Variability of a Sample of Potential meerKAT/SKA Calibrators Faith Hungwe – RU/HartRAO Advisers: R.Ojha – United States Naval Observatory (USNO)‏ (Alan Fey)‏ R. Booth – HartRAO/Rhodes University

Outline Background Interferometry/VLBI USNO/RRFID Results

Background Project was born out of the need for more calibrators in the southern hemisphere Project aims to improve the calibrator grid and characterise the departure of calibrators from point sources Ideally, a calibrator should be a point source, unresolved, should not be variable or have structure The flux density should be constant in time to facilitate removal of systematic atmospheric and instrumental variations

Background At radio wavelengths, the primary flux calibrators are mostly powerful radio galaxies i.e. active galactic nuclei (AGN)‏

Quasars Quite luminous at radio wavelengths (M B < log h o )‏ Most compact of the AGN They are also the most distant and so have stable positions in the sky Well distributed around the sky

Variability Quasars are variable at every wavelength at which they have been studied Mostly due to apparent motion which are related to the existence of jets originating from the source. These apparent motions can cause the departure of the calibrators from being point sources

Interferometry/VLBI θ≈λ/D The resolution of a telescope is limited by its diameter For higher resolution, need to move to larger diameter Mechanical limitations

Interferometry Technique whereby simultaneous observations of the same source are made using several telescopes and the data is combined to produce a useful scientific image

VLBI

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USNO/RRFID The USNO is responsible for maintaining the International Celestial Reference Frame They do this by monitoring a set of quasars which define the ICRF They are continuously looking to densify their grid

USNO/RRFID All source were selected from the United States Naval Observatory’s Radio Reference Frame Image Database (RRFID)‏ The RRFID is a database of about 4000 images of 500 sources compiled from geodetic and astrometric VLBI experiments since 1994

Observations Observations were made using the 10 antennas of the VLBA and for some epochs, the VLBA, Hartebeesthoek + up to 6 geodetic VLBI antennas

Data Analysis The data was correlated at the Arrays Operation Centre in Socorro,New Mexico The correlated data was calibrated using NRAO's Astronomical Image Processing System (AIPS)‏

Data Analysis Imaging and modelfitting were done using the Caltech VLBI Imaging software, DIFMAP Calculations for superluminal motion were done using a program provided by Glen Piner of the Jet Propulsion Lab

Results Classification Scheme A - Point source morphology with little variation in core flux density, (very good calibrator).‏ B - Point source morphology but core varies significantly, (good calibrator)‏. C - Source has components which show proper motion but most flux density is still in the core, (use with caution). D - source has components which show motion and most of the flux density is outside the core, (unsuitable as calibrator).

Results – Superluminal Motion

Superluminal Motion

Summary of Work Done Calibration of Data (100 sources from Jan 2008 epoch at both X and S-band) using AIPS Imaging (for about 66 sources from 2008 epoch) (Difmap)‏ Modelfitting (for 33 sources for all epochs, an average of 19 epochs per source.)(Difmap)‏ Calculations for flux density variation, radial extend of flux and compactness of sources and plotting of graphs (for 33 sources, all epochs, average of 19 epochs per source.)‏ Calculation of proper motion for a few of the sources Classification scheme Just finished paper for Proceeding of EVN meeting attended in Bologna, Italy Have started write up of thesis

Discussion/Future Work We have detected superluminal motion at 2.3GHz in some of the sources Although sources exhibiting superluminal motion do not make good calibrators, they make interesting science cases We find that both the apparent component location and speeds at 2.3GHz do not reproduce well those obtained at 8.4GHz Will continue working on AGN as part of the TANAMI program (Tracking Active galactic Nuclei with Australian-south african Milliarcsecond Interferometry

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