ALMACAL IV: a catalogue of ALMA calibrator continuum observations

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 (∼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 . 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.

Characterizing the Gamma-Ray Variability of the Brightest Flat Spectrum Radio Quasars Observed with the Fermi LAT

“…We search for time lags between the Fermi -LAT light curves and radio light curves obtained with the Owens Valley Radio Observatory (OVRO) at 15 GHz, the At-acama Large submillimeter/millimeter Array (ALMA) between 84 and 116 GHz (Band 3, 3.6-2.6 mm), and the Submillimeter Array (SMA) at 230 GHz (1.3 mm). All of the studied FSRQs are included in an ongoing blazar monitoring program at OVRO (Richards et al 2011) and SMA (Gurwell et al 2007), and also serve as calibrators for SMA and ALMA (Bonato et al 2018) at millimeter wavelengths. 20 We show all radio and γ-ray light curves in Figure 16.…”

ALMA photometry of extragalactic radio sources

“…Each calibrator is generally observed several times, in connection with different targets, on different days, in various bands and array configurations. Bonato et al (2018) have published a catalogue of ALMA flux density measurements of 754 calibrators for a total of 16,263 observations in different bands and epochs (ALMACAL catalogue). These flux densities were uniformly measured from a collection of ALMA images, thus obtaining robust measurements for both resolved and non-resolved sources.…”

“…These flux densities were uniformly measured from a collection of ALMA images, thus obtaining robust measurements for both resolved and non-resolved sources. The calibration and imaging analyses are described in Bonato et al (2018). Almost all sources (∼97%) were found to be blazars.…”

“…About 41% (1391) of our sources are listed in the 5th edition of the Roma Multi-frequency Catalogue of Blazars 2 (BZCAT; Massaro et al 2009) where they are classified into 5 sub-classes: FSRQs, BL Lacs, BL Lacs-galaxy dominated, Blazars of uncertain type, BL Lac candidates. We have classified the remaining objects following Bonato et al (2018).…”

“…An accurate calibration is difficult to achieve due to variability of the emissive and absorptive properties of the Earth's atmosphere and to the lack of astronomical sources that could serve as accurate flux standards. Following Bonato et al (2018) we adopt at 5% calibration uncertainty for ALMACAL sources. This value was found to be consistent with the median absolute differences among measurements in Bands 3 and 6 within short time-spans (30 d in the source frame), expected to be only weakly affected by variability (see Sect.…”