TITLE:   GCN CIRCULAR
NUMBER:  36300
SUBJECT: LIGO/Virgo/KAGRA S240422ed: Upper limits from GECAM Observations
DATE:    24/04/25 12:36:16 GMT
FROM:    tanwj@ihep.ac.cn

Wen-Jun Tan, Ce Cai, Shao-Lin Xiong, Chen-Wei Wang, Wang-Chen Xue, Chao Zheng,
Hao-Xuan Guo, Wen-Long Zhang, Cheng-Kui Li, Xiao-Bo Li, Shu-Xu Yi, Ping Wang and Bing Li
report on behalf of the GECAM team:

At the event time 2024-04-22T21:35:13.417 (UTC) of S240422ed (GCN 36236), GECAM-C was observing normally and monitored the full localization region of this GW event, while GECAM-B detectros were turned off due to the power supply limitation. 

The incident angle of GECAM-C GRD detectors is down to about 12 deg for the center region (reference location RA = 122.38°, Dec = -20°) of the probability sky map of this GW event. This small incident angle indicates a good detector sensitivity to this event.

There was no GECAM-C in-flight trigger around the event time of S240422ed. An automated, blind search for gamma-ray burst of GECAM-C data also found no burst candidates. Thus we implemented a targeted search [1] within +/-30 s around event time, and also identified no candidates.

Considering three typical GRB spectral models, three timescales and the reference location metioned above, the 3 sigma upper limits of the GRB energy flux (10 keV-2000 keV, in units of 10^-7 erg/s/cm^2) are reported below:


Timescale (s)   Soft      Normal    Hard
0.1             2.18      4.15     10.26
1          	0.66      1.28      3.07
10       	0.20      0.40      0.97

Assuming the median luminosity distance of 188 Mpc from the GW observation, we can further estimate the 3 sigma upper limits of the GRB istropic luminosity (1 keV-10 MeV, in units of 10^48 erg/s):

Timescale (s)   Soft      Normal    Hard
0.1     	1.36       2.17     11.70
1               0.42       0.67     3.48
10              0.13       0.21     1.15

We note that all these results are preliminary and refined analysis will be reported.

Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission originally consists of two micro-satellites (GECAM-A and GECAM-B) launched in Dec. 2020. As the third member of GECAM constellation, GECAM-C was launched onboard SATech-01 experimental satellite in July 2022. GECAM mission is funded by the Chinese Academy of Sciences (CAS).

[1] C. Cai et al. MNRAS 508, 3910–3920 (2021) https://doi.org/10.1093/mnras/stab2760