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To: stefano@bicar.it
From: atel@astronomerstelegram.org
Subject: ATel  17689, 17690, 17691
Message-Id: <20260220071729.0ECFD288079B@server.astronomerstelegram.org>
Date: Fri, 20 Feb 2026 02:17:29 -0500 (EST)

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ATEL #17689							     ATEL #17689

Title:	No bright optical counterpart to the Galactic plane gamma-ray
		transient Fermi J1820-1648
Author:	Kirill Sokolovsky (TTU), Oxana Chernyshenko, Alexander Getmansky,
		Ekaterina Saveleva, Stanislav Korotkiy (Astroverty, Ka-Dar), Elias
		Aydi (TTU)
Queries:	kirx@kirx.net
Posted:	19 Feb 2026; 17:28 UT
Subjects:Optical, Gamma Ray, Transient

Following the report of the Galactic plane gamma-ray transient  
Fermi J1820-1648 detected with Fermi-LAT in a week-long integration  
from 2026-02-09.0 to 2026-02-16.0 (ATel #17688), we inspected  
wide-field optical images of the region obtained with the New Milky  
Way (NMW) survey cameras around that time. No new optical sources  
brighter than magnitude 13 appear within the Fermi error circle  
during the gamma-ray transient detection time. The dates and  
approximate limiting magnitudes of the NMW survey images of  
the Fermi J1820-1648 region are listed below:

# Date_(UTC) lim.mag. Camera 
2026-01-29.144 >12.5 ST-8300M 
2026-01-30.142 >13.0 ST-8300M 
2026-02-10.112 >13.3 ST-8300M 
2026-02-10.130 >13.0 STL-11000M 
2026-02-12.107 >13.5 ST-8300M 
2026-02-12.124 >13.1 STL-11000M 
2026-02-16.096 >13.3 ST-8300M

The images were obtained with two unfiltered CCD cameras,  
ST-8300M and STL-11000M, attached to identical Canon 135mm f/2.0  
telephoto lenses providing 8x6 degrees (8.35"/pix) and  
15x10 degrees (13.8"/pix) fields of view, respectively. The cameras  
are located in a rolling-roof pavilion at the Astroverty astrofarm  
in Nizhnii Arkhyz, Karachay-Cherkessia, Russia. The magnitude scale  
zero-point is set using V magnitudes of Tycho-2 stars in the field.

The optical non-detections largely exclude the possibility of  
Fermi J1820-1648 being associated with a classical nova.  
The faintest Fermi-detected classical nova in the sample of  
Craig et al. (arXiv:2508.15900) was V1707 Sco, which peaked  
at V=11.8, while the other novae in that sample peaked at V  
magnitudes brighter than 10. An association with a classical  
nova would require either an unusually optically faint gamma-ray  
nova or a nova with a very fast decline time (fast enough to fall  
within the gaps between NMW observations) to have escaped  
optical detection.

Plate-solved FITS images of the Fermi J1820-1648 region obtained with the
NMW wide-field cameras: http://scan.sai.msu.ru/~kirx/img/FermiJ1820-1648/
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Password Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)
https://www.astronomerstelegram.org/?read=17689
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ATEL #17690							     ATEL #17690

Title:	Fermi/GBM detects outburst from Be/X-ray binary A 1118-616.
Author:	P. Jenke (UAH), Colleen Wilson-Hodge (NASA/MSFC), C. Malacaria
		(INAF/OAR)
Queries:	peter.a.jenke@nasa.gov
Posted:	19 Feb 2026; 18:32 UT
Subjects:X-ray, Gamma Ray, Binary, Neutron Star, Transient, Pulsar

The GBM Accreting Pulsar Project (GAPP) detected pulsations from the transient
Be/X-ray binary A 1118-616 between January 16, 2026 and February 13, 2026.
The outburst, on January 25, 2026, reached a peak RMS pulsed flux of 2.42
+/- 0.03 keV cm<sup>-2</sup> s<sup>-1</sup> (238 mCrab). The frequency,
uncorrected for orbit, was 2.4500 mHz by the end of the outburst.  GAPP
last detected this source in 2009 between January 5 and February 22 when
it reached a peak flux of 123 mCrabs in the 12-50 keV band. The two outbursts
detected with GBM were separated by 17 years. Previous outbursts reported
in literature were in 1975 (Elyes, C. J., et al. 1975) and 1992 (Coe, M.
J., et al. 1994), also showing a 17 year separation. This suggests a long-term
period of ~17 years in this system (See ATeL #17615, Coe. M. J.). Other
observations of A 1118-616 may be found at ATeLs #17613, #17616, #17626,
and #17627.

GAPP observation of A 118-616: https://gammaray.nsstc.nasa.gov/gbm/science/pulsars/lightcurves/a1118_fig0.png
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Password Certification: Peter Jenke (peter.a.jenke@nasa.gov)
https://www.astronomerstelegram.org/?read=17690
==============================================================================
ATEL #17691							     ATEL #17691

Title:	Einstein Probe Follow-up X-ray Telescope (FXT) Detection of
		AT2026dbl
Author:	Rahul Jayaraman (Cornell), Ning Jiang (USTC), Anna Y. Q. Ho
		(Cornell), Hui Sun (NAO, CAS), Xinxiang Sun (NAO, CAS), Y. L. Wang
		(NAO, CAS; ICE, CSIC-IEEC), Y. Q. Zhao (USTC, PRIC), Weimin Yuan (NAO,
		CAS), Genevieve Schroeder (Cornell)
Queries:	ep_ta@bao.ac.cn
Posted:	20 Feb 2026; 04:02 UT
Subjects:X-ray, Transient

The Follow-up X-ray Telescope on board the Einstein Probe conducted a 5ks
observation at the position of AT2026dbl (Wise et al.; AstroNote 2026-36)
beginning at UTC 2026-02-18T17:57:32. An uncatalogued X-ray source was
detected at R.A. = 135.3221, DEC = 18.6029 (J2000) with an uncertainty
of 10 arcsec (radius, 90% C.L. statistical and systematic), consistent
with the optical position.

We performed a preliminary analysis of the FXT spectrum in the 0.5-10 keV
energy range. We fit this with an absorbed power-law with a line-of-sight
hydrogen column density of 3.19 x 10^20 cm^-2 (calculated using the nh
utility). We find a photon index of 2.5(+/-0.6), with an unabsorbed flux
of 5.9 (+1.6/-1.2) x 10^-14 erg/s/cm^2. Uncertainties in these parameters
are reported at the 90% C.L. More detailed analysis is ongoing.

This flux measurement would correspond to a luminosity at the redshift
of AT2026dbl (z = 0.19; Wise et al., AstroNote 2026-36) of 6.4 (+1.7/-1.3)
x 10^42 erg/s, which is similar to the early-time X-ray flux of other LFBOTs
such as AT2018cow at a similar epoch (Margutti et al. 2019). We encourage
additional multiwavelength follow-up.

Launched on January 9, 2024, EP is a space X-ray observatory to monitor
the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook
of X-ray and Gamma-ray Astrophysics).

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Password Certification: Yuan Liu (liuyuan@bao.ac.cn)
https://www.astronomerstelegram.org/?read=17691
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