GRB 230911C

E. Sonbas (Adiyaman Univ.), R. Brivio (INAF-OAB), S. Dichiara (PSU),
M. Ferro (INAF-OAB), J.D. Gropp (PSU), A. Y. Lien (U Tampa),
A. Melandri (INAF-OAR), K. L. Page (U Leicester), D. M. Palmer (LANL),
T. Sakamoto (AGU), C. Salvaggio (INAF-OAB) and T. Sbarrato (INAF-OAB)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 08:47:25 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 230911C (trigger=1191188).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 226.782, -41.334 which is 
   RA(J2000) = 15h 07m 08s
   Dec(J2000) = -41d 20' 01"
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 10 sec.  The peak count rate
was ~700 counts/sec (15-350 keV), at ~3 sec after the trigger. 

The XRT began observing the field at 08:49:24.4 UT, 119.0 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 226.79804, -41.32973 which is
equivalent to:
   RA(J2000)  = 15h 07m 11.53s
   Dec(J2000) = -41d 19' 47.0"
with an uncertainty of 4.2 arcseconds (radius, 90% containment). This
location is 46 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.  We
cannot determine whether the source is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (9.36 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 4
(+3.90/-3.22) x 10^21 cm^-2 (90% confidence). 

Processed UVOT data were not available at low latency, with the images
affected by the attitude issues reported in GCN Circ. 34633
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 . 

Burst Advocate for this burst is E. Sonbas (edasonbas AT yahoo.com). 
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/)

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

Using 1217 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 230911C, 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 = 226.79850, -41.33017 which is equivalent
to:

RA (J2000): 15h 07m 11.64s
Dec (J2000): -41d 19' 48.6"

with an uncertainty of 2.1 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.

D. O’Neill; A. Kumar; B. P. Gompertz; K. Ulaczyk; G. Ramsay; R. Starling; K. Ackley; M. J. Dyer; J. Lyman; F. Jiminez-Ibarra; D. Steeghs; D. K. Galloway; V. Dhillon; P. O'Brien; K. Noysena; R. Kotak; R. P. Breton; L. K. Nuttall; E. Pall'e and D. Pollacco report on behalf of the GOTO collaboration:

We report on observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022) in response to GRB 230911C (Sonbas et al., GCN 34655
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 ). Three targeted observations were performed by GOTO-South, located at Siding Spring Observatory, between 08:52:42 UT and 09:14:52 UT on 2023-09-11 (5.28, 16.08 and 27.47 minutes after trigger). Each observation consisted of 4x90s exposures in the GOTO L-band (400-700 nm). Images were processed immediately after acquisition using the GOTO pipeline. We identify no candidate optical counterparts within the enhanced XRT localisation region (Evans et al., GCN 34658
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 ). The 5-sigma limiting magnitudes were 20.7, 20.4 and 21.0 mag, respectively.

GOTO (https://goto-observatory.org) is a network of telescopes operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester and the Instituto de Astrofisica de Canarias (IAC). We note that this is the first scientific contribution from GOTO-South.

A. A. Breeveld (UCL-MSSL) and E. Sonbas (Adiyaman Univ.) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 230911C 122 s after the BAT trigger (Sonbas et al., GCN Circ. 34655
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 ).
No optical afterglow consistent with the XRT position (Evans et al., GCN Circ. 34658
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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           122          272          147         >20.4
white              122         4926          289         >21.0
u_FC               280          530          246         >19.7
u                  280         4572          708         >20.2
b                  537         4778          235         >19.8
w1                 661         4368          216         >19.0
w2                 587          781           39         >18.2

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.101 in the direction of the burst (Schlegel et al. 1998).

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

We observed the Swift GRB 230911C (Sonbas et al., GCN 34655
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 ) field with the LCOGT 1-meter Sinistro instrument at the South African Astronomical Observatory site, on September 11, from 17:20 to 17:52 UT (corresponding to 8.55 to 9.08 hours from 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 a source within the Swift-XRT enhanced error region (Evans et al., GCN 34658
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 ) in either band.

The following upper limits are calculated using the USNO-B1.0 catalog as reference:

r > 21.6
i > 21.1

These magnitudes are not corrected for galactic extinction.

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-OAFA robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Argentina (OAFA observatory of San Juan National University) was pointed to the Swift GRB 230911C ( E. Sonbas et al., GCN 34655
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 ) errorbox  51970 sec after notice time and 52039 sec after trigger time at 2023-09-11 23:14:45 UT, with upper limit up to  17.3 mag. Observations started at twilight.  The observations began at zenith distance = 37 deg. The sun  altitude  is -11.6 deg. 

The galactic latitude b = 14 deg., longitude l = 329 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2268945

We obtain a following upper limits.  

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

   52130 |         MASTER-OAFA |   C |   180 | 16.3 |        
   52362 |         MASTER-OAFA |   C |   180 | 16.5 |        
   52795 |         MASTER-OAFA |   C |   180 | 17.3 |        
Filter C is a clear (unfiltred) band. 


