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VLA Ionospheric and Transient Experiment (VLITE)

Transients

The transient radio sky at megahertz frequencies is a new frontier in radio science. Because VLITE operates commensally with the VLA, it observes the sky > 6000 hours each year. A single pointing of VLITE images 5.5 deg², enabling tens of thousands of square degrees to be imaged over the course of a month. Integration times for these observations vary from tens of seconds to several hours. Further, repeated visits to the same field give cadences of day-long to year-long timescales. Using existing catalogs and archival data, we can increase the cadence to decade-long timescales. Consequently, VLITE provides a unique tool for exploration of the transient radio sky.

VLITE sky coverage — Mollweide projection in Galactic coordinates showing the distribution of the VLITE pointings over a 30-month interval. Fields are colored by their total integration time across all images. This is an updated figure from Polisensky et al. (2016).

The above image shows the sky coverage after 30 months of VLITE observations. Some of the 30-month milestones include:

3C 286 observed > 720 hr
33% sky observed > 45 min.
80% sky observed > 30 sec

The transient search component of the VLITE project is divided into:

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.

Fast Transients

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.

Slow Transients

The VLITE slow transient mode has made several detections of known, non-radio selected transients. An example of one such detection, the low mass X-ray binary system V404 Cyg, is shown below.

some text — The low mass X-ray binary V404 Cyg was detected in outburst in June 2015. VLITE was used to trace the source from pre-outburst, through the burst to the postburst quiescence. VLITE flux measurements for 5 epochs are shown (red) with insets showing the VLITE image of the region pre-outburst (no detected source), during the outburst (V404 Cyg clearly detected), and an artist's concept of the system. This figure is featured in Taylor et al. (2017).

While VLITE has not yet found any new radio-selected transients, VLITE observations do allow us to set stringent upper limits on transients at mJy levels and at low frequencies where comparatively few transient surveys have been carried out to date. VLITE is not expected to probe the known populations of slow transients such as NSM-Magnetars or dMe flare stars. However, VLITE has been designed with the intention of expanding it from 10 to all 27 VLA antennas and transitioning from narrow-band (64 MHz passband) to the full bandwidth of P-band (256 MHz). This new LOw Band Observatory (LOBO) will provide significantly increased imaging fidelity, much lower rms noise levels, and broader spectral coverage. The expansion of VLITE to LOBO will have a major impact on megahertz transient detection because it will be possible to detect known classes of megahertz transients. On 10 minute timescales, LOBO is expected to detect dMe flare stars. For longer (≥ 6 hr) timescales, LOBO will be sensitive to neutron star mergers that create magnetars as well as off-axis TDEs.

Modified on Monday, 16-Oct-2017 13:50:37 MDT