GRB 261020A

T. Kato, M. Serino(AGU), W. Iwakiri (Chiba U.),
H. Negoro, M. Nakajima, K. Saito, T. Yokoyama, M. Wada (Nihon U.), 
T. Mihara, A. Aoyama, M. Ota, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), 
T. Sakamoto, S. Sugita, Y. Kawakubo, H. Hiramatsu,  A. Yoshida (AGU), 
Y. Tsuboi, Y. Ishihara, H. Ogaki (Chuo U.), 
M. Shidatsu, C. Kang, M. Uenishi, T. Usuki, S. Yatsuzuka, H. Aoe (Ehime U.),
I. Takahashi, Y. Yatsu (Science Tokyo), 
S. Nakahira, S. Ueno, H. Tomida, M. Kurihara (JAXA), 
Y. Ueda, K. Fujiwara, S. Kobayashi (Kyoto U.), 
M. Yamauchi, M. Nishio, C. Hiraizumi (Miyazaki U.), 
K. Yamaoka (Nagoya U.),
M. Sugizaki (Kanazawa U.),
T. Kawamuro (Osaka U.), 
S. Yamada (Tohoku U),
S. Ogawa (TUS)
report on behalf of the MAXI team:

The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient 
source at 05:42:38 UT on 20 October 2026.
Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (290.658 deg, -35.016 deg) = (19 22 37, -35 00 57) (J2000)
with a statistical 90% C.L. elliptical error region
with long and short radii of 0.53 deg and 0.45 deg, respectively.
The roll angle of long axis from the north direction is 61.0 deg counterclockwise.
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).
The X-ray flux averaged over the scan was 71 +- 18 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:
(289.257, -34.240) deg = (19 17 01, -34 14 24) (J2000)
(288.926, -35.140) deg = (19 15 42, -35 08 24) (J2000)
(292.410, -35.950) deg = (19 29 38, -35 57 00) (J2000)
(292.706, -35.040) deg = (19 30 49, -35 02 23) (J2000)
There was no significant excess flux in the previous transit at 04:09 UT
and in the next transit at 07:15 UT with an upper limit of 20 mCrab for each.

Fermi/GBM triggered GRB 261020A at 05:37:08 UT. MAXI/GSC detected X-ray emission approximately 330 s 
later, during its first scan of the region after the trigger.

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

At 05:37:08 UT on 20 Oct 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 261020A (trigger 814167433.941196 / 261020234).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 284.4, Dec = -33.9 (J2000 degrees, equivalent to J2000 18h 57m, -33d 53'), with a statistical uncertainty of 2.5 degrees.

The angle from the Fermi LAT boresight is 86.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn261020234/quicklook/glg_skymap_all_bn261020234.png

The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn261020234/quicklook/glg_healpix_all_bn261020234.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn261020234/quicklook/glg_lc_medres34_bn261020234.gif