Gamma-ray Burst 250628B

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN Circular #40862
D. Dornic, J-G. Ducoin, J-L. Atteia, O. Godet, C. Plasse, report on behalf of the SVOM mission team:
At 2025-06-28T16:29:30 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 250628B (SVOM burst-id sb25062804).
The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network.
The burst was only detected by the Image Trigger (IMT), which produced a sequence of 1 alert. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 6.90 in the [5-20] keV energy band over a time window of 20.48 seconds starting at 2025-06-28T16:29:10.
The localization of the best alert is R.A., Dec. 333.1563, -45.5663 degrees:
```
R.A. (J2000) = 22h12m37.51s
Dec. (J2000) = -45d33m58.52s
```
with a 90% confidence level (C.L.) radius of 11.33 arcmin (including systematic error of 2 arcmin added in quadrature).
The SVOM/ECLAIRs light curve showed a single/multiple broad/narrow peak structure with a T90 duration of about 33.51s (-3.02 +4.13).
Due to the detection significance being below the slew threshold, no immediate slew was performed on this burst. A SVOM ToO has been programmed for follow-up.
No X-ray observation could be performed by SVOM/MXT for the time being.No optical observation could be performed by SVOM/VT for the time being.
The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. SVOM/ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. SVOM/GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. SVOM/MXT was developed jointly by CNES, CEA-IRFU, CNRS-IJCLab, University of Leicester, MPE.
The Burst Advocate (BA) on shift for this alert is Dornic Damien: dornic@cppm.in2p3.fr. Please contact the BA by email if you require additional information.
GCN Circular #40874
W. J. Zhang, X. Mao (NAO, CAS), Y. C. Fu (BNU), H. W. Pan (NAO, CAS) on behalf of the Einstein Probe (EP) team:
EP-FXT performed a follow-up observation of the SVOM/ECLAIRs-detected burst GRB 250628B (SVOM/sb25062804, Dornic et al. GCN #40862) at 2025-06-28T18:51:59 (UTC), about 140 minutes after the SVOM/ECLAIRs trigger, with an exposure time of ~5000s. Five uncatalogued sources are detected within the ECLAIRs error circle. Preliminary analysis of these sources has been conducted automatically, and the details are listed below.
Source 1: EPF_J221230.7-453117
```
RA (J2000): 333.1278
Dec (J2000): -45.5213
Flux: 6.57 x 10^-14 erg/s/cm2 (FXT-A observed, 0.5-10 kev)
Flux_err: 2.48 x 10^-14 erg/s/cm2 (1 sigma)
```
Source 2: EPF_J221214.3-453344
```
RA (J2000): 333.0595
Dec (J2000): -45.5621
Flux: 2.13 x 10^-13 erg/s/cm2 (FXT-A observed, 0.5-10 kev)
Flux_err: 4.30 x 10^-14 erg/s/cm2 (1 sigma)
```
Source 3: EPF_J221327.1-453729
```
RA (J2000): 333.3628
Dec (J2000): -45.6247
Flux: 1.89 x 10^-13 erg/s/cm2 (FXT-A observed, 0.5-10 kev)
Flux_err: 4.06 x 10^-14 erg/s/cm2 (1 sigma)
```
Source 4: EPF_J221218.2-453223
```
RA (J2000): 333.0758
Dec (J2000): -45.5397
Flux: 9.71 x 10^-14 erg/s/cm2 (FXT-A observed, 0.5-10 kev)
Flux_err: 2.74 x 10^-14 erg/s/cm2 (1 sigma)
```
Source 5: EPF_J221226.9-453909
```
RA (J2000): 333.1120
Dec (J2000): -45.6525
Flux: 4.23 x 10^-14 erg/s/cm2 (FXT-B observed, 0.5-10 kev)
Flux_err: 1.54 x 10^-14 erg/s/cm2 (1 sigma)
```
Note: Sources 1-3 were simultaneously detected by both FXT-A and FXT-B, while Source 4 and Source 5 were individually detected by FXT-A and FXT-B, respectively.
The position uncertainties of the sources are about 10 arcsec in radius (90% C.L. statistical and systematic).
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).
GCN Circular #40875
H. L. Li, Y. L. Qiu, L. P. Xin, C. Wu, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, Z. H. Yao, Y. N. Ma, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA), D. Dornic (CPPM)，J-G. Ducoin（CPPM）report on behalf of the SVOM/VT team.
SVOM/VT performed ToO follow-up observations on the GRB 250628B (SVOM burst-id sb25062804, Dornic et al., GCN 40862) in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. The observation started on 2025-06-28T20:33:10 UT, ie. 4.06 hours after the SVOM T0 time.
No uncatalogued sources are detected in single or stacked images at the position of the Swift/XRT (https://www.swift.ac.uk/SVOM/SVOM_FIELD00021/) and EP (Zhang et al., GCN 40874), compared to the Legacy survey. The 3 sigma upper limit magnitudes are derived as below:
```
Mid_time   |   Band     |    Exposure Time  |     Upper limit (AB)
5.8 hour       VT_B            50*70 sec             23.6 mag                         
5.8 hour       VT_R            49*70 sec             23.4 mag                           
```
Also no apparent variation is found for the catalogued sources that lie within the errorbox of the Swift/XRT (https://www.swift.ac.uk/SVOM/SVOM_FIELD00021/) and EP (Zhang et al., GCN 40874).
The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC),CAS.

