GRB 250108B

N. J. Klingler (GSFC/UMBC/CRESSTII), R. Brivio (INAF-OAB),
J. J. DeLaunay (PSU), R. Gupta (NASA GSFC), K. L. Page (U Leicester),
C. Salvaggio (INAF-OAB) and T. Sbarrato (INAF-OAB) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 10:23:06 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250108B (trigger=1280056).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 201.297, +25.613 which is 
   RA(J2000) = 13h 25m 11s
   Dec(J2000) = +25d 36' 46"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 30s sec.  The peak count rate
was ~1210 counts/sec (15-350 keV), at ~2 sec after the trigger.

The XRT began observing the field at 10:24:54.0 UT, 107.8 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source with an enhanced position: RA, Dec 201.32686,
25.61541 which is equivalent to:
   RA(J2000)  = 13h 25m 18.45s
   Dec(J2000) = +25d 36' 55.5"
with an uncertainty of 2.0 arcseconds (radius, 90% containment). This
location is 97 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at https://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (1.16 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.6
(+2.37/-2.05) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 9.22e-10 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 250 seconds with the U filter starting
274 seconds after the BAT trigger. No credible afterglow candidate has been
found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the
XRT error circle. The typical 3-sigma upper limit has been about 19.2 mag. The
8'x8' region for the list of sources generated on-board covers 100% of the XRT
error circle. The list of sources is typically complete to about 18.0 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.012.

Burst Advocate for this burst is N. J. Klingler (noelklin AT umbc.edu).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)

J.P. Osborne, A.P. Beardmore, P.A. Evans and M.R. Goad (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 558 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 250108B, we find an astrometrically corrected X-ray
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 201.32654, +25.61519 which is equivalent
to:

RA (J2000): 13h 25m 18.37s
Dec (J2000): +25d 36' 54.7"

with an uncertainty of 2.2 arcsec (radius, 90% confidence).

This position may be improved as more data are received. The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).

This circular was automatically generated, and is an official product of the
Swift-XRT team.

A. A. Breeveld (UCL-MSSL) and N. J. Klingler (GSFC/UMBC/CRESSTII) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250108B 117 s after the BAT trigger (Klingler et al., GCN Circ. 38847
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 ).
No optical afterglow consistent with the enhanced XRT position (Osborne et al., GCN Circ. 38848
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 ) is detected in the initial UVOT exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

white_FC           117          267          147         >20.7
white              117          980          247         >21.2
u_FC               275          525          246         >20.2

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.012 in the direction of the burst (Schlegel et al. 1998).

Jillian Rastinejad (Northwestern), Andrew J. Levan (Radboud), Wen-fai Fong, Charlie Kilpatrick (Northwestern), Daniele B. Malesani (DAWN/ NBI and Radboud), Nial R. Tanvir (U. Leicester), Gavin P Lamb (LJMU), Benjamin P. Gompertz (U. Birmingham) and Brian D. Metzger (Columbia/CCA) report on behalf of a larger collaboration:

We observed the location of the X-ray afterglow of GRB 250108B (Klinger et al., GCN 38847
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 ; Osborne et al., GCN 38848
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 ) with the Gemini Multi-Object Spectrograph (GMOS-N) mounted on Gemini-North under Program GN-2024B-Q-107. We obtained 15x120-s imaging in r-band at a mid-time of 2025-01-08 12:42:45.8 UT (2.33 hours post-burst), at a median airmass of 1.7 and seeing < 1.2''. 

We clearly detect a faint source not present in archival Legacy Survey DR10 r-band imaging but within the XRT localization at a position of

RA (J2000) = 13:25:18.50
Dec (J2000) = 25:36:55.82

with an uncertainty of 0.7''. We suggest that this is the afterglow of GRB 250108B.

We note that the candidate afterglow is 16.8'' offset from a bright galaxy (r = 19.5 AB mag). This galaxy has a Legacy Survey DR9 photometric redshift of z = 0.29 +/- 0.04 (Zhou et al., 2021) and an SDSS photometric redshift of z = 0.22 +/- 0.04 (Alam et al., 2015), corresponding to projected separations of 74 kpc and 60 kpc from the transient, respectively. We determine a probability of chance coincidence between this galaxy and the candidate afterglow of Pcc = 0.13 (Bloom et al., 2002). With present data we cannot rule out the presence of a faint, underlying host at the position of the transient.

Calibrated to SDSS, we measure a preliminary brightness for the optical source of r = 23.4 +/- 0.1 AB mag, not corrected for Galactic extinction. At a redshift of z = 0.29, this corresponds to an optical luminosity of 2.1 x 10^42 erg/s.

