Gravitational Wave Followup Overview
Welcome to the NED Gravitational Wave Follow-up (GWF) Service
The purpose of this NED service is to facilitate searches for electromagnetic (EM) counterparts to gravitational wave (GW) events. Within minutes after the Laser Interferometer Gravitational-Wave Observatory (LIGO)-Virgo collaboration issues an alert using the Gamma-ray Coordinates Network (GCN) operated by the NASA GSFC (https://gcn.gsfc.nasa.gov/), this NED service responds by cross-matching in 3D the event's HEALPix map with the galaxies in the local Universe and the following results are provided: an all-sky image of the probability contours, the location of all galaxies in NED within the LIGO 90% probability volume, and the top 20 galaxies sorted by 2MASS absolute Ks-band magnitude. To get started, click on "Events" in the menu above or simply click on the (blue) button below for the most recent GW event.
Note: The NED and Census of the Local Universe (CLU) projects are issuing a joint notice of candidate galaxies for each LIGO GW trigger. While a large fraction of the galaxies in CLU catalog come from NED, there are galaxies in CLU that have not yet been ingested into NED, and NED is continually updating its holdings. Thus, some of the CLU top 20 galaxies listed in the GCN notices may not be in the NED list on this website. The NED service sorts its galaxies by absolute K-band magnitude. However, the overall composition of the CLU and NED galaxy lists will be similar.
Overview
Gravitational Waves have opened an exciting new window into the Universe by enabling the direct study of the merger of massive, compact objects. Our understanding of these events is greatly enhanced by identifying and studying their EM counterparts in a new field known as multi-messenger astronomy. The sky localizations for GW events detected by LIGO are generally quite large (> 100 square degrees; see Figure 1), making this task very challenging. However, an effective strategy to increase the efficiency of observational follow-up campaigns is to search for GW afterglows by targeting galaxies in the highest probability volume of the GW event.
This service exploits the rich and constantly growing content of the NASA/IPAC Extragalactic Database (NED) to help astronomers around the world to efficiently search for the EM counterparts to GW events. An overview of the process is as follows: A list of galaxies in the local Universe is constructed by using available redshift and redshift-independent distances. Components of galaxies, galaxy groups, and clusters are omitted to keep the sample confined to candidate host galaxies for the GW event. Currently redshift-independent distances are selected (when available) over redshift measurements, and the median of the individual redshift-independent distances is used as the final distance when multiple measurements are available.
Prior to January 2020, a distance of 200 Mpc was used to define the NED Local Universe Sample in order to match the targeted sensitivity of LIGO to merging neutron stars. However, several NS-BH events occurred during the LIGO/Virgo O3 run beyond 200 Mpc. Consequently, we have increased the distance limit of the NED Local Universe Sample to 1 Gpc to provide coverage of future NS-BH events. We remind users that the availability of galaxy distances and redshifts are increasingly incomplete at greater distances. Characterization of the completeness of redshifts in NED is in progress. As of January 2020, the NED Local Universe Sample contains 1,351,208 galaxies, with 141,824 having redshift-independent distances.
The NED GW follow-up service listens to public alerts on the Gamma-ray Coordinates Network (GCN), which is used by LIGO to distribute GW event information. After each GW event, the HEALPix map is cross-matched in 3D to the NED galaxy list. During this process an all-sky image of the probability contours, the location of all galaxies in the 90% volume, and the top 20 galaxies sorted by 2MASS absolute Ks-band magnitude are displayed on each event's page. The list is sorted by the absolute Ks magnitude because evidence to date suggests that binary mergers that contain a neutron star (NS-NS or BN-NS) are likely to occur in more evolved, more massive galaxies, and because the NIR light more closely traces the older stellar population of a galaxy and hence is a proxy for stellar mass. If an event's mean distance minus 1 standard deviation is greater than the NED-GWF distance cut of 1 Gpc the galaxy cross-match is not performed and no galaxies are provided.
Latest Update (January 2020)
We have increased the distance of the local Universe galaxy sample to 1 Gpc (z~0.24) to provide coverage of future NS-NS and NS-BH events that occur past 200 Mpc which were observed during the LIGO/Virgo O3 run.
Acknowlegments
This service makes use of the Gamma-ray Coordinates Network (GCN) operated by NASA, and of the python package ligo.skymap written by Leo Singer (NASA/GSFC). We thank Leo Singer for his helpful comments and feedback.
Last update: 2020-1-15 17:58:13 PST
