In August 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and sister instrument, the interferometric gravitational-wave antenna (Virgo), detected a gravitational wave signature from the colliding stars which they designated GW170817. The stars are located in NGC 4993, an elliptical galaxy in the constellation Hydra discovered in 1789 by Wilhelm Herschel.
“The radio counterpart to the LIGO detection was highly anticipated because it is by far the longest lasting electromagnetic [EM] signature from the event, allowing us to track its evolution for weeks, months, and even years into the future,” said Dr. Simona Giacintucci, radio astronomer, NRL.
The GROWTH collaboration continues to track the source with radio telescopes around the world. The evolution of the radio emission across the dimensions of time, frequency and space is critical to constrain and distinguish between competing theoretical models of the event. These include the presence of on- and off-axis jets and a cocoon, as well as standard dynamical ejecta models. In addition to its longevity, radio is important because it constrains both the energy and physical surroundings of the event.
As an international leader in sub-GHz radio interferometry, NRL’s most powerful radio astronomy asset is a new VHF/UHF receiver system developed on the National Radio Observatory (NRAO) Very Large Array (VLA). Recognizing the power of the new receiver, NRL and NRAO researchers developed the VLA Low Band Ionosphere and Transient Experiment (VLITE) to continuously tap into the new broadband low frequency receivers.
“VLITE collects data during nearly all normal VLA observations at higher frequencies, and has been monitoring the ionosphere for travelling ionospheric disturbances since 2014. It is also an excellent tool for detecting EM transients,” said Dr. Tracy Clarke, radio astronomer and VLITE Project Scientist, NRL. “As part of its standard operations, VLITE piggybacks on all of the VLA programs that target the NGC 4993 system at higher frequencies.”
“VLITE provides a statistically powerful constraint on the VHF/UHF emission from the gravitational wave source, anchoring the spectrum at lower frequencies,” said Dr. Namir Kassim, VLITE principal investigator and Head, NRL Radio Astrophysics and Sensing Section. “VLITE measurements are enhanced by its recent expansion completed last month.”
In August, VLITE was expanded (eVLITE) from 10 to 15 of the VLA’s 27 total antennas, more than doubling the number of baselines from the original 45 to 105. The VLITE limits on the LIGO event in NGC 4993 appear with other radio data in the Oct. 20, 2017 edition of Science Magazine.