ALMACAL IV: A catalogue of ALMA calibrator continuum observations

TL;DR

This paper presents a comprehensive catalogue of ALMA calibrator flux densities from 754 sources over five years, enabling studies of variability, spectral properties, and blazar characteristics across multiple frequencies.

Contribution

It introduces a new, extensive catalogue of ALMA calibrator observations with reprocessed flux measurements and redshift data, facilitating diverse astrophysical analyses.

Findings

Median variability increases with time lag from 100 to 800 days.

BL Lacs have flatter frequency spectra than flat-spectrum radio quasars.

The catalogue covers 17 orders of magnitude in frequency for spectral energy distributions.

Abstract

We present a catalogue of ALMA flux density measurements of 754 calibrators observed between August 2012 and September 2017, for a total of 16,263 observations in different bands and epochs. The flux densities were measured reprocessing the ALMA images generated in the framework of the ALMACAL project, with a new code developed by the Italian node of the European ALMA Regional Centre. A search in the online databases yielded redshift measurements for 589 sources ($\sim$78 per cent of the total). Almost all sources are flat-spectrum, based on their low-frequency spectral index, and have properties consistent with being blazars of different types. To illustrate the properties of the sample we show the redshift and flux density distributions as well as the distributions of the number of observations of individual sources and of time spans in the source frame for sources observed in bands 3 (84$-$116 GHz) and 6 (211$-$275 GHz). As examples of the scientific investigations allowed by the catalogue we briefly discuss the variability properties of our sources in ALMA bands 3 and 6 and the frequency spectra between the effective frequencies of these bands. We find that the median variability index steadily increases with the source-frame time lag increasing from 100 to 800 days, and that the frequency spectra of BL Lacs are significantly flatter than those of flat-spectrum radio quasars. We also show the global spectral energy distributions of our sources over 17 orders of magnitude in frequency.