GCN Circular 39787

Jin-Peng Zhang, Yan-Qiu Zhang, Wang-Chen Xue, Shao-Lin Xiong, Yue Wang (IHEP) and Ce Cai (HEBNU) report on behalf of the Insight-HXMT team:

At the event time 2025-03-16T08:33:50.842 UTC (T0) of FRB 20250316A (Ng et al., ATel 17081; Leung et al., ATel 17086), Insight-HXMT/HE was observing normally and monitored the full location error region of FRB 20250316A.

The routine blind search of Insight-HXMT/HE data found no burst candidate around the time of FRB 20250316A. Thus, we implemented a targeted search [1] from T0 - 100 s to T0 + 5 s, and identified no candidate above 3 sigma.

Considering three typical spectral models (i.e. soft, normal and hard Band functions), three timescales and the FRB localization (RA = 182.476 deg, Dec = +58.8494 deg (J2000)), the 3 sigma upper limits of the gamma-ray transient energy flux (10 keV-1 MeV, in units of 10^-7 erg/s/cm^2) are reported below:

Timescale (s)    Soft    Normal    Hard
0.1            	 7.17    7.48      3.27
1          	 2.26    2.36      1.03
10       	 0.72    0.75      0.33

With the distance of 40.8 Mpc from NGC 4141, the probable host galaxy of this FRB, we further calculate the following upper limits of the gamma-ray transient intrinsic isotropic luminosity (1 keV-10 MeV, in units of 10^47 erg/s):

Timescale (s)    Soft    Normal    Hard
0.1     	 1.43    1.49      0.65
1                0.45    0.47      0.21
10               0.14    0.15      0.07

We note that the above results are preliminary. Refined results will be reported.

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://hxmten.ihep.ac.cn/.

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