S240615dg: an extremely well localized GW event

The GW event S240615dg has been detected by the LIGO and Virgo interferometers on 2024-06-15 11:36:20. It is classified as a merger of two black holes at a distance of 1321 ± 238 Mpc.

What makes the event special, is its extremly small localisation uncertainty of only 8 deg². This is clearly a new record and facilitates follow-up observations searching for possible associated electromagnetic emission. Given the nature of the event (i.e. two black holes merging), it is unfortunately not very likely that such emission exists. But let’s better check…

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An updated analysis of the GW data resulted in an even better constrained localization of the event. The 90% confidence region is now down to 5 deg² !