GRB 240421B

M. A. Williams (PSU), J. J. DeLaunay (PSU), S. Dichiara (PSU),
J.D. Gropp (PSU) and T. Sakamoto (AGU) report on behalf of the Neil
Gehrels Swift Observatory Team:

At 09:42:54 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 240421B (trigger=1223470).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 299.686, -14.879 which is 
   RA(J2000) = 19h 58m 45s
   Dec(J2000) = -14d 52' 42"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 20 sec.  The peak count rate
was ~9000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 09:45:00.4 UT, 126.3 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 299.70011, -14.86012
which is equivalent to:
   RA(J2000)  = 19h 58m 48.03s
   Dec(J2000) = -14d 51' 36.4"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 83 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 (1.24 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 3.8
(+2.62/-2.25) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 129 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
0.183. 

Burst Advocate for this burst is M. A. Williams (mjw6837 AT 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/)

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

Using 1017 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 240421B, 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 = 299.70085, -14.85962 which is equivalent
to:

RA (J2000): 19h 58m 48.20s
Dec (J2000): -14d 51' 34.6"

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.

N. P. M. Kuin (UCL-MSSL) and M. Williams (PSU)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 240421B
130s after the BAT trigger (Williams et al., GCN Circ. 36208
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 ).  No
optical afterglow consistent with the XRT position Beardmore et al.
(GCN Circ. 36213
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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           130          279          148         >20.6
u_FC               343          592          246         >20.2
white              130         1708          218         >20.9
v                  672         1764           78         >18.4
b                  598         1854           84         >19.8
u                  343         1827          339         >20.2
w1                 721         1813           78         >19.4
w2                 648         1568           78         >18.8

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.182 in the direction of the burst
(Schlegel et al. 1998).

Viraj Karambelkar (Caltech), Geoffrey Mo (MIT), Robert Stein (Caltech),
Danielle Frostig (MIT), Tomas Ahumada (Caltech), Nathan Lourie (MIT),
Robert Simcoe (MIT), Mansi Kasliwal (Caltech)

We observed the field of the long GRB 240421B (GCN 36208
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 , 36213
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 , 36214
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 ) in
the near-IR J-band with the WINTER camera on the Palomar 40-inch telescope.
Our observations started at UTC 2024-04-19T11:16:00 (~1.5 hours since
trigger) and lasted for an hour.

The images were processed through the WINTER data reduction pipeline
implemented using mirar (https://zenodo.org/records/10888437), with image
subtraction performed relative to reference images built from the VISTA
Hemisphere Survey (McMahon et al. 2013).



We do not detect any source at the location of the X-ray counterpart
(reported in GCN 36213
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 ) to a limit of m_J = 19.5 mag (AB, not corrected for
Galactic extinction). No other new sources are found within the 3 arcmin
localization of the GRB.

WINTER (Wide-field INfrared Transient ExploreR) is a partnership between
MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF
AAG, the David and Lucille Packard Foundation, and the MIT Kavli Institute
for Astrophysics and Space Research

E. Fernandez-Garcia, I. Perez-Garcia, M.D. Caballero-Garcia, R. Sanchez-Ramirez, S. Guziy, S.-Y. Wu and A. J. Castro-Tirado (IAA-CSIC, Granada), Y.-D. Hu (INAF-OAB), D. Hiriart and W. H. Lee (UNAM), C. J. Perez del Pulgar (UMA), I. M. Carrasco-Garcia (SMA) and I. H. Park (SKKU), on behalf of a larger collaboration, report:

Following the detection of GRB 240421B by Swift (Williams et al. GCNC 36208), the BOOTES-5/JGT 0.6m robotic telescope at Observatorio Astronomico Nacional in San Pedro Martir (Mexico) automatically observed the GRB location starting on April. 21, 09:43:44 UT (~ 50 s after trigger). No new optical source is detected on the co-added images (20 x 1 s, clear-filter) within the enhanced Swift/XRT error box (Beardmore et al. GCNC 36213) down to 18.4 mag.  which is consistent with both reports from UVOT (Kuin et al., GCNC 36214) and WINTER (Karambelkar et al. GCNC 36215).

We thank the staff at Observatorio Astronomico Nacional in San Pedro Martir for their excellent support.

