GRB 240523A

M. H. Siegel (PSU), J.D. Gropp (PSU), R. Gupta (NASA/GSFC),
J. A. Kennea (PSU), N. J. Klingler (GSFC/UMBC/CRESSTII),
H. A. Krimm (NSF), K. L. Page (U Leicester), D. M. Palmer (LANL) and
M. A. Williams (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 14:43:11 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 240523A (trigger=1230423).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 33.567, -22.020 which is 
   RA(J2000) = 02h 14m 16s
   Dec(J2000) = -22d 01' 12"
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 40 sec.  The peak count rate
was ~800 counts/sec (15-350 keV), at ~-1 sec after the trigger. 

The XRT began observing the field at 14:45:39.6 UT, 147.7 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 33.56813,
-22.03320 which is equivalent to:
   RA(J2000)  = 02h 14m 16.35s
   Dec(J2000) = -22d 01' 59.5"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 47 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 150 seconds with the White filter
starting 151 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 25% of
the BAT 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
BAT 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
0.015. 

Burst Advocate for this burst is M. H. Siegel (siegel AT swift.psu.edu). 
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 3015 s of XRT Photon Counting mode data and 7 UVOT
images for GRB 240523A, 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 = 33.56798, -22.03336 which is equivalent
to:

RA (J2000): 02h 14m 16.31s
Dec (J2000): -22d 02' 00.1"

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.

R. Brivio (INAF-OAB), D.N. Burrows (PSU), J. D. Gropp (PSU), S.
Dichiara (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester),
A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAR) and P.A. Evans report on
behalf of the Swift-XRT team:

We have analysed 6.5 ks of XRT data for GRB 240523A, from 132 s to 27.6
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 series of power-law decays. The
initial decay index is alpha=2.8 (+0.6, -0.4). At T+400 s  the decay
flattens to an alpha of -0.4 (+0.3, -0.5) before breaking again at
T+2137 s to a final decay with index alpha=1.41 (+0.26, -0.24).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.06 (+0.24, -0.22). The
best-fitting absorption column is  1.1 (+0.6, -0.5) x 10^21 cm^-2, in
excess of the Galactic value of 1.6 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 (4.2 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.1 (+0.6, -0.5) x 10^21 cm^-2
Galactic foreground: 1.6 x 10^20 cm^-2
Excess significance: 2.9 sigma
Photon index:	     2.06 (+0.24, -0.22)

If the light curve continues to decay with a power-law decay index of
1.41, the count rate at T+24 hours will be 2.0 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.5 x
10^-14 (8.2 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01230423.

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

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), M. H. Siegel (PSU)
(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 240523A (trigger #1230423)
(Siegel et al., GCN Circ. 36536
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 ).  The BAT ground-calculated position is
RA, Dec = 33.537, -22.053 deg which is
   RA(J2000)  =  02h 14m 08.9s
   Dec(J2000) = -22d 03' 10.6"
with an uncertainty of 1.6 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 47%.

The mask-weighted light curve shows a multi-peaked structure that
starts at ~T-6 s and ends at ~T+46 s. The main peak occurs at ~T+1 s.
T90 (15-350 keV) is 41.93 +- 8.66 sec (estimated error including
systematics).

The time-averaged spectrum from T-6.37 to T+46.30 sec is best fit by a
simple
power-law model.  The power law index of the time-averaged spectrum is
2.07 +- 0.20.  The fluence in the 15-150 keV band is 1.1 +- 0.1 x 10^-6
erg/cm2.
The 1-sec peak photon flux measured from T+0.18 sec in the 15-150 keV band
is 1.0 +- 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/1230423