GRB 241213B

H. Negoro (Nihon U.), M. Serino, Y. Kawakubo (AGU), W. Iwakiri (Chiba U.),
M. Nakajima, Y. Kudo, H. Shibui, K. Takagi, H. Takahashi, K. Tatano, H. Nishio (Nihon U.), 
T. Mihara, T. Kawamuro, S. Yamada, S. Wang, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), 
T. Sakamoto, S. Sugita, H. Hiramatsu, H. Nishikawa, Y. Kondo, S. Sasao, A. Yoshida (AGU), 
Y. Tsuboi, H. Sugai, N. Nagashima (Chuo U.), M. Shidatsu, Y. Niida (Ehime U.),
I. Takahashi, M. Niwano, N. Higuchi, Y. Yatsu (Tokyo Tech), S. Nakahira, S. Ueno, H. Tomida,
 M. Ishikawa, S. Ogawa, M. Kurihara (JAXA), Y. Ueda, Y. Okada, K. Fujiwara (Kyoto U.),  
M. Yamauchi, Y. Otsuki, T. Hasegawa, M. Nishio (Miyazaki U.), K. Yamaoka (Nagoya U.), 
and M. Sugizaki (Kanazawa U.)

The MAXI/GSC nova alert system triggered on a bright uncatalogued X-ray transient source 
at 12:56:30 UT on 2024 December 13. Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (331.450 deg, 4.282 deg) = (22 05 48, +04 16 55) (J2000) 
with a statistical 90% C.L. elliptical error region 
with long and short radii of 0.16 deg and 0.15 deg, respectively. 
The roll angle of the long axis from the north direction is 141.0 deg counterclockwise. 
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).
The X-ray flux averaged over the scan was 465 +- 58 mCrab
(4.0-10.0keV, 1 sigma error).
Without assumptions on the source constancy, we obtain a rectangular error
box for the transient source with the following corners:
(R.A., Dec) = (331.162, 4.788) deg = (22 04 38, +04 47 16) (J2000)
(R.A., Dec) = (330.936, 4.585) deg = (22 03 44, +04 35 05) (J2000)
(R.A., Dec) = (331.900, 3.518) deg = (22 07 35, +03 31 04) (J2000)
(R.A., Dec) = (332.126, 3.721) deg = (22 08 30, +03 43 15) (J2000)
There was no significant excess flux in the previous transit at 11:23 UT 
with an upper limit of 20 mCrab.

P. S. Marrocchesi (U of Siena), A. Yoshida, T. Sakamoto, S. Sugita,
Y. Kawakubo (AGU), K. Yamaoka (Nagoya U), S. Nakahira (JAXA), 
Y. Asaoka (ICRR), S. Torii, Y. Akaike, K. Kobayashi (Waseda U), Y. Shimizu,
T. Tamura (Kanagawa U), N. Cannady (GSFC/UMBC), M. L. Cherry (LSU),
S. Ricciarini (U of Florence),
and the CALET collaboration:

The GRB 241213B (MAXI/GSC detection: Negoro et al., GCN Circ. 
38555
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 ) was detected in the ground analysis of the CALET Gamma-ray 
Burst Monitor (CGBM) data around 12:56:30 on 13 December 2024 (referenced
to the MAXI detection: GCN Circ. 38555
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 ).
(https://cgbm.calet.jp/cgbm_trigger/flight/1418129753/index.html).
The burst signal was seen by all CGBM detectors.  

The burst light curve shows a single pulse that starts
at T+1.8 sec, peaks at T+2.4 sec, and ends at T+4.5 sec.
The T90 and T50 durations measured by the SGM data are 2.4 +/- 0.5 sec
and 1.2 +/- 0.4 sec (7-100 keV), respectively.

The ground-processed light curve is available at

https://cgbm.calet.jp/cgbm_trigger/ground/1418129753/

The CALET data used in this analysis are provided by
the Waseda CALET Operation Center located at Waseda University.

P. A. Evans (U. Leicester) reports on behalf of the Swift team:

Swift has initiated a series of observations, tiled on the sky, of the
MAXI GRB 241213B. Automated analysis of the XRT data will
be presented online at http://www.swift.ac.uk/xrt_products/TILED_GRB00130

Any uncatalogued X-ray sources detected in this analysis will be
reported on this website and via GCN COUNTERPART notices. The probability of finding
serendipitous sources, unrelated to the MAXI event is high: any X-ray source
considered to be a probable afterglow candidate will be reported via a GCN Circular
after manual consideration.

Details of the XRT automated analysis methods are detailed in Evans et
al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8).

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

A.P. Beardmore (U. Leicester), M. Capalbi (INAF-IASFPA), M. Perri (SSDC
& INAF-OAR), V. D'Elia (SSDC & INAF-OAR), J.A. Kennea (PSU), D.N.
Burrows (PSU), M. A. Williams (PSU), J.P. Osborne (U. Leicester) and
P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the MAXI-detected
burst GRB 241213B in a series of observations tiled on the sky.  The
total exposure time is	2.2 ks, distributed over 7 tiles; the maximum
exposure at a single sky location in the tiling was 822 s. The data
were collected between T0+54.1 ks and T0+60.2 ks, and are entirely in
Photon Counting (PC) mode. 

One uncatalogued X-ray source has been detected, it is below the RASS
limit and shows no definitive signs of fading. Therefore, at the
present time we cannot confirm this as the afterglow. Details of this
source are given below:

Source 1:
  RA (J2000.0):  331.7177  =  22:06:52.25
  Dec (J2000.0): +4.0971  =  +04:05:49.6
  Error: 9.0 arcsec (radius, 90% conf.)
  Count-rate: 0.0185 [+0.0089, -0.0068] ct s^-1   
  Distance: 1169 arcsec from MAXI position.
  Flux: (1.21 [+0.59, -0.44])e-9 erg cm^-2 s^-1 (observed, 0.3-10 keV)

The results of the XRT-team automatic analysis of the tiled XRT
observations, including a position-specific upper limit calculator, are
available at https://www.swift.ac.uk/xrt_products/TILED_GRB00130.

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