GRB 240811A

K. L. Page (U Leicester) and R. Gupta (NASA GSFC) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 14:07:33 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 240811A (trigger=1248132).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 226.373, +28.854 which is 
   RA(J2000) = 15h 05m 30s
   Dec(J2000) = +28d 51' 15"
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 50 sec.  The peak count rate
was ~3000 counts/sec (15-350 keV), at ~11 sec after the trigger. 

The XRT began observing the field at 14:09:08.3 UT, 95.1 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
226.36218, 28.89573 which is equivalent to:
   RA(J2000)  = 15h 05m 26.92s
   Dec(J2000) = +28d 53' 44.6"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 154 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 (1.98 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.5
(+3.65/-3.05) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 7.46e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 102 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
0.023. 

Burst Advocate for this burst is K. L. Page (klp5 AT leicester.ac.uk). 
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/)

V. Lipunov, V.Kornilov, E. Gorbovskoy, K. Zhirkov, N.Tyurina, P.Balanutsa, A.Kuznetsov, V.Senik,  D. Vlasenko,
G.Antipov, D.Zimnukhov, E.Minkina, A.Chasovnikov, V.Topolev, D.Kuvshinov, D. Cheryasov, Ya.Kechin, Yu.Tselik, A. Sosnovskij
(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. Gress, N.M. Budnev, O.Ershova
(Irkutsk State University, API),

L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez
(INAOE, Guillermo Haro Astrophysics Observatory),

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

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

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 240811A ( K. L. Page et al., GCN 37136
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 ) errorbox  10112 sec after notice time and 10127 sec after trigger time at 2024-08-11 16:56:20 UT, with upper limit up to  16.7 mag. Observations started at twilight.  The observations began at zenith distance = 61 deg. The sun  altitude  is -10.5 deg. 

The galactic latitude b = 60 deg., longitude l = 44 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2558765

We obtain a following upper limits.  

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

   10218 |         MASTER-SAAO |   C |   180 | 16.7 |        
Filter C is a clear (unfiltred) band. 


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

Guowang Du, Yuan Fang, Xufeng Zhu, Yehao Cheng, Yuze Zhao, Brajesh Kumar, Weikang Lin, Yuanpei Yang, Xinzhong Er, Xiangkun Liu and Xiaowei Liu (SWIFAR, YNU) report on behalf of Mephisto team:

We performed simultaneous multi-band photometric observations of GRB 240811A (Page & Gupta, GCN 37136
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 ) using 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory. The observations were started ~37 min after the notice and multiple frames in uvgriz bands were acquired. The upper limits of our initial frames are the following:

UTC(2024-08-11)   Exptime(s)  filter   Mag(AB)
14:55               180          v     >21.07
14:55                60          r     >20.96
14:55               120          z     >19.98
15:08               180          u     >21.03
15:08                60          g     >20.97  
15:08               120          i     >20.86

----------------------------------------------------------------------------------
Mephisto (Multi-channel Photometric Survey Telescope) is a 1.6m wide-field multi-channel telescope, the first of its type in the world, capable of imaging the same field of view in three optical bands simultaneously. It provides real-time, high-quality colors of stellar objects. The on-site telescope assemblage and commissioning were carried out in September 2022. The first light in all three channels was achieved on 2023 December 21. ----------------------------------------------------------------------------------

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

Using 1050 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 240811A, 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 = 226.36231, +28.89615 which is equivalent
to:

RA (J2000): 15h 05m 26.96s
Dec (J2000): +28d 53' 46.1"

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.

R. Strausbaugh (Eastern Illinois University), A. Cucchiara (NASA) report on
behalf of a larger collaboration:

We observed the Swift GRB 240811A (Beardmore et al., GCN 37136
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 ) field with the LCOGT 1-meter Sinistro instrument at the Teide Observatory, Tenerife site, on August 11, from 20:43 to 21:13 UT (corresponding to 6.60 to 7.10 hours after the GRB trigger time) with the sdss r and i filters.

We performed a series of 3x300s exposures in each band.  We do not detect an uncataloged source within the XRT error region in either band.  This result is consistent with other optcal upper limits (Lipunov et al., GCN 37137
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 ; Du et al., GCN 37140
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 ).

The following 5-sigma upper limits are calculated using the PanSTARRS catalog as reference:

r > 22.6
i > 20.5

These magnitudes are not corrected for galactic extinction.

N. P. M. Kuin (UCL-MSSL) and K. L. Page (U. Leicester)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB
240811A 102 s after the BAT trigger (Page et al., GCN Circ. 37136
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 ).
No optical afterglow consistent with the XRT position (Beardmore
et al. GCN Circ. 37141
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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           102          252          147         >21.0
u_FC               315          564          246         >20.7
white              102         1191          353         >21.9
v                  644         5026          255         >18.7
b                  570         1167           58         >20.0
u                  315         1142          285         >20.7
w1                1098         5251           34         >19.2
m2                5031         5231          197         >18.8
w2                4622         4821          197         >19.6

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).

