1 of 1

OneRing: The Observing Plan to Rule them All

Authors: Elise Kesler¹, Nora Sherman¹, Isaac McMahon², André Santos³, Thomas Ruch¹, Sean MacBride¹, Marcelle Soares-Santos² 1. The University of Michigan 2. University of Zurich 3. Centro Brasileiro de Pesquisas Físicas

​

References

  1. James Annis. “Observations of GW170817 by DESGW and the DECam GW-EM Collaboration”. In: Proceedings of the International Astronomical Union 13.S338 (2017), pp. 72–79. DOI: 10.1017/S1743921318000935.
  2. Herner, K., et al. (2020). The updated DESGW Processing Pipeline for the third LIGO/Virgo Observing Run. EPJ Web of Conferences, 245, 01008. doi:10.1051/epjconf/202024501008
  3. W. Freedman (2021) “Measurements of the Hubble Constant: Tensions in Perspective”, ApJ, doi:10.3847/1538-4357/ac0e95.
  4. Abbott B P. “A gravitational-wave standard siren measurement of the Hubble constant”. In: Nature 551.7678 (2017), pp. 85–88. DOI: 10 . 1038 /nature24471

​

Abstract

Information from gravitational wave (GW) events, such as binary neutron star and black hole mergers, will help resolve one of the most pressing issues in cosmology – the Hubble tension. By finding optical counterparts to GW events, we will obtain more precise measurements of the Hubble constant photometrically independent of the cosmic distance ladder. Here, we discuss our method of searching for optical counterparts to GW observations from the LIGO-Virgo-KAGRA (LVK) collaboration utilizing DECam (the Dark Energy Camera). Updated from previous versions, our new OneRing script produces an observing plan based on probability, efficiency, and visibility factors. The code has performed well with previous LVK events and simulations, but has not yet been utilized for observations in LVK observing run 4.

Why observe optical

counterparts?

  • Measuring precise distances in astronomy is difficult– cosmic distance ladder brings uncertainties (indirect measurement system)
  • Optical counterparts provide context: can measure distance from GW signal, redshift from EM counterpart
  • Bright standard sirens (mergers with EM and GW components) thus have both distances and redshifts, allowing for new measurements of H₀ (the Hubble Constant) from the relationship between them
  • H₀ describes the current expansion of the universe, and measuring it is vital to validating our current cosmology and characterising dark energy

Visualization of the Hubble tension, the disagreement between different measurements of the Hubble constant. Plot from Freedman(2021)[3]

​

DECam image of the optical counterpart to GW170817, the first ever observed kilonova, prepared in 2017 by the search-and-discovery pipeline. From this merger, DESGW measured H₀=70 (+12, -8) km/s/Mpc [1].

OneRing & the Awesomeness Factor

  • Complete overhaul of observation plan creation with OneRing for O4
  • Now rank observation hexes based on Awesomeness Factor, which takes into account probability, airmass, slew time, hex visibility, lunar separation, and coverage
  • Sorts hexes into inner (high probability) and outer (low probability), then, starting at sunset, finds the hex with the highest Awesomeness Factor and adds it to the observing plan. Prioritizes inner hexes.
  • Includes dithers, observing a hex multiple times to cover camera gaps
  • Performs well in testing; finds GW170817 and consistently covers high probability percentage of test sky maps. No real data to test on yet, but a new event will hopefully come soon (O4 is currently underway)

Sample skymap (this one is from GW170817). The colors in the zoomed plot represent probabilities.

Observing Process

Act on trigger from GW Detector, obtain skymap

Strategy Code (determines DECam observation parameters)

OneRing determines optimal observing plan

Left: QR code to sample observing plan for GW170817. Right: Shape of a hex. Each box is a DECam CCD.