TITLE:   GCN CIRCULAR
NUMBER:  35785
SUBJECT: Insight-HXMT/HE Sub-Threshold Detection of Transient EPW20240219aa 
DATE:    24/02/24 07:20:16 GMT
FROM:    yqzhang_cl@163.com

Ce Cai (HEBNU), Shao-Lin Xiong, Li-Ming Song, Shuang-Nan Zhang (IHEP) 
report on behalf of the Insight-HXMT team:

Einstein Probe detected a bright X-ray flare EPW20240219aa, which started 
from 2024-02-19T06:21:42 UTC (denoted as T0) with a duration about 200 s 
(ATel 16463; ATel 16472). There is no Insight-HXMT/HE on-ground automated 
blind search trigger around this event. Thus we implemented a targeted search 
for sub-threshold burst signals within T0+/-100 s, using an improved coherent 
search algorithm [1]. 

From the targeted search, we identified a gamma-ray transient most significant 
on the 4 s timescale, with the peak time of about T0+40 s and duration of about 50 s, 
matching the observation by Fermi GBM (GCN 35773, GCN 35776) and Swift/BAT 
(GCN 35784). Its location is also consistent with the EP (ATel 16463) and Swift/BAT 
positions (GCN #35784). The spectrum is relatively hard (Band function with 
Epeak = 1 MeV, alpha = 0, beta = -1.5).

We note that these results are very preliminary. Refined analysis will be reported later.

All measurements above are made with the CsI detectors operating in the
regular mode with the energy range of about 80-800 keV (deposited energy).
Only gamma-rays with energy greater than about 200 keV can penetrate
the spacecraft and leave signals in the CsI detectors installed inside
the telescope.

Insight-HXMT is the first Chinese space X-ray telescope, which was
funded jointly by the China National Space Administration (CNSA) and
the Chinese Academy of Sciences (CAS).
More information about it could be found at:
http://www.hxmt.org.

[1] Cai, C., Xiong, S. L., Li, C. K., et al. 2021, MNRAS, 508, 3910S