Further observations are planned to monitor the variability of the source. We thank Jen Miller, Leila Alamos, Brian Lemaux and additional Gemini staff for excellent support in the rapid planning and execution of these observations.

V.Lipunov, E.Gorbovskoy, N.Tiurina,  P.Balanutsa, , D.Vlasenko, I.Panchenko,
A.Kuznetsov,  G.Antipov,  A.Sankovich, A.Sosnovskij, Yu.Tselik, M.Gulyaev, 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. 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  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the Swift GRB 250108B ( N. J. Klingler et al., GCN 38847
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 ) errorbox  35528 sec after notice time and 35573 sec after trigger time at 2025-01-08 20:16:00 UT, with upper limit up to  17.8 mag. The observations began at zenith distance = 84 deg. The sun  altitude  is -65.6 deg. 

The galactic latitude b = 82 deg., longitude l = 23 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2737810

We obtain a following upper limits.  

Tmid-T0  |          Site       |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________

   35664 |   MASTER-Kislovodsk |   C |   180 | 16.7 |        
   35664 |   MASTER-Kislovodsk |   C |   180 | 16.7 |        
   35864 |   MASTER-Kislovodsk |   C |   180 | 16.7 |        
   35864 |   MASTER-Kislovodsk |   C |   180 | 16.7 |        
   36908 |   MASTER-Kislovodsk |   C |   180 | 17.5 |        
   36908 |   MASTER-Kislovodsk |   C |   180 | 17.5 |        
   38307 |   MASTER-Kislovodsk |   C |   180 | 17.6 |        
   38307 |   MASTER-Kislovodsk |   C |   180 | 17.6 |        
   38757 |   MASTER-Kislovodsk |   C |   120 | 17.6 |        
   38757 |   MASTER-Kislovodsk |   C |   120 | 17.6 |        
   39177 |   MASTER-Kislovodsk |   C |   120 | 17.5 |        
   39177 |   MASTER-Kislovodsk |   C |   120 | 17.5 |        
   39547 |   MASTER-Kislovodsk |   C |   180 | 17.8 |        
   39547 |   MASTER-Kislovodsk |   C |   180 | 17.8 |        
Filter C is a clear (unfiltred) band. 


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

M. Capalbi (INAF-IASFPA), D.N. Burrows (PSU), M. A. Williams (PSU), S.
Dichiara (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester),
M. Perri (SSDC & INAF-OAR), V. D'Elia (SSDC & INAF-OAR) and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 8.5 ks of XRT data for GRB 250108B, from 97 s to 33.7
ks after the  BAT trigger. The data comprise 608 s in Windowed Timing
(WT) mode (the first 9 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The late-time light curve (from T0+5.5 ks) can be modelled with a
power-law decay with a decay index of alpha=0.35 (+/-0.13).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.37 (+/-0.04). The
best-fitting absorption column is  1.47 (+/-0.09) x 10^21 cm^-2, in
excess of the Galactic value of 1.2 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.07 (+0.17, -0.16)
and a best-fitting absorption column of 1.3 (+0.5, -0.4) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.3 x 10^-11 (4.4 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.3 (+0.5, -0.4) x 10^21 cm^-2
Galactic foreground: 1.2 x 10^20 cm^-2
Excess significance: 4.8 sigma
Photon index:	     2.07 (+0.17, -0.16)

If the light curve continues to decay with a power-law decay index of
0.35, the count rate at T+24 hours will be 0.045 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.5 x
10^-12 (2.0 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01280056.

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

R. Brivio, M. Ferro, P. D’Avanzo, S. Covino, D. Fugazza (INAF-OAB) report on behalf of the REM team:

We observed the field of GRB 250108B detected by Swift (Klingler et al., GCN 38847
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 ) with the REM 60cm robotic telescope located at the ESO observatory of La Silla (Chile). The observations were carried in the g, r, i, z, J, H, K bands, starting on 2025 January 09 at 07:02:48 UT (i.e. 21.07 hours after the Swift trigger), and lasting for about 1 hour.
Within the XRT-enhanced position (Osborne et al., GCN 38848
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 ), from preliminary photometry we do not detect any optical or NIR counterpart consistent with the candidate optical afterglow (Rastinejad et al., GCN 38855
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 ), down to the following 3sigma magnitude upper limits:

r > 20.1 (AB; calibrated against the PanSTARRS catalogue)
at a mid-time of t - t0 = 21.2 hr after the trigger,

H > 17.0 (Vega; calibrated against the 2MASS catalogue) 
at a mid-time of t - t0 = 21.1 hr after the trigger.