J.P. Osborne (U. Leicester), B. Sbarufatti (INAF-OAB), C. Salvaggio
(INAF-OAB), M. Ferro (INAF-OAB), S. Dichiara (PSU), J.A. Kennea (PSU),
A. Tohuvavohu (U. Toronto), 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 6.6 ks of XRT data for GRB 240421B, from 110 s to 36.0
ks after the  BAT trigger. The data comprise 42 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=0.50 (+/-0.07), followed by a break at T+2500 s to an
alpha of 1.20 (+/-0.10).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.87 (+/-0.13). The
best-fitting absorption column is  6.5 (+1.0, -0.9) x 10^21 cm^-2, in
excess of the Galactic value of 1.2 x 10^21 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 4.6 x 10^-11 (7.3 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.5 (+1.0, -0.9) x 10^21 cm^-2
Galactic foreground: 1.2 x 10^21 cm^-2
Excess significance: 9.6 sigma
Photon index:	     1.87 (+/-0.13)

If the light curve continues to decay with a power-law decay index of
1.20, the count rate at T+24 hours will be 0.023 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-12 (1.7 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/01223470.

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

C. Fletcher (USRA) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 09:42:41.91 UT on 21 April 2024, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 240421B (trigger 735385366/240421405).
which was also detected by Swift BAT (M. A. Williams et al. 2024, GCN 36208
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 ).
The Fermi GBM on-ground location is consistent with the Swift BAT position.

The angle from the Fermi LAT boresight is 29 degrees.

The GBM light curve consists of multiple peaks with a duration (T90)
of about 12.3 s (50-300 keV). The time-averaged spectrum
from T0+0.003 to T0+18.4 s is best fit by
a Band function with Epeak = 141 +/- 8 keV,
alpha = -0.91 +/- 0.04, and beta = -2.31 +/- 0.09.

The event fluence (10-1000 keV) in this time interval is
(1.28 +/- 0.02)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+12 s in the 10-1000 keV band is 31.9 +/- 0.5 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-Kislovodsk robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the Swift GRB 240421B ( M. A. Williams et al., GCN 36208
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 ) errorbox  52346 sec after notice time and 52368 sec after trigger time at 2024-04-22 00:15:42 UT, with upper limit up to  16.5 mag. The observations began at zenith distance = 70 deg. The sun  altitude  is -20.3 deg. 

The galactic latitude b = -22 deg., longitude l = 27 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2430219

We obtain a following upper limits.  

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

   52459 |   MASTER-Kislovodsk |   C |   180 | 15.8 |        
   52459 |   MASTER-Kislovodsk |   C |   180 | 16.5 |        
Filter C is a clear (unfiltred) band. 


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

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-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) started inspect of the Fermi GRB 240421B ( C. Fletcher (USRA) and C. Meegan (UAH) report on behalf of et al., GCN 36218
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 ) errorbox  19358 sec after notice time and 56607 sec after trigger time at 2024-04-22 01:26:08 UT, with upper limit up to  17.1 mag. Observations started at twilight.  The observations began at zenith distance = 67 deg. The sun  altitude  is -14.1 deg. 

The galactic latitude b = -22 deg., longitude l = 22 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2430649

We obtain a following upper limits.  

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

   56637 | 2024-04-22 01:26:08 |      MASTER-Tavrida | (19h 37m 21.45s , -18d 16m 09.9s) |   C |    60 | 16.4 |        
   56785 | 2024-04-22 01:28:36 |      MASTER-Tavrida | (19h 53m 26.05s , -18d 16m 57.8s) |   C |    60 | 14.1 |        
   56858 | 2024-04-22 01:29:50 |      MASTER-Tavrida | (19h 49m 21.85s , -22d 05m 18.0s) |   C |    60 | 14.2 |        
   57563 | 2024-04-22 01:41:35 |      MASTER-Tavrida | (19h 50m 44.40s , -20d 11m 20.7s) |   C |    60 | 15.8 |        
   57636 | 2024-04-22 01:42:47 |      MASTER-Tavrida | (19h 59m 17.11s , -20d 10m 02.0s) |   C |    60 | 16.6 |        
   57709 | 2024-04-22 01:44:00 |      MASTER-Tavrida | (19h 37m 21.08s , -18d 16m 19.0s) |   C |    60 | 17.1 |        
   57782 | 2024-04-22 01:45:13 |      MASTER-Tavrida | (19h 45m 45.17s , -18d 15m 26.6s) |   C |    60 | 17.0 |        
   57926 | 2024-04-22 01:47:38 |      MASTER-Tavrida | (19h 43m 02.18s , -20d 11m 33.0s) |   C |    60 | 16.8 |        
   57999 | 2024-04-22 01:48:50 |      MASTER-Tavrida | (19h 53m 25.96s , -18d 17m 25.9s) |   C |    60 | 16.7 |        
   58144 | 2024-04-22 01:51:16 |      MASTER-Tavrida | (19h 49m 20.84s , -22d 05m 40.1s) |   C |    60 | 14.5 |        
Filter C is a clear (unfiltred) band. 


