GRB 260206A

H. Negoro, M. Nakajima (Nihon U.), J.A. Kennea (PSU), T. Mihara (RIKEN)

This is a cross-posting of ATel 17682.

In Swift-XRT followup observations of GRB 260206A detected with the MAXI-GSC (GCN Circ. 43624
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 ), 
the XRT marginally detected a faint, uncatalogued X-ray source near the center of the
MAXI error region (Lenava et al. GCN Circ. 43630
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 ). The XRT also detected a bright known X-ray source, 
LSXPS J051406.5-400237, of which a count rate was 7.3 +/- 0.3 cts/s, approximately 70% higher than 
the mean count rate (Source 1: Evans et al. GCN Circ. 43625
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 ). 

We analyzed the GSC energy spectrum of the burst, and found that a power-law model gave
a better fit to the spectrum than a blackbody model. This is because many hard X-ray photons 
were detected above 10 keV. We fixed the equivalent hydrogen column density to 3.84e20 
(obtained with the nH Calculator at NASA/HEASARC for the direction of 4U 0513-40), and obtained
a photon index of 1.7 (+0.7, -0.6 at 90% confidence interval) and a blackbody temperature 
of 1.5 (+0.6, - 0.4) keV in each fit. 

The burst exhibited a double-peaked-like structure with an e-folding decay time of
5.8 +/- 1.0 sec at 2-20 keV without any collimator response correction. The peak X-ray flux is 
around 1.22e-8 erg/cm2/s at 2-10 keV, which corresponds to 2.2e38 erg/s
if the source is in the globular cluster NGC 1851 hosting 4U 0513-40 (aka X0512-401) (12.2 kpc away).
MAXI detected more than ten X-ray bursts with a variety of time profiles originating from 
the direction of NGC 1851. These strongly suggest that this hard energetic burst also came from 
4U 0513-40 (or ABK A, see Homer et al. 2001, ApJ, L155), within the error circle of LSXPS 
J051406.5-400237, though 4U 0513-40 is just outside the MAXI error box if we take account of 
the systematic uncertainty of the localization, 0.1 deg (GCN Circ. 43624
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 ).

S. Lanava (PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester), K.L.
Page (U. Leicester), A.P. Beardmore (U. Leicester), A. Melandri
(INAF-OAR), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), J.A. Kennea
(PSU) 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 260206A (Negoro et al. GCN Circ. 43624
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 ) in a series of
observations tiled on the sky.	The total exposure time is  1.6 ks,
distributed over 5 tiles; the maximum exposure at a single sky location
in the tiling was 787 s. The data were collected between T0+32.1 ks and
T0+38.5 ks, and are entirely in Photon Counting (PC) mode. 

One uncatalogued X-ray source has been detected, with a rank of
"reasonable" (i.e. a ~2 sigma detection); however, it is neither above
historical upper limits nor shows evidence of fading. Therefore, at the
present time we cannot say whether or not it is the afterglow. The
source details are:

Source 3:
  RA (J2000.0):  78.4839  =  05h 13m 56.13s
  Dec (J2000.0): -40.5311  =  -40d 31' 51.8"
  Error: 6.7 arcsec (radius, 90% conf.)
  Count-rate: 0.0117 [+0.0077, -0.0055] ct s^-1   
  Distance: 523 arcsec from MAXI position.

Two catalogued sources were also detected, including the LMXB 4U
0513-40 noted by Negoro et al. (GCN Circ. 43624
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 ); as well two 'poor'
sources (~1 sigma detections).


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_GRB00142.

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

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 260206A. Automated analysis of the XRT data will
be presented online at https://www.swift.ac.uk/xrt_products/TILED_GRB00142

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.

H. Negoro, M. Nakajima, K. Saito (Nihon U.), T. Usuki (Ehime U.),
K. Takagi, H. Nishio (Nihon U.), T. Mihara, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), 
T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, H. Hiramatsu, Y. Kondo, D. Iijima, A. Yoshida (AGU), 
Y. Tsuboi, H. Sugai, N. Nagashima, Y. Ishihara (Chuo U.), M. Shidatsu, C. Kang, T. Nakamoto, 
M. Uenishi, S. Yatsuzuka (Ehime U.),I. Takahashi, Y. Yatsu (Science Tokyo), 
S. Nakahira, S. Ueno, H. Tomida, S. Ogawa, 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.), W. Iwakiri (Chiba U.), T. Kawamuro (Osaka U.), 
and S. Yamada (Tohoku U),

The MAXI/GSC nova alert system triggered on a bright uncatalogued hard X-ray transient source
at 08:47:33 UT on February 6, 2026.
Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (78.322 deg, -40.534 deg) = (05 13 17, -40 32 02) (J2000) 
with a statistical 90% C.L. elliptical error region 
with long and short radii of 0.33 deg and 0.23 deg, respectively. 
The roll angle of the long axis from the north direction is 38.0 deg counterclockwise. 
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).
Without assumptions on the source constancy, we obtain a rectangular error
box for the transient source with the following corners:
(77.845, -39.759) deg = (05 11 22, -39 45 32) (J2000)
(77.278, -40.246) deg = (05 09 06, -40 14 45) (J2000)
(79.005, -41.397) deg = (05 16 01, -41 23 49) (J2000)
(79.569, -40.903) deg = (05 18 16, -40 54 10) (J2000)
The X-ray flux averaged over the scan was 244 +- 33 mCrab
(4.0-10.0keV, 1 sigma error).
There was no significant excess flux in the previous transit at 07:36 UT 
and in the next transit at 10:22 UT with an upper limit of 20 mCrab for each.

We note that the X-ray burster 4U 0513-40 is close to the above position, but out of the error box.
If the systematic error is large this time, the burst may come from 4U 0513-40.