GRB 211018A

M. Arimoto (Kanazawa Univ.), M. Axelsson (KTH & Stockholm Univ.), N. Di Lalla (Stanford Univ.), N. Omodei (Stanford Univ.), F. Longo (Univ. and INFN, Trieste) and D. Tak (DESY) report on behalf of the Fermi-LAT Collaboration:

On October 18th, 2021, Fermi-LAT detected high-energy emission from GRB 211018A, which was also detected by Fermi GBM (trigger 656288898 / 211018936). The best LAT on-ground location is found to be

RA, Dec = 19.68, -3.60 (degrees, J2000)

with an error radius of 0.19 deg (90% containment, statistical error only).

The position was 68 deg from the LAT boresight at the time of the GBM trigger:

T0 = 22:28:13.9 UT.

The data from the Fermi-LAT show a significant increase in the event rate after the trigger that is temporally and spatially correlated with the GBM emission with high significance. The photon flux above 100 MeV in the time interval 0-300 s after the trigger is 1.2 (-/+ 0.3) e-04 ph/cm2/s.

The estimated photon index above 100 MeV is -2.0 (-/+ 0.2).

The highest-energy photon is a 3.3 GeV event which is observed 25 seconds after the GBM trigger.

A Swift ToO has been requested for this burst.

The Fermi-LAT point of contact for this burst is Donggeun Tak (takdg123@gmail.com).

The Fermi-LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.

M. Ohno (Eotvos U./Konkoly Observatory/Hiroshima U.),  J. Ripa (Masaryk
U.), H. Takahashi (Hiroshima U.), A. Pal, L. Meszaros, B. Csak (Konkoly
Observatory), N. Werner, M. Topinka, F. Munz, 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.), Yuusuke Uchida, Helen Poon, H. Matake, N.
Uchida (Hiroshima U.), 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.) -- the GRBAlpha
collaboration.

The bright/long duration GRB 211018A (Fermi-GBM detection trigger
656288898/211018936, Fermi-LAT detection; Arimoto et al., GCN Circ. 30943
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 )
was detected by the GRBAlpha 1U CubeSat (Pal et al. Proc. SPIE 2020).

The light curve shows a very long multiple-peaked structure with a duration
of about 168 s (T90 = 140 s). At 22:29:34 UTC, our detection reached a SNR
~ 46.

The light curve obtained by GRBAlpha is available here:
https://grbalpha.konkoly.hu/static/share/GRB211018A_GCN.pdf

GRBAlpha is a demonstration mission for a future CubeSats 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. GRBAlpha was launched on 2021 March 22
from Baikonur. After its commissioning phase, the scientific observations
are now under way. 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.

O.J Roberts (USRA/NASA-MSFC) and C. Meegan (UAH)
report on behalf of the Fermi GBM Team:

"At 22:28:13.94 UT on 18 October 2021, the Fermi Gamma-Ray Burst Monitor (GBM)
triggered and located GRB 211018A (trigger 656288898 / 211018936),
which was also detected by the Fermi LAT (M. Axelsson et al. 2021, GCN 30943
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 ),
and GRBAlpha (M. Ohno, GCN 30945
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 ). The GBM on-ground location is consistent
with the Fermi-LAT position.

The angle from the Fermi LAT boresight at the GBM trigger time is 68 degrees.

The GBM light curve consists of multiple peaks
with a duration (T90) of 123.9 s (50-300 keV).
The time-averaged spectrum from T-2.4s to T0+142.0 s is
well fit by a power law function with an exponential
high-energy cutoff. The power law index is -0.75 +/- 0.01 and
the cutoff energy, parameterized as Epeak, is 351.4 +/- 4.9 keV.

The Band function also fits the burst equally well, with parameters,
alpha= -0.74 +/- 0.01, beta= -3.13 +/- 0.28 and Epeak of 344.6 +/- 6.2 keV.

