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GRB 250109A, EP250109a

GCN Circular 38920

Subject
GRB 250109A / EP250109a: GRBAlpha detection
Date
2025-01-13T11:54:38Z (9 months ago)
From
Andras Pal at Konkoly Observatory <apal@szofi.net>
Via
Web form

A. Pal (Konkoly Observatory), M. Dafcikova, J. Ripa, M. Kolar, N. Werner (Masaryk U.), M. Ohno, H. Takahashi (Hiroshima U.), L. Meszaros, B. Csak (Konkoly Observatory), N. Husarikova, F. Munz , M. Topinka, M. Duriskova, L. Szakszonova, J.-P. Breuer, F. Hroch (Masaryk U.), T. Urbanec, M. Kasal, A. Povalac (Brno U. of Technology), J. Hudec, J. Kapus, M. Frajt (Spacemanic s.r.o), R. Laszlo, M. Koleda (Needronix s.r.o), M. Smelko, P. Hanak, P. Lipovsky (Technical U. of Kosice), G. Galgoczi (Wigner Research Center/Eotvos U.), Y. Uchida, H. Poon, H. Matake (Hiroshima U.), N. Uchida (ISAS/JAXA), T. Bozoki (Eotvos U.), G. Dalya (Eotvos U.), T. Enoto (Kyoto U.), Zs. Frei (Eotvos U.), G. Friss (Eotvos U.), Y. Fukazawa, K. Hirose (Hiroshima U.), S. Hisadomi (Nagoya U.), Y. Ichinohe (Rikkyo U.), K. Kapas (Eotvos U.), L. L. Kiss (Konkoly Observatory), T. Mizuno (Hiroshima U.), K. Nakazawa (Nagoya U.), H. Odaka (Univ of Tokyo), J. Takatsy (Eotvos U.), K. Torigoe (Hiroshima U.), N. Kogiso, M. Yoneyama (Osaka Metropolitan U.), M. Moritaki (U. Tokyo), T. Kano (U. Michigan) -- the GRBAlpha collaboration.

The long-duration GRB 250107D (EP/WXT detection: GCN 38864

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; Fermi/GBM detection: GCN 38873; Swift/BAT-GUANO detection: GCN 38900) was observed by the GRBAlpha 1U CubeSat (Pal et al. 2023, A&A, 677, 40; https://ui.adsabs.harvard.edu/abs/2023A%26A...677A..40P/abstract).

The detection was confirmed at the peak time 2025-01-09 06:17:11.3 UTC. The T90 duration measured by GRBAlpha is 0.5 s and the overall significance during T90 reaches 6.6 sigma in the ~120-400 keV band.

The light curve obtained by GRBAlpha is available here: https://grbalpha.konkoly.hu/static/share/GRB250109A_GCN.pdf

All GRBAlpha detections are listed at: https://monoceros.physics.muni.cz/hea/GRBAlpha/ GRBAlpha, launched on 2021 March 22, is a demonstration mission for a future CubeSat constellation (Werner et al. Proc. SPIE 2018). The detector of GRBAlpha consists of a 75 x 75 x 5 mm3 CsI scintillator read out by a SiPM array, covering the energy range from ~50 keV to ~1000 keV. To increase the duty cycle and the downlink rate, the upgrade of the on-board data acquisition software stack is in progress. The ground segment is also supported by the radio amateur community and it takes advantage of the SatNOGS network for increased data downlink volume.

GCN Circular 38915

Subject
GRB 250109A / EP250109a: Mephisto optical upper limits
Date
2025-01-12T04:25:54Z (9 months ago)
From
Chenxu Liu at Mephisto Team <cxliu@ynu.edu.cn>
Via
Web form
Xingzhu Zou, Chenxu Liu, Guowang Du, Brajesh Kumar, Tao Wang, Edoardo Lagioia, Yuan Fang, Xinlei Chen, Yu Pan (all SWIFAR, YNU), Xuhui Han, Pinpin Zhang, Liping Xin, Chao Wu (all NAOC), Xiangkun Liu, Xiaowei Liu (all SWIFAR, YNU) report on behalf of the Mephisto Team:

The Mephisto team observed the field of the GRB 250109A / EP250109a (EP Team , GCN 38864, GCN 38889; Fermi GBM team, GCN 38873, GCN 38887; the Swift XRT team, GCN 38867, GCN 38899; James DeLaunay et al., GCN 38900; A. Moskvitin et al. GCN 38901; the SVOM team, GCN 38904) with the 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory. Simultaneous uvgriz band photometric observations were conducted starting from 12:27:02 2025-01-09 UT (6.15 hr after the EP-WXT trigger) under fair observational conditions. Several frames with different exposure time were taken. No new uncatalogued optical sources were detected within the EP-WXT error circle in our stacked images of uvgriz bands. The preliminary photometry and 3 sigma upper limits are below.

Start_Time(UT)      | Band | Exp(s) | LimMag (AB)
--------------------|------|--------|------------
2025-01-09T12:27:02 | u    | 180*2  | >20.02
2025-01-09T12:34:13 | v    | 180*2  | >20.15
2025-01-09T12:27:03 | g    |  50*6  | >20.64
2025-01-09T12:34:14 | r    |  50*6  | >20.73
2025-01-09T12:27:02 | i    |  79*4  | >20.14
2025-01-09T12:34:14 | z    |  79*4  | >19.19

-------------------------------------------------------------------------------------------------
Mephisto (Multi-channel Photometric Survey Telescope) is a 1.6-m wide-field multi-channel telescope, the first of its type in the world, capable of imaging the same field of view in three optical bands simultaneously. It provides real-time, high-quality colors of stellar objects. The on-site telescope assemblage and commissioning were carried out in September 2022. The first light in all three channels was achieved on 2023 December 21.
-------------------------------------------------------------------------------------------------

GCN Circular 38904

Subject
EP250109a/GRB 250109A:SVOM/VT optical counterpart
Date
2025-01-11T02:50:01Z (9 months ago)
From
Liping Xin at NAOC, SVOM <xlp@nao.cas.cn>
Via
Web form
SVOM/VT commissioning team: Y. L. Qiu, H. L. Li, L. P. Xin, C. Wu, X. H. Han, J. Wang, W. J. Xie, H. B. Cai, Y. Xu, Y. J. Xiao, P. P. Zhang, J. S. Deng, L. Lan, X. M. Lu, R. S. Zhang, Z. H. Yao (NAOC), J. Zhang, L. J. Dan, G. Y. Zou, C. J. Wang, Y. F. Du, C. Huang (XIOPM)

SVOM JSWG: Jian-Yan Wei (NAOC), Bertrand Cordier (CEA), Shuang-Nan Zhang (IHEP), Stéphane Basa (LAM), Arnaud Claret (CEA), Zi-Gao Dai (USTC), Frédéric Daigne (IAP), Jin-Song Deng (NAOC), Olivier Godet (IRAP), Andrea Goldwurm (APC), Diego Götz (CEA), Xu-Hui Han (NAOC), Cyril Lachaud (APC), En-Wei Liang (GXU), Yu-Lei Qiu (NAOC), Susanna Vergani (Obs.Paris), Jing Wang (NAOC), Chao Wu (NAOC), Li-Ping Xin (NAOC), Shao-Lin Xiong (IHEP), Bing Zhang (UNLV)

report on behalf of the SVOM team:

The further analysis of the candidate(Qiu et al., GCN 38872) for the first ToO observation EP250109a / GRB 250109A(Li et al., GCN 38864; Fermi GBM team, GCN 38873, DeLaunay et al., GCN 38900) showed that it was fading for about 0.5 mag from 4.13 hours to 9.65 hours. 

The SVOM/VT conducted the second ToO follow-up observations for the transient in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channel simultaneously, from 31.27 hours to 38.79 hours after the burst. 

The candidate was still clearly detected in VT_R stacked image with VT_R=23.2+/-0.2 in AB magnitude at the mid time of 35.2 hours post the burst, with a total exposure time of 102*100 seconds. 

