GRB 230903A

The Fermi GBM team reports the detection of a likely LONG GRB

At 17:22:58 UT on 3 Sep 2023, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 230903A (trigger 715454583.81493 / 230903724).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 13.4, Dec = -13.5 (J2000 degrees, equivalent to J2000 00h 53m, -13d 30'), with a statistical uncertainty of 16.6 degrees.

The angle from the Fermi LAT boresight is 70.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2023/bn230903724/quicklook/glg_skymap_all_bn230903724.png

The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2023/bn230903724/quicklook/glg_healpix_all_bn230903724.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2023/bn230903724/quicklook/glg_lc_medres34_bn230903724.gif

A. D'Ai (INAF-IASFPA), E. Ambrosi (INAF-IASFPA), J.D. Gropp (PSU),
J. A. Kennea (PSU), A. Y. Lien (U Tampa), A. Melandri (INAF-OAR),
M. J. Moss (GWU), D. M. Palmer (LANL) and A. Tohuvavohu (U Toronto)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 17:22:56 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 230903A (trigger=1189514).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 9.926, -40.924 which is 
   RA(J2000) = 00h 39m 42s
   Dec(J2000) = -40d 55' 25"
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 ~1500 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 17:24:47.9 UT, 111.7 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 9.91169,
-40.94930 which is equivalent to:
   RA(J2000)  = 00h 39m 38.81s
   Dec(J2000) = -40d 56' 57.5"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 99 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 (2.68 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 2.6
(+2.17/-1.93) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
117 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. 

Burst Advocate for this burst is A. D'Ai (antonino.dai 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/)

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

We observed the Swift GRB 230903A (D'ai et al., GCN 34610
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 ) field with the LCOGT 1-meter Sinistro instrument at the South African Astronomical Observatory site, on September 3,  from 20:08 to 20:41 UT (corresponding to 2.77 to 3.32 hours from the GRB trigger time) with the Bessel R and I filters.

We performed a series of 3x300s exposures in each band.  We do not detect a source within the Swift-XRT error region (D'Ai et al., GCN 34610
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 ) in either band.

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

R > 20.5
I > 19.0

These magnitudes are not corrected for galactic extinction.

O.J. Roberts (USRA), B. Mailyan (Florida Tech) and C. Meegan (UAH)
report on behalf of the Fermi GBM Team:

"At 17:22:58.81 UT on the 3rd September 2023 , the Fermi Gamma-Ray Burst Monitor (GBM)
triggered and located GRB 230903A (trigger 715454583 / 230903724) which was also detected 
by the Swift/BAT (A. D'ai et al. 2023, GCN 34610
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 ). The Fermi GBM Final Real-time Localization 
(GCN 34608
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 ) is consistent with the Swift position.

The angle from the Fermi LAT boresight at the GBM trigger time is 98 degrees.

The GBM light curve consists of one pulse
with a duration (T90) of about 3 s (50-300 keV).
The time-averaged spectrum from T0-4.0 s to T0+1.0 s is
best fit by a simple power law function with index -1.78 +/- 0.09.

A power law function with an exponential high-energy cutoff fits
the spectrum equally well. The power law index is -1.27 +/- 0.33 
and the cutoff energy, parameterized as Epeak, is 95 +/- 31 keV.

The event fluence (10-1000 keV) in this time interval is
(5.87 +/- 0.85)E-07 erg/cm^2. The 1-sec peak photon flux measured
starting from T0-1s in the 10-1000 keV band
is 2.0 +/- 0.2 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html

For Fermi GBM data and info, please visit the official Fermi GBM Support Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"

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) started inspect of the Fermi GRB 230903A ( Fermi GBM team, GCN 34608
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 ) errorbox  49439 sec after notice time and 49475 sec after trigger time at 2023-09-04 07:07:34 UT, with upper limit up to  18.3 mag. The observations began at zenith distance = 24 deg. The sun  altitude  is -47.1 deg. 

MASTER-Amur robotic telescope  located in Russia (Blagoveshchensk State Pedagogical University) started inspect of the Fermi GRB 230903A errorbox  83217 sec after notice time and 83253 sec after trigger time at 2023-09-04 16:30:32 UT, with upper limit up to  16.5 mag. The observations began at zenith distance = 50 deg. The sun  altitude  is -31.3 deg. 

The galactic latitude b = -76 deg., longitude l = 128 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2264968

We obtain a following upper limits.  

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

   49505 | 2023-09-04 07:07:34 |         MASTER-OAFA | (23h 45m 07.73s , -21d 40m 56.2s) |   C |    60 | 18.3 |        
   50337 | 2023-09-04 07:21:25 |         MASTER-OAFA | (23h 45m 12.23s , -21d 40m 04.4s) |   C |    60 | 18.3 |        
   83283 | 2023-09-04 16:30:32 |         MASTER-Amur | (00h 45m 02.38s , +01d 03m 22.9s) |   C |    60 | 16.4 |        
   83363 | 2023-09-04 16:31:51 |         MASTER-Amur | (00h 52m 59.10s , +01d 05m 05.8s) |   C |    60 | 16.5 |        
   83442 | 2023-09-04 16:33:11 |         MASTER-Amur | (00h 44m 55.77s , +02d 58m 32.3s) |   C |    60 | 16.5 |        
   83522 | 2023-09-04 16:34:30 |         MASTER-Amur | (00h 52m 50.71s , +02d 58m 14.6s) |   C |    60 | 16.2 |        
   83941 | 2023-09-04 16:41:29 |         MASTER-Amur | (00h 44m 58.62s , +01d 04m 26.3s) |   C |    60 | 16.2 |        
   84021 | 2023-09-04 16:42:49 |         MASTER-Amur | (00h 52m 58.54s , +01d 03m 22.6s) |   C |    60 | 16.2 |        
   84100 | 2023-09-04 16:44:08 |         MASTER-Amur | (00h 44m 54.16s , +02d 58m 06.4s) |   C |    60 | 16.2 |        
   84180 | 2023-09-04 16:45:28 |         MASTER-Amur | (00h 52m 50.10s , +02d 57m 42.0s) |   C |    60 | 15.9 |        
Filter C is a clear (unfiltred) band. 


