EP260509a
GCN Circular 44546
Subject
EP260509a: LCO optical upper limits
Date
2026-05-11T12:37:13Z (4 days ago)
From
Rob Eyles-Ferris at U of Leicester <raje1@leicester.ac.uk>
Via
Web form
R. A. J. Eyles-Ferris (Leicester), G. Corcoran (UCD), P. G. Jonker (Radboud), J. A. Chácon (PUC), F. E. Bauer (SSI and UTA), report on behalf of a larger collaboration:
We observed the field of the X-ray transient EP260509a (Wang et al., GCN 44517; Fu et al., GCN 44539) using LCO 1m telescopes located at the Cerro Tololo Inter-American Observatory (Chile) and McDonald Observatory (USA) equipped with SINISTRO instruments. A series of 6x300 s exposures were taken in each of the SDSS-r and SDSS-z filters, starting at 2026-05-10 08:04:39 UT and 2026-05-10 06:53:47 UT respectively (~17.2 and 18.3 hr after the first X-ray detection).
No new source is detected within or at the border of the EP/WXT uncertainty region (Wang et al., GCN 44517) nor do we identify any significant residuals in subtractions using PanSTARRS DR1 templates. From the stacked images, we obtain the following 3-sigma upper limits:
r > 22.8
z > 21.4
These upper limits are in AB magnitudes, calibrated using nearby stars from the PanSTARRS catalogue, and are not corrected for Galactic extinction.
GCN Circular 44539
Subject
EP260509a: EP-FXT non-detection and upper limit
Date
2026-05-11T09:39:15Z (5 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
S.-Y. Fu (HUST), A. Li (BNU), C.-Y. Wang (THU), R. Shi (PMO), Y. Liu (NAOC, CAS) on behalf of the Einstein Probe (EP) team:
EP-FXT observed the field of EP260509a (detected by EP-WXT, Wang et al., GCN 44517; and followed by MASTER-Net observations, Lipunov et al., GCN 44525) for 5475 seconds starting at 2026-05-10T10:15:44, around 20.5 hours after the detection by EP-WXT. No significant X-ray source is detected within the 2.78 arcmin radius of the EP-WXT position (Wang et al., GCN 44517) to a preliminary 0.5-10 keV upper limit of 3e-14 erg/cm2/s.
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).
GCN Circular 44525
Subject
EP260509a: Global MASTER-Net observations report
Date
2026-05-10T21:36:20Z (5 days ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko,
G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A.Sosnovskij (CrAO),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory)
MASTER-Kislovodsk robotic telescope [1] located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the EP260509a ( EP Team et al., GCN 44517) errorbox 24957 sec after notice time and 1 days 27906 sec after trigger time at 2026-05-10 21:29:08 UT, with upper limit up to 17.9 mag. The observations began at zenith distance = 61 deg. The sun altitude is -29.1 deg.
The galactic latitude b = 25 deg., longitude l = 353 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3305450
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
114336 | 2026-05-10 21:29:08 | MASTER-Kislovodsk | (15h 55m 47.13s , -18d 04m 15.2s) | C | 60 | 12.6 |
114336 | 2026-05-10 21:29:08 | MASTER-Kislovodsk | (15h 55m 58.90s , -18d 27m 08.8s) | C | 60 | 17.8 |
114426 | 2026-05-10 21:30:38 | MASTER-Kislovodsk | (15h 55m 42.18s , -18d 05m 11.2s) | C | 60 | 12.1 |
114426 | 2026-05-10 21:30:38 | MASTER-Kislovodsk | (15h 55m 53.95s , -18d 28m 02.6s) | C | 60 | 17.9 |
114514 | 2026-05-10 21:32:06 | MASTER-Kislovodsk | (15h 55m 55.18s , -18d 27m 07.8s) | C | 60 | 17.9 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
[1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023, Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp.
http : // www.pereplet.ru/lipunov/625.html
GCN Circular 44517
Subject
EP260509a: Einstein Probe detection of an X-ray transient
Date
2026-05-10T14:32:36Z (5 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
C.-Y. Wang (THU), S.-Y. Fu (HUST), A. Li (BNU)Y, R. Shi (PMO), Y. Liu (NAOC, CAS) on behalf of the Einstein Probe (EP) team:
We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP260509a. The transient was identified during ground-based telemetry analysis. It was started at 2026-05-09T13:44:02 (UTC). The WXT position of the source is R.A. = 239.4813 deg, DEC = -18.4328 deg (J2000) with an uncertainty of 2.78 arcmin in radius (90% C.L. statistical and systematic). The duration of the event is ~ 250 s. The WXT spectrum can be fitted with an absorbed power-law, with the absorption fixed at the Galactic value of 1.05 x 10^21 cm^-2, and a photon index of 2.0 (-0.8, +0.9). The unabsorbed 0.5-4 keV flux is 6.0 (-2.0, +3.1) x 10^-11 erg/s/cm2.
A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed at 2026-05-10T09:52:54. Further information will be updated when the telemetry data is received.
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).