Given the continuous fading of about 0.9 mag during VT_R total observations, as well as the optical detection of Zeiss-2000 (Moskvitin et al., GCN 38901) and the XRT detection (Kennea et al., GCN 38899), it is confirmed that this source is the optical afterglow of the burst. 

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC),CAS.

GCN Circular 38901

Subject
GRB 250109A / EP250109a: Terskol (INASAN) and SAO RAS observations
Date
2025-01-10T22:51:16Z (9 months ago)
From
Moskvitin Alexander at SAO RAS <mosk@sao.ru>
Via
Web form
A. Moskvitin, O. Spiridonova (SAO), A. Pozanenko (IKI), N. Kapitanov (INASAN), N. Pankov (IKI, HSE) report on behalf of  larger GRB follow-up GRB IKI FuN collaboration.

We observed the field of the GRB 250109A / EP250109a (Li et al., 
GCN 38864; Fermi GBM team, GCN 38873) with two telescopes,
2-m Zeiss-2000 of Terskol observatory and 1-m Zeiss-1000 of SAO RAS on January 9. We obtained 100 x 60 sec. images R band on Jan. 09,  19:36:42--21:28:07 UT with 2-m telescope and 11 x 300 sec. images  in Rc band on Jan. 09, 21:12:33--22:23:56 UT with 1-m telescope.

In the stacked image of  Zeiss-2000 observation we marginally detect the  afterglow candidate (Qiu et al.,  GCN 38872) within XRT error circle (Kennea et al., GCN 38899). Photometry of the staked images is the following:

Date    UT start  t-T0, h(mid)   Filter  Exp.,s   OT,   err,  UL(3sigma)   Telescope
25-01-09 19:36:42  14.254  R       100 x 60     22.5    0.4  22.4   Zeiss-2000
25-01-09 21:12:33  15.518  Rc      11 x 300    n/d      n/d 22.2   Zeiss-1000

The photometry is based on nearby stars of the PS1 catalog (Lupton 2005 transformations) and not corrected for the Galactic extinction.

GCN Circular 38900

Subject
GRB 250109A / EP250109A: Swift/BAT-GUANO localization skymap of a burst
Date
2025-01-10T21:06:43Z (9 months ago)
From
Jimmy DeLaunay at Penn State <delauj2@gmail.com>
Via
Web form

James DeLaunay (PSU), Samuele Ronchini (PSU), Aaron Tohuvavohu (Caltech), Gayathri Raman (PSU), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC) report:

Swift/BAT did not localize GRB 250109A onboard (T0: 2025-01-09T06:17:08.60 UTC, Fermi trig 758096233)

The Fermi notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; Tohuvavohu et al. 2020, ApJ, 900, 1).

Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.

The BAT likelihood search, NITRATES (DeLaunay + Tohuvavohu 2022, ApJ, 941, 169), performed on the temporal window [T0-20 s, T0+20 s], detects the burst with a sqrt(TS) of 12.1 in a 16.384 s analysis time bin, starting at T0 - 8.192 s.

Using the NITRATES analysis, parameter estimation was performed to obtain the localization of this burst in the form of a HEALPIX Multi-Order Coverage (MOC) skymap. This localization accounts for both statistical and systematic errors. More details in the creation and calibration of these maps will soon be published (DeLaunay et al. 2025. in prep)

The 90% credible area is 0.06 deg2 and the 50% credible area is ~0.01 deg2. The integrated probability inside the coded field of view is 98%.

55% of the NITRATES localization probability is contained within a 0.1 deg radius circle around the maximum probability position. The majority of the remaining probability is contained within similarly sized peaks of probability across the BAT coded field of view.

The maximum probability position of the NITRATES skymap is,

RA, Dec = 88.806, -12.458 deg which is

RA(J2000) = 05h 55m 13.4s

Dec(J2000) = -12d 27′ 28.8″

The NITRATES skymap is consistent with the Fermi localization reported in the final position notice (GCN 38873

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). It is also consistent with the position of the X-ray transient EP250109a (GCN 38864, GCN 38889), with the position lying on the 0.28 credible region contour. The spatial and temporal coincidence of GRB 250109A and EP250109a make it very likely that they originate from the same cosmic event.

