GRB 251208D

R. Konno (WIS), S. Garrappa (WIS), E. A. Zimmerman (WIS), A. Horowicz (WIS), E. O. Ofek (WIS), S. Ben-Ami (WIS), D. Polishook (WIS), O. Yaron (WIS), S. Fainer (WIS), A. Krassilchtchikov (WIS), Y. M. Shani (WIS), E. Segre (WIS), A. Gal-Yam (WIS), and S. Spitzer (WIS) report on behalf of the LAST Collaboration.

We report observations of GRB 251208D, detected by SVOM (Lin et al., GCN 43032
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 ). Observations were conducted with the Large Array Survey Telescope (LAST; Ofek et al. 2023, PASP 135, 5001; Ben-Ami et al. 2023, PASP 135, 5002).

We observed the field of GRB 251208D using 4 divergent telescopes (each with a 7.4 deg^2 FoV) in clear band (similar to the Gaia Bp band) over 5 sequential epochs. Each epoch consists of 20x20s exposures.

We began observations at 2025-12-08 17:22:15 UTC (T-T0 = 0.53h). We coadd a total of 100x20s exposure images and perform image subtraction using a reference image of the field. We do not detect any new optical source up to a limiting magnitude of 20.33 (AB) within the error region reported in Lin et al., GCN 43032
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 .

LAST is a survey telescope array of the Weizmann Astrophysical Observatory (https://www.weizmann.ac.il/wao/).

D. B. Malesani (DAWN/NBI and Radboud), D. Turpin, A. Saccardi (CEA/Irfu), Z.-Y. Lin, F. Cangemi (APC), report on behalf of the SVOM mission team:

Following the report of two X-ray sources detected by Swift/XRT (Evans et al., GCN 43043
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 ), we inspected again our images taken with LCO (Turpin et al., GCN 43037
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 ) of the field of the SVOM GRB 251208A (Lin et al., GCN 43032
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 ). 

* XRT source #1 matches the high proper-motion star TYC 5226-156-1 (distance ~500 pc). We measure a magnitude of r = 11.34 +- 0.02, consistent with the APASS archival value r = 11.34 +- 0.06.

* XRT source #2 is consistent with SDSS J223038.66-003108.3, a known QSO at z = 1.32 with magnitude r = 20.34 (from SDSS) and r = 20.65 (from Pan-STARRS), possibly indicating some long-term variability. We measure r = 20.73 +- 0.12, indicating that the object is not in a particularly active state.

Our observations suggest that neither sources are likely related to the high-energy emission observed by SVOM.

This project is funded by the SVOM collaboration.

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 source
sb25120806 (GCN 43032
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 ), collecting 2.4 ks of Photon Counting (PC) mode data between
T0+5.2 ks and T0+11 ks after the trigger. We have detected 2 sources.  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 J223038.6-003107):
  ==================================
    RA (J2000.0):   337.6610  =  22 30 38.64
    Dec (J2000.0):  -0.5186  =	-00 31 07.0
    Error:	    5.8 (arcsec, radius, 90% confidence).
    Detect flag:    GOOD
    Distance:	    10.7 arcmin from the SVOM/ECLAIRs position.
    Mean rate:	    (4.6 [+2.2, -1.7])e-3 ct s^-1
    Mean flux:	    (4.6 [+2.2, -1.7])e-13 erg cm^-2 s^-1
    Peak rate:	    (4.6 [+2.2, -1.7])e-3 ct s^-1
    Peak flux:	    (4.6 [+2.2, -1.7])e-13 erg cm^-2 s^-1
    ECF:	    1.02e-10 erg cm^-2 ct^-1
		      assuming NH=3.34e+21 cm^-2, gamma=0.47
		      determined from a spectral fit.
    RASS UL:	    1.6e-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.
    A SIMBAD object `SDSS J223038.66-003108.3' is 1.2" away.


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

  Source 1 (SWIFT J222951.0-001338):
  ==================================
    RA (J2000.0):   337.4626  =  22 29 51.02
    Dec (J2000.0):  -0.2273  =	-00 13 38.3
    Error:	    5.2 (arcsec, radius, 90% confidence).
    Detect flag:    GOOD
    Distance:	    10.8 arcmin from the SVOM/ECLAIRs position.
    Mean rate:	    (5.6 [+2.4, -1.9])e-3 ct s^-1
    Mean flux:	    (2.41 [+1.05, -0.81])e-13 erg cm^-2 s^-1
    Peak rate:	    (5.6 [+2.4, -1.9])e-3 ct s^-1
    Peak flux:	    (2.41 [+1.05, -0.81])e-13 erg cm^-2 s^-1
    This matches a catalogued X-ray source 4XMM J222950.9-001339
    in the XMM-NEWTON/XMMSSC catalogue. Details:
    Separation:     2.0" from the XRT source
    Cat Rate:	    5.9e-02 +/- 8.2e-03 ct s^-1 
    Cat Flux:	    6.0e-13 +/- 8.3e-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 5226-156-1' is 1" 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.

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/SVOM_FIELD00056.

This circular is an official product of the Swift-XRT team.

