Gamma-ray Burst 240402B

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN Circular #36016
```
SUBJECT: X-ray transient LXT 240402A: LEIA detection
DATE:    24/04/03 16:20:48 GMT
FROM:    EP Team at NAOC/CAS 
```
X. P. Xu , Z. X. Ling , Y. Liu , C. Zhang , W. Chen , H. Q. Cheng , C. Z. C= ui , D. W. Fan , H. B. Hu , J. W. Hu , M. H. Huang , C. C. Jin , D. Y. Li ,= T. Y. Lian , H. Y. Liu , M. J. Liu , Z. Z. Lv, X. Mao , H. W. Pan , X. Pa= n , H. Sun , W. X. Wang , Y. L. Wang , Q. Y. Wu , Y. F. Xu , H. N. Yang , M= . Zhang , W. D. Zhang , W. J. Zhang , , D. H. Zhao (NAOC, CAS), D. M. Li, = Q. X. Li (BNU)=EF=BC=8CC. Y. Wang, Y. J. Zhang (THU), and W. Yuan (NAOC, CA= S) on behalf of the LEIA and Einstein Probe team
We report on a fast X-ray transient LXT 240402A detected by LEIA (Lobster E= ye Imager for Astronomy) in 0.5-4 keV. The position of the source is R.A. = =3D 245.438 deg, DEC =3D 25.800 deg with an uncertainty of 1.5 arcmin (radi= us, 90%C.L. statistical and systematic). The source was detected by LEIA at= 2024-04-02T08:47:41 (UTC). The averaged 0.5-4 keV spectrum can be fitted w= ith an absorbed power law (with the column density fixed to the galactic va= lue of 3.4e20 cm^-2) with a photon index of 0.8 (-0.2/+0.2), giving an unab= sorbed flux of ~2.6 (-0.4/+0.5) e-9 erg/s/cm2 in the 0.5-4 keV band. There are two peaks observed in the light curve. The transient event lasted for a= pproximately 200 seconds with a peak flux of ~3e-8 erg/s/cm2 in 0.5-4 keV b= and.
Within the error circle of LXT 240402A, there is a radio galaxy (NVSS J1621= 46+254915) at a redshift of ~0.05 with a ROSAT flux of 2.3e-12 erg/s/cm2, w= hich is fainter than the peak flux detected by LEIA by more than three orde= rs of magnitude. Assuming that LXT 240402A is associated with NVSS J162146+= 254915, the average luminosity of the X-ray transient is esitimated to be 1= .5e46 erg/s. More follow-up observations are encouraged to help explore the nature of this transient.
LEIA (Zhang et al. 2022, ApJL, 941, 2; Ling et al. 2023, RAA, 23, 095007) i= s a soft X-ray Lobster-eye imager (0.5 - 4.0keV) with a FoV of 340 square d= egrees aboard the experimental satellite SATech-01 of the CAS, launched on July 27, 2022. The above result is preliminary and the final result will be published elsewhere.
GCN Circular #36017
Wang-Chen Xue, Shao-Lin Xiong, Chao Zheng, Yanqiu Zhang, and Chengkui Li, report on behalf of the GECAM team:
GECAM-C observed a long burst, GRB 240402A at 2024-04-02T08:47:46 UTC (denoted as T0), which has been detected by LEIA (GCN 36016) and Konus-Wind.
According to the observation data of GECAM-C, this burst mainly consists of two peaks with a duration (T90) of 6.55 +/- 0.52 sec (15-350 keV).
The GECAM-C localization is consistent with the LEIA localization (GCN 36016) within the error.
The time-averaged spectrum of GECAM-C data from T0-7.0 to T0+7.0 s could be adequately fit by a cut-off power-law with photon index alpha of -0.62 (-0.14, +0.13), and peak energy Ep of 66 (-3, +3) keV. The corresponding burst fluence is 7.69 (-0.24, +0.25) * 10^-7 erg/cm^2 in 15-350 keV.
We note that these results are preliminary and refined analysis will be reported later.
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).
