GRB 250106A

D.-H. ZHAO, C. WU, W.-J. XIE (NAOC, CAS), D. Turpin (CEA), S. Guillot (IRAP), D. Götz (CEA), C-W. Wang (IHEP) report on behalf of the SVOM team.

The SVOM/ECLAIRs telescope triggered and located the long duration GRB 250106A (sb25010601) starting at 2025-01-06T02:29:19.972 UTC (Tb).
 
The following trigger information was received on the ground with low-latency by the SVOM VHF Alert Network.
 
The burst was detected by both the on-board Count-Rate Trigger (CRT) and Image Trigger (IMT) and 8 alerts were received. The best detection is obtained by IMT with a signal-to-noise ratio of 8.94 in the 8-50 keV energy band over a time window of 20.48 s starting at Tb.
 
The localization of the best Alert is RA, Dec = 117.152, 63.770 (J2000), which is
RA(J2000) = 07h 48m 36.480s
Dec(J2000)= +63d 46' 12.000'
The uncertainty on this position is 8.84 arcminutes at 90% C.L., which includes a systematic uncertainty of 2 arcminutes in quadrature.

The light curve shows one broad peak in 5-120 keV in ECLAIRs with a preliminary T90 duration of 93.64 s.

The SVOM/GRM light curve can be found here:
 https://www.bursthub.cn//admin/static/svgrb250106A.png

SVOM slewed automatically on this burst. The information about MXT and VT observations will be reported later. 

The Space 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.
 
The SVOM point of contact for this burst is: Donghua ZHAO(zhaodh@bao.ac.cn)
 

P. A. Evans (U. Leicester) reports on behalf of the Swift team:

Swift has initiated a ToO observation of the SVOM/ECLAIRs GRB 250106A. 
Automated analysis of the XRT data will be presented online at
https://www.swift.ac.uk/ToO_GRBs/00021753

Any uncatalogued X-ray sources detected in this analysis will be
reported on this website and via GCN COUNTERPART notices. These are
not necessarily related to the SVOM/ECLAIRs event. Any X-ray source
considered to be a probable afterglow candidate will be reported via a 
GCN Circular after manual consideration.

Details of the XRT automated analysis methods are detailed in Evans et
al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8).

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

SVOM/VT commissioning team: Y. L. Qiu, H. L. Li, C. Wu, D. H. Zhao, L. P. Xin, X. H. Han, J. Wang, W. J. Xie, H. B. Cai, Y. Xu, Y. J. Xiao, P. P. Zhang, J. S. Deng, L. Lan, X. M. Lu, R. S. Zhang, Z. H. Yao (NAOC), J. Zhang, L. J. Dan, G. Y. Zou, C. J. Wang, Y. F. Du, C. Huang (XIOPM)

SVOM JSWG: Jian-Yan Wei (NAOC), Bertrand Cordier (CEA), Shuang-Nan Zhang (IHEP), Stéphane Basa (LAM), Arnaud Claret (CEA), Zi-Gao Dai (USTC), Frédéric Daigne (IAP), Jin-Song Deng (NAOC), Olivier Godet (IRAP), Andrea Goldwurm (APC), Diego Götz (CEA), Xu-Hui Han (NAOC), Cyril Lachaud (APC), En-Wei Liang (GXU), Yu-Lei Qiu (NAOC), Susanna Vergani (Obs.Paris), Jing Wang (NAOC), Chao Wu (NAOC), Li-Ping Xin (NAOC), Shao-Lin Xiong (IHEP), Bing Zhang (UNLV)

report on behalf of the SVOM team:

The SVOM/VT conducted observations of GRB 250106A (Zhao et al., GCN 38826
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 ) after the automatic slew of the platform. The observations were carried out in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channel simultaneously.

In the stacked VT_R image with an exposure time of 14*100 sec, an uncatalogued source was detected in the errorbox of XRT source #2(P. A. Evans et al., GCN 38828
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 ), while no any new source was detected within the errorbox of XRT source #1, compared to Pan-STARRS catalog.  

The brightness of the uncatalogued source was VT_R=22.5+/-0.3 mag in AB miniguide at the mid time of 0.965 hours after the burst. 
The source was not detected in VT_B stacked image with a 3 sigma limit of 23.3 mag at the mid time of 0.966 hours after the burst. 


The source is located at RA, Dec = 117.37078 63.70817 deg, which is equivalent to:
RA (J2000) : 07:49:28.99
Dec (J2000): +63:42:29.4
with an uncertainty of 0.5 arcsec.

More deep follow-ups are encouraged for the nature of the source.