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

E. Fernandez-Garcia, Y.-D. Hu, I. Perez-Garcia, M.D. Caballero-Garcia, R. Sanchez-Ramirez, S. Guziy, S.-Y. Wu and A. J. Castro-Tirado (IAA-CSIC, Granada), P. J. Meintjes and H. J. van Heerden (UFS, South Africa), A. Martin-Carrillo and L. Hanlon (UCD, Ireland) and C. J. Perez del Pulgar (UMA, Malaga), on behalf of a larger collaboration, report:

Following the detection of GRB 230911C by Swift (Sonbas et al. GCNC 34655), the BOOTES-6/DPRT 0.6m robotic telescope at Boyden Observatory in Maselspoort (South Africa) automatically observed the GRB location starting on Sep. 11, 17:26 UT (~ 8.7 h after trigger). No new optical source is detected on the co-added images (3 x 60 s, i-filter & 3 x 60 s, Z-filter) within the enhanced Swift/XRT error region (Evans et al. GCNC 34658) down to 19.2 mag and 18.5 mag respectively, which are consistent with reports from GOTO (O'Neill et al. GCNC 34662), UVOT (Breeveld et al. GCNC 34665), LCOGT (Strausbaugh et al. GCNC 34667) and MASTER (Lipunov et al. GCNC 34671).

We thank the staff at Boyden Observatory for their excellent support.

T. Sakamoto (AGU), S. D. Barthelmy (GSFC),
H. A. Krimm (NSF), S. Laha (GSFC/UMBC),
A. Y. Lien (U Tampa), C. B. Markwardt (GSFC),
D. M. Palmer (LANL), T. Parsotan (GSFC/UMBC),
E. Sonbas (Adiyaman Univ.), M. Stamatikos (OSU)
(i.e. the Swift-BAT team):

Using the data set from T-61 to T+243 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 230911C (trigger #1191188)
(Sonbas et al., GCN Circ. 34655
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 ).  The BAT ground-calculated position is
RA, Dec = 226.796, -41.356 deg which is
   RA(J2000)  =  15h 07m 11.0s
   Dec(J2000) = -41d 21' 21.2"
with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 95%.

The mask-weighted light curve shows multiple weak overlapping pulses that
start at ~T-9 s and end at ~T+10 s. T90 (15-350 keV) is 16.21 +- 2.89 sec
(estimated error including systematics).

The time-averaged spectrum from T-8.63 to T+9.58 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.26 +- 0.28.  The fluence in the 15-150 keV band is 3.8 +- 0.6 x 10^-7
erg/cm2.
The 1-sec peak photon flux measured from T+2.07 sec in the 15-150 keV band
is 0.7 +- 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/1191188/BA/

A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB),
J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), D.N. Burrows (PSU), J.P.
Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U.
Leicester) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 7.2 ks of XRT data for GRB 230911C, from 103 s to 74.1
ks after the  BAT trigger. The data comprise 106 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+4.2 ks) can be modelled with a
power-law decay with a decay index of alpha=0.48 (+0.22, -0.18).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.86 (+0.26, -0.21). The
best-fitting absorption column is  1.3 (+0.9, -0.3) x 10^21 cm^-2,
consistent with the Galactic value of 9.4 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 3.6 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 (+0.9, -0.3) x 10^21 cm^-2
Galactic foreground: 9.4 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.86 (+0.26, -0.21)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01191188.

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

S. Chastain (UNM),  G. E. Anderson (Curtin), J. K. Leung (UofT/HUJI), S. D. Ryder (Macquarie), A. J. van der Horst (GWU), A. Gulati (USyd), and L. Rhodes (Oxford) on behalf of the ATCA PanRadio GRB collaboration

The Australia Telescope Compact Array (ATCA) observed long GRB 230911C (E. Sonbas et al., GCN 34655
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 ) as part of the ATCA "PanRadio GRB" Large Project C3542 (PI: G. Anderson) on 2023-09-11 UT, 2023-09-13 UT, 2023-09-26 UT, and 2023-10-16 UT.

We detect a possible radio afterglow at 9 GHz in the second epoch, which continues to brighten in the third, and fourth epoch. At 5.5 GHz this source is not detected in any of the observations.
In the fourth epoch at 5.5 GHz, we find 3-sigma upper limit of 105 microJy/beam. In the fourth epoch at 9.0 GHz, the source detected in the previous observations is detected at 150 +/- 35 microJy/beam.
The fitted position is:
RAJ2000  = 15:07:13 +/- 8 arcseconds
DECJ2000 = -41:19:51 +/- 1 arcsecond
This is approximately 18 arcseconds away from the reported Swift/XRT enhanced position (P. A. Evans et al., GCN 34658
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 ).

 

Further observations are planned.

We thank the CSIRO Space and Astronomy staff for supporting these observations.
We acknowledge the Gomeroi people as the traditional owners of the Observatory site. The Australia Telescope Compact Array is part of the Australia Telescope National Facility (https://ror.org/05qajvd42) which is funded by the Australian Government for operation as a National Facility managed by CSIRO.