GCN Circular #36018
L.P.Xin(NAOC), Y.G.Yang(HNU), C.WU(NAOC), Y.L.Qiu(NAOC), J.Wang(GXU/NAOC), L.H.Li(NAOC), C.Wu(NAOC), E.W.Liang(GXU), X.H.Han (NAOC), X.M.Lu(NAOC), R.S.Zhang(NAOC), Y.Xu(NAOC), Y.J.Xiao(NAOC), P.P.Zhang(NAOC), L.Lan(NAOC), J.Y.Wei(NAOC) on behalf of the SVOM/GWAC team:
We began to observe X-ray transient LXT 240402A (Xu et all., GCN 36106) detected by LEIA, which was also detected by GECAME-C as GRB 240402B (Xue et al., GCN 36107), with GWAC-F50A telescope, at Xinglong station, China, at 19:22:13 (UT), 03. April. 2024, about 34.57 hours after the burst. The weather was not good.
After stacking 5*200 seconds, no any new sources were detected in our stacked image down to a limit magnitude of 18.0 mag in R band comparing to several nearby USNO B1.0 stars.
We acknowledge the excellent support from observation assistant Yangtong Zheng.

GCN Circular #36019
V.Lipunov, P.Balanutsa (Lomonosov MSU), D.Buckley (SAAO), O.Gress, N.Budnev(ISU), K.Zhirkov, A.Kuznetsov, G.Antipov, N.Tiurina, E.Gorbovskoy, Ya.Kechin, D.Vlasenko, Yu.Tselik, I.Gorbunov, V.Vladimirov,D.Zimnukhov, V.Senik, A.Yudin,A.Chasovnikov, D.Cheryasov(Lomonosov MSU,SAI,PhysicsDepartment), A.Sosnovskij (Crimean Astrophysical Observatory, RAS), C.Francile. F. Podesta, R.Podesta (Observatorio Astronomico Felix AguilarOAFA), R. Rebolo, M. Serra(The Instituto de Astrofisica de Canarias), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)
MASTER Global robotic net (http://observ.pereplet.ru Lipunov et al.,2010,Advances in Astronomy,2010,30L) started LXT 240402A (Xu et al. GCN 36016, Triggertime=2024-04-02 08:47:41UT, Notice=24/04/03 16:20:48; Center R.A.,Dec. = 245.438 +25.800 +-1.5 arcmin; also detected by GECAM-C (GCN 36017 Wang-Chen Xue et al.)) observation at 2024-04-03 02:48:45 (optical observations also started by GWAC-F50A (Xin et al. GCN 36018))
MASTER-SAAO robotic telescope located in South Africa was pointed to errorbox 64864 sec after trigger time at 2024-04-03 02:48:45 UT (by own observational program), with upper limit up to 19.3 mag. The observations began at zenith distance = 60 deg. The sun altitude is -26.7 deg.
MASTER-Tunka robotic telescope (Russia, Baikal) was pointed 1 days 28326 sec after trigger time at 2024-04-03 16:39:47 UT, with upper limit up to 20.0 mag. The observations began at zenith distance = 52 deg. The sun altitude is -33.1 deg.
MASTER-Amur robotic telescope (Russia, Blagoveschensk) was pointed 1 days 28367 sec after trigger time at 2024-04-03 16:40:28 UT, with upper limit up to 19.0 mag. The observations began at zenith distance = 37 deg. The sun altitude is -32.6 deg.
The analysis of LERG AGN (Comerford et al. 2020), locate in 75" frin the errir-box center) light curve in MASTER database will be continued.
The galactic latitude b = 43 deg., longitude l = 44 deg.
Real time updated cover map and possibly OT, that can be discovered, will be available here: https://master.sai.msu.ru/site/master2/observ.php?id=2413927
We obtain the following upper limits.

