GRB 250805A

P. Jaquiery,  R. Hellot (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), M. Pillas (ULiege), S. Antier (OCA/IJCLAB) on behalf of the GRANDMA/Kilonova-Catcher collaboration:

We observed the field of GRB 250805A (Ambrosi et al., GCN 41233
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 ) detected by Swift/BAT with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with a Celestron C14 telescope located at Beverly-Begg Observatory and operated by P. Jaquiery and the CDK17 telescope located at AITP San Pedro Chile Observatory operated by R. Hellot. Our observations started at T0+1.2hr. 

In our stacked frames, subtracted from the Legacy Survey DR10 template image, we do not detect any optical counterpart inside the Swift/XRT position (Kennea et al., GCN 41234
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 ; Beardmore et al., GCN 41238
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 ) and at the position of the optical afterglow candidate reported by the VLT/X-shooter (Izzo et al., GCN 41241
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 ) and SVOM/VT (Li et al., GCN 41242
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 ) teams.

We report our follow-up results in the table below:
+---------------+-----------+------------+----------------------+-------------+
| Tmid-TGRB (hr)| Exp (s)   | Filter     | Magnitude            | Instrument  |
+===============+===========+============+======================+=============+
| 1.9           | 28 x 120s | sdssr (AB) | 18.3 (U.L., 5 sigma) | C14-BBO     | 
| 26.1          |  5 x 600s | sdssr (AB) | 19.5 (U.L., 5 sigma) | CDK17-AITP  | 
+---------------+-----------+------------+----------------------+-------------+

These upper limits are compatible with the magnitudes and upper limits reported by (Liu et al., GCN 41236
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 ; Izzo et al., GCN 41241
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 ; Li et al., GCN 41242
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 ; Siegel et al., GCN 41265
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 )

All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained with the sloan filters were calibrated using the SkyMapper DR4 catalog. 

We use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023). 

GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/).

M. H. Siegel (PSU) and Ambrosi (INAF-IASFPA)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250805A
3536 s after the BAT trigger (Ambrosi et al., GCN Circ. 41233
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 ).
No optical afterglow consistent with the XRT position (Beardmore et al., 
GCN Circ. 41238
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 ) or optical afterglow (Izzo et al., GCN Circ. 41241
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 ; Li et
al., GCN Circ. 41241
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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          3536         3686          147         >21.3
white             3536         4306          344         >21.7
b                 3902         4101          197         >20.9
u                 3696         3895          197         >20.6
w2                4312         4460          146         >20.7

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.092 in the direction of the burst
(Schlegel et al. 1998).

T. Sakamoto (AGU), E. Ambrosi (INAF-IASFPA),
S. D. Barthelmy (GSFC), R. Gupta (GSFC),
H. A. Krimm (NSF), S. Laha (GSFC/UMBC),
A. Y. Lien (U Tampa), C. B. Markwardt (GSFC),
M. J. Moss (GSFC), D. M. Palmer (LANL),
T. Parsotan (GSFC), D. Sadaula (GSFC/UMBC)
(i.e. the Swift-BAT team):
 
Using the data set from T-240 to T+962 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 250805A (trigger #1339848)
(Ambrosi, et al., GCN Circ. 41233
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 ).  The BAT ground-calculated position is
RA, Dec = 24.124, -81.400 deg which is 
   RA(J2000)  =  01h 36m 29.8s 
   Dec(J2000) = -81d 23' 59.0" 
with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 65%.
 
The mask-weighted light curve displays a main emission period between T0-25 sec 
and T0+30 sec composed of multiple pulses with the strongest pulse beginning at T0. 
There is dim but significant emission beginning at T0-60 sec. The T90 (15-350 keV)
is 79.37 +- 11.98 sec (estimated error including systematics).
 
The time-averaged spectrum from T-60.66 to T+36.25 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 0.58 +- 0.53, 
and Epeak of 68.0 +- 15.8 keV (chi squared 54.47 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 2.2 +- 0.2 x 10^-06 erg/cm2
and the 1-sec peak flux measured from T+0.46 sec in the 15-150 keV band is
1.7 +- 0.2 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.59 +- 0.11 (chi squared 67.84 for 57 d.o.f.).  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/1339848

H. L. Li, Y. N. Ma, L. P. Xin, Y. L. Qiu, C. Wu, Z. H. Yao, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA), A. Li (BNU), Y. D. Hu (GXU) report on behalf of the SVOM/VT team.

SVOM performed a Target of Opportunity observation of GRB 250805A detected by Swift/BAT (Ambrosi et al., GCN 41233
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 ; Evans et al., GCN 41234
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 ; Page et al., GCN 41240
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 ). SVOM/VT began observing the field at 2025-08-05 07:00:42 UTC, 50.4 min after the Swift trigger time, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.

An uncatalogued source, compared to Lagacy survey, is found using VT X-band data, with a distance of 2.7 arcsec to Swift/XRT enhanced position (Beardmore et al., GCN 41238
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 ). The optical source is at R.A., Dec 24.216108, -81.402177 degrees:
 
  RA  (J2000) =  01:36:51.86
  Dec (J2000) = -81:24:07.84
with an uncertainty of 0.5 arcsec.
 
