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VLA Low Band Ionospheric and Transient Experiment (VLITE)

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The Naval Research Laboratory and NRAO have successfully collaborated to develop the VLA Low Band Ionospheric and Transient Experiment (VLITE). VLITE is a 10-antenna commensal system capable of continuously accessing 64 MHz from the new 236-492 MHz Low Band system deployed on the VLA. The VLITE backend includes dedicated samplers, fiber optics, and a DiFX-based software correlator. By harvesting data from the VLA's prime focus, VLITE will effectively provide more than 3000 hours of additional observing time every year, effectively making the VLA "two telescopes in one". VLITE first fringes were obtained on July 17, 2014; for a description see this NRAO eNews article. VLITE has been operational since the VLA moved into its B-configuration in January of 2015. A recent press release describes the instrument and shows an image produced from the data, which is reproduced below. VLITE is an NRL-funded project supported by NRAO; the PI is Namir Kassim and the Astrophysics lead scientist and technical point of contact is Tracy Clarke.

                Radio (VLITE) and optical (SDSS) image showing the giant radio galaxy IC 711 and companions IC 708 and IC 712. All three systems are part of the distant galaxy cluster Abell 1314 and were serendipitously located in a field pointed at an unrelated low redshift galaxy. The radio data were fully processed through the VLITE pipeline and show the power of this new instrument. The field shown is the size of a full moon. (Credit: Radio (blue) from VLA Low Band Ionospheric and Transient Experiment on the NRAO VLA. Optical (red and green) from the Sloan Digital Sky Survey. U.S. Naval Research Laboratory/Dr. Tracy Clarke)                

VLITE has three direct scientific drivers: ionospheric studies; transient science; and astronomical imaging. The ionospheric studies will provide continuous, near real-time monitoring of ionospheric waves over the VLA. VLITE is significantly more sensitive to tiny fluctuations in ionospheric total electron content than the Global Positioning System (GPS), opening a new field of ionospheric remote sensing. The triansient science will be achieved through a continuous, blind search for astronomical transients. VLITE plays to the strength of transient observations by accessing a wide field-of-view (~5 square degrees) nearly continuously. Using astronomical imaging, VLITE will challenge the paradigm of targeted observations catering exclusively to a priori science. Its exploration-driven model seeks to reinforce the complimentary value of serendipity in a landscape increasingly focused on "transformational" science goals.

VLITE is a multi-year project commencing in Fall 2014; contingent on scientific productivity, community interest, and the availability of funding, it could pave the road towards a full VLA LOw Band Observatory, or LOBO system, in the future. With a 27-antenna LOBO tapping into the full available low frequency bandwidth, the commensal data stream becomes much more interesting for spectroscopy and imaging, whether as a compliment to targeted, higher frequency observations, or independently.

Ionospheric Remote Sensing: Waves and Turbulence

The ionospheric analysis portion of VLITE is dedicated to the study of fine-scale (~1-10 km) ionospheric dynamics and the relationship to larger structures (hundreds of km). The VLA low-band systems have virtually unmatched sensitivity to fluctuations in the ionospheric total electron content (TEC), the integrated density of free electrons along a line of sight. When observing a bright cosmic source, these systems can be used to characterize TEC fluctuations more than two orders of magnitude weaker than those detectable with similar GPS-based methods. Such fluctuations are prevalent on smaller scales, making the VLA an excellent instrument for probing fine-scale ionospheric dynamics. Many continuously operating GPS receivers within New Mexico will also be used to simultaneously study larger-scale fluctuations. The (nearly) continuous data stream that will be yielded by VLITE, when combined with this GPS data, will constitute a singular data set for the study of coupling mechanisms among fine-, medium-, and large-scale ionospheric dynamics. In addition, such a continuous flow of data will allow for the characterization of the fine-scale ionospheric response to relatively rare atmospheric and/or seismic events such as large storms, earthquakes, and explosions that would be missed by proposal-based, low-band observing.

Radio Transients

The transient search component of the VLITE project is divided across slow (greater than 1 second, e.g. supernovae, gamma-ray bursts) and fast (less than 1 second, e.g. pulsars and fast radio bursts) time domains. Transient searches on timescales from 1 second to 1 hour will proceed through a dedicated, real-time pipeline being developed by NRL for VLITE visibility data generated by its software correlator at the VLA. Search algorithms will operate on both visibility data and images, and will include comparisons with existing sky catalogs. Searches for transients on timescales longer than 1 hour will be conducted off-line at the NRL VLITE computing facility, and will leverage VLITE’s self-generated sky models in addition to existing catalogs. This component of the transient search will share data and resources with the astronomical imaging component of the project. For transients shorter than 1 second, dedicated hardware is being developed to sample the raw voltages upstream of the correlator. This part of the project is still under development, with a goal towards realizing the capabilities to search for highly dispersed, msec timescale transients.

Astronomical Imaging

The astronomical imaging component of the VLITE project will proceed through a dedicated pipeline being developed by NRL and run on the NRL VLITE computing facility. The pipeline will produce calibrated uv data, self-calibrated images, and a source catalog for a significant fraction of the VLITE data. The images and source catalog will be used to build a sky model that will feed back into the transient search components of the project. While limited to 10 antennas, with the aid of bandwidth synthesis across its 64 MHz passband, VLITE can produce useful imaging products that will be explored for their scientific benefits across a broad range of astrophysical study.

Data Rights and Archiving

When fully operational, VLITE data will be taken simultaneously with all VLA observing, with the following exceptions:

  • All TAC-approved P-band observations
  • Test observations not directly related to VLITE
  • Moving source observations (e.g. planets and the Sun)
  • Non-fixed Ra and Dec observations such as On-The-Fly Mosaicing (OTFM)
While VLITE data are proprietary, NRL invites Universities and others in the community to approach us for access to the data through collaboration. Only by tapping the potential of a wider community can we realize the full scientific value of VLITE and stimulate its expansion to LOBO. If you are interested in learning more about VLITE including inquiries about collaborations, please contact us directly as follows: 1) For slow transients and for general questions about VLITE, Namir Kassim; 2) For fast transients or pulsar-related phenomena, Paul Ray; 3) for ionospheric science, Joseph Helmboldt; 4) for astronomical imaging, source catalogs, and technical questions or suggestions, Tracy Clarke.

Modified on Monday, 16-Mar-2015 21:33:44 MDT