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)
          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

   64894 | 2024-04-03 02:48:45 |         MASTER-SAAO | (16h 19m 12.57s , +27d 40m 03.0s) |   C |    60 | 18.2 |
   65856 | 2024-04-03 03:04:47 |         MASTER-SAAO | (16h 19m 18.33s , +27d 39m 39.9s) |   C |    60 | 18.1 |
   66812 | 2024-04-03 03:20:43 |         MASTER-SAAO | (16h 19m 23.87s , +27d 41m 06.6s) |   C |    60 | 18.4 |
   67771 | 2024-04-03 03:36:41 |         MASTER-SAAO | (16h 17m 58.29s , +25d 59m 53.5s) |   C |    60 | 18.3 |
   67831 | 2024-04-03 03:36:41 |         MASTER-SAAO | (16h 17m 58.29s , +25d 59m 53.3s) |   C |   180 | 17.3 |  Coadd
   67771 | 2024-04-03 03:36:41 |         MASTER-SAAO | (16h 19m 52.88s , +25d 46m 41.7s) |   C |    60 | 18.5 |
   67831 | 2024-04-03 03:36:41 |         MASTER-SAAO | (16h 19m 52.89s , +25d 46m 41.7s) |   C |   180 | 17.4 |  Coadd
   68729 | 2024-04-03 03:52:39 |         MASTER-SAAO | (16h 18m 01.18s , +26d 01m 18.6s) |   C |    60 | 17.8 |
   68729 | 2024-04-03 03:52:39 |         MASTER-SAAO | (16h 19m 56.33s , +25d 48m 07.4s) |   C |    60 | 18.2 |
   69688 | 2024-04-03 04:08:39 |         MASTER-SAAO | (16h 18m 00.35s , +26d 02m 44.4s) |   C |    60 | 16.0 |
   69688 | 2024-04-03 04:08:39 |         MASTER-SAAO | (16h 19m 55.57s , +25d 49m 31.9s) |   C |    60 | 16.1 |
  114741 | 2024-04-03 16:39:47 |        MASTER-Tunka | (16h 21m 42.47s , +25d 05m 39.5s) |   C |    30 | 18.5 |
  114742 | 2024-04-03 16:39:47 |        MASTER-Tunka | (16h 22m 52.93s , +25d 47m 55.4s) |   C |    30 | 19.0 |
  114772 | 2024-04-03 16:39:47 |        MASTER-Tunka | (16h 22m 52.93s , +25d 47m 55.4s) |   C |    90 | 19.7 |  Coadd
  114783 | 2024-04-03 16:40:28 |         MASTER-Amur | (16h 23m 12.57s , +25d 41m 12.4s) |   C |    30 | 18.4 |
  114783 | 2024-04-03 16:40:28 |         MASTER-Amur | (16h 22m 57.71s , +26d 02m 22.2s) |   C |    30 | 17.4 |
  114816 | 2024-04-03 16:41:02 |        MASTER-Tunka | (16h 21m 36.76s , +25d 04m 43.0s) |   C |    30 | 18.6 |
  114817 | 2024-04-03 16:41:02 |        MASTER-Tunka | (16h 22m 47.20s , +25d 46m 58.9s) |   C |    30 | 19.1 |
  114846 | 2024-04-03 16:41:32 |         MASTER-Amur | (16h 23m 07.00s , +25d 40m 05.4s) |   C |    30 | 18.4 |
  114846 | 2024-04-03 16:41:32 |         MASTER-Amur | (16h 22m 52.04s , +26d 01m 14.7s) |   C |    30 | 17.2 |
  114891 | 2024-04-03 16:42:16 |        MASTER-Tunka | (16h 21m 42.59s , +25d 05m 02.4s) |   C |    30 | 18.5 |
  114891 | 2024-04-03 16:42:16 |        MASTER-Tunka | (16h 22m 53.03s , +25d 47m 18.5s) |   C |    30 | 19.2 |
  114910 | 2024-04-03 16:42:36 |         MASTER-Amur | (16h 22m 57.77s , +26d 01m 20.4s) |   C |    30 | 17.3 |
  114910 | 2024-04-03 16:42:36 |         MASTER-Amur | (16h 23m 12.81s , +25d 40m 11.5s) |   C |    30 | 18.5 |
  114960 | 2024-04-03 16:43:25 |        MASTER-Tunka | (16h 21m 39.39s , +25d 06m 34.5s) |   C |    30 | 18.4 |