A plot of the probability skymap can be viewed here:

skymap_plot

The probability skymap file can be downloaded from the link here

skymap_fits_file

Instructions on how to read and manipulate this map can be found here:

https://guano.swift.psu.edu/documentation

More details about this burst can be found on the trigger report page here:

https://guano.swift.psu.edu/trigger_report?id=758096263

GUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches.

A live reporting of Swift/BAT event data recovered by GUANO can be found at: https://www.swift.psu.edu/guano/

GCN Circular 38899

Subject
GRB 250109A / EP250109a: Swift detection of the X-ray afterglow
Date
2025-01-10T20:57:21Z (9 months ago)
From
Jamie Kennea at Penn State <jak51@psu.edu>
Via
Web form
J. A. Kennea (PSU), P. A. Evans and K. L. Page (Leicester) report on behalf of the Swift/XRT team:

Swift/XRT performed a further 3.1ks of exposure on GRB 250109A (GCN #38887) AKA EP250109a (GCN #38864), following on from a rapid 1ks observation which did not detect the counterpart (GCN #38867). Combining the two datasets, we detect one point source inside the EP WXT error circle, at the following position: RA/Dec(J2000) = 88.80698, -12.4958, which is equivalent to:

RA(J2000)  = 05h 55m 13.67s,
Dec(J2000) = -12° 29′ 44.8″,

with an estimated uncertainty of 5.8 arc-seconds radius (90% containment). This position lies 6 arc-seconds from the EP FXT position, and 1.1 arc-seconds from the SVOM-VT reported optical position (GCN #38872). Given the localization coincidence, this source is likely to be the afterglow of GRB 250109A.

The combined XRT flux from this afterglow is 9(+5,-4) x 10^-14 erg/s/cm^2 (0.3-10 keV).

GCN Circular 38889

Subject
EP250109a / GRB 250109A: refined EP-WXT analysis and EP-FXT follow-up observation
Date
2025-01-10T09:43:01Z (9 months ago)
Edited On
2025-01-10T21:02:26Z (9 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
R.-Z. Li (YNAO, CAS), X.-L. Chen (YNU), K. Chatterjee (YNU), Y.-L. Hua (PMO, CAS) and Y. Liu (NAO, CAS) report on behalf of the Einstein Probe team:

Since 2025-01-09T06:17:58 (UTC, Trigger time), EP250109a (Li et al., GCN 38864) has been detectable with high significance using the WXT onboard the EP, peaking about 60 seconds post-trigger. The observed peak flux in the 0.5–4 keV range is about 2.5 x 10^-9 erg/s/cm^2. The flux then rapidly decayed to background levels within 200 seconds post-trigger. The average 0.5-4 keV WXT spectrum can be fitted with an absorbed power law with a photon index of 2.9 +/- 1.0 (with a fixed Galactic equivalent hydrogen column density of 3.04 x 10^21 cm^-2). The derived intrinsic column density is 5.0 (-0.3, +0.4) x 10^21 cm^-2 when the redshift is fixed at 0. The derived average unabsorbed 0.5-4 keV flux is 2.5 (-1.2, +4.6) x 10^-9 erg/s/cm^2.

We conducted a follow-up observation of EP250109a using the FXT onboard the EP. The observation began at 2025-01-09T08:06:40 (UTC, about 1.8 hours post-trigger), with an exposure time of approximately 4.2 ks. The average 0.5–4 keV FXT spectrum can be modeled with an absorbed power law, featuring a photon index of 1.9 (-0.7, +1.9) and a fixed Galactic equivalent hydrogen column density of 3.04 x 10^21 cm^-2. The derived intrinsic column density is about 3 x 10^20 cm^-2 when the redshift is fixed at 0. The estimated average unabsorbed 0.5–4 keV flux is about 4.9 (-1.2, +2.0) x 10^-13 erg/s/cm^2.

The detection of EP-WXT/FXT (GCN 38864) and Fermi-GBM (GCN 38873 and GCN 38887), combined with the non-detection by Swift-XRT (GCN 38867), indicates that the flux of this transient has decreased very rapidly.