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

M. J. Moss (GSFC),
S. D. Barthelmy (GSFC), 
A. D'Ai (INAF-IASFPA),
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),
T. Sakamoto (AGU),
M. Stamatikos (OSU)
(i.e. the Swift-BAT team):
 
Using the data set from T-239 to T+963 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 230903A (trigger #1189514)
(D'Ai, et al., GCN Circ. 34610
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 ).  The BAT ground-calculated position is
RA, Dec = 9.898, -40.910 deg which is 
   RA(J2000)  =  00h 39m 35.5s 
   Dec(J2000) = -40d 54' 34.5" 
with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 40%.
 
The mask weighted light curve shows a single short pulse. T90 (15-350 keV) is 2.54 +- 0.27 sec (estimated error including systematics). 
The T90 and the spectral hardness of this burst places it on the border of the long-soft and hard-short burst classifications. 
 
The time-averaged spectrum from T+0.20 to T+2.93 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index -0.66 +- 1.49, 
and Epeak of 63.4 +- 17.8 keV (chi squared 44.41 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 2.2 +- 0.4 x 10^-07 erg/cm2
and the 1-sec peak flux measured from T+0.50 sec in the 15-150 keV band is
1.3 +- 0.3 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.40 +- 0.21 (chi squared 54.66 for 57 d.o.f.).  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/1189514/BA/

D. Pieterse (Radboud University), D. B. Malesani (Radboud and DAWN/NBI), E. Le Floc’h (CEA-Saclay), A. Saccardi (GEPI, Obs. de Paris) and N. R. Tanvir (Univ. of Leicester) report on behalf of the Stargate collaboration :

We observed the field of GRB230903A (Fermi GBM team, GCN 34608
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  ; D’Ai et al., GCN 34610
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 ) using the ESO VLT UT3 (Melipal) equipped with the X-shooter instrument. Three 60s exposures were obtained in the r-band on 2023 September 4.097 UT (T-To ~8.95h). The observations were performed under poor weather conditions, with a seeing of 3.2 arcsec measured in the data. We do not detect any source coinciding with the location of the X-ray afterglow detected by Swift/XRT (D’Ai et al., GCN 34610
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 ). We obtain the following upper limit, calibrated using a faint, nearby object from the Legacy Survey (Dey et al. 2019, ApJ, 257, 29).

r > 22.7 mag (AB)

This estimate is not corrected for Galactic extinction and is consistent with the non-detection reported earlier by LCOGT (Strausbaugh et al., GCN 34611
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 ).

We acknowledge excellent support from the ESO observing staff in Paranal, in particular Claudia Paladini and Jonathan Smoker.

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

Using 832 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 230903A, 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 = 9.91061, -40.95001 which is equivalent
to:

RA (J2000): 00h 39m 38.55s
Dec (J2000): -40d 57' 00.0"

with an uncertainty of 2.2 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.

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

We have analysed 2.3 ks of XRT data for GRB 230903A, from 97 s to 29.3
ks after the  BAT trigger. The data comprise 8 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=1.17 (+/-0.09).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.00 (+0.24, -0.22). The
best-fitting absorption column is  8.2 (+5.9, -5.0) x 10^20 cm^-2, in
excess of the Galactic value of 2.7 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 3.3 x 10^-11 (3.9 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     8.2 (+5.9, -5.0) x 10^20 cm^-2
Galactic foreground: 2.7 x 10^20 cm^-2
Excess significance: 1.8 sigma
Photon index:	     2.00 (+0.24, -0.22)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01189514.

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

R. Jayaraman (MIT), M.M. Fausnaugh (TTU/MIT), R. Vanderspek (MIT), and G.R. Ricker (MIT) report:

The Transiting Exoplanet Survey Satellite (TESS; Ricker et al. JATIS 2015) observed the location of the gamma-ray burst GRB230903A (GCN 34608
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 , Fermi GBM Team et al.; GCN 34610
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 , D’Ai et al.) at a 200 second cadence continuously from 2.8 days before the trigger to 3.75 days after the trigger. The GRB occurred during TESS observational Sector 69, in the field of view of Camera 2, CCD 3.

We extracted a light curve from the calibrated TESS Full-Frame Images (FFIs) produced by TICA (Fausnaugh et al. RNAAS 2020), using difference imaging and forced photometry at the enhanced location provided by Swift-XRT in GCN 34625
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  (Beardmore et al.). Our analysis procedure is described in Fausnaugh et al. 2023 (arXiv:2307.11815
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 ).

TESS observations show a flux excess of 5.1-sigma above the background in the 200s FFI exposure that encompasses the Fermi trigger time, corresponding to an apparent magnitude of 17.20 ± 0.05 in the TESS band (600 nm–1000 nm). Further analysis is ongoing.

The TICA data are publicly available on the MAST archive at https://archive.stsci.edu/hlsp/tica.