Intellectual Merit:  The CHILES VERDES survey of the radio time-domain sky piggy-backs on another project's (CHILES) huge expenditure of observing time to make optimal use of the Karl G. Jansky Very Large Array---one of the world's most sensitive radio telescopes, located in New Mexico. CHILES VERDES stares at the same field for 1002 hours spread over 6 years, obsrving how this small portion of the sky changes on timescales of minutes to years at radio wavelengths. Because of the large investment of telescope time, the survey reaches very good sensitivities---similar sensitivities will not be reached for another ~decade, when next generation radio telescopes come online.

Based on theoretical models and early observations, CHILES VERDES expected to see significant numbers of radio transients, including supernovae, stars torn apart by black holes, and gamma-ray bursts. However, the program team found that the radio sky in CHILES VERDES is quiet. In those years of data, they only found a single source come and go---a flare from a Galactic flare star. It appears that models were a bit optimistic in predicting how many radio transient events could be expected. Meanwhile, other wider and shallower surveys of the radio time domain sky have also come up largely empty-handed---although a few radio transients have recently been found. Putting all these results together, it appears that radio transients are relatively faint, and at the sensitivities of our current telescopes, relatively rare. It also appears that a wider and shallower survey strategy is better for finding radio transients than a deep and narrow strategy.

CHILES VERDES also allows us to characterize which sources vary at radio wavelengths. We find a host of radio-variable active galactic nuclei --- super-massive black holes at the centers of distant galaxies that are accreting gas from their surroundings. Small changes in the infall rate of gas into the black hole can change the radio luminosity of these sources. We observe that some active galactic nuclei appear to vary randomly on timescales of days, while others vary smoothly and gradually over timescales of years. The CHILES VERDES light curves sample a much wider range of black hole and accretion properties than previous studies, and with much better times coverage. Further study will enable the team to test which  active galactic nuclei  determine the radio variability properties.

Broader Impacts: The public's imagination is sparked when events transpire on human timescales in what is usually a pristine and immutable sky. The program team has worked with students at Michigan State University and the mid-Michigan public to instill an understanding of how we observe and understand explosions, mergers, and jets. PI L. Chomiuk runs the MSU Campus Observatory, where she leads monthly public nights, hosts community groups, and facilitates student-led research. She has expanded public offerings at the Observatory to include hands-on family-friendly activity nights and updated exhibits. She has also expanded and diversified opportunities for MSU students by engaging ~25 undergradutes each year in hands-on observations and research at the Observatory.

Last Modified: 11/19/2019
Modified by: Laura B Chomiuk