The event fluence (10-1000 keV) in the T90 interval is
(1.53347 +/- 0.0003)E-04 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+134.5 s in the 10-1000 keV band
is 15.6 +/- 0.3 ph/s/cm^2.

We note that owing to the large total fluence of this burst,
the 1-sec peak photon flux begins after the T90.

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/"

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

Swift has initiated a ToO observation of the Fermi/LAT GRB 211018A. 
Automated analysis of the XRT data will be presented online at
https://www.swift.ac.uk/ToO_GRBs/00021463

Any uncatalogued X-ray sources detected in this analysis will be
reported on this website and via GCN COUNTERPART notices. These are
not necessarily related to the Fermi/LAT event. 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.

S. Dichiara (PSU), B. Sbarufatti (PSU), K.L. Page (U. Leicester), A.P.
Beardmore (U. Leicester), P. D'Avanzo (INAF-OAB), M.G. Bernardini
(INAF-OAB), E. Ambrosi	(INAF-IASFPA) , D.N. Burrows (PSU), J. D. Gropp
(PSU) and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT
team:

Swift-XRT has performed follow-up observations of the
Fermi/LAT-detected burst GRB 211018A (Arimoto et al. GCN Circ. 30943
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 ),
collecting 4.4 ks of Photon Counting (PC) mode data between T0+304.3 ks
and T0+340.2 ks. 

Five uncatalogued X-ray sources have been detected, however none of
them is above the RASS limit or shows definitive signs of fading.
Therefore, at the present time we cannot identify which, if any, is the
afterglow. We notice that Source 2 and Source 4 are consistent with
catalogued quasars at redshift z = 0.64 and z=1.09, respectively. 
Details of these sources are given below:

Source 1:
  RA (J2000.0):  19.5986  =  01:18:23.66
  Dec (J2000.0): -3.6511  =  -03:39:04.1
  Error: 6.1 arcsec (radius, 90% conf. [Enhanced position])
  Count-rate: (2.68 [+1.16, -0.92])e-3 ct s^-1	 
  Distance: 338 arcsec from Fermi/LAT position.
  Flux: (9.3 [+4.0, -3.2])e-14 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 2:
  RA (J2000.0):  19.5656  =  01:18:15.74
  Dec (J2000.0): -3.3800  =  -03:22:48.2
  Error: 5.1 arcsec (radius, 90% conf. [Enhanced position])
  Count-rate: (9.6 [+3.9, -3.1])e-3 ct s^-1   
  Distance: 884 arcsec from Fermi/LAT position.

Source 3:
  RA (J2000.0):  19.4544  =  01:17:49.06
  Dec (J2000.0): -3.6662  =  -03:39:58.4
  Error: 7.2 arcsec (radius, 90% conf.)
  Count-rate: (3.6 [+1.4, -1.1])e-3 ct s^-1   
  Distance: 835 arcsec from Fermi/LAT position.
  Flux: (1.85 [+0.70, -0.56])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)


Source 4:
  RA (J2000.0):  19.5576  =  01:18:13.83
  Dec (J2000.0): -3.7295  =  -03:43:46.2
  Error: 3.2 arcsec (radius, 90% conf. [Enhanced position])
  Count-rate: (6.6 +/- 1.6)e-3 ct s^-1	 
  Distance: 636 arcsec from Fermi/LAT position.
  Flux: (1.70 +/- 0.40)e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 5:
  RA (J2000.0):  19.6548  =  01:18:37.16
  Dec (J2000.0): -3.6066  =  -03:36:23.9
  Error: 8.0 arcsec (radius, 90% conf.)
  Count-rate: (2.46 [+1.08, -0.85])e-3 ct s^-1	 
  Distance: 84 arcsec from Fermi/LAT position.
  Flux: (5.3 [+2.3, -1.8])e-14 erg cm^-2 s^-1 (observed, 0.3-10 keV)

The results of the XRT-team automatic analysis of the XRT observations,
including a position-specific upper limit calculator, are available at
https://www.swift.ac.uk/ToO_GRBs/00021463.

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