  114960 | 2024-04-03 16:43:26 |        MASTER-Tunka | (16h 22m 49.83s , +25d 48m 50.6s) |   C |    30 | 19.0 |
  114990 | 2024-04-03 16:43:26 |        MASTER-Tunka | (16h 22m 49.83s , +25d 48m 50.7s) |   C |    90 | 19.4 |  Coadd
  114973 | 2024-04-03 16:43:39 |         MASTER-Amur | (16h 23m 09.88s , +25d 41m 33.3s) |   C |    30 | 18.5 |
  114973 | 2024-04-03 16:43:39 |         MASTER-Amur | (16h 22m 54.77s , +26d 02m 41.6s) |   C |    30 | 17.4 |
  115036 | 2024-04-03 16:44:42 |         MASTER-Amur | (16h 23m 10.10s , +25d 40m 13.1s) |   C |    30 | 18.5 |
  115037 | 2024-04-03 16:44:42 |         MASTER-Amur | (16h 22m 54.96s , +26d 01m 20.9s) |   C |    30 | 17.3 |
  115038 | 2024-04-03 16:44:44 |        MASTER-Tunka | (16h 21m 39.42s , +25d 05m 05.1s) |   C |    30 | 18.2 |
  115038 | 2024-04-03 16:44:44 |        MASTER-Tunka | (16h 22m 49.83s , +25d 47m 21.3s) |   C |    30 | 18.8 |
  115100 | 2024-04-03 16:45:46 |         MASTER-Amur | (16h 23m 12.16s , +25d 41m 27.8s) |   C |    30 | 18.4 |
  115100 | 2024-04-03 16:45:46 |         MASTER-Amur | (16h 22m 56.99s , +26d 02m 35.4s) |   C |    30 | 17.4 |
  115113 | 2024-04-03 16:45:58 |        MASTER-Tunka | (16h 21m 43.49s , +25d 06m 49.9s) |   C |    30 | 18.3 |
  115113 | 2024-04-03 16:45:59 |        MASTER-Tunka | (16h 22m 53.89s , +25d 49m 05.9s) |   C |    30 | 18.8 |
  115163 | 2024-04-03 16:46:49 |         MASTER-Amur | (16h 23m 07.27s , +25d 40m 36.5s) |   C |    30 | 18.5 |
  115163 | 2024-04-03 16:46:49 |         MASTER-Amur | (16h 22m 52.06s , +26d 01m 43.8s) |   C |    30 | 17.3 |
  115189 | 2024-04-03 16:47:14 |        MASTER-Tunka | (16h 21m 35.99s , +25d 05m 59.2s) |   C |    30 | 18.3 |
  115189 | 2024-04-03 16:47:15 |        MASTER-Tunka | (16h 22m 46.37s , +25d 48m 15.2s) |   C |    30 | 18.8 |
  115219 | 2024-04-03 16:47:15 |        MASTER-Tunka | (16h 22m 46.37s , +25d 48m 15.2s) |   C |    90 | 19.3 |  Coadd

GCN Circular #36022
S. M. Jia (IHEP, CAS), H. Sun (NAOC, CAS), C. K. Li, Y. Chen, W. W. Cui, H. Feng, J. Guan, D. W. Han, W. Li, C. Z. Liu, F. J. Lu, L. M. Song, J. Wang, J. J. Xu, J. Zhang, S. N. Zhang, H. S. Zhao, X. F. Zhao, (IHEP, CAS), E. Kuulkers, A. Santovincenzo (ESA), P. O'Brien (Univ. of Leicester), K. Nandra, A. Rau (MPE), B. Cordier (CEA), X. F. Wu (PMO, CAS), X. P. Xu, Z. X. Ling , Y. Liu , C. Zhang , C. C. Jin , J. W. Hu , D. Y. Li , H. Y. Liu , H. W. Pan , W. Yuan (NAOC, CAS), on behalf of the LEIA and Einstein Probe team
Following the detection of the fast X-ray transient LXT 240402A (Xu et al., GCN 36016) and the associated GRB 240402B (Wang et al., GCN 36017), we performed observations of LXT 240402A with the Follow-up X-ray Telescope (FXT) on board Einstein Probe (EP). The observations began at 2024-04-03T19:45:00, about 35 hours after the LEIA detection. The exposure time was 12 ks. An X-ray source was detected at R.A. = 245.451 deg, DEC = 25.763 deg, with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic), 2.4 arcmin away from the LEIA source.
