GRB 250530C

R. Caputo (GSFC), R. Caputo (NASA/GSFC), R. Gupta (NASA GSFC),
J. A. Kennea (PSU), M. J. Moss (GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL), T. M. Parsotan (GSFC), C. Salvaggio (INAF-OAB),
T. Sbarrato (INAF-OAB) and B. Sbarufatti (INAF-OAB) report on behalf
of the Neil Gehrels Swift Observatory Team:

At 18:58:42 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250530C (trigger=1319125). Swift slewed immediately to the burst.
The BAT on-board calculated location is RA, Dec 55.143, -22.459 which is 
   RA(J2000) = 3h 40m 34s
   Dec(J2000) = -22d 27' 32"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  Due to a telemetry gap, the immediately-available
BAT light curve does not start until T+8s.  This shows possible activity out
to T+40 s.  The count rate was ~200 counts/sec (15-350 keV), at
the start of available light curve. A more complete description of
the emission, including the trigger time, will require the full
downlinked data from the next ground pass.

The XRT began observing the field at 19:00:29.8 UT, 106.9 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 55.17029,
-22.44825 which is equivalent to:
   RA(J2000)  = 03h 40m 40.87s
   Dec(J2000) = -22d 26' 53.7"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 98 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 (1.70 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.6
(+2.00/-1.76) x 10^21 cm^-2 (90% confidence).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 111 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 19.6 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 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.025.

Burst Advocate for this burst is R. Caputo (regina.caputo AT nasa.gov).
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 2203 s of XRT Photon Counting mode data and 5 UVOT
images for GRB 250530C, 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 = 55.17085, -22.44801 which is equivalent
to:

RA (J2000): 03h 40m 41.00s
Dec (J2000): -22d 26' 52.8"

with an uncertainty of 2.0 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.

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-SAAO robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in South Africa (South African Astronomical Observatory) was pointed to the Swift GRB 250530C ( R. Caputo et al., GCN 40585
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 ) errorbox  31007 sec after notice time and 31055 sec after trigger time at 2025-05-31 03:36:18 UT, with upper limit up to  18.6 mag. The observations began at zenith distance = 79 deg. The sun  altitude  is -23.6 deg. 

The galactic latitude b = -51 deg., longitude l = 216 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2887090

We obtain a following upper limits.  

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

   31086 |         MASTER-SAAO |   C |    60 | 18.6 |        
   31086 |             MASTER- |   C |    60 | 13.1 |        
Filter C is a clear (unfiltred) band. 


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

M.A. Williams (PSU), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), A.P. Beardmore (U. Leicester), M. Ferro (INAF-OAB), R.
Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), S. Dichiara (PSU), J.A.
Kennea (PSU) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 7.7 ks of XRT data for GRB 250530C, from 90 s to 52.0
ks after the   trigger. The data comprise 9 s in Windowed Timing (WT)
mode (taken while Swift was slewing), with the remainder in Photon
Counting (PC) mode. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.80 (+/-0.04).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.55 (+0.20, -0.19). The
best-fitting absorption column is  2.8 (+1.1, -0.9) x 10^21 cm^-2, in
excess of the Galactic value of 1.7 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 5.0 x 10^-11 (6.1 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.8 (+1.1, -0.9) x 10^21 cm^-2
Galactic foreground: 1.7 x 10^20 cm^-2
Excess significance: 4.7 sigma
Photon index:	     1.55 (+0.20, -0.19)

If the light curve continues to decay with a power-law decay index of
0.80, the count rate at T+24 hours will be 0.012 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.0 x
10^-13 (7.3 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01319125.

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

I. Perez-Garcia, E. Fernandez-Garcia, S.-Y. Wu, M. D. Caballero-Garcia, R. Sanchez-Ramirez, S. Guziy, and A. J. Castro-Tirado (IAA-CSIC, Granada), C. Perez del Pulgar (Univ. of Malaga), L. Hernandez-Garcia (Univ. of Valparaiso), M. Gritsevich (Univ. of Helsinki), Y.-D. Hu (GXU), B.-B. Zhang (Nanjing Univ.), and A. Maury (Space Obs., San Pedro de Atacama), on behalf of a larger collaboration, report:

Following the detection of GRB 250530C by Swift (Osborne et al., GCNC [40586](https://gcn.nasa.gov/circulars/40586
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 )), the 0.6m BOOTES-7 robotic telescope at Space Observatory (San Pedro de Atacama, Chile) automatically responded to this high-energy event starting on May 21, 09:42:59 UT (i.e., 14.73 hour after detection), observing at low elevation (airmass ≈ 5). No optical transient is detected down to 16.4 mag (clear filter). On a coadd (11) of images at 09:51:43 UT (mid exposure time; i.e. 14.88 post burst), nothing is detected down to 18.2 mag.

We would like to thank the staff at San Pedro de Atacama Space Observatory for their excellent support.

D. Sadaula (GSFC/UMBC), S. D. Barthelmy (GSFC), 
R. Caputo (NASA/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), T. Sakamoto (AGU)
(i.e. the Swift-BAT team):
 
Using the data set from T-240 to T+885 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 250530C (trigger #1319125)
(Caputo, et al., GCN Circ. 40585
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 ).  The BAT ground-calculated position is
RA, Dec = 55.146, -22.457 deg which is 
   RA(J2000)  =  03h 40m 35.1s 
   Dec(J2000) = -22d 27' 26.1" 
with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 96%.
 
The mask-weighted light curve displays a complex structure with multiple dim pulses. 
The T90 (15-350 keV) is 42.86 +- 6.04 sec (estimated error including systematics).
 
The time-averaged spectrum from T-29.77 to T+21.26 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.32 +- 0.17.  The fluence in the 15-150 keV band is 8.5 +- 0.9 x 10^-07 erg/cm2.
The 1-sec peak photon flux measured from T-0.28 sec in the 15-150 keV band
is 0.8 +- 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/1319125