D. Turpin, A. Saccardi (CEA/Irfu), D. B. Malesani (DAWN/NBI and Radboud), Z.-Y. Lin, F. Cangemi (APC), report on behalf of the SVOM mission team:

We observed the field of the SVOM GRB 251208D (Lin et al., GCN 43032
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 ) with the LCO 1m telescope at South African Astronomical Observatory equipped with the Sinistro instrument. 

Our observation started on 2025-12-08 at 19:02:27 UT (about 2.18 hr after the trigger) and we obtained 3x200 s exposures in the SDSS r and 3x200 s exposures in the Pan-STARRS z filters. In the stacked image, we do not detect any new source within the ECLAIRs error circle.

We measure the following upper limits calibrated against the PanSTARRS DR1 catalog, and not corrected for Galactic extinction:

r > 21.6 AB (mid-time 2.27 hr after the trigger);
z > 19.7 AB (mid-time 2.65 hr after the trigger).

This project is funded by the SVOM collaboration.

V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, 
G.Antipov,  A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
C.Francile,  F. Podesta, R.Podesta, E. Gonzalez  (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A.Sosnovskij (CrAO),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) 

MASTER-Kislovodsk robotic telescope  [1]  located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the SVOM GRB251208.70 (trigger No 1765212621,22h 30m 08.33s , -00d 23m 30.5s, R=0.1962) errorbox  6047 sec after notice time and 7322 sec after trigger time at 2025-12-08 18:52:23 UT, with upper limit up to  13.3 mag. The observations began at zenith distance = 73 deg. The sun  altitude  is -56.3 deg. 

MASTER-SAAO robotic telescope  located in South Africa (South African Astronomical Observatory) was pointed to the SVOM GRB251208.70 errorbox  7366 sec after notice time and 8641 sec after trigger time at 2025-12-08 19:14:22 UT, with upper limit up to  19.0 mag. The observations began at zenith distance = 56 deg. The sun  altitude  is -17.9 deg. 

The galactic latitude b = -47 deg., longitude l = 66 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=3064681

We obtain a following upper limits.  

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

    7382 | 2025-12-08 18:52:23 |   MASTER-Kislovodsk | (22h 29m 37.06s , -00d 21m 55.6s) |   C |   120 | 12.6 |        
    7382 | 2025-12-08 18:52:23 |   MASTER-Kislovodsk | (22h 29m 37.94s , -00d 00m 13.8s) |   C |   120 | 13.2 |        
    8671 | 2025-12-08 19:14:22 |         MASTER-SAAO | (22h 28m 00.88s , -00d 37m 02.0s) |   C |    60 | 18.9 |        
    8671 | 2025-12-08 19:14:22 |         MASTER-SAAO | (22h 26m 18.28s , -00d 21m 13.1s) |   C |    60 | 18.5 |        
    8764 | 2025-12-08 19:15:54 |         MASTER-SAAO | (22h 27m 55.90s , -00d 37m 31.3s) |   C |    60 | 19.0 |        
    8764 | 2025-12-08 19:15:54 |         MASTER-SAAO | (22h 26m 13.40s , -00d 21m 40.9s) |   C |    60 | 18.6 |        
    8867 | 2025-12-08 19:17:38 |         MASTER-SAAO | (22h 26m 12.54s , -00d 20m 34.9s) |   C |    60 | 18.5 |        
    8892 | 2025-12-08 19:18:03 |   MASTER-Kislovodsk | (22h 29m 36.09s , -00d 22m 32.0s) |   C |    60 | 12.2 |        
    8892 | 2025-12-08 19:18:03 |   MASTER-Kislovodsk | (22h 29m 39.94s , -00d 01m 05.5s) |   C |    60 | 13.2 |        
    8972 | 2025-12-08 19:19:22 |   MASTER-Kislovodsk | (22h 29m 33.23s , -00d 02m 12.0s) |   C |    60 | 13.3 |        
Filter C is a clear (unfiltred) band. 


The observation and reduction will continue. 
The message may be cited.

[1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023,  Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp.
http : // www.pereplet.ru/lipunov/625.html

Z.-Y. Lin, F. Cangemi (APC), D. Turpin, S. Schanne (CEA/Irfu), F. Daigne (IAP)

At 2025-12-08T16:51:43 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 251208D (SVOM burst-id sb25120806).

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 6.64 in the [8-120] keV energy band over a time window of 81.92 seconds starting at 2025-12-08T16:50:21.

The localization of the best alert is R.A., Dec. 337.5347, -0.3918 degrees:
R.A. (J2000) = 22h30m08.33s
Dec. (J2000) = -0d23m30.32s
 with a 90% confidence level (C.L.) radius of 11.77 arcmin (including systematic error of 2 arcmin added in quadrature).

Due to the detection significance being below the slew threshold, no immediate slew was performed on this burst.

A SVOM ToO has been requested. MXT and VT ToO observation results will be reported in dedicated GCN Circulars.

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 Zhe-Yu Lin: zheyu.lin@apc.in2p3.fr.
Please contact the BA by email if you require additional information.