On 2019 August 14 , the Advanced LIGO and Virgo interferometers detected the gravitational wave ( GW ) signal S190814bv with a false alarm rate of 1 in 10 ^ { 25 } years .
The GW data indicated ( with > 99 % probability ) that the event had M _ { 1 } \geq 5 _ { \odot } and M _ { 2 } \leq 3 M _ { \odot } , suggesting that it resulted from a neutron star–black hole ( NSBH ) merger ( or potentially a low-mass binary black hole merger ) .
Due to the low false alarm rate and the precise localization ( 23 deg ^ { 2 } at 90 % ) , S190814bv presented the community with the best opportunity yet to directly observe an optical/near-infrared counterpart to an NSBH merger .
To search for potential counterparts , our collaboration ( GROWTH ) performed real-time image subtractions on 6 nights of public Dark Energy Camera ( DECam ) i - and z -band images that were acquired in the three weeks following the merger .
The images covered > 98 % of the integrated probability area .
Using a worldwide network of follow-up facilities , we systematically undertook spectroscopy and imaging of potential counterpart candidates discovered in the DECam data .
Combining these data with a photometric redshift catalog that is > 97 % complete in the volume of interest , we ruled out each candidate as the counterpart to S190814bv .
Here we present deep and uniform photometric limits on the optical emission associated with the event .
For the nearest consistent GW distance , radiative transfer simulations of NSBH mergers constrain the ejecta mass of S190814bv to be M _ { \mathrm { ej } } < 0.04 M _ { \odot } at polar viewing angles , or M _ { \mathrm { ej } } < 0.03 M _ { \odot } if the opacity is \kappa < 2 cm ^ { 2 } g ^ { -1 } .
Assuming a tidal deformability for the neutron star compatible with GW170817 results , our limits would constrain the BH spin component aligned with the orbital momentum to be \chi < 0.7 for mass ratios Q < 6 .