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.

J.P. Osborne (U. Leicester), P. D'Avanzo (INAF-OAB), M.G. Bernardini
(INAF-OAB), E. Ambrosi	(INAF-IASFPA) , M. A. Williams (PSU), S.
Dichiara (PSU), J.A. Kennea (PSU), K.L. Page (U. Leicester), A.P.
Beardmore (U. Leicester) and P.A. Evans (U. Leicester) reports on
behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the
SVOM/ECLAIRs-detected burst GRB 250106A (Zhao et al., GCN Circ. 38826
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 ),
collecting  2.9 ks of Photon Counting (PC) mode data between T0+19.9 ks
and T0+27.1 ks. 

Four uncatalogued X-ray sources have been detected, however none of
them is above the RASS limit or shows definitive signs of fading.
However, Source 2 was reported by Qiu et al. (GCN Circ. 38829
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 ) to be
consistent with the position of a SVOM/VT optical candiate. Details of
the Swift-XRT sources are given below:

Source 1:
  RA (J2000.0):  117.2387  =  07:48:57.28
  Dec (J2000.0): +63.8123  =  +63:48:44.3
  Error: 3.0 arcsec (radius, 90% conf. [Enhanced position])
  Count-rate: 0.0209 +/- 0.0032 ct s^-1   
  Distance: 205 arcsec from SVOM/ECLAIRs position.
  Flux: (5.99 +/- 0.91)e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 2:
  RA (J2000.0):  117.3686  =  07:49:28.47
  Dec (J2000.0): +63.7079  =  +63:42:28.6
  Error: 5.9 arcsec (radius, 90% conf.)
  Count-rate: (5.7 [+2.1, -1.7])e-3 ct s^-1   
  Distance: 411 arcsec from SVOM/ECLAIRs position.
  Flux: (4.1 [+1.5, -1.2])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 3:
  RA (J2000.0):  116.9049  =  07:47:37.18
  Dec (J2000.0): +63.9115  =  +63:54:41.5
  Error: 6.5 arcsec (radius, 90% conf. [Enhanced position])
  Count-rate: (4.5 [+1.9, -1.5])e-3 ct s^-1   
  Distance: 642 arcsec from SVOM/ECLAIRs position.
  Flux: (3.1 [+1.3, -1.0])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 4:
  RA (J2000.0):  117.2096  =  07:48:50.31
  Dec (J2000.0): +63.8218  =  +63:49:18.4
  Error: 7.1 arcsec (radius, 90% conf.)
  Count-rate: (2.4 [+1.4, -1.0])e-3 ct s^-1   
  Distance: 207 arcsec from SVOM/ECLAIRs position.
  Flux: (7.3 [+4.2, -3.1])e-14 erg cm^-2 s^-1 (observed, 0.3-10 keV)

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/ToO_GRBs/00021753.

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

L. Izzo (INAF/OACn and DARK/NBI), D. B. Malesani (DAWN/NBI and Radboud Univ.), D. Xu (NAOC), Z.P. Zhu (NAOC), X. Liu (NAOC), T. Pursimo (NOT), report on behalf of a larger collaboration:

We observed the field of GRB 250106A (Zhao et al., GCN 38826
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 ) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera, using the SDSS z filter. The instrument was rotated in order to include XRT sources #1 and #2 (Osborne et al., GCN 38830
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 ), as well as the candidate SVOM/VT optical afterglow (which is consistent with the position of XRT source #2; Qiu et al., GCN 38829
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 ). The observation mid time was 2025 Jan 6.982 UT (10.13 hr after the SVOM trigger), and the exposure time was 2000 s.

No source is detected within the localization circle of XRT source #1, down to a limiting magnitude z > 23.3 (AB).


Two sources are detected within the localization circle of XRT source #2.

Source A (coincident with the SVOM/VT optical candidate):
RA = 07:49:29.04
Dec = +63:42:29.4
mag z = 22.23 +- 0.10 AB

Source B:
RA = 07:49:27.99
Dec = +63:42:31.5
mag z = 19.90 +- 0.02 AB

At the locations of both sources, objects are visible in the Legacy Survey, for which we measure AB magnitudes z = 22.24 +- 0.11 and z = 19.84 +- 0.02 for sources A and B, respectively. Our measured values are thus fully consistent with those in the archival images.

Source B is also detected by Gaia, which provides a QSO classification. This opens the question whether object B could be responsible for the X-ray emission detected by Swift/XRT.

Source A could be the host galaxy of GRB 250106A, though we note that its archival r-band magnitude (r = 23) is only marginally fainter than the SVOM/VT magnitude (R = 22.5 +- 0.3 mag; Qiu et al., GCN 38829
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 ).

