSS field is a key field to study because of the wealth of ancillary information, encompassing most of the electromagnetic spectrum. The field was observed for a total of 12 hours from three four-hour LOFAR tracks using the Dutch array. The final image presented encompasses ∼ 27 deg 2 , which is the region of the observations with a >50% primary beam response. Once combined, the observations reach a central rms of 280 µJy beam −1 at 144 MHz and have an angular resolution of 7.5 × 8.5 . We present our catalogue of detected sources and investigate how our observations compare to previous radio observations. This includes investigating the flux scale calibration of these observations compared to previous measurements, the implied spectral indices of the sources, the observed source counts and corrections to obtain the true source counts, and finally the clustering of the observed radio sources.

The contribution of discrete sources to the sky temperature at 144 MHz

“…We see that there is good agreement at the bright end with the shape and normalisation of the 6C counts of Hales et al (1988), and reasonable agreement with the fitted functional form of Intema et al (2017) using counts from the TIFR GMRT Sky Survey Alternative Data Release (TGSS), which, however, seems systematically low by around 20 per cent with respect to both LoTSS and 6C at flux densities around 1 Jy, and of course deviates from observations below the flux density limit of 5 mJy used by Intema et al A crossmatch between TGSS and 6C suggests a flux scale offset of around 6 per cent for bright sources (in the sense that TGSS fluxes are systematically higher than 6C) which may account for some of the discrepancy. The source counts we present here are generally consistent with both brightend and faint-end source counts from LOFAR already presented in the literature or shortly to be published (Williams et al 2016;Hardcastle et al 2016;Retana-Montenegro et al 2018;Hale et al 2019;Siewert et al 2020) and, as the source counts are not the main topic of this paper, we do not discuss them in more detail here: the reader is referred to the paper by Mandal et al (in prep. ), who make use of the full area of the deep fields, for discussion of the implications of the deep-field counts.…”

The study of the angular and spatial distribution of radio-selected AGNs and star-forming galaxies in the ELAIS N1 field

“…The black line is the best fitted power law to w(θ). We also compare our findings with previous observations of XMM-LSS(Hale et al 2019), FIRST(Lindsay et al 2014),COSMOS Hale et al (2018) and TGSS (Rana & Bagla 2019).…”

The LOFAR Two-metre Sky Survey

“…The source populations being recovered in our images span a wide range of flux densities (with substantial numbers of sources from 0.2 mJy< S I < 5000 mJy), and as shown by e.g. Williams et al (2016) and Hale et al (2019), even an individual LoTSSdepth pointing has sufficient area and sensitivity to accurately constrain radio source counts over several orders of magnitude in flux density (from approximately 1 mJy to 1 Jy). However, recent longer duration and more sensitive observations at lowfrequency (LoTSS-deep) allow for better statistics to constrain fainter populations that are largely beyond the reach of LoTSS depth observations (Mandal et al 2021 present counts down to 0.25 mJy).…”