Hintze Centre for Astrophysical Surveys

Abstract:

Context. Apertif is a phased-array feed system for the Westerbork Synthesis Radio Telescope, providing forty instantaneous beams over 300 MHz of bandwidth. A dedicated survey program utilizing this upgrade started on 1 July 2019, with the last observations taken on 28 February 2022. The imaging survey component provides radio continuum, polarization, and spectral line data.
Aims. Public release of data is critical for maximizing the legacy of a survey. Toward that end, we describe the release of data products from the first year of survey operations, through 30 June 2020. In particular, we focus on defining quality control metrics for the processed data products.
Methods. The Apertif imaging pipeline, Apercal, automatically produces non-primary beam corrected continuum images, polarization images and cubes, and uncleaned spectral line and dirty beam cubes for each beam of an Apertif imaging observation. For this release, processed data products are considered on a beam-by-beam basis within an observation. We validate the continuum images by using metrics that identify deviations from Gaussian noise in the residual images. If the continuum image passes validation, we release all processed data products for a given beam. We apply further validation to the polarization and line data products and provide flags indicating the quality of those data products.
Results. We release all raw observational data from the first year of survey observations, for a total of 221 observations of 160 independent target fields, covering approximately one thousand square degrees of sky. Images and cubes are released on a per beam basis, and 3374 beams (of 7640 considered) are released. The median noise in the continuum images is 41.4 uJy beam−1, with a slightly lower median noise of 36.9 uJy beam−1 in the Stokes V polarization image. The median angular resolution is 11.6″/sin δ. The median noise for all line cubes, with a spectral resolution of 36.6 kHz, is 1.6 mJy beam−1, corresponding to a 3-σ H I column density sensitivity of 1.8 × 1020 atoms cm−2 over 20 km s−1 (for a median angular resolution of 24″ × 15″). Line cubes at lower frequency have slightly higher noise values, consistent with the global RFI environment and overall Apertif system performance. We also provide primary beam images for each individual Apertif compound beam. The data are made accessible using a Virtual Observatory interface and can be queried using a variety of standard tools.

Abstract:

We present an observational study of the luminous red nova (LRN) AT\,2021biy in the nearby galaxy NGC\,4631. The field of the object was routinely imaged during the pre-eruptive stage by synoptic surveys, but the transient was detected only at a few epochs from $\sim 231$\,days before maximum brightness. The LRN outburst was monitored with unprecedented cadence both photometrically and spectroscopically. AT\,2021biy shows a short-duration blue peak, with a bolometric luminosity of $\sim 1.6 \times 10^{41}$\,erg\,s$^{-1}$, followed by the longest plateau among LRNe to date, with a duration of 210\,days. A late-time hump in the light curve was also observed, possibly produced by a shell-shell collision. AT\,2021biy exhibits the typical spectral evolution of LRNe. Early-time spectra are characterised by a blue continuum and prominent H emission lines. Then, the continuum becomes redder, resembling that of a K-type star with a forest of metal absorption lines during the plateau phase. Finally, late-time spectra show a very red continuum ($T_{\mathrm{BB}} \approx 2050$ K) with molecular features (e.g., TiO) resembling those of M-type stars. Spectropolarimetric analysis indicates that AT\,2021biy has local dust properties similar to those of V838\,Mon in the Milky Way Galaxy. Inspection of archival {\it Hubble Space Telescope} data taken on 2003 August 3 reveals a $\sim 20$\,\msun\ progenitor candidate with log\,$(L/{\rm L}_{\odot}) = 5.0$\,dex and $T_{\rm{eff}} = 5900$\,K at solar metallicity. The above luminosity and colour match those of a luminous yellow supergiant. Most likely, this source is a close binary, with a 17--24\,\msun\ primary component.Comment: 21 pages, 14 figures. Accepted by Astronomy and Astrophysic