# S240422ed: a neutron star + black hole merger **URL:** \ **Category:** Latest transients\ **Tags:** gw\ **Created:** [April 23, 2024, 7:35am UTC](https://forum.astro-colibri.science/t/s240422ed-a-neutron-star-black-hole-merger/154 "2024-04-23T07:35:52Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Fabian.Schussler](https://yyz1.discourse-cdn.com/flex007/user_avatar/forum.astro-colibri.science/fabian.schussler/32/25_2.png) [@Fabian.Schussler](https://forum.astro-colibri.science/u/Fabian.Schussler)\ **Post date:** [April 23, 2024, 7:35am UTC](https://forum.astro-colibri.science/t/s240422ed-a-neutron-star-black-hole-merger/154/1 "2024-04-23T07:35:53Z")
All three currently running gravitational wave interferometers jointly detected a very interesting event last night: named **[S240422ed](https://astro-colibri.com/event: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](https://gcn.nasa.gov/circulars/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%. > **[Astro-COLIBRI](https://astro-colibri.com/event:S240422ed)** > > Enabling real-time multi-messenger astrophysics discoveries ---
**Author:** ![jeanpascal.vignes](https://avatars.discourse-cdn.com/v4/letter/j/e19b73/32.png) [@jeanpascal.vignes](https://forum.astro-colibri.science/u/jeanpascal.vignes)\ **Post date:** [April 23, 2024, 2:21pm UTC](https://forum.astro-colibri.science/t/s240422ed-a-neutron-star-black-hole-merger/154/2 "2024-04-23T14:21:23Z")
we are on the spot ---
**Author:** ![Fabian.Schussler](https://yyz1.discourse-cdn.com/flex007/user_avatar/forum.astro-colibri.science/fabian.schussler/32/25_2.png) [@Fabian.Schussler](https://forum.astro-colibri.science/u/Fabian.Schussler)\ **Post date:** [April 26, 2024, 4:36pm UTC](https://forum.astro-colibri.science/t/s240422ed-a-neutron-star-black-hole-merger/154/3 "2024-04-26T16:36:21Z")
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: - [AT 2024hfq](https://astro-colibri.com/event:TNS2024hfq): an optical source detected by DECam ([GCN #36298](https://gcn.nasa.gov/circulars/36298)) and later also detected in X-rays by the FXT onboard Einstein Probe ([GCN #36319](https://gcn.nasa.gov/circulars/36319)) - [AT 2024hhs](https://astro-colibri.com/event:TNS2024hhs): an X-ray source detected by FXT onboard Einstein Probe and named [EP240226a](https://astro-colibri.com/event:EP240426a) ([GCN #36313](https://gcn.nasa.gov/circulars/36313)) and later also found in previously taken optical observations by DECam ([GCN #36317](https://gcn.nasa.gov/circulars/36317)) Nice interplay between Einstein Probe (which has only been launched recently and is still in commissioning, see more info [here](https://www.esa.int/Science_Exploration/Space_Science/Einstein_Probe_factsheet)) and DECam. Both sources have been submitted to the [BHTOM](https://bh-tom2.astrolabs.pl/) network for follow-up. If you have a (sizeable) telescope: joint the hunt… ---
**Author:** ![Fabian.Schussler](https://yyz1.discourse-cdn.com/flex007/user_avatar/forum.astro-colibri.science/fabian.schussler/32/25_2.png) [@Fabian.Schussler](https://forum.astro-colibri.science/u/Fabian.Schussler)\ **Post date:** [April 28, 2024, 7:59am UTC](https://forum.astro-colibri.science/t/s240422ed-a-neutron-star-black-hole-merger/154/4 "2024-04-28T07:59:13Z")
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](https://gcn.nasa.gov/circulars/36325)) and in radio observations from the Australian SKA Pathfinder (ASKAP) Variable And Slow Transients Survey (VAST), see [GCN #36329](https://gcn.nasa.gov/circulars/36329). Both analyses point to an AGN origin of the detected emission, which is thus unrelated to the GW event S240422ed. ---
**Author:** ![Fabian.Schussler](https://yyz1.discourse-cdn.com/flex007/user_avatar/forum.astro-colibri.science/fabian.schussler/32/25_2.png) [@Fabian.Schussler](https://forum.astro-colibri.science/u/Fabian.Schussler)\ **Post date:** [May 2, 2024, 7:30pm UTC](https://forum.astro-colibri.science/t/s240422ed-a-neutron-star-black-hole-merger/154/5 "2024-05-02T19:30:39Z")
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](https://gcn.nasa.gov/circulars/36367) of z=0.148 (i.e. about 700Mpc) is incompatible with the distance estimate provided by the GW detection (188 ± 43 Mpc). ---
**Author:** ![Fabian.Schussler](https://yyz1.discourse-cdn.com/flex007/user_avatar/forum.astro-colibri.science/fabian.schussler/32/25_2.png) [@Fabian.Schussler](https://forum.astro-colibri.science/u/Fabian.Schussler)\ **Post date:** [July 3, 2024, 4:41pm UTC](https://forum.astro-colibri.science/t/s240422ed-a-neutron-star-black-hole-merger/154/6 "2024-07-03T16:41:58Z")
Further analysis of the gravitational wave data has unfortunately led to a re-classification of the event. The signal is not significant any longer (lowest false-alarm-rate is once every 35days) and the type of event is (in order of descending probability): Terrestrial Noise (93%), Binary Neutron Star merger (5%), … [https://gcn.nasa.gov/circulars/36812](https://gcn.nasa.gov/circulars/36812)