S240422ed: a neutron star + black hole merger

All three currently running gravitational wave interferometers jointly detected a very interesting event last night: named S240422ed, it is classified as a merger of a neutron star and a black hole. The event is well localized (the 90% area has a size of 258 deg²) but is very close to the Galactic Plane in the Southern hemisphere (around Dec=-26deg), which makes follow-up observations searching for an associated kilonova difficult. No gamma-ray burst was detected in coincidence with the event.

Technical details from the LVK GCN circular #36240: Based on posterior support from parameter estimation, under the assumption that the candidate S240422ed is astrophysical in origin, the probability that the lighter compact object is consistent with a neutron star mass (HasNS) is >99%. Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is >99%. Both HasNS and HasRemnant consider the support of several neutron star equations of state. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 34%.


we are on the spot

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The follow-up of this exciting event is still extremely active in all wavelengths. The situation changes very rapidly with additional data, but at this stage two possible counterparts to the NS-BH merger stick out:

Nice interplay between Einstein Probe (which has only been launched recently and is still in commissioning, see more info here) and DECam.

Both sources have been submitted to the BHTOM network for follow-up.
If you have a (sizeable) telescope: joint the hunt…

Update: The second source mentioned above (EP240226a) is no longer considered a viable counterpart to the GW event. It is already present in archival data of the VISTA Variables in Via Lactea (VVV) survey by the ESO/VISTA telescope (GCN #36325) and in radio observations from the Australian SKA Pathfinder (ASKAP) Variable And Slow Transients Survey (VAST), see GCN #36329.
Both analyses point to an AGN origin of the detected emission, which is thus unrelated to the GW event S240422ed.

Another update: the first source mentioned above (AT 2024hfq) is now also excluded as counterpart to the GW event: the redshift reported in GCN #36367 of z=0.148 (i.e. about 700Mpc) is incompatible with the distance estimate provided by the GW detection (188 ± 43 Mpc).