Right ascension and declination values - Sky Position


I am trying to get the ascension and declination values of all the GW events. I have downloaded HDF file (Example - IGWN-GWTC3p0-v1-GW191103_012549_PEDataRelease_mixed_cosmo) and am using HDFview (software) to get the values .

Is there a alternate method?

My objective is, what is the sky location of each GW events with respect to the Earth and am interested to know if the source is coming from southern hemisphere or northern hemisphere with respect to the Earth

Could you please help me on this?

Note: I am a beginner and your help would be much appreciated.


Hi @gopistudy,

Apologies for the late reply. The HDF files released usually contain ‘‘posterior samples’’, i.e. sample points that represent where the GW is expected to be in sky, what masses have etc. The file also contains posterior samples of Right Ascension and Declination for the event. You can use those points with healpy to see where they fall on the sky.

The skymaps can be read with healpy, there is a small example here but also with ligo.skymap, that is a package developed to plot, handle and visualize skymaps.

I hope this helps,

Thank you for your reply. However, i am struggling to get the right ascension and declination for the event. For example, I have downloaded “IGWN-GWTC3p0-v1-GW200316_215756_PEDataRelease_mixed_cosmo” file and used “HDFview” to get those values. I have attached the HDF file structure and if you could let me know where i can find those files, please?

Hi @gopistudy !

Thank you for your question.

There are a few different ways you could do this.

For the file you have now, the RA and DEC values are in the data object “posterior_samples”. So, you could load in that data object using h5py and make plots with them. There is a tutorial showing how to do this on Day 3 of the Open Data Workshop.

An alternate approach would be to use the skymap files instead, which can be downloaded from the same zenodo repository. Then, you could use ligo.skymap or healpy to read them in.

Good luck!