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

GCN Circular 38887

Subject
GRB 250109A: Fermi GBM Observation
Date
2025-01-10T06:40:17Z (9 months ago)
From
Utkarsh Pathak at IIT Bombay <utkarshpathak.07@gmail.com>
Via
Web form
U. Pathak (IITB) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 06:17:08.60 UT on 09 January 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250109A (trigger 758096233/250109262).
which was also detected by EP-WXT and EP-FXT (Li et al. 2025, GCN 38864).
The Fermi GBM on-ground location is consistent with the EP-FXT position.
A possible optical counterpart was also detected by SVOM/VT (Qiu et al. 2025, GCN 38872).

The angle from the Fermi LAT boresight is 72 degrees.

The GBM light curve consists of multiple weak spikes from a single emission episode 
with a duration (T90) of about 15 s (50-300 keV). The time-averaged spectrum
from T0-8.2 to T0+9.2 s is best fit by a power law function. 
The power law index is -1.57 +/- 0.05.

The event fluence (10-1000 keV) in this time interval is
(1.9 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+2.9 s in the 10-1000 keV band is 1.3 +/- 0.2 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html

For Fermi GBM data and info, please visit the official Fermi GBM Support Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"

GCN Circular 38884

Subject
EP-WXT trigger EP250109a：Xinglong optical follow-up observations
Date
2025-01-10T04:52:16Z (9 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form

Report on behalf of the Xinglong collaboration: Junjie-Jin(NAOC), Haiyang-Mu(NAOC), Feng-Xiao(NAOC), Pengliang-Du(NAOC), Ying-Wu(NAOC), Zhou-Fan(NAOC), Hong-Wu(NAOC), Jie-Zheng (NAOC) We observed the field of the X-ray transient, EP 250109a by the Xinglong 2.16-m telescope located at Xinglong, Hebei, China. A total of 6 x 600 s N-band exposures were taken , with a median observation time of 2025-01-09 T13:06:56, approximately 6.5 hours after the EP trigger. No optical counterpart was detected, with a 5-sigma limiting magnitude of 22.48, calibrated with Pan-STARRS sources in the field.

GCN Circular 38883

Subject
GRB 250109a: GMG Optical Upper Limit
Date
2025-01-10T03:05:29Z (9 months ago)
From
wangbaiting@ynao.ac.cn
Via
Web form
B.-T. Wang, R.-Z. Li, F.-F. Song, J. Mao, X.-L. Zhang and J.-M. Bai (YNAO, CAS) report:

We observed the field of EP250109a (EP team, GCN 38864; Kennea et. al, GCN 38867; Kumar et. al, GCN 38868; Lipunov et. al, GCN 38871; Fermi GBM team, GCN 38873）with the GMG-2.4m telescope at the Lijiang Observatory. The observation began at 2025-01-09T13:30:50 (UT), about 7.2 hours after the trigger.

No new uncataloged optical counterpart was detected within the EP/FXT error circle (GCN 38864).

The preliminary photometry is as follows:
-----------------------------------------------------------------------------
UT                          EXP(s)	   filter	              mag
-----------------------------------------------------------------------------
2025-01-09T13:30:50      	1800	    sdss-z	            >23.0
-----------------------------------------------------------------------------

The magnitudes were calibrated using nearby stars of Pan-STARRS DR1 field and without corrections for Galactic extinction. 

We acknowledge the staff at the Lijiang Observatory for conducting the observation.

GCN Circular 38882

Subject
EP250109a: Swift/UVOT non-detection
Date
2025-01-09T23:46:29Z (9 months ago)
From
Sam Shilling at Lancaster University <shilling.sam@gmail.com>
Via
Web form
S. P. R. Shilling (Lancaster U.), S. R. Oates (Lancaster U.), A. A. Breeveld (UCL-MSSL)
and N. P. M. Kuin (UCL-MSSL) report on behalf of the Swift UVOT team:

Swift/UVOT observed the field of EP250109a (Li et al., GCN 38864) for 938 seconds
in the U-band starting at 09:22:16 UT, 3 hours after the detection by Einstein Probe WXT.

