EP241113a

Z. Y. Liu, M. Q. Huang (USTC), C. Y. Dai (NJU), X. P. Xu, M. H. Zhang, W. Yuan (NAO, CAS) report on behalf of the Einstein Probe team:

We report on the detection of an X-ray transient detected by EP-WXT, EP241113a, which triggered the on-board processing unit (trigger ID: 01709120178) at 2024-11-13T19:12:53 (UTC). The telemetry data show that the flare started at 2024-11-13T19:09:19 (UTC) . The 0.5-4 keV spectrum can be fitted using an absorbed power law with a photon index of 1.3 +/- 0.2 (with column density fixed to the Galactic value of 2.6e20 cm^-2). The derived average flux is estimated to be around 5.57 (+1.26/-0.76 ) ×10^(-10) erg/s/cm2 in 0.5-4 keV.

An autonomous observation on the X-ray transient was performed by the EP-FXT about 2 minutes later, which detected an uncatalogued X-ray source at R.A. = 131.9964 deg, DEC = 52.3815 deg (J2000) with an uncertainty of about 10 arcsecs (radius, 90% C.L. statistical and systematic), consistent with the position of the WXT transient within the uncertainties. During the 5000-second exposure, a significant decline is observed in the light curve. We fit an absorbed power law model to the 0.5-10 keV spectra with N_H fixed at the Galactic value. The average flux is 2.5 (+0.3/-0.2)×10^(-11) erg/s/cm^2, and the photon index is 2.40+/-0.17. 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).

Tomas Ahumada (Caltech), Geoffrey Mo (MIT), Benjamin Schneider (MIT), Viraj Karambelkar (Caltech), Danielle Frostig (CfA), Robert Stein (UMD), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:

We observed the field of the EP-WXT and EP-FXT X-ray transient EP241113a (Liu et al., GCN 38211
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 ) in the near-infrared J-band with the Palomar 1-m telescope, equipped with the 1 square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024). 

Observations began at 2024-11-14T11:44:10.899 UTC (~16.5 hrs after the X-ray transient) and consisted of 30 x 120 s exposures. The images were processed using the WINTER data reduction pipeline (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565
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 ). 

No source is detected in the FXT error region (Liu et al., GCN 38211
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 ), setting a 5-sigma upper limit of 18.5 mag (AB) in the J band.

WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research.

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 EP241113a ( EP Team et al., GCN 38211
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 ) errorbox  30873 sec after notice time and 63871 sec after trigger time at 2024-11-14 12:57:24 UT, with upper limit up to  15.2 mag. The observations began at zenith distance = 71 deg. The sun  altitude  is -33.9 deg. 

MASTER-Kislovodsk robotic telescope  located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the EP241113a errorbox  62177 sec after notice time and 1 days 8776 sec after trigger time at 2024-11-14 21:39:09 UT, with upper limit up to  15.3 mag. The observations began at zenith distance = 46 deg. The sun  altitude  is -63.7 deg. 

The galactic latitude b = 39 deg., longitude l = 166 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2669629

We obtain a following upper limits.  

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

   63901 | 2024-11-14 12:57:24 |        MASTER-Tunka | (08h 47m 28.22s , +52d 20m 09.3s) |   C |    60 | 15.3 |  Coadd 
   64205 | 2024-11-14 13:02:28 |        MASTER-Tunka | (08h 47m 25.95s , +52d 21m 42.1s) |   C |    60 | 14.6 |  Coadd 
   64422 | 2024-11-14 13:06:04 |        MASTER-Tunka | (08h 47m 26.36s , +52d 19m 37.9s) |   C |    60 | 14.5 |  Coadd 
   64632 | 2024-11-14 13:09:35 |        MASTER-Tunka | (08h 47m 30.22s , +52d 19m 47.5s) |   C |    60 | 14.7 |  Coadd 
   64998 | 2024-11-14 13:15:41 |        MASTER-Tunka | (08h 47m 27.35s , +52d 21m 30.0s) |   C |    60 | 14.0 |  Coadd 
   65244 | 2024-11-14 13:19:47 |        MASTER-Tunka | (08h 47m 29.00s , +52d 19m 25.6s) |   C |    60 | 14.7 |  Coadd 
   65497 | 2024-11-14 13:24:00 |        MASTER-Tunka | (08h 47m 32.84s , +52d 19m 41.7s) |   C |    60 | 15.2 |  Coadd 
   65860 | 2024-11-14 13:30:02 |        MASTER-Tunka | (08h 47m 31.41s , +52d 21m 17.9s) |   C |    60 | 13.6 |  Coadd 
   66303 | 2024-11-14 13:37:25 |        MASTER-Tunka | (08h 47m 35.63s , +52d 19m 41.6s) |   C |    60 | 13.1 |  Coadd 
   95206 | 2024-11-14 21:39:09 |   MASTER-Kislovodsk | (08h 46m 45.12s , +52d 31m 54.4s) |   C |    60 | 15.1 |        
   95685 | 2024-11-14 21:47:08 |   MASTER-Kislovodsk | (08h 46m 40.57s , +52d 32m 18.9s) |   C |    60 | 15.3 |        
   95965 | 2024-11-14 21:51:48 |   MASTER-Kislovodsk | (08h 46m 45.53s , +52d 30m 29.5s) |   C |    60 | 14.6 |        
Filter C is a clear (unfiltred) band. 