There is no cataloged X-ray source within the error circle, consistent with this source being the afterglow of LXT240402A. The spectrum can be fitted with an absorbed power-law model with NH fixed at the Galactic value of 4.3e20 cm-2 and a photon index of 1.7(+/-0.1) (90% C.L.). The derived flux in 0.5-10 keV is 7.8(+/-0.8)e-13 erg/s/cm2 (90% C.L.). Further monitoring of this source with FXT is planned for the next few days.
Please note that EP-FXT is currently undergoing in-flight calibration. The derived source parameters may be subject to larger uncertainties, so please use them with caution. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.
GCN Circular #36023
Following the detection of LXT 240402A by Leia (Xu et al. GCN 36016) and FXT on board =E2=80=8BEinstein Probe (Jia et al. GCN 36022) , we use Antarctic Survey Telescope 3-3 at YaoAn Astronomy Observation Station (China, Yunnan) for follow-up. Our observation started on 2024-04-03T16:28:21.084 . No new optical source was detected within the error box (Jia et al. GCN 36022) down to 22.1 mag in the coadd image (total 60x180s, g-band).
Our non-detection is consistent with the results from the GWAC-F50A (Xin et al., GCN 36018), MASTER (Lipunov et al. GCN 36019) limits.
We thank the staff at YaoAn Astronomy Observation Station for their excellent support.
GCN Circular #36025
A.J. Levan (Radboud), D.B. Malesani (DAWN/NBI & Radboud), P.G. Jonker (Radb= oud), N.R. Tanvir (Leicester), M. E. Ravasio (Radboud), A. Martin-Carrillo = (UCD), G. Pugliese (Amsterdam), S. Campana (INAF/Brera), D. Xu (NAOC/CAS) r= eport on behalf of the Stargate collaboration:=20
We obtained observations of FXT/GRB 240402B (Xu et al., GCN 36016; Xue et a= l., GCN 36017) with the FORS2 imager on the VLT UT1 (Antu). Observations be= gan at 06:44 UT on 4 April 2024, approximately 1.9 days after the high-ener= gy detection. We obtained a total of 1400 s of observations in the R band. = Within the 10=E2=80=9D error box of the X-ray transient located by Einstein= Probe follow-up observations (Jia et al., GCN 36022), we identify a new so= urce not present in archival images retrieved via the Legacy Survey. This s= ource lies at
```
RA = 16:21:48.247
Dec = +25:45:47.34
```
We suggest this is the optical afterglow of FXT/GRB240402B. Photometry is c= omplicated by the presence of a nearby star just 1=E2=80=9D away (a confirm= ed star, given a Gaia measured proper motion), but the magnitude of the tra= nsient at the time of the observations was approximately r ~ 22 calibrated to Pan-STARRS. The offset from the nearby galaxy CGCG 138-001 (z = 0.047) is approximately 200 kpc in projection, at the higher end of those expecte= d for ejected compact object mergers.
We thank the staff at the VLT for the rapid execution of these observations= , in particular Julia Seidel.
