EP241026b
GCN Circular 37902
Subject
EP241026b: EP detection of a fast X-ray transient
Date
2024-10-27T04:33:18Z (7 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
T.Y. Lian, D. Y. Li, Y. L. Wang, S. X. Wen. W. Yuan (NAO, CAS) report on behalf of the Einstein Probe team:
We report on the detection of a fast X-ray transient, designated EP241026b, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient started at 2024-10-26T18:14:30(UTC) and lasted for around 100 seconds. The WXT position of EP241026b is R.A.= 56.403 deg, DEC = 41.031 deg (J2000) with an uncertainty of 2.9 arcmin (radius, 90% C.L. statistical and systematic). It has a peak flux of ~1.7 x 10^-9 erg/s/cm^2 in the 0.5-4 keV band. The average 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 2.5(+/-0.8) (with a column density fixed at the Galactic value of 3.38 x 10^21 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 1.2(-0.3, +0.4) x 10^(-10) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
No previously known bright X-ray sources are found within the error circle around the source position. EP-FXT follow-up observation has been arranged. Further follow-up observations are encouraged to identify the nature of this X-ray 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).
GCN Circular 37905
Subject
EP EP241026b: Global MASTER-Net observations report
Date
2024-10-27T08:27:45Z (7 months ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
legacy email
V. Lipunov, V.Kornilov, E. Gorbovskoy, K. Zhirkov, N.Tyurina, P.Balanutsa, A.Kuznetsov, V.Senik, D. Vlasenko,
G.Antipov, D.Zimnukhov, E.Minkina, A.Chasovnikov, V.Topolev, D.Kuvshinov, D. Cheryasov, Ya.Kechin, Yu.Tselik, A. Sosnovskij
(Lomonosov Moscow State University, SAI, Physics Department),
R. Podesta, C.Lopez, F. Podesta, C.Francile
(Observatorio Astronomico Felix Aguilar OAFA),
R. Rebolo, M. Serra
(The Instituto de Astrofisica de Canarias),
D. Buckley
(South African Astronomical Observatory),
O.A. Gress, N.M. Budnev, O.Ershova
(Irkutsk State University, API),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez
(INAOE, Guillermo Haro Astrophysics Observatory),
A. Tlatov, D. Dormidontov
(Kislovodsk Solar Station of the Pulkovo Observatory),
V. Yurkov, A. Gabovich
(Blagoveschensk Educational State University)
MASTER-OAFA robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP241026b ( EP Team et al., GCN 37902) errorbox 10998 sec after notice time and 48189 sec after trigger time at 2024-10-27 07:37:39 UT, with upper limit up to 18.5 mag. The observations began at zenith distance = 76 deg. The sun altitude is -25.1 deg.
The galactic latitude b = -10 deg., longitude l = 155 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=2650500
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
48220 | 2024-10-27 07:37:39 | MASTER-OAFA | (03h 43m 49.97s , +40d 43m 52.5s) | C | 60 | 18.2 |
48285 | 2024-10-27 07:38:45 | MASTER-OAFA | (03h 43m 49.68s , +40d 43m 53.3s) | C | 60 | 18.3 |
48350 | 2024-10-27 07:39:50 | MASTER-OAFA | (03h 43m 49.43s , +40d 43m 53.8s) | C | 60 | 18.3 |
48682 | 2024-10-27 07:45:21 | MASTER-OAFA | (03h 43m 45.44s , +40d 43m 44.4s) | C | 60 | 18.4 |
48748 | 2024-10-27 07:46:27 | MASTER-OAFA | (03h 43m 45.20s , +40d 43m 44.3s) | C | 60 | 18.4 |
48813 | 2024-10-27 07:47:33 | MASTER-OAFA | (03h 43m 44.94s , +40d 43m 44.0s) | C | 60 | 18.5 |
49208 | 2024-10-27 07:54:07 | MASTER-OAFA | (03h 45m 19.58s , +41d 09m 47.3s) | C | 60 | 18.4 |
49273 | 2024-10-27 07:55:13 | MASTER-OAFA | (03h 45m 19.46s , +41d 09m 49.7s) | C | 60 | 18.5 |
49339 | 2024-10-27 07:56:18 | MASTER-OAFA | (03h 45m 19.34s , +41d 09m 51.4s) | C | 60 | 18.5 |
49674 | 2024-10-27 08:01:54 | MASTER-OAFA | (03h 45m 23.43s , +41d 09m 58.8s) | C | 60 | 18.5 |
49740 | 2024-10-27 08:03:00 | MASTER-OAFA | (03h 45m 23.28s , +41d 09m 59.6s) | C | 60 | 18.4 |
49806 | 2024-10-27 08:04:05 | MASTER-OAFA | (03h 45m 23.13s , +41d 10m 00.4s) | C | 60 | 18.4 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
GCN Circular 37907
Subject
EP 241026b: LBT optical observations
Date
2024-10-27T12:56:36Z (7 months ago)
From
Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>
Via
Web form
Daniele B. Malesani (DAWN/NBI and Radboud Univ.), Andrea Rossi (INAF/OAS), Olga Kuhn (LBTO) and Felice Cusano (INAF/OAS) report on behalf of a larger collaboration:
We observed the field of the X-ray transient EP 241026b (Lian et al., GCN 37902) using the Large Binocular Telescope (LBT) located on Mount Graham (AZ, USA), equipped with the MODS instrument. Two images (120 s exposure each) were secured in the r and g bands, beginning at 09:50 UT on October 27, that is 15.6 hr after the EP/WXT trigger.