The source is detected in both VT_B and VT_R images. It decayed by 0.7 mag  within 1.89 hours of VT observation. The magnitudes are derived as below:
 
Mid time            | exposure time (s) | band | mag (AB) | mag err
--------------------|-------------------|------|----------|--------
1.24 hours          | 37*70             | VT_B | 22.79    | 0.10   
1.24 hours          | 37*70             | VT_R | 21.16    | 0.04    

Our photometry was not corrected for Galactic extinction.

Given the fading of its brightness, the uncatalogued source is proposed to be the optical afterglow of the burst. Our result is consistent with the report by ESO/VLT (Izzo et al., GCN 41241
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 ). 

The color of VT_B-VT_R~1.5 mag suggests that it shall be a low or intermediate redshift GRB.

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.

L. Izzo (INAF/OACn & DARK/NBI), I. F. Giudice (INAF/OACn), N. Tanvir (U. Leicester), A. Levan (Radboud U.), A. Martin-Carrillo (UCD), M. De Pasquale (U. Messina), A.L Thakur (INAF-IAPS), V. D’Elia (ASI/SSDC) report on behalf of the Stargate collaboration:

We observed the region of the X-ray counterpart (Kennea et al., GCN #42134
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 ) of GRB 250805A (Ambrosi et al., GCN #41233
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 ) using the ESO/VLT UT3 (Melipal) equipped with the X-Shooter spectrograph.

A 30-second acquisition image in the r-band, initiated on August 05, 2025, at approximately 09:30 UT (3.3 hours post-burst), revealed no optical counterpart within the initial Swift-XRT error region (Kennea et al., GCN #42134
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 ). Due to the absence of an optical source within this region, spectroscopic observations were not executed.

However, we have identified the presence of an uncatalogued optical source outside the initial XRT error region, situated approximately 6 arcsec away from the XRT centroid. This source aligns with the enhanced Swift-XRT position reported later by Beardmore et al. (GCN #41238
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 ), indicating that it is likely the optical counterpart of GRB 250805A.

We acknowledge the excellent support from the ESO observing staff in Paranal, in particular Robert Klement and Luca Sbordone.

K.L. Page (U. Leicester), M.G. Bernardini (INAF-OAB), E. Ambrosi 
(INAF-IASFPA) , M. Capalbi (INAF-OAR), S. Dichiara (PSU), J.A. Kennea
(PSU), D.N. Burrows (PSU), A.P. Beardmore (U. Leicester) and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 6.1 ks of XRT data for GRB 250805A, from 3.5 ks to
27.4 ks after the   trigger. The data are entirely in Photon Counting
(PC) mode. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.89 (+/-0.08).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.63 (+/-0.14). The
best-fitting absorption column is  1.4 (+/-0.5) x 10^21 cm^-2, in
excess of the Galactic value of 8.4 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 4.4 x 10^-11 (5.1 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.4 (+/-0.5) x 10^21 cm^-2
Galactic foreground: 8.4 x 10^20 cm^-2
Excess significance: 2.0 sigma
Photon index:	     1.63 (+/-0.14)

If the light curve continues to decay with a power-law decay index of
0.89, the count rate at T+24 hours will be 0.042 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.8 x
10^-12 (2.1 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/01339848.

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

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

Using 912 s of XRT Photon Counting mode data and 3 UVOT
images for GRB 250805A, 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 = 24.21207, -81.40174 which is equivalent
to:

RA (J2000): 01h 36m 50.90s
Dec (J2000): -81d 24' 06.3"

with an uncertainty of 2.4 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.

J.A. Kennea (PSU), G. Cusumano (INAF-IASF PA) and P.A. Evans (U.
Leicester) report on behalf of the Swift-XRT team:

The XRT began observing the field of GRB 250805A at 07:08:01.8 UT,
3532.9 seconds after the BAT trigger. Using promptly downlinked data we
find an uncatalogued X-ray source located at RA, Dec 24.20214,
-81.40161 which is equivalent to:
   RA(J2000)  = 01h 36m 48.51s
   Dec(J2000) = -81d 24' 05.8"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 30 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 (8.44 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.6
(+3.34/-2.82) x 10^21 cm^-2 (90% confidence). 

E. Ambrosi (INAF-IASFPA), S. Dichiara (PSU), R. Gupta (NASA GSFC),
K. L. Page (U Leicester) and M. H. Siegel (PSU) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 06:09:08 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250805A (trigger=1339848).  
The BAT on-board calculated location is 
RA, Dec 24.153, -81.406 which is 
   RA(J2000) = 01h 36m 37s
   Dec(J2000) = -81d 24' 22"
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 30 sec.  The peak count rate
was ~1800 counts/sec (15-350 keV), at ~0 sec after the trigger.

Due to an observing constraint, Swift will not slew until ~ 57 minutes. There will be no XRT or UVOT data until this time.

Burst Advocate for this burst is E. Ambrosi (elena.ambrosi AT inaf.it).
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/)