GCN Circular #36027
S. Yang (HNAS), T.-W. Chen, H.-Y. Hsiao, W.-J. Hou, A. Sankar.K, Y.-C. Pan,= M.-H. Lee, C.-H. Lai, C.-C. Ngeow, C.-S. Lin, H.-C. Lin, J.-K. Guo (all NC= UIA), S. J. Smartt, J. Gillanders, L. Rhodes, S. Srivastav (all Oxford) an= d T. Moore (QUB) report:
Following the detection of an X-ray transient (LXT 240402A) by LEIA (Xu et = al. GCN 36016) and FXT on board =E2=80=8BEinstein Probe (Jia et al. GCN 360= 22), and an associated GRB 240402A by GECAM-C (Xue et al. GCN 36017), we us= ed several telescopes at the Lulin Observatory, Taiwan as part of the Kinde= r collaboration (Chen et al., AstroNote 2021-92) for optical counterpart se= arches. Our followup facilities includes the Lulin One-meter Telescope (LOT= ) with g and r bands, and the 40cm Super Light Telescope (SLT) with r and i= bands, and our observing details were below:
```
Telescope | Filter | MJD | t-t0 | Explosure | avg. Seeing | med. Airmass
SLT | r | 60403.644 | 1.28 day | 300 sec * 15 | 1".8 | 1.33=20
SLT | i | 60403.713 | 1.35 day | 300 sec * 44 | 1".8 | 1.03=20
LOT | r | 60403.720 | 1.35 day | 300 sec * 12 | 1".2 | 1.08
LOT | g | 60403.771 | 1.40 day | 300 sec * 10 | 1".2 | 1.04=20
```
(1). LXT 240402A
We used the Kinder pipeline (Yang et al. A&A 646, A22) to stack the images = with same pointing and subtract the stacked images with the LEGACY SURVEY t= emplate images. We do not detect significant optical counterpart candidates= within the 1.5 arcmin error area of LXT 240402A. In 2.5 sigma level, we ob= tained the following preliminary upper limits (in the AB system):=20
```
SLT r > 22.74,=20
SLT i > 22.61,=20
LOT r > 23.57 and=20
LOT g >  23.58 mag.
```
Our result is consistent with GWAC-F50A (Xin et al., GCN 36018), MASTER (Li= punov et al., GCN 36019) and AST3-3 (Sun et al., GCN 36023), and our limits= are deeper than these reports.
(2). FXT/GRB 240402B
Furthermore, we noticed that EP-FXT announced the detection of the X-ray afterglow (Jia et al., GCN 36022) of LXT240402A, which is 2.4 arcmin away from the LEIA source. We thus repeat the procedure detailed above on this spec= ific position and found a clear optical counterpart at the location of RA = = 16:21:48.225, Dec =3D 25:45:47.57 in the differencing images. We measured the PSF magnitudes of this source after subtracted with the LEGACY SURVEY templates and obtained (all in the AB system):
```
g =3D 22.86 +/- 0.09 and
r =3D  22.28 +/-  0.10 mag.
```
No deep i-band template images were available in the LEGACY SURVEY archive,= so we do not report the measurements in this note. Further analysis is on-= going.
The given magnitudes are derived based on calibrating against SDSS field st= ars and is not corrected for the expected Galactic foreground extinction co= rresponding to a reddening of E_(B-V) =3D 0.03 mag in the direction of the = burst (Schlafly & Finkbeiner 2011).
We note that this source is difficult to resolve from a star (i.e. SDSS J16= 2148.18+254546.5, r =3D 20.6 mag) in our images. Upon visual inspection, th= e star appears to have a broader profile.
We independently identified this optical counterpart candidate while drafti= ng this report, and we noticed that this counterpart has also been reported= in Levan et al., GCN 36025.
We thank the Einstein Probe team for useful communication.
GCN Circular #36028
A. Ridnaia, D. Frederiks, A. Lysenko, D. Svinkin, A. Tsvetkova, M. Ulanov, and T. Cline, on behalf of the Konus-Wind team, report:
The long-duration GRB 240402B (GECAM-C detection: Xue et al., GCN 36017) associated with the LEIA fast X-ray transient LXT 240402A (Xu et al., GCN 36016) triggered Konus-Wind at T0=31668.797 s UT (08:47:48.797).
The burst light curve shows a multipeaked structure which starts at ~T0-0.2 s and has a total duration of ~5.1 s. The emission is seen up to ~3 MeV.
The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB240402_T31668/
As observed by Konus-Wind, the burst had a fluence of 7.56(-0.43,+0.50)x10^-6 erg/cm2, and a 64-ms peak flux, measured from T0+2.704 s, of 5.07(-0.66,+0.68)x10^-6 erg/cm2/s (both in the 20 keV - 10 MeV energy range).
The time-averaged spectrum of the burst (measured from T0 to T0+8.448 s) is best fit in the 20 keV - 4 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.71(-0.17,+0.19), the high energy photon index beta = -3.58(-1.30,+0.44), the peak energy Ep = 86(-4,+5) keV (chi2 = 57/68 dof).