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

A. Rossi (INAF/OAS), A.J. Levan (Radboud Univ.), D. B. Malesani (DAWN/NBI and Radboud Univ.), P.G. Jonker (Radboud Univ.), J. Rastinejad (Northwestern), E. Maiorano (INAF/OAS), report on behalf of a larger collaboration:

We observed the field of EP 241113a (Liu et al., GCN 38211
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 ) with the LBC camera mounted on LBT (Mt. Graham, AZ, USA) in the r' band (22 min of exposure time) with approximate midtime 07:45:00 UT on 2024-11-15, or 1.52 days after the burst. Observations were performed under an average seeing of ~1.5".

We detect a very faint source within the EP/WXT error circle, which is not present in Legacy Survey r-band images, and is located at coordinates (J2000):

RA =  08:47:59.39
Dec =  +52:22:54.7

We measure a preliminary AB magnitude of  

r' = 23.35 +- 0.15,

calibrated against Pan-STARRS field stars, and not corrected for the foreground Galactic extinction.

We note that the candidate afterglow is offset ~20” from the galaxy 2MASX J08480182+5222544 with a measured spectroscopic redshift of z=0.115. This corresponds to ~40 kpc in projection and the probability of chance alignment is approximately 3%. 

We acknowledge excellent support from the LBTO and LBT-INAF staff, particularly A. Becker and D. G. Huerta.

E. Burns (LSU), M. E. Ravasio (Radboud Univ.) and P.G. Jonker (Radboud Univ.) report on behalf of the Fermi-GBM Team:


Fermi-GBM had full spatial and temporal coverage of the EP WXT signal of EP241113a (Liu et al., GCN 38211
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 ).

There was no Fermi-GBM onboard trigger around the EP-WXT trigger time T0=2024-11-13T19:12:53 UTC, nor at the earlier starting time of the transient (as reported in the EP-WXT GCN 38211
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 , 2024-11-13T19:09:19 UTC). An automated, blind search for a short gamma-ray burst below the onboard triggering threshold in Fermi-GBM also identified no counterpart candidates. The GBM targeted search, the most sensitive, coherent search for GRB-like signals, was run from 50 s before the EP trigger time until 500 s after both reported times, seeking signals between 64 ms and 32.768 s in duration. A candidate was found, but its localization is not consistent with the EP transient and appears to be a soft Galactic source. No signal consistent both temporally and spatially is identified, as confirmed by visual inspection of the data. 

P.G. Jonker (Radboud Univ.), D. Malesani (DAWN/NBI and Radboud Univ.) and A. Levan (Radboud Univ. & Warwick Univ.) report on behalf of a larger collaboration:

We observed the field of EP241113a (Liu et al., GCN 38221
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 ) with Swift/XRT starting on 2024-11-15 at 21:10:03 (UTC) for ~3325 seconds. We detect a faint X-ray source at an astrometrically corrected position RA, Dec = 131.9998, 52.3819 (=08 47 59.95, +52 22 55.0) with an error radius of 14.2′′ (90% confidence) using the XRT UK build products website (Goad et al. 2007, A&A, 476, 1401; Evans et al. 2009, MNRAS, 397, 1177). This position is consistent with that of the Einstein Probe Follow-up X-ray Telescope position (Liu et al., GCN 38221
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 ). This position is also consistent with the object reported by Rossi et al. (GCN 38233
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 ), but also with the galaxy 2MASX J08480182+5222544 with a measured spectroscopic redshift of z=0.115. 

