Imaging Sources in the Third Realization of the International Celestial Reference Frame

Abstract

The third iteration of the International Celestial Reference Frame (ICRF3) is made up of 4536 quasars observed at S/X bands using very-long-baseline interferometry. These sources are high-redshift quasars, typically between 1 < z < 2, that are believed to host active galactic nuclei at their centers. The position of compact radio sources can be determined better than sources with large amounts of extended radio structure. Here we report information on a series of 20 observations from 2017 January through December 2017 that were designed for precise astrometry and to monitor the structure of sources included in the ICRF3. We targeted 3627 sources over the one year campaign and found the median flux density of 2697 detected sources at S band is 0.13 Jy, and the median flux density of 3209 sources detected at X band is 0.09 Jy. We find that 70% of detected sources in our campaign are considered compact at X band and ideal for use in the ICRF and 89% of the 2615 sources detected at both frequencies have a flat spectral index, α > 0.5

Overview and Status of the International Celestial Reference Frame as Realized by VLBI

“…Since the spring 2018 cutoff date for sessions to be included in the ICRF-3, observations and analysis have continued with very positive results at S/X and K-bands [20], and at X/Ka-band [21]. Imaging work, to assess and monitor the astrometric source suitability, has also continued, and images of ICRF sources from astrometric VLBI observations at S, X, and K-bands are continuously updated, e.g., as part of the Bordeaux VLBI Image Database (BVID) [25,101], the Radio Fundamental Catalog (RFC) [102] of the Astrogeo Center, the USNO Fundamental Reference Image Data Archive (FRIDA) [26,103], and as part of the K-band CRF collaboration [27]. The current status, including the accuracy and frequency specific limiting errors, for each of the S/X, K, and X/Ka frames, are detailed below.…”

“…Precision of the 4536 original ICRF-3 S/X sources has been improved by ∼25% to ∼95 µas in right ascension and ∼160 µas in declination. Imaging of the VLBA S/X sessions is also being made at the USNO and will serve to characterize the structure of ∼4500 sources at S/X band (e.g., [26]).…”

“…Looking to the future, the international community is developing road maps for continued improvement of the ICRF, e.g., [4] (Section 8), and [22][23][24], in particular dedicated strategies to improve the astrometric observing programs in the South, ongoing efforts to monitor source structure [8,[25][26][27][28] and to mitigate its effect on the measured astrometric source positions [19,29,30], and plans for a fully integrated multi-wavelength realization of the ICRF, including also the optical Gaia reference frame [31,32].…”

The Fundamental Reference AGN Monitoring Experiment (FRAMEx)

“…• ICRF Source Structure Imaging Database. Associated with FRAMEx, and in support of USNO agreements with the International VLBI Service for Geodesy and Astrometry (IVS) and IERS agreements, we are in the process of developing a data archive of VLBA and VLBI images of AGN (the Fundamental Reference Image Data Archive, or FRIDA (Hunt 2019(Hunt , 2020. These high-resolution images will be made available to the public, and will eventually be supplemented by other data products (including any visible, IR, or x-ray observations) associated with the relevant RFOs.…”

Resolving the Inner Parsec of the Blazar J1924–2914 with the Event Horizon Telescope

“…The first EHT millimeter images of linearly polarized emission in M87 * were published recently (EHTC et al 2021a(EHTC et al , 2021b. In this paper, we present the first total-intensity and linear-polarization images of J1924-2914 at 230 GHz obtained with the EHT and make comparisons to the near-contemporaneous GMVA results from Issaoun et al (2019) and Very Long Baseline Array (VLBA) observations at 2.3 and 8.7 GHz (Hunt et al 2021).…”