This spectrum is nearly-well fitted by a power law with exponential cutoff model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with alpha = -0.78(-0.15,+0.16) and Ep = 88(-4,+4) keV (chi2 = 61/69 dof).
All the quoted errors are at the 90% confidence level. All the quoted values are preliminary.
GCN Circular #36030
C.C. Cheung, R. Woolf, M. Kerr, J.E. Grove (NRL), A. Goldstein (USRA), C.A.= Wilson-Hodge, D. Kocevski (MSFC), and M.S. Briggs (UAH) report:
The Glowbug gamma-ray telescope [1,2], operating on the International Space= Station, reports the detection of LXT 240402A/GRB 240402B, which was also = detected by LEIA (GCN 36016), GECAM-C (GCN 36017), and Konus/Wind (GCN 3602= 8).
Using an adaptive window with a resolution of 32-ms, the burst onset is det= ermined to be 2024-04-02 08:47:45.240 with a duration of 4.1 s and a total = significance of about 18.3 sigma. The light curve comprises two primary pe= aks at ~T0+0.5s and ~T0+3s.
The analysis results presented here are preliminary and use a response func= tion that lacks a detailed characterization of the surrounding passive stru= cture of the ISS.
Glowbug is a NASA-funded technology demonstrator for sensitive, low-cost ga= mma-ray transient telescopes developed, built, and operated by the U.S. Nav= al Research Laboratory (NRL) with support from the University of Alabama in= Huntsville, USRA, and NASA MSFC. It was launched on 2023 March 15 aboard = the Department of Defense Space Test Program=E2=80=99s STP-H9 to the ISS. = The detector comprises 12 large-area (15 cm x 15 cm) CsI:Tl panels covering= the surface of a half cube, and two hexagonal (5-cm diameter, 10-cm length= ) CLLB scintillators, giving it a large field of view (instantaneous FoV ~2= /3 sky) over a wide energy band of 50 keV to >2 MeV.
```
[1] Grove, J.E. et al. 2020, Proc. Yamada Conf. LXXI, arXiv:2009.11959
[2] Woolf, R.S. et al. 2022, Proc. SPIE, 12181, id. 121811O
[3] Goldstein, A. et al. 2020, ApJ 895, 40, arXiv :1909.03006
```
Distribution Statement A: Approved for public release. Distribution is unl= imited.
GCN Circular #36034
Wangchen Xue, Shaolin Xiong, Chenwei Wang, Chengkui Li, Shuxu Yi on behalf of GECAM team
Following our initial analysis reported in GCN 36017, here we analyzed the Amati relation for this burst. We find that, if this burst had a redshift of z~0.047 (Levan et al.; GCN 36025, Xu et al., GCN 36016), it falls in the short burst region, supporting the compact object merger origin as also evident by the large offset from the galaxy (Levan et al., GCN 36025).
The small spectral lag compared to its luminosity also indicates the compact object merger origin.
The associated gravitational wave would likely have been observed if LVK was observing at the time of this burst. The gravitational wave of such kind of GRB with small redshift would be potentially observed in O4b of LVK.
With the probable compact object merger origin of this GRB, follow-up observations are encouraged.
GCN Circular #36041
A. Ridnaia, D. Frederiks, A. Lysenko, D. Svinkin, A. Tsvetkova, M. Ulanov, and T. Cline, on behalf of the Konus-Wind team, report:
Assuming the spectrum and the observer-frame energetics of GRB 240402B measured by KW (GCN 36028); the redshift of the probable host galaxy z=3D0.047 (Xu et al., GCN 36016; Levan et al., GCN 36025; Xue at al., GCN 36034); and a standard cosmology with H_0 =3D 67.3 km/s/Mpc, Omega_M =3D 0.315, and Omega_Lambda =3D 0.685 (Planck Collaboration, 2014); we estimate the burst isotropic energy release E_iso to (4.06 =C2=B1 0.27)x10^49 erg, the isotropic peak luminosity L_iso to (2.85 =C2=B1 0.38)x10^49 erg/s, the rest-frame peak energy of the time-integrated spectrum Ep,i,z to (90 =C2=B1 5) keV.