No optical afterglow is detected within a 5" radius of the EP FXT position (Li et al., GCN 38864)
to a preliminary 3-sigma U-band upper limit of >20.6, calculated using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373).

The non-detection reported here is consistent with the non-detections reported in other optical
observations using the GOTO (Kumar et al., GCN 38868), MASTER (Lipunov et al., GCN 38871), and
the SVOM/VT in one of the two filters (Qiu et al., GCN 38872).

GCN Circular 38873

Subject
GRB 250109A /EP250109a: Fermi GBM Final Localization
Date
2025-01-09T16:02:55Z (9 months ago)
From
Utkarsh Pathak at IIT Bombay <utkarshpathak.07@gmail.com>
Via
Web form
The Fermi GBM team reports the detection of a likely LONG GRB

At 06:17:08.60 UT on 9 January 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250109A (trigger 758096233/250109262).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 84.43, Dec = 2.26 (J2000 degrees, equivalent to J2000 05h 37m, 02d 15'), with a statistical uncertainty of 13.52 degrees. This event is associated with EP250109a (Li et al. 2025, GCN 38864).

The angle from the Fermi LAT boresight is 69.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250109262/quicklook/glg_skymap_all_bn250109262.png

The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250109262/quicklook/glg_healpix_all_bn250109262.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250109262/quicklook/glg_lc_medres34_bn250109262.gif

GCN Circular 38872

Subject
EP250109a: SVOM/VT optical candidate
Date
2025-01-09T13:56:18Z (9 months ago)
From
Liping Xin at NAOC, SVOM <xlp@nao.cas.cn>
Via
Web form
SVOM/VT commissioning team: Y. L. Qiu, H. L. Li, H. Sun, L. P. Xin, C. Wu, X. H. Han, J. Wang, W. J. Xie, H. B. Cai, Y. Xu, Y. J. Xiao, P. P. Zhang, J. S. Deng, L. Lan, X. M. Lu, R. S. Zhang, Z. H. Yao (NAOC), J. Zhang, L. J. Dan, G. Y. Zou, C. J. Wang, Y. F. Du, C. Huang (XIOPM)

SVOM JSWG: Jian-Yan Wei (NAOC), Bertrand Cordier (CEA), Shuang-Nan Zhang (IHEP), Stéphane Basa (LAM), Arnaud Claret (CEA), Zi-Gao Dai (USTC), Frédéric Daigne (IAP), Jin-Song Deng (NAOC), Olivier Godet (IRAP), Andrea Goldwurm (APC), Diego Götz (CEA), Xu-Hui Han (NAOC), Cyril Lachaud (APC), En-Wei Liang (GXU), Yu-Lei Qiu (NAOC), Susanna Vergani (Obs.Paris), Jing Wang (NAOC), Chao Wu (NAOC), Li-Ping Xin (NAOC), Shao-Lin Xiong (IHEP), Bing Zhang (UNLV)

report on behalf of the SVOM team:

The SVOM/VT conducted a ToO follow-up observations of EP250109a triggered by EP-WXT(Li et al., GCN 38864) in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channel simultaneously, from 4.13 hours to 4.80 hours after the burst. 

An uncatalogued source was detected within the errorbox of FXT (Li et al.,GCN 38864) in VT_R stacked image, compared to the Pan-STARRS catalog. This source is the only one source within the error box. The brightness of the source was estimated to be VT_R=22.34+/-0.15 in AB magnitude at the mid time of 4.47 hours post the burst, with a total exposure time of 38*100 seconds. The source was not detected in VT_B stacked image down to a 3 sigma upper limit of 23.6 mag with a total exposure time of 21*100 sec.

The source is located at RA, Dec = 88.80672, -12.49599 deg, which is equivalent to:
RA (J2000) : 05:55:13.61
Dec (J2000): -12:29:45.6
with an uncertainty of 0.5 arcsec.

We are waiting for more data to confirm the fading of the source. 

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC),CAS.