An approximate X-ray flux of 1E-13 erg cm^-2 s^-1 (0.3-10 keV) was derived using a powerlaw with an index fixed to 2 and Galactic absorption fixed to 3E20 cm^-2 using C-stat. 

J. A. Quirola-Vasquez (Radboud Univ.), P. G. Jonker (Radboud Univ.), D. Stern (JPL/Caltech), F. Harrison (Caltech), A. Rodriguez (Caltech), J. van Dalen (Radboud Univ.), D. B. Malesani (DAWN/NBI & Radboud Univ.), A. J. Levan (Radboud Univ. & Warwick Univ.), J. C. Rastinejad (Northwestern Univ.), A. van Hoof (Radboud Univ.), J. Sanchez-Sierras (Radboud Univ.), F. E. Bauer (PUC), A. Rossi (INAF/OAS), M. E. Ravasio (Radboud Univ.) report on behalf of a larger collaboration:

We observed the optical counterpart (Rossi et al., GCN 38233
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 ) of the X-ray transient EP241113a (Liu et al., GCN 38211
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 )  with the Low-Resolution Imaging Spectrometer (LRIS; Oke et al. 1995) on the Keck I 10 m telescope. Observations were performed on 2024 Nov. 30 starting at 13:24:00 UT (i.e., ~16.76 days after the trigger), with the 600/4000 grism and 400/8500 grating for the blue and red arms, respectively. A total of 3 observations of 1000 s each were secured. The covered wavelength range is 3200 to 11000 AA.

The LRIS spectrum at the position of EP241113a shows a low signal-to-noise trace, likely corresponding to the host galaxy of the transient. A single, weak emission line is detected at 9420 Å. Among the possible interpretations, we favor [O II] at z = 1.53, due to the lack of any other lines which would be expected if the line were [O III] (at z = 0.88) or H-alpha (at z = 0.44). The resolution of the slit-grating combination (6.9 Å FWHM) is such that we do not expect to resolve the two components of the [O II] doublet for a low S/N line.

We caution that, due to the lack of detection of other emission or absorption lines, the redshift z = 1.53 should be considered tentative.

Finally, our slit covers the galaxy 2MASX J08480182+5222544, 20" away from the transient (in projection), and for that object, we confirm the redshift of z = 0.115 using, among other lines, the detection of [O II], H-alpha, [N II], [S II]. No lines are detected at this redshift at the transient position, which makes an association between these two objects unlikely. 

The data presented herein were obtained at the W. M. Keck Observatory, which operates as a scientific partnership between the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. We thank the staff of the Keck Observatory for their excellent support.

Ailing Wang (IHEP, China), Tao An (SHAO, China), Cuiyuan Dai, Xiang-Yu Wang (Nanjing University, China), on behalf of the EP radio follow-up team:
 
We report radio follow-up observations of EP241113a, which was initially discovered by the EP team (Liu et al., GCN 38211
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 ). A candidate optical counterpart was subsequently identified by Rossi et al. (GCN 38233
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 ), with a tentative redshift of z ~ 1.5 (Quirola-Vasquez et al., GCN 38449
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 ). We conducted follow-up observations using the enhanced Multi Element Remotely Linked Interferometer Network (eMERLIN).

Our observations targeted the EP/FXT position at R.A. = 131.9964°, DEC = 52.3815° (J2000). The observations were carried out in two epochs:
1. 2024-11-18 20:00 UTC to 2024-11-19 12:21 UTC
2. 2024-11-23 08:30 UTC to 2024-11-27 14:49 UTC

No significant radio emission was detected at 5 GHz in either epoch. We derive 3σ upper limits of:
- First epoch: 1.0 mJy
- Second epoch: 0.2 mJy

We acknowledge the eMERLIN TAC for approving this DDT proposal and the operations team for their rapid scheduling and excellent support.