With the obtained estimates, GRB 240402B is a low-luminosity outlier in both 'Amati' and 'Yonetoku' relations for the sample of >300 long KW GRBs with known redshifts (Tsvetkova et al., 2017; Tsvetkova et al., 2021), see http://www.ioffe.ru/LEA/GRBs/GRB240402_T31668/GRB240402B_rest_frame.pdf
All the quoted errors are estimated at the 90% confidence level. All the presented results are preliminary.
GCN Circular #36114
A. Martin-Carrillo (UCD), S. Campana (INAF/Brera), D. B. Malesani (DAWN/NBI & Radboud Univ.), A. J. Levan (Radboud Univ.), G. Pugliese (Amsterdam), D. Xu (NAOC/CAS), N. R. Tanvir (Leicester), P. G. Jonker (Radboud Univ.), A. Rossi (INAF-OAS), P. O=E2=80=99Brien (Leicester) report on behalf of the Stargate collaboration:
Following up the LEIA trigger LXT 240402A (Xu et al., GCN 36016) associated= with the Konus-Wind GRB 240402B (Ridnaia et al., GCN 3628) and the detecti= on of its X-ray counterpart (Jia et al., GCN 36022) a series of Swift follo= w-up observations (PI Xu, ID: 203030 and PI Martin-Carrillo, ID: 20382) wer= e performed between April 7 and April 14, with a total exposure time of 8 k= s.
Combining all the data, a source is well detected with a count rate of 1.9 = (+0.7, -0.6) x 10^-3 count s^-1 at the coordinates (J2000):
```
RA: 16:21:47.75 (245.4490);
Dec: +25:45:45.9=E2=80=99 (25.7627);
Error: 5.8=E2=80=9D (90% confidence).
```
Using the spectral parameters of the EP-FXT detection (Jia et al., GCN 3602= 2), this corresponds to an observed flux of 7.7*10^-14 erg cm^-2 s^-1. Assu= ming a mid-point for the Swift observations of 8.5 days after the LEIA trig= ger, and using a single power-law decay, the X-ray flux decays with an appr= oximate index -1.2, typical for a GRB afterglow.
The source centroid lies within 1.5" from the optical counterpart reported by Levan et al. (GCN 36025) and Yang et al. (GCN 36027). The reduced locali= sation error strengthens its association with the proposed optical counterpart.
We thank the Neil Gehrels Swift Observatory team for making these ToO observations possible.
GCN Circular #36385
N. R. Tanvir (U. Leicester), J. Palmerio (GEPI/Obs. de Paris), D. B. Malesani (DAWN/NBI & Radboud U.), A. J. Levan (Radboud U.), A. Saccardi (GEPI/Obs. de Paris), A. Martin-Carrillo (UCD), P. G. Jonker (Radboud U.), K. Wiersema (U. Hertfordshire), Y. Julakanti (U. Leicester) report on behalf of the Stargate collaboration:
We obtained observations of the location of the X-ray transient LXT 240402A (Xu et al., GCN 36016; Jia et al., GCN 36022; Levan et al., GCN 36025), coincident with GRB 240402B detected by GECAM-C (Xue et al., GCN 36017) and Konus-Wind (Ridnaia et al., GCN 36028), using the X-shooter spectrograph at the VLT. Observations began at 04:04 (UT) on 03 May 2024, approximately 31 days after the trigger. The spectrum covers a wavelength range roughly 0.31 to 2.1 mum, with a total integration time of 4800 s.
A weak continuum is seen throughout the spectrum, and we clearly detect emission lines of [O II], H-beta, [O III], H-alpha, and tentatively Ly-alpha, at a common redshift of z = 1.551. We propose that these lines are from the host of the transient. Based on the gamma-ray fluence seen by Konus-Wind in the 20 keV - 10 MeV band (Ridnaia et al., GCN 36028) we conclude an isotropic-equivalent energy release in gamma-rays of 4.7x10^52 erg. This places it very close to the mean Amati relationship (Amati et al. 2006 MNRAS 372 233), and thus is consistent with a long-GRB origin.
We thank ESO support staff Zahed Wahhaj and Francisco Caceres for undertaking these observations.

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