GCN Circular 38871

Subject
EP250109a: Global MASTER-Net observations report
Date
2025-01-09T13:10:18Z (9 months ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
V.Lipunov, E.Gorbovskoy, N.Tiurina,  P.Balanutsa, , D.Vlasenko, I.Panchenko,
A.Kuznetsov,  G.Antipov,  A.Sankovich, A.Sosnovskij, Yu.Tselik, M.Gulyaev, 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. 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-Tunka robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in Russia (Applied Physics Institute, Irkutsk State University) was pointed to the EP250109a ( EP Team et al., GCN 38864) errorbox  10841 sec after notice time and 22040 sec after trigger time at 2025-01-09 12:25:18 UT, with upper limit up to  16.2 mag. The observations began at zenith distance = 77 deg. The sun  altitude  is -28.1 deg. 

The galactic latitude b = -18 deg., longitude l = 218 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2739026

We obtain a following upper limits.  

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

   22049 | 2025-01-09 12:25:18 |        MASTER-Tunka | (05h 55m 17.63s , -12d 32m 20.4s) |   C |    15 | 15.5 |        
   22083 | 2025-01-09 12:25:53 |        MASTER-Tunka | (05h 55m 17.99s , -12d 31m 16.2s) |   C |    15 | 15.4 |        
   22112 | 2025-01-09 12:26:21 |        MASTER-Tunka | (05h 55m 24.27s , -12d 32m 15.0s) |   C |    15 | 15.4 |        
   22139 | 2025-01-09 12:26:48 |        MASTER-Tunka | (05h 55m 19.06s , -12d 33m 13.7s) |   C |    15 | 14.9 |        
   22165 | 2025-01-09 12:27:15 |        MASTER-Tunka | (05h 55m 24.58s , -12d 32m 56.1s) |   C |    15 | 14.9 |        
   22191 | 2025-01-09 12:27:41 |        MASTER-Tunka | (05h 55m 21.14s , -12d 31m 40.0s) |   C |    15 | 14.8 |        
   22503 | 2025-01-09 12:32:57 |        MASTER-Tunka | (05h 55m 20.91s , -12d 32m 45.4s) |   C |     5 | 15.5 |        
   22522 | 2025-01-09 12:33:16 |        MASTER-Tunka | (05h 55m 24.47s , -12d 31m 13.3s) |   C |     5 | 15.4 |        
   22540 | 2025-01-09 12:33:33 |        MASTER-Tunka | (05h 55m 17.12s , -12d 31m 49.4s) |   C |     8 | 15.3 |        
   22560 | 2025-01-09 12:33:53 |        MASTER-Tunka | (05h 55m 18.59s , -12d 30m 47.9s) |   C |     8 | 15.3 |        
   22582 | 2025-01-09 12:34:16 |        MASTER-Tunka | (05h 55m 23.95s , -12d 31m 47.5s) |   C |     8 | 15.2 |        
   22604 | 2025-01-09 12:34:37 |        MASTER-Tunka | (05h 55m 18.35s , -12d 32m 43.7s) |   C |     8 | 15.2 |        
   22626 | 2025-01-09 12:35:00 |        MASTER-Tunka | (05h 55m 23.61s , -12d 32m 10.6s) |   C |     8 | 15.3 |        
   22648 | 2025-01-09 12:35:21 |        MASTER-Tunka | (05h 55m 20.84s , -12d 31m 08.9s) |   C |     8 | 14.8 |        
   22671 | 2025-01-09 12:35:43 |        MASTER-Tunka | (05h 55m 20.86s , -12d 32m 22.5s) |   C |    10 | 14.6 |        
   22694 | 2025-01-09 12:36:07 |        MASTER-Tunka | (05h 55m 23.91s , -12d 31m 03.4s) |   C |    10 | 14.3 |        
   22718 | 2025-01-09 12:36:30 |        MASTER-Tunka | (05h 55m 18.05s , -12d 31m 34.6s) |   C |    10 | 14.6 |        
   22741 | 2025-01-09 12:36:55 |        MASTER-Tunka | (05h 55m 18.67s , -12d 30m 33.0s) |   C |     5 | 14.6 |        
   22774 | 2025-01-09 12:37:28 |        MASTER-Tunka | (05h 55m 23.56s , -12d 31m 31.8s) |   C |     5 | 14.9 |        
   22794 | 2025-01-09 12:37:48 |        MASTER-Tunka | (05h 55m 17.70s , -12d 32m 31.2s) |   C |     5 | 15.0 |        
   22814 | 2025-01-09 12:38:07 |        MASTER-Tunka | (05h 55m 24.18s , -12d 32m 28.8s) |   C |    10 | 15.3 |        
   24449 | 2025-01-09 13:05:18 |        MASTER-Tunka | (05h 55m 20.96s , -12d 29m 25.1s) |   C |    15 | 16.2 |        
   24477 | 2025-01-09 13:05:46 |        MASTER-Tunka | (05h 55m 20.83s , -12d 30m 45.1s) |   C |    15 | 15.0 |        
Filter C is a clear (unfiltred) band. 