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

B. Gendre, E. Moore, F. Panther, D. Coward, J. A. Moore (OzGrav-UWA),
and A. Klotz (IRAP-CNRS-OMP),
report:

We imaged the field of GRB 240421B detected by SWIFT
(trigger 1223470) with the Zadko robotic telescope (D=100cm)
located at the observatory - Gingin, Australia.

The observations started 7.97h after the GRB trigger.
The elevation of the field increased from
25 degrees above horizon and weather conditions
were good.

The date of trigger : t0 = 2024-04-21T09:42:54.050

The first image is 60.0s exposure in tracking mode:
t0+28754.8s to t0+28814.8s : R > 16.2

We co-added a series of exposures:
t0+28754.8s to t0+29599.3s : R > 17.7

Magnitudes were estimated with the nearby USNO-B1 stars
and are not corrected for galactic dust extinction.

N.B. Galactic coordinates are lon= 26.7039 lat=-21.4736
and the galactic extinction in R band is 0.7 magnitudes
estimated from D. Schlegel et al. 1998ApJ...500..525S.

This message may be cited.

G. Waratkar (IITB),  J. Joshi (IUCAA), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:

Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a long-duration GRB 240421B which was also detected by Swift-BAT (Williams et al., GCN Circ. 36208
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 ) and Fermi-GBM (Fletcher et al., GCN Circ. 36218
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 ).

The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2024-04-21 09:42:54.5 UTC. The measured peak count rate associated with the burst is 368 (+44, -52) counts/s above the background in the combined data of all quadrants, with a total of 843 (+148, -223) counts. The local mean background count rate was 474 (+5, -4) counts/s. Using cumulative rates, we measure a T90 of 6.3 (+3.2, -2.7) s.

It was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2024-04-21 09:42:53.8 UTC. The measured peak count rate is 745 (+82, -87) counts/s above the background in the combined Veto data of all quadrants, with a total of 1802 (+268, -278) counts. The local mean background count rate was 1740 (+7, -8) counts/s. We measure a T90 of 5.6 (+1.6, -1.7) s from the cumulative Veto light curve.

CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project.

CZTI GRB detections are reported regularly on the payload site at:
http://astrosat.iucaa.in/czti/?q=grb

G. E. Anderson (Curtin University), L. Rhodes (U. Oxford), J. K. Leung (U. Toronto/HUJI), S. Chastain (UNM), A. Gulati (U. Sydney), A. J. van der Horst (GWU), on behalf of the PanRadio GRB collaboration

The Australia Telescope Compact Array (ATCA) performed rapid-response observations of GRB 240421B (Williams et al., GCN 36208
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 ) at 5.5, 9, 16.7 and 21.2 GHz as part of the Long-term ATCA "PanRadio GRB" follow-up programme C3542 (PI. Anderson). ATCA observed from 2024-04-21 17:00 UT (7.3 hours post-burst) to 2024-04-22 01:40 UT. In our preliminary analysis, we find no radio source coincident with the enhanced XRT position (Beardmore et al., GCN 36213
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 ) with 3 sigma upper limits of 57, 54, 60, 90 microJy/beam at 5.5, 9.0, 16.7 and 21.2 GHz, respectively.

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.