Amit K. Ror, Anshika Gupta, RKS Yadav, Shashi B. Pandey, Kuntal Mishra
(ARIES) report:

We observed the field of GRB 250108B detected by Swift (Swift Observatory
Team, Klingler et al. 2025, GCN 38847
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 ) with the 1.3m Devasthal Fast Optical
Telescope (DFOT), located at the Devasthal Observatory of the Aryabhatta
Research Institute of Observational Sciences (ARIES), India. The
observations were started on 2025-01-08 at 21:58:57 UT, i.e., ~ 0.48 days
after the Swift-BAT trigger. We have taken multiple frames with an exposure
time of 300s in the R filter. We stacked the images after the alignment. We
could not detect the optical emission reported by Rastinejad et al. (2025,
GCN 38855
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 ) in our stacked image within the error box enhanced Swift-XRT
position (Osborne et al. 2025, GCN 38848
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 ). We obtain the following 3-sigma
upper limit in the stacked image:

Date Start_UT T_start-T0 (days) Filter  Exp time (s)  Magnitude
=========================================================
2025-01-08 21:58:57    ~ 0.48     R     300s*12     >21.8

The non-detection of the burst is consistent with the upper limits reported
by Breeveld et al. 2025, GCN 38850
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  and Lipunov et al. 2025, GCN 38858
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 .

The magnitude is not corrected for the Galactic extinction in the direction
of the burst. Photometric calibration is performed using the standard stars
from the USNO-B1.0 catalogue. This circular may be cited.

S. Laha (GSFC/UMBC), S. D. Barthelmy (GSFC), 
R. Gupta (GSFC), N. J. Klingler (GSFC/UMBC/CRESSTII),
H. A. Krimm (NSF), A. Y. Lien (U Tampa), 
C. B. Markwardt (GSFC), M. J. Moss (GSFC), 
D. M. Palmer (LANL), T. Parsotan (GSFC), 
D. Sadaula (GSFC/UMBC), T. Sakamoto (AGU) 
(i.e. the Swift-BAT team):
 
Using the data set from T-239 to T+792 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 250108B (trigger #1280056)
(Klinger et al., GCN Circ. 38847
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 ).  The BAT ground-calculated position is
RA, Dec = 201.312, 25.622 deg which is 
   RA(J2000)  =  13h 25m 14.8s 
   Dec(J2000) = +25d 37' 19.4" 
with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 89%.
 
The mask-weighted light curve exhibits multi-peaked structures, including evidence of precursor emission prior to the main burst. T90 (15-350 keV) is 229.66 +- 43.30 sec (estimated error including systematics).
 
The time-averaged spectrum from T-63.14 to T+218.21 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.63 +- 0.19.  The fluence in the 15-150 keV band is 1.7 +- 0.2 x 10^-06 erg/cm2.
The 1-sec peak photon flux measured from T+1.56 sec in the 15-150 keV band
is 0.6 +- 0.2 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level. 
 
The results of the batgrbproduct analysis are available at
https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1280056

Daniele B. Malesani (DAWN/ NBI and Radboud), Jillian C. Rastinejad (Northwestern), Andrew J. Levan (Radboud), Wen-fai Fong, Charlie Kilpatrick (Northwestern), Gavin P. Lamb (LJMU), and Nial R. Tanvir (Leicester) report on behalf of a larger collaboration:

Following the detection of the optical afterglow (Rastinejad et al., GCN 38855
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 ) of GRB 250108B  (Klinger et al., GCN 38847
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 ), we obtained optical spectroscopy using the Gemini Multi-Object Spectrograph (GMOS-N) mounted on Gemini-North, under Program GN-2024B-Q-107 (P.I. Rastinejad). The mean time of the observation was 15:12 UT on 2025 Jan 8 (4.81 hr post-burst). The total exposure time was 4x400 s using the R400 grating, which covers the wavelength range 4650-9450 AA.

Given the faintness of the target (r ~ 23.4; Rastinejad et al., GCN 38855
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 ), only a weak trace is detected, with most of the signal apparent at the red end of the spectrum. A few absorption features are weakly detected. A possible redshift solution, based on the detection of Mg I 2852, Mg II 2796,2803, Fe II 2600, and Fe II 2586, yields z = 2.197, which we suggest to be the redshift of GRB 250108B.

The slit was aligned in order to cover also the nearby galaxy (Rastinejad et al., GCN 38855
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 ) at coordinates RA = 13:25:18.72, Dec = +25:37:12.7, with photometric redshift z = 0.2-0.3, resulting in a bright trace. We detect Halpha, [N II], and [S II] at z = 0.302, consistent with the Legacy Survey photometric redshift of z = 0.29 +/- 0.04 (Zhou et al., 2021). Our observations indicate that this object is unlikely to be physically connected to the GRB.

We thank Jen Miller, Leila Alamos, Brian Lemaux and additional Gemini staff for excellent support in the rapid planning and execution of these observations.