We encourage further observations with both SVOM/VT and Swift/XRT in order to establish variability of both the optical and X-ray sources.

SVOM/VT commissioning team: Y. L. Qiu, H. L. Li, C. Wu, D. H. Zhao, L. P. Xin, X. H. Han, J. Wang, W. J. Xie, H. B. Cai, Y. Xu, Y. J. Xiao, P. P. Zhang, J. S. Deng, L. Lan, X. M. Lu, R. S. Zhang, Z. H. Yao (NAOC), J. Zhang, L. J. Dan, G. Y. Zou, C. J. Wang, Y. F. Du, C. Huang (XIOPM)

SVOM JSWG: Jian-Yan Wei (NAOC), Bertrand Cordier (CEA), Shuang-Nan Zhang (IHEP), Stéphane Basa (LAM), Arnaud Claret (CEA), Zi-Gao Dai (USTC), Frédéric Daigne (IAP), Jin-Song Deng (NAOC), Olivier Godet (IRAP), Andrea Goldwurm (APC), Diego Götz (CEA), Xu-Hui Han (NAOC), Cyril Lachaud (APC), En-Wei Liang (GXU), Yu-Lei Qiu (NAOC), Susanna Vergani (Obs.Paris), Jing Wang (NAOC), Chao Wu (NAOC), Li-Ping Xin (NAOC), Shao-Lin Xiong (IHEP), Bing Zhang (UNLV)

report on behalf of the SVOM team:

The SVOM/VT conducted TOO observations of GRB 250106A (Zhao et al., GCN 38826
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 ) at 8.88 hr after the trigger. The observations were carried out in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channel simultaneously.

The candidate reported by  Qiu et al. (GCN 38829
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 ) were detected in both of stacked images of VT_B and VT_R. The brightness are, 
channel  (t-T0)_mid(hr)    exptime      mag(AB)
VT_B     13.25       149x100s    23.5 +/-0.2 
VT_R     13.26       142x100s    22.4 +/-0.1

No definite fading was found compared to brightness reported (Qiu et al., GCN 38829
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 ). This observation is consistent with the result (Izzo et al. GCN 38834
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 ). Thus, this source is not a good candidate for the burst. 

A new source with a low S/N with a magnitude of VT_R~24.0 mag was tentatively detected near the errorbox of XRT source #1 (Osborne et al. GCN 38830
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 ), with a distance of 4.0 arcseconds from the XRT source #1. Its position is RA=117.2372, DEC=63.8132 deg, which is equivalent to:
RA (J2000) :  07:48:56.93
Dec (J2000): +63:48:47.5
with an uncertainty of 0.5 arcsec.

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.

J.P. Osborne (U. Leicester), P. D'Avanzo (INAF-OAB), M.G. Bernardini
(INAF-OAB), E. Ambrosi	(INAF-IASFPA) , M. A. Williams (PSU), S.
Dichiara (PSU), J.A. Kennea (PSU), K.L. Page (U. Leicester), A.P.
Beardmore (U. Leicester) and P.A. Evans (U. Leicester) reports on
behalf of the Swift-XRT team:

Swift-XRT has conducted further observations of the field of the
SVOM/ECLAIRs-detected burst GRB 250106A. The observations now extend
from T0+20.2 ks to T0+579.8 ks and have a total exposure time of 7.9
ks.  

Of the sources previously reported, "Source 1" is fading with >3-sigma
significance, and is therefore likely the GRB afterglow. Using 2981 s
of PC mode data and 3 UVOT images, we find an enhanced XRT position
(using the XRT-UVOT alignment and matching UVOT field sources to the
USNO-B1 catalogue): RA, Dec = 117.23873, +63.81242 which is equivalent
to:

RA (J2000): 07h 48m 57.29s
Dec(J2000): +63d 48' 44.7"

with an uncertainty of 3.0 arcsec (radius, 90% confidence). This
position is 3.4 arcmin from the SVOM/ECLAIRs position.	The source is
fading with alpha >0.6.

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.4 (+0.8, -0.7). The
best-fitting absorption column is  3.7 (+3.4, -2.4) x 10^21 cm^-2, in
excess of the Galactic value of 4.5 x 10^20 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.1 x 10^-11 (6.2 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.7 (+3.4, -2.4) x 10^21 cm^-2
Galactic foreground: 4.5 x 10^20 cm^-2
Excess significance: 2.2 sigma
Photon index:	     2.4 (+0.8, -0.7)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00021753.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021753.

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