EP260502a
GCN Circular 44483
Subject
EP260502a is likely a flaring star
Date
2026-05-06T10:24:37Z (4 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
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T.Y. Liu (NAO,CAS), Y. Q. Zhao(PRIC,USTC), Y. F. Liang (PMO, CAS), Y. Wang (PMO, CAS; UCB), H. W. Pan (NAO,CAS) on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP260502a (GCN #44440) at the time of 2026-05-02T06:08:51, is likely a stellar flare associated with SIPS J1809-7613. The estimated flux of the flare is around 1.1×10^(-9) erg/s/cm^2 in 0.5-4.0 keV, corresponding to an X-ray luminosity of around 9.8×10^(30) erg/s.
The telemetry WXT data show that the flare started around 2026-05-02T05:58:08 (UTC) and exhibits no obvious flux variation later in the WXT exposure. The average WXT 0.5-4 keV spectrum of the burst period can be fitted with an absorbed power law with a hydrogen column density of 3.68×10^(-9)（-3.68×10^(-9)/+0.361 ）×10^22 cm^-2 and a photon index of 0.52(-0.52/+1.70). The derived average unabsorbed 0.5-4 keV flux is 9.96 (-3.89/+11.51) ×10^(-11) erg/s/cm^2.
The Follow-up X-ray Telescope (FXT) on board EP observed this source at 2026-05-02T06:19:45 (UTC). The exposure time of this observation is 3.0ks. The flux show a rapid fading of around 5 times in 1ks. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 0.66 (-0.21/+0.25) × 10^22 cm^-2 and a photon index of 5.44 (-1.02/+1.23). The derived average unabsorbed 0.5-10 keV flux is 8.29 (-4.61/+15.56) × 10^(-12) erg/s/cm^2.
The optical follow-up observations were performed by Li et al. (GCN 44441), Sharma et al. (GCN 44442, GCN 44444), and catch a fading of 0.7mag of SIPS J1809-7613 in 30min starting from 2026-05-02 06:24:42, consistent with the X-ray fading detected by the FXT autumatic follow-up.
A follow-up observation around 3ks with the EP/FXT was carried out starting at 2026-05-02T06:19:45, and no FXT trigger was received during the observation.
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 44444
Subject
EP260502a: BOOTES-7 observations of a fading catalogued source close to the X-ray position
Date
2026-05-02T15:48:54Z (7 days ago)
From
I. Perez-Garcia at Instituto de Astrofísica de Andalucía <ipg@iaa.es>
Via
Web form
S.-Y. Wu, I. Perez-Garcia, E. Fernandez-Garcia, M. D. Caballero-Garcia, R. Sanchez-Ramirez, S. Guziy, M. Gritsevich and A. J. Castro-Tirado (IAA-CSIC, Granada), C. Perez del Pulgar, A. Castellon, A. Reina (Univ. de Malaga),Y.-D. Hu (GXU), L. Hernandez-Garcia (Univ. de Valparaiso), D. R. Xiong (Yunnan Observatories of CAS), B.-B. Zhang (Nanjing Univ.) and A. Maury (Space, San Pedro de Atacama), on behalf of a larger collaboration, report:
Further to our previous report on EP260502a (Perez-Garcia et al., GCN 44442), and following a comment by D. B. Malesani, we have inspected the BOOTES-7 images at the position of a bright catalogued source close to the EP/FXT position, located at
RA(J2000) = 18:09:07.01
Dec(J2000) = -76:13:26.79.
This source is reported as an eROSITA source and lies close to the boundary of the EP/WXT error region. Relative photometry of this source in the BOOTES-7 r'-band images shows a fading trend during the first observing sequence. In the 9 s r'-band exposures, the source faded from r' = 13.8 +/- 0.06 at 2026-05-02 06:24:42 UT to r' = 14.5 +/- 0.08 at 2026-05-02 06:57:36 UT, corresponding to a decline of 0.7 mag over ~30 min. No significant fading trend is observed in the BOOTES-7 i' and Z band images over the same time interval.
The final measured value is consistent with the archival SkyMapper magnitude of the source, r = 14.5.
We thank D. B. Malesani for pointing out this source.
GCN Circular 44442
Subject
EP260502a: BOOTES-7: early optical upper limit
Date
2026-05-02T09:00:37Z (8 days ago)
Edited On
2026-05-04T18:12:18Z (5 days ago)
From
I. Perez-Garcia at Instituto de Astrofísica de Andalucía <ipg@iaa.es>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of I. Perez-Garcia at Instituto de Astrofísica de Andalucía <ipg@iaa.es>
Via
Web form
I. Perez-Garcia, S.-Y. Wu, E. Fernandez-Garcia, M. D. Caballero-Garcia, R. Sanchez-Ramirez, S. Guziy, M. Gritsevich and A. J. Castro-Tirado (IAA-CSIC, Granada), C. Perez del Pulgar, A. Castellon, A. Reina (Univ. de Malaga),Y.-D. Hu (GXU), L. Hernandez-Garcia (Univ. de Valparaiso), D. R. Xiong (Yunnan Observatories of CAS), B.-B. Zhang (Nanjing Univ.) and A. Maury (Space, San Pedro de Atacama), on behalf of a larger collaboration, report:
Following the detection of the fast X-ray transient EP260502a by the Einstein Probe (Liu et al., GCN 44440), the 0.6m BOOTES-7 robotic telescope at San Pedro de Atacama (Chile) observed the fast X-ray transient location starting on May 2, 06:23:31 UT (~14.7 min after trigger) in different optical bands. No new optical source is detected within the EP/WXT 3 arcmin error circle on the initial 10s image or on the first co-added clear-filter images (mid exposure time 2026-05-02 07:34 UT) down to 15.3 mag or 18.3 mag respectively, in agreement with the LCO upper limit (Li et al. GCN 44441).
We thank the staff at San Pedro de Atacama Celestial Explorations observatory for their excellent support.
GCN Circular 44441
Subject
EP260502a: Las Cumbres upper limit
Date
2026-05-02T07:43:21Z (8 days ago)
From
Wenxiong Li at NAOC <liwenxiong1992@gmail.com>
Via
Web form
Wenxiong Li (NAOC), Iair Arcavi (TAU), Runduo Liang (NAOC), Ido Keinan (TAU), David Sand (U of Arizona)
We observed the position of EP260502a (Liu et al., GCN 44440) with a Las Cumbres 1m telescope at Cerro Tololo Interamerican Observatory, Chile, 19 mins after the Einstein Probe WXT trigger. We took 2x300s exposures in each visit in the broad optical w band.
We didn’t detect any uncataloged source within the EP-FXT error circle down to a 3-sigma limiting r-band mag of 20.0 mag.
GCN Circular 44440
Subject
EP260502a: Einstein Probe detection of an X-ray transient
Date
2026-05-02T07:20:32Z (8 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
T.Y. Liu (NAO,CAS), Y. Q. Zhao(PRIC,USTC), Y. F. Liang (PMO, CAS), Y. Wang (PMO, CAS; UCB), H. W. Pan (NAO,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 EP260502a. The transient triggered EP-WXT (ID: 01709261349) at 2026-05-02T06:08:51 (UTC). The WXT position of the source is R.A. = 272.190 deg, DEC = -76.244 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).
A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 272.2762 deg, DEC =-76.2297 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic).
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).