GRB 210725B

M. G. Bernardini (INAF-OAB), P. D'Avanzo (INAF-OAB),
J. A. Kennea (PSU), N. J. Klingler (GSFC/UMBC/CRESSTII),
F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB),
D. M. Palmer (LANL), T. Sakamoto (AGU) and T. Sbarrato (INAF-OAB)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 12:00:48 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210725B (trigger=1061554).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 192.932, +17.125 which is 
   RA(J2000) = 12h 51m 44s
   Dec(J2000) = +17d 07' 30"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The immediately-available BAT light curve
(extending to T+8 seconds) shows a complex peak of at least 5 seconds
duration.  The peak count rate was ~1000 counts/sec (15-350 keV), 
at ~2 sec after the trigger. 

The XRT began observing the field at 12:02:18.8 UT, 90.9 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 192.9219,
17.1080 which is equivalent to:
   RA(J2000)  = 12h 51m 41.26s
   Dec(J2000) = +17d 06' 28.9"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 70 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. No
spectrum from the promptly downlinked event data is yet available to
determine the column density. 

UVOT took a finding chart exposure of 250 seconds with the U filter starting 91
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
0.023. 

Burst Advocate for this burst is M. G. Bernardini (grazia.bernardini AT brera.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/)

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

Using 273 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 210725B, 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 = 192.92217, +17.10809 which is equivalent
to:

RA (J2000): 12h 51m 41.32s
Dec (J2000): +17d 06' 29.1"

with an uncertainty of 2.5 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, V.Kornilov, N.Tyurina, P.Balanutsa, A.Kuznetsov, F.Balakin, 
V.Vladimirov, D. Vlasenko, I.Gorbunov, D.Zimnukhov, V.Senik, T.Pogrosheva,
D.Kuvshinov,  D. Cheryasov
(Lomonosov Moscow State University, SAI, Physics Department),

R. Podesta, C.Lopez, F. Podesta, C.Francile 
(Observatorio Astronomico Felix Aguilar OAFA),

R. Rebolo, M. Serra 
(The Instituto de Astrofisica de Canarias),

D. Buckley 
(South African Astronomical Observatory),

O.A. Gres, N.M. Budnev, O.Ershova 
(Irkutsk State University, API),

A. Tlatov, D. Dormidontov 
(Kislovodsk Solar Station of the Pulkovo Observatory),

V. Yurkov, A. Gabovich, Yu. Sergienko 
(Blagoveschensk Educational State University)




MASTER-Tavrida robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Russia (Lomonosov MSU, SAI Crimea astronomical station) was pointed to the Swift GRB 210725B ( M. G. Bernardini et al., GCN 30508
Loading...
 
 ) errorbox  22581 sec after notice time and 22657 sec after trigger time at 2021-07-25 18:18:25 UT, with upper limit up to  16.5 mag. Observations started at twilight.  The observations began at zenith distance = 56 deg. The sun  altitude  is -10.3 deg. 

The galactic latitude b = 80 deg., longitude l = 307 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=1669944

We obtain a following upper limits.  

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

   22747 |      MASTER-Tavrida |   C |   180 | 16.5 |        
Filter C is a clear (unfiltred) band. 


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

C. B. Markwardt (GSFC), S. D. Barthelmy (GSFC),
M. G. Bernardini (INAF-OAB), J. R. Cummings (CPI),
H. A. Krimm (NSF), S. Laha (GSFC/UMBC),
A. Y. Lien (GSFC/UMBC), D. M. Palmer (LANL),
T. Sakamoto (AGU), M. Stamatikos (OSU),
T. N. Ukwatta (LANL) (i.e. the Swift-BAT team):

Using the data set from T-239 to T+400 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 210725B (trigger #1061554)
(Bernardini et al., GCN Circ. 30508
Loading...
 
 ).  The BAT ground-calculated position is
RA, Dec = 192.914, 17.130 deg which is
   RA(J2000)  =  12h 51m 39.3s
   Dec(J2000) = +17d 07' 48.5"
with an uncertainty of 2.7 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 98%.

The mask-weighted light curve shows a main pulse that starts at ~T0, peaks
at ~T+1 s, and ends at ~T+5 s. In addition, there are some weak emission
that starts at ~T-30 s and ends at ~T+15 s. T90 (15-350 keV) is
48.00 +- 22.63 sec (estimated error including systematics).

The time-averaged spectrum from T-31.46 to T+16.54 sec is best fit
by a simple power-law model.  The power law index of the time-averaged
spectrum is 1.16 +- 0.43.  The fluence in the 15-150 keV band is
4.9 +- 1.1 x 10^-7 erg/cm2. The 1-sec peak photon flux measured from
T+0.97 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
http://gcn.gsfc.nasa.gov/notices_s/1061554/BA/

V. D'Elia (SSDC & INAF-OAR), A. D'Ai (INAF-IASFPA), D.N. Burrows (PSU),
J. D. Gropp (PSU), J.A. Kennea (PSU), K.L. Page (U. Leicester), A.P.
Beardmore (U. Leicester), P.A. Evans (U. Leicester), A. Melandri
(INAF-OAB) and M.G. Bernardini report on behalf of the Swift-XRT team:

We have analysed 4.9 ks of XRT data for GRB 210725B (Bernardini et al.
GCN Circ. 30508
Loading...
 
 ), from 94 s to 107.1 ks after the  BAT trigger. The
data comprise 68 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Goad et al. (GCN Circ. 30510
Loading...
 
 ).

The light curve can be modelled with a power-law decay with a decay
index of alpha=3.43 (+0.20, -0.18).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.77 (+0.29, -0.27). The
best-fitting absorption column is  6.6 (+2.7, -2.1) x 10^21 cm^-2, in
excess of the Galactic value of 2.0 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 4.5 x 10^-11 (6.7 x 10^-11) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     6.6 (+2.7, -2.1) x 10^21 cm^-2
Galactic foreground: 2.0 x 10^20 cm^-2
Excess significance: 5.2 sigma
Photon index:	     1.77 (+0.29, -0.27)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01061554.

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

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

The Swift/UVOT began settled observations of the field of GRB 210725B
92 s after the BAT trigger (Bernardini et al., GCN Circ. 30508
Loading...
 
 ).
No optical afterglow consistent with the XRT position (Goad et al. GCN Circ. 30510
Loading...
 
 )
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

u (fc)               92          341          246         >20.5
b                  347          439           91         >20.0
u                   92          341          246         >20.5
w2                3722         3905          180         >19.4

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.023 in the direction of the burst
(Schlegel et al. 1998).

C. Fletcher (USRA) reports on behalf of the Fermi-GBM Team:

Swift-BAT detected GRB 210725B at 12:00:48 UT (GCN 30508
Loading...
 
 ). There was no
Fermi-GBM onboard trigger around the event.

An automated, blind search for short gamma-ray bursts below the onboard
triggering threshold in Fermi-GBM identified a transient source at this time.

The GBM targeted search [1], the most sensitive, coherent search for
GRB-like signals identified a transient most significantly on the 8.192 s
timescale, with a false alarm rate of 2.6e-05 Hz and a location consistent with
the Swift-BAT event, using the standard search protocol with a S/N of 9.
The GBM targeted search event was found with the highest
significance with a "normal" spectrum (Band function with
Epeak = 230 keV, alpha = -1.0, beta = -2.3) for a GRB.

[1] Goldstein et al. 2019 arXiv:1903.12597
Loading...