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To: stefano@bicar.it
From: atel@astronomerstelegram.org
Subject: ATel  17775, 17776, 17777
Message-Id: <20260507070822.4E94F288087F@server.astronomerstelegram.org>
Date: Thu,  7 May 2026 03:08:22 -0400 (EDT)

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Posted: Within the last 24 hours
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ATEL #17775							     ATEL #17775

Title:	Einstein Probe detection of the brightening of the Galaxy LEDA
		735555
Author:	J. W. Hu, X. Mao (NAO, CAS), Y. F. Liang (PMO, CAS), Z. X.
		Ling (NAO, CAS) on behalf of the Einstein Probe team
Queries:	ep_ta@bao.ac.cn
Posted:	6 May 2026; 10:31 UT
Subjects:X-ray, AGN

We report on the detection of the brightening of Galaxy LEDA 735555 by
the Einstein Probe (EP). The source was first detected by the Wide-field
X-ray Telescope (WXT) on board EP in a single pointing observation (with
an exposure time of 4.7 ks) at 2026-05-01T21:53:57 (UTC). The position
of the source is R.A. = 222.744 deg, Dec. = -28.965 deg, with an uncertainty
of 2.8 arcmin in radius (90% C.L. statistical and systematic). A backwards
search by data stacking finds that the source has brightened since 22nd
April.

The WXT spectrum can be fitted by an absorbed power law with a photon index
of 2.07(+2.08/-1.77), with the NH fixed at the Galactic value of 1.3e+21
cm^-2. The derived unabsorbed flux in 0.5-4 keV is 1.09(+1.26, -0.60) x
10^-11 ergs/s/cm^2.

With the observation of the Follow-up X-ray Telescope (FXT) on board EP
performed at 2026-05-05T12:00:58, an X-ray source was detected at R.A.=
222.7393 deg, Dec.= -28.9394 deg (with an uncertainty of 10 arcsec), consistent
with the WXT source position. This position is consistent with that of
the Galaxy LEDA 735555, a previously known X-ray source, with an angular
separation of 3.2 arcsec. Preliminary analysis shows that the FXT spectrum
can be modeled by an absorbed power law with a photon index of 1.88 (+/-0.22)
and an absorption column density of NH=1.72(+0.84, -0.8) e+21 cm^-2. The
derived unabsorbed 0.5-10 keV flux is 9.06 (+0.87, -0.75) x 10^-12 ergs/s/cm^2,
which is more than one order of magnitude higher than that from the historical
eROSITA detection (1eRASS J145057.1-285620; ~ 2e-13 ergs/s/cm^2). Given
the photometric redshift of z_ph = 0.093047, this corresponds to a luminosity
of ~2 x 10^44 erg/s.

Recent optical data also indicate contemporaneous brightening of the Galaxy
LEDA 735555. We note that a previous optical brightening event from this
source was reported in 2021 and announced as AT 2021jbt via the Transient
Name Server.

Multi-wavelength follow-up observations are encouraged.

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). EP is a mission of the Chinese Academy
of Sciences in collaboration with ESA, MPE and CNES.

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Password Certification: Yuan Liu (liuyuan@bao.ac.cn)
https://www.astronomerstelegram.org/?read=17775
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ATEL #17776							     ATEL #17776

Title:	Fermi LAT detection of renewed gamma-ray activity from the
		BL Lac S5 1803+784
Author:	 F. Giacchino (USAL - Universidad de Salamanca, Spain, and
		INFN - Rome Tor Vergata, Sapienza University Rome) and P. Monti-Guarnieri
		(University of Trieste and INFN Trieste), on behalf of the Fermi Large
		Area Telescope Collaboration
Queries:	federica.giacchino@roma2.infn.it
Posted:	6 May 2026; 19:24 UT
Subjects:Gamma Ray, >GeV, AGN, Blazar, Quasar, Fast Radio Burst

The Large Area Telescope (LAT), one of the two instruments on the Fermi
Gamma-ray Space Telescope, has observed renewed gamma-ray activity from
a source positionally consistent with the BL Lac S5 1803+78, also known
as 4FGL J1800.6+7828 (The Fermi-LAT collaboration 2020, ApJS, 247, 33),
with coordinates R.A. = 270.19035 deg, Decl. = +78.46778 deg (J2000; Johnston
et al. 1995, AJ, 110, 880), and redshift z=0.684 (Stickel et al. 1993,
A&AS, 98, 393).

Preliminary analysis indicates that this source was in an elevated gamma-ray
emission state on  May 05, 2026 with a daily averaged gamma-ray flux (E>100MeV)
of (1.3+/-0.1) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only).
This corresponds to a flux increase of a factor of more than 10 relative
to the average flux reported in the fourth dara release of the fourth Fermi-LAT
catalog (4FGL-DR4, Ballet et al. 2023, arXiv:2307.12546). This is the highest
daily flux ever observed for this source by the LAT. The corresponding
photon index is 2.27+/-0.09, and is consistent with the 4FGL value of 2.24+/-0.01.
The Fermi-LAT Collaboration has previously reported flaring activity from
this source in ATels #15292, #13633, #3322 and #2386.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray
monitoring of this source will continue. This source is one of the "LAT
Monitored Sources" and consequently a preliminary estimation of the daily
gamma-ray flux observed by Fermi-LAT is publicly available at https://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/source/S5_1803p78.
A preliminary light curve can be accessed via the Fermi-LAT Light-Curve
Repository at https://fermi.gsfc.nasa.gov/ssc/data/access/lat/LightCurveRepository/source.html?source_name=4FGL_J1800.6+7828.
We encourage multifrequency observations of this source. For this source,
the Fermi-LAT contact person is Tonia Venters (tonia.m.venters@nasa.gov).

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.

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Password Certification: Federica Giacchino (federica.giacchino@roma2.infn.it)
https://www.astronomerstelegram.org/?read=17776
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ATEL #17777							     ATEL #17777

Title:	LEDA 735555: eROSITA detection of a flare in  2020-2022
Author:	Arne Rau, Pietro Baldini, Andrea Merloni (all MPE)
Queries:	arau@mpe.mpg.de
Posted:	7 May 2026; 06:57 UT
Subjects:X-ray, AGN, Transient

We report on the eROSITA (Predehl et al. 2021) observations of LEDA 735555,
for which a recent significant brightening has been reported by Einstein
Probe (Hu et al., ATel#17775).

The source was observed during the five eROSITA all-sky surveys between
2020 and 2022 with typical net exposure times of ~250s per survey spread
over approximately 1 day each. The source was detected in all observations,
showing an initially steady flux increase by a factor of 9 over the first
1.5 years, but then faded again:

#MJD, UT, Flux

58895, 2020-02-06, (0.9+/-0.4) E-13 cgs

59075, 2020-08-14, (2.5+/-0.5) E-13 cgs

59246, 2021-02-01, (5.7+/-0.9) E-13 cgs

59435, 2021-08-09,(9.5+/-1.0) E-13 cgs

59617, 2022-02-07, (6.0+/-1.0) E-13 cgs

The fluxes are reported in the 0.5-10 keV band, and are derived by fitting
the corresponding X-ray spectra with a powerlaw, taking into account Galactic
absorption, and modeling the background.

When compared to optical lightcurves (e.g., ATLAS) the eROSITA lightcurve
appears to show a flare coincident with AT 2021jbt (https://www.wis-tns.org/object/2021jbt/discovery-cert),
but on broader timescales.

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Password Certification: Arne Rau (arau@mpe.mpg.de)
https://www.astronomerstelegram.org/?read=17777
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