The observation and reduction will continue. 
The message may be cited.

GCN Circular 38868

Subject
EP250109a: GOTO optical upper limits
Date
2025-01-09T12:35:49Z (9 months ago)
From
Amit Kundu at Royal Holloway - UoL/ U of Warwick, UK <amitkundu515@gmail.com>
Via
Web form
A. Kumar, D. O'Neill, G. Ramsay, B. P. Gompertz, R. Starling, M. Kennedy, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, F. Jimenez-Ibarra, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, E. Palle and D. Pollacco report on behalf of the GOTO collaboration:

We report on observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to EP-WXT detected EP250109a (Li et al., GCN 38864); also observed by the Swift-XRT (Kennea et al., GCN 38867). Targeted observations were performed using the GOTO-South at 11:42:03 UT on 2025-01-09 (5.401 hours post-trigger). The stacked image is composed of 3x90s exposures in the GOTO L-band (400-700 nm).

Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations of the same pointings.

No new optical source within the EP-FXT localisation region (Li et al., GCN 38864) is identified to a 3-sigma limiting magnitude of L > 17.8. The shallower upper limit is attributed to the unfavourable weather conditions at the Siding Spring Observatory (SSO). 

Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are not corrected for Galactic extinction.

GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester and the Instituto de Astrofisica de Canarias (IAC).

GCN Circular 38867

Subject
EP250109a: Swift/XRT non-detection
Date
2025-01-09T12:12:58Z (9 months ago)
From
Jamie Kennea at Penn State <jak51@psu.edu>
Via
Web form
J. A. Kennea (PSU) and P. A. Evans (Leicester) on behalf of the Swift XRT Team report:

On 09:19:41UT Swift XRT began a 950s exposure of the field containing the recently reported transient EP250109a (GCN #38864), 3 hours after detection by Einstein Probe WXT. Examination of the XRT data does not reveal any new X-ray sources, with an upper limit on count rate of 0.01 c/s at the EP FXT position. For a typical GRB-like spectrum this equates to an upper limit on flux of 3 x 10^-13 erg/s/cm^2 (0.3-10 keV). 

Although GCN #38864 does not report flux, the Swift/XRT non-detection suggests that this transient has rapidly faded.

GCN Circular 38864

Subject
EP250109a: EP-WXT detection of an X-ray transient
Date
2025-01-09T09:20:17Z (9 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
R.-Z. Li (YNAO, CAS), X.-L. Chen (YNU), K. Chatterjee (YNU), Y.-L. Hua (PMO, CAS), H. Sun and Y. Liu (NAO, CAS) report on behalf of the Einstein Probe team:

We report on the detection of an X-ray transient detected by EP-WXT, which triggered the on-board processing unit at 2025-01-09 06:17:58 (UTC) (trigger ID: 01709130082). The source position is R.A. = 88.806 deg, DEC = -12.500 deg (J2000) with an uncertainty of about 3 arcmin (radius, 90% C.L. statistical and systematic).

A follow-up observation on the X-ray transient was performed by the EP-FXT, which detected an uncatalogued X-ray source at R.A. = 88.8055 deg, DEC = -12.4966 deg (J2000) with an uncertainty of about 20 arcsec (radius, 90% C.L. statistical and systematic), consistent with the position of the WXT transient within the uncertainties.

More information will be updated when the telemetry data is received. Further observations are encouraged to explore the origin of the transient.

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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