M. Dafcikova, J. Ripa, M. Kolar (Masaryk U.), A. Pal (Konkoly Observatory), N. Werner (Masaryk U.), M. Ohno, H. Takahashi (Hiroshima U.), L. Meszaros, B. Csak (Konkoly Observatory), N. Husarikova, F. Munz , M. Topinka, M. Duriskova, L. Szakszonova, J.-P. Breuer, F. Hroch (Masaryk U.), T. Urbanec, M. Kasal,  A. Povalac (Brno U. of Technology), J. Hudec, J. Kapus, M. Frajt (Spacemanic s.r.o), R. Laszlo, M. Koleda (Needronix s.r.o), M. Smelko, P. Hanak, P. Lipovsky (Technical U. of Kosice), G. Galgoczi (Wigner Research Center/Eotvos U.), Y. Uchida, H. Poon, H. Matake (Hiroshima U.), N. Uchida (ISAS/JAXA), T. Bozoki (Eotvos U.), G. Dalya (Eotvos U.), T. Enoto (Kyoto U.), Zs. Frei (Eotvos U.), G. Friss (Eotvos U.), Y. Fukazawa, K. Hirose (Hiroshima U.), S. Hisadomi (Nagoya U.), Y. Ichinohe (Rikkyo U.), K. Kapas (Eotvos U.), L. L. Kiss (Konkoly Observatory),  T. Mizuno (Hiroshima U.), K. Nakazawa (Nagoya U.), H. Odaka (Univ of Tokyo), J. Takatsy (Eotvos U.), K. Torigoe (Hiroshima U.), N. Kogiso, M. Yoneyama (Osaka Metropolitan U.), M. Moritaki (U. Tokyo), T. Kano (U. Michigan) -- the GRBAlpha collaboration.

The long-duration GRB 240421B (Swift/BAT detection: GCN 36208
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 ; Fermi/GBM detection: GCN 36218
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 ; AstroSat detection: GCN 36223
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 ; Konus/Wind detection trigger at 2024-04-21 09:42:53.511 UT; INTEGRAL/SPI-ACS peak detection at 2024-04-21 ~09:42:54 UT) was observed by the GRBAlpha 1U CubeSat (Pal et al. 2023, A&A, 677, 40; https://ui.adsabs.harvard.edu/abs/2023A%26A...677A..40P/abstract).

The detection was confirmed at the peak time 2024-04-21 09:42:53.5 UTC. The T90 duration measured by GRBAlpha is 5.0 s and the overall significance during T90 reaches 11 sigma.

The light curve obtained by GRBAlpha is available here: https://grbalpha.konkoly.hu/static/share/GRB240421B_GCN.pdf

All GRBAlpha detections are listed at: https://monoceros.physics.muni.cz/hea/GRBAlpha/
GRBAlpha, launched on 2021 March 22, is a demonstration mission for a future CubeSat constellation (Werner et al. Proc. SPIE 2018). The detector of GRBAlpha consists of a 75 x 75 x 5 mm3 CsI scintillator read out by a SiPM array, covering the energy range from ~50 keV to ~1000 keV. To increase the duty cycle and the downlink rate, the upgrade of the on-board data acquisition software stack is in progress. The ground segment is also supported by the radio amateur community and it takes advantage of the SatNOGS network for increased data downlink volume. 

S. D. Barthelmy (GSFC), H. A. Krimm (NSF),
S. Laha (GSFC/UMBC),A. Y. Lien (U Tampa),
C. B. Markwardt (GSFC),M. J. Moss (GSFC),
D. M. Palmer (LANL), T. Parsotan (GSFC),
T. Sakamoto (AGU) (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 240421B (trigger #1223470)
(Williams, et al., GCN Circ. 36208
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 ).  The BAT ground-calculated position is
RA, Dec = 299.685, -14.878 deg which is
   RA(J2000)  =  19h 58m 48.03s
   Dec(J2000) =  -14d 51' 36.4"
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 4%.

The BAT light curve shows a complex structure with a duration of ~ 40 sec.
T90 (15-350 keV) is 24.00 +- 9.22 sec (estimated error including systematics).

The time-averaged spectrum from T-30.4 to T+7.6 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 0.63 +- 0.52,
and Epeak of 81.3 +- 22.3 keV (chi squared 50.17 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 6.4 +- 0.5 x 10^-06 erg/cm2
and the 1-sec peak flux measured from T-0.36 sec in the 15-150 keV band is
14.1 +- 1.6 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.55 +- 0.10 (chi squared 61.45 for 57 d.o.f.).  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/1223470