This web tool is to support rapid galaxy-targeted observations following gravitational wave events (Salmon et al. 2019). A list of galaxies inside the contours enclosing 50%, 90% or 99% of the probability within the gravitational wave localisation region is retrieved and displayed. The galaxies are ranked in order of probability of merger, calculated using the approach in Gehrels et al. 2016 and the prioritisation algorithm laid out by Arcavi et al. 2017. The back-end galaxy ranking algorithm is open source and can be found on Github. The probability is calculated based on the galaxy’s location within the contours enclosing 50%, 90% or 99% of the probability within the gravitational wave localisation region, the distance of the galaxy and the mass (B magnitude) of the galaxy. The sum of probabilities in each list adds up to 1. The galaxy list can be downloaded in json or ascii format for further use.
Retrieve all galaxies within a region
The 3 options are:
Retrieve only the galaxies within a region that are visible from a specific location at a user-specified time
Retrieve only the galaxies within a region above a user-specified limiting magnitude that are visible from a specific location at a user-specified time
Each pixel in the LIGO/Virgo computed skymap contains probability and distance parameters that are computed at that specific sky location. The distance to the merger computed by LIGO/Virgo is extracted at the location of the galaxies within the selected localisation regions in the sky map. This distance is compared to the distance of each galaxy extracted from the GLADE V2 catalogue to calculate the distance probability measure \(p_{dist}\) \[p_{dist}=N_{dist}exp(\frac{-[D-\mu_{dist}]^2}{2\sigma_{dist}^2})\] where \(N_{dist}\) is a normalisation factor specified in the pixel at the galaxy's position in the sky map; D is distance to the galaxy from the GLADE V2 catalogue; \(\mu_{dist}\) is the distance estimate given in the pixel at the position of the galaxy in the sky map and \(\sigma_{dist}\) is the uncertainty in distance contained within the pixel at the galaxy's position in the sky map.
The probability that the galaxy is at a certain location, \(p_{loc}\), is obtained from the pixel at that location in the skymap. This is combined with the distance probability measure to obtain the following location probability measure, \(S_{loc}\). \[S_{loc}=p_{loc}p_{dist}\]
The B magnitude of the galaxy is used as a proxy for the galaxy mass. The luminosity probability measure \(S_{lum}\) is given by: \[S_{lum}=\frac{L_{B}}{\sum L_{B}}\]
The overall probability of the merger occurring in that galaxy is given by \[S=S_{loc}S_{lum}\]
The detectability of the source by the observatory is an important factor which depends on kilonovae light curves and the limiting magnitude of the observatory . The minimum detectable B magnitude \(m_{lim}\) is set by the observer. This is converted to absolute magnitude using the galaxy’s distance, D. The limiting luminosity is then calculated. The maximum and minimum luminosity of a kilonova is calculated using the minimum and maximum absolute magnitude of a kilonova, \(M_{knmin}\)=-17 and \(M_{knmax}\)=-12. If the limiting luminosity is greater than the minimum luminosity expected from a kilonova, the deteactability is set to Yes. Else, it is set to No.
The galaxies are ranked from highest to lowest probability and the information is displayed in a table.