T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), M.G. Bernardini
(INAF-OAB), J. D. Gropp (PSU), S. Dichiara (PSU), J.A. Kennea (PSU),
J.P. Osborne (U. Leicester), K.L. Page (U. Leicester) and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 6.7 ks of XRT data for GRB 240811A, from 84 s to 39.6
ks after the  BAT trigger. The data comprise 41 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 light curve can be modelled with an initial power-law decay with an
index of alpha=4.2 (+/-0.3), followed by a break at T+240 s to an alpha
of 0.42 (+/-0.04).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.97 (+0.19, -0.17). The
best-fitting absorption column is  5.1 (+4.6, -3.1) x 10^20 cm^-2,
consistent with 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 3.4 x 10^-11 (3.8 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     5.1 (+4.6, -3.1) x 10^20 cm^-2
Galactic foreground: 2.0 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.97 (+0.19, -0.17)

If the light curve continues to decay with a power-law decay index of
0.42, the count rate at T+24 hours will be 0.036 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x
10^-12 (1.4 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/01248132.

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

U.Quadri, L.Strabla and P.Madurini (Bassano Bresciano Astronomical Observatory - Italy),

Members of:
AAVSO - American Association of Variable Star Observers.
UAI/SSV - Unione Astrofili Italiani/SSV-GRB section.
GAC - Gruppo Astrofili Cremonesi.

In a large collaboration with:
M.G. Dainotti (National Astronomical Observatory of Japan),
Y. Niino (Tokyo University, Institute of Astronomy),
K. Kalinowski (Aarhus University, Department of Physics and Astronomy),
B. De Simone (Universita' degli Studi Di Salerno)
report:

We imaged the field of GRB 240811A detected by SWIFT(trigger 1248132) with the robotic telescope of (IAU station 565) Bassano Bresciano Observatory, Italy.
The observations started 5.56 hour after the GRB trigger, At the end of twilight with our Newton telescope D=450 mm F/D=4.5.

Weather conditions were good.

We co-added 150 exposures of 20 sec each (total 50 min).

Start T0+      End T0+      Vlim
5.56 hour     6.47 hour      21

We did not found any optical counterpart in the error box of the XRTcandidate (K. L. Page & R. Gupta, GCN 37136
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 ; v. Lipunov et al., GCN 37137
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 ; Guowang Du et al., GCN 37140
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 ; A.P. Beardmore et al., GCN 37141
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 ; R. Strausbaugh and A. Cucchiara, GCN 37142
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 ; N. P. M. Kuin and K. L. Page, GCN 37147
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 ; T. Sbarrato et al., GCN 37149
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 ).

Magnitudes were estimated with the pan-STARRS cat and are derived using Lupton (2005) equations.

Not corrected for galactic dust extinction.

Reference:
http://www.osservatoriobassano.org/grb.asp

The message may be cited.

A. Y. Lien (U Tampa), S. D. Barthelmy (GSFC),

R. Gupta (GSFC), H. A. Krimm (NSF),

S. Laha (GSFC/UMBC), 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-60 to T+243 sec from the recent telemetry downlink,

we report further analysis of BAT GRB 240811A (trigger #1248132)

(Page, et al., GCN Circ. 37136
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 ).  The BAT ground-calculated position is

RA, Dec = 226.328, 28.884 deg which is

   RA(J2000)  =  15h 05m 18.8s

   Dec(J2000) = +28d 53' 02.5"

with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment).

The partial coding was 71%.



The mask-weighted light curve shows a complex structure with many peaks and

potential precursor emission ~30 seconds before the trigger time.

T90 (15-350 keV) is 66.32 +- 5.22 sec (estimated error including systematics).



The time-averaged spectrum from T-42.95 to T+41.32 sec is best fit by a simple

power-law model.  The power law index of the time-averaged spectrum is

1.71 +- 0.08.  The fluence in the 15-150 keV band is 3.0 +- 0.1 x 10^-06 erg/cm2.

The 1-sec peak photon flux measured from T+11.30 sec in the 15-150 keV band

is 2.9 +- 0.3 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/1248132

A. Ridnaia, D. Frederiks, A. Lysenko, D. Svinkin,
A. Tsvetkova,  M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:

The long-duration GRB 240811A
(Swift detection: Page & Gupta, GCN 37136
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 ; Lien et al., GCN 37155
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 )
was detected by Konus-Wind (KW) in the waiting mode.

A Bayesian block analysis of the KW waiting mode data in the 20-300 keV band
reveals a >18 sigma count rate increase in the interval
from T0-34.908 s to T0+35.748 s where T0 = T0(BAT) = 14:07:33.190 UT.

The KW light curve of this burst is available at
http://www.ioffe.ru/LEA/GRBs/GRB240811A/

Modeling a time-integrated spectrum of the burst
(from T0-34.908 s to T0+35.748 s)
by a power law with exponential cutoff (CPL) model
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep):
yields alpha = -1.22 (-0.27, + 0.80) and Ep = 126(-33,+51) keV.

The total burst fluence is 4.65(-0.30,+1.35)x10^-6 erg/cm^2,
and the 2.944 s peak energy flux, measured from T0+3.364 s,
is 2.43(-0.33,+0.76)x10^-7 erg/cm^2.
(both in the 20 keV - 10 MeV energy range).

All the quoted errors are estimated at the 68% confidence level.
All the presented results are preliminary.