GCN Circular #40877
P.A. Evans (U. Leicester), J.A. Kennea (PSU), E. Ambrosi (INAF-IASFPA) , A.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), R. Brivio (INAF-OAB), D.N. Burrows (PSU), S. Campana (INAF-OAB), M. Capalbi (INAF-OAR), A. D'Ai (INAF-IASFPA), P. D'Avanzo (INAF-OAB), V. D'Elia (SSDC & INAF-OAR), S. Dichiara (PSU), M. Ferro (INAF-OAB), A. Melandri (INAF-OAR), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M. Perri (SSDC & INAF-OAR), C. Salvaggio (INAF-OAB), T. Sbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), M.A. Williams (PSU) report on behalf of the Swift-XRT team:
Swift-XRT has performed follow-up observations of the SVOM/ECLAIRs-detected GRB 250628B/sb25062804, collecting 1.4 ks of Photon Counting (PC) mode data between T0+2.9 ks and T0+8.9 ks after the trigger. We have detected 2 sources, outside the SVOM/ECLAIRs error circle. These have been automatically classified as follows:

  * 0 likely counterparts
  * 0 candidate counterparts
  * 1 uncatalogued X-ray source
  * 1 known X-ray source

Uncatalogued X-ray sources
--------------------------

  Source 2 (SWIFT J221311.1-454427):
  ==================================
    RA (J2000.0):  333.2963  =	22h 13m 11.11s
    Dec (J2000.0): -45.7408  =	-45d 44' 26.9"
    Error:	   6.8 (arcsec, radius, 90% confidence).
    Detect flag:   GOOD
    Distance:	   12.0 arcmin from the SVOM/ECLAIRs position.
    Mean rate:	   (7.7 [+3.9, -3.0])e-3 ct s^-1
    Mean flux:	   (2.9 [+1.5, -1.1])e-13 erg cm^-2 s^-1
    Peak rate:	   (7.7 [+3.9, -3.0])e-3 ct s^-1
    Peak flux:	   (2.9 [+1.5, -1.1])e-13 erg cm^-2 s^-1
    ECF:	   3.78e-11 erg cm^-2 ct^-1, assuming NH=2.76e+22 cm^-2,
		   gamma=2.97; determined from a spectral fit.
    RASS UL:	   1.0e-01 ct s^-1 (converted to XRT; 0.3-10 keV)
	 so the source is not above this 3-sigma upper limit.
    There is no evidence for fading.

Known X-ray sources
-------------------

  Source 1 (SWIFT J221222.2-452220):
  ==================================
    RA (J2000.0):  333.0927  =	22h 12m 22.25s
    Dec (J2000.0): -45.3724  =	-45d 22' 20.6"
    Error:	   6.3 (arcsec, radius, 90% confidence).
    Detect flag:   GOOD
    Distance:	   11.9 arcmin from the SVOM/ECLAIRs position.
    Mean rate:	   0.0141 [+0.0050, -0.0041] ct s^-1
    Mean flux:	   (3.16 [+1.12, -0.91])e-13 erg cm^-2 s^-1
    Peak rate:	   0.0141 [+0.0050, -0.0041] ct s^-1
    Peak flux:	   (3.16 [+1.12, -0.91])e-13 erg cm^-2 s^-1
    ECF:	   2.24e-11 erg cm^-2 ct^-1, assuming NH=1.46e+20 cm^-2,
		   gamma=3.23; determined from a spectral fit.
    This matches a catalogued X-ray source LSXPS J221222.5-452221
    in the LSXPS catalogue. Details:
      Separation:  3.6" from the XRT source
      Cat Rate:    2.4e-02 +/- 2.1e-03 ct s^-1 
      Cat Flux:    5.3e-13 +/- 4.7e-14 erg cm^-2 s^-1 (0.3-10 keV)
	     so the source is not above the catalogued flux.
    There is no evidence for fading.
    A SIMBAD object `TYC 8438-268-1' is 3.7" away.
    There is 1 2MASS object within the source's 3-sigma error radius.

All fluxes are 0.3-10 keV, observed. For all flux conversions and comparisons with catalogues and upper limits from other missions, we assumed a power-law spectrum with NH=3x10^20 cm^-2 and photon index (Gamma)=1.7 unless otherwise stated.
We did not detect any sources consistent with the five EP-FXT candidates reported by Zhang et al. (GCN #40874).
The results of the XRT-team automatic analysis of the XRT observations, including a position-specific upper limit calculator, are available at https://www.swift.ac.uk/SVOM.
This circular is an official product of the Swift-XRT team.

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Jochen Greiner, last update: 30-Jun-2025 [Disclaimer]