EP241109a

A.Li (BNU), C.-Y. Wang (THU), H.-Y. Liu (NAO, CAS), Y.-C. Fu (BNU), X.-L. Chen, Kaushik Chatterjee (YNU), and H. W. Pan (NAO,CAS) report on behalf of the Einstein Probe team:

We report on the detection of an X-ray transient detected by EP-WXT, EP241109a, which triggered the on-board processing unit at 2024-11-09T06:01:55Z (UTC) (trigger ID: 01709118383). An autonomous observation on the X-ray transient was performed by the EP-FXT, which detected an X-ray source at R.A. = 18.3599 deg, DEC = 0.0184 deg (J2000) with an uncertainty of about 15 arcsecs (radius, 90% C.L. statistical and systematic), consistent with the position of the WXT transient within the uncertainties.

In the GAIA DR3 database, there is a close star (effective temperature is about 3200 K, distance is about 71.83 pc) within the EP-FXT error circle of EP241109a. Hence, the possibility that EP241109a is a stellar flare event cannot be ruled out. Further follow-up observations are encouraged to explore the orgin of EP241109a.

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).

I. Perez-Garcia, S.-Y. Wu, A. J. Castro-Tirado, E. J. Fernandez-Garcia, M.D. Caballero-Garcia, S. Guziy,  G. Garcia-Segura, R. Sanchez-Ramirez (IