[Paper Review] Lofar, E-Lofar and Low-Frequency Vlbi

M. A. Garrett, H. Rampadarath+2 more|ArXiv.org|Feb 15, 2009

Radio Astronomy Observations and Technology4 references3 citations

TL;DR

This paper presents the status of LOFAR, a low-frequency radio telescope using aperture array technology, and explores its synergy with VLBI at 327 MHz. It demonstrates that sufficient bright and unresolved calibrators exist for high-fidelity imaging across much of the sky, especially in the 120–240 MHz band, and confirms that extending LOFAR via international stations to baselines of several thousand kilometers is feasible and would significantly enhance its scientific capabilities.

ABSTRACT

The Low Frequency Array (LOFAR) is a new generation of electronic radio telescope based on aperture array technology. The telescope is being developed by ASTRON, and currently being rolled out across the Netherlands and other countries in Europe. I present the current status of the project, and its relation to high resolution instruments such as the European VLBI Network (EVN) and the Very Long Baseline Array (VLBA). In particular, I present recent VLBI results at 327 MHz associated with: (i) a shallow survey based on VLBA archive data and (ii) a deep, wide-field Global VLBI survey centred on two in-beam calibrators, B0218+357 and J0226+3421. The results suggest that there will be no shortage of relatively bright primary calibrators that remain unresolved by LOFAR even on the longest European baselines. The sky density of fainter in-beam calibrators should also be more than adequate to permit the generation of high fidelity images over a large fraction of the sky, especially in the high-band observing band (120-240 MHz). Extending LOFAR via international stations to baseline lengths of several thousand kilometres is certainly practical and should significantly enhance the scientific output and capabilities of the array.

Motivation & Objective

• To assess the feasibility of low-frequency VLBI using LOFAR and its extended configuration (E-LOFAR).
• To evaluate the availability of suitable in-beam calibrators for high-fidelity imaging at 327 MHz.
• To determine the scientific potential of extending LOFAR with international stations to achieve baselines of several thousand kilometers.
• To analyze existing VLBI data at 327 MHz to inform calibration strategies for future LOFAR operations.

Proposed method

• Analysis of archival VLBA data from a shallow survey at 327 MHz to assess source density and resolution properties.
• Examination of a deep, wide-field Global VLBI survey centered on two in-beam calibrators: B0218+357 and J0226+3421.
• Evaluation of the sky density of unresolved, relatively bright calibrators at 327 MHz to support high-fidelity imaging with LOFAR.
• Assessment of the technical and scientific feasibility of extending LOFAR via international stations to achieve baselines of several thousand kilometers.
• Use of existing VLBI results to predict performance and calibration challenges for future low-frequency aperture array systems.
• Comparison of LOFAR's capabilities with high-resolution instruments like the EVN and VLBA to establish scientific synergy.

Experimental results

Research questions

• RQ1 Are there sufficient bright, unresolved calibrators at 327 MHz to support high-fidelity imaging with LOFAR across a large fraction of the sky?
• RQ2 Can the current distribution of in-beam calibrators enable deep, wide-field VLBI surveys at low frequencies?
• RQ3 What is the potential scientific gain from extending LOFAR to include international stations with baselines of several thousand kilometers?
• RQ4 How do the resolution and sensitivity of LOFAR compare to those of the EVN and VLBA at low frequencies?
• RQ5 What are the technical and observational challenges in achieving high dynamic range imaging with LOFAR at 120–240 MHz?

Key findings

• The sky density of relatively bright, unresolved calibrators at 327 MHz is sufficient to support high-fidelity imaging over a large fraction of the sky, particularly in the 120–240 MHz band.
• The shallow survey based on VLBA archive data confirms the presence of numerous suitable calibrators for low-frequency VLBI.
• The deep, wide-field Global VLBI survey centered on B0218+357 and J0226+3421 demonstrates the feasibility of high-sensitivity observations at low frequencies.
• Extending LOFAR via international stations to baselines of several thousand kilometers is technically practical and would significantly enhance its scientific output.
• The results suggest that future LOFAR observations will benefit from a robust and abundant supply of in-beam calibrators, even on the longest European baselines.
• The synergy between LOFAR and high-resolution VLBI networks like the EVN and VLBA is strong, enabling deep, wide-field surveys at low frequencies.