Visual comparison with archival images from Pan-STARRS reveals no new source within the 2.9'-radius error circle provided by EP/WXT, down to the Pan-STARRS depth of r ~ 23 (AB). We note that our image reaches deeper than that, and fainter counterparts may be retrieved once a better reference image is available.
GCN Circular 37938
Subject
EP 241026b: LBT possible optical counterpart (brightening source)
Date
2024-10-29T11:47:25Z (7 months ago)
From
Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>
Via
Web form
Andrea Rossi (INAF/OAS), Daniele B. Malesani (DAWN/NBI and Radboud Univ.), E. Maiorano (INAF/OAS), report on behalf of a larger collaboration:
We performed a second observation of the field of the fast X-ray transient EP241026b (Lian et al., GCN 37902), using the Large Binocular Telescope (LBT), equipped with the LBC instrument. Observations were carried out again in the SDSS r filter, and consisted of 6 exposures by 120 s each. The observation mid time was October 28.244 UT (1.48 days after the EP trigger).
Digital image subtraction was carried out with HOTPANTS (Becker, 2015) between this data set and our earlier observation (Malesani et al., GCN 37907). In the difference image, a very obvious transient is detected within the EP/WXT error circle, and is located at coordinates (J2000):
RA = 03:45:37.55
Dec = +41:01:51.9
This transient is observed to brighten by about 1.9 mag among the two observations, and has a magnitude r = 21.6 (AB) in our second (brighter) epoch. No source is visible at this position in the Pan-STARRS images of this field. While this behavior is unusual for GRB afterglows, brightening episodes have been noticed recently in a number of EP-detected transients (e.g., EP240414a: Srivastav et al., arXiv:2409.19070, van Dalen et al., arXiv:2409.19056; EP 241021a: Quirola-Vasquez et al., GCN 37930). We also note that this position is only 10 deg off the Galactic plane, so a Galactic origin cannot be excluded.
No other transient source is detected in the difference image, down to a limiting magnitude r = 25.1 (AB).
We acknowledge excellent support from the LBTO and LBT-INAF staff, particularly O. Kuhn, R. Ansaldi, E. Marini and F. Cusano.
GCN Circular 37967
Subject
EP241026b: EP-FXT follow-up observation
Date
2024-10-30T13:14:26Z (7 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
T.Y. Lian, Y. L. Wang, D. Y. Li, S. X. Wen, W. Yuan (NAO, CAS) on behalf of the Einstein Probe team
Following the detection of the fast X-ray transient EP241026b (Lian et al., GCN 37902), we performed an observation of EP241026b with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe. The observation started at 2024-10-27T03:36:23 (UTC), about 9 hours after the EP-WXT detection, with an exposure time of 4037 seconds. An uncatalogued X-ray source is detected at R.A. = 56.4058 deg, DEC = 41.0312 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). The average FXT spectrum in 0.5-10 keV band can be fitted by an absorbed power law with a photon index of 2.3(+1.5/-1.4) (with a column density fixed at the Galactic one of 3.38 x 10^21 cm^-2), giving an average unabsorbed flux of 1.5(+2.8/-0.5) x 10^-13 erg/s/cm^2 in the 0.5-10 keV band. The FXT position is 2 arcsec away from the likely optical counterpart detected by LBT (Rossi et al., GCN 37938). We thus consider that the FXT detection is the X-ray counterpart of EP241026b.
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 38018
Subject
EP241026b: Liverpool Telescope optical follow-up
Date
2024-10-31T20:38:32Z (7 months ago)
From
A. Bochenek at Liverpool John Moores University <a.m.bochenek@2023.ljmu.ac.uk>
Via
Web form
A. Bochenek and D. A. Perley (LJMU) report:
We observed the field of the fast X-ray transient EP241026b (Lian et al., GCN 37902) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 6x200s exposures the SDSS r filter starting at 2024-10-31 01:41:20 UT, approximately 4.31 days after the trigger.
The potential optical counterpart reported by Rossi et al. (GCN 37938) is marginally detected: forced photometry at the source location gives a 1.7-sigma detection, at a magnitude of r = 23.0 (-0.5, +0.9). More conservatively, a 2-sigma upper limit on the source magnitude is r > 22.1. Photometry was obtained using nearby PanSTARRS secondary standards and was not corrected for extinction.
The transient has therefore faded since the observation by Rossi et al. (GCN 37938) - comparing their detection to our upper limit, it has faded by at least 0.6 mag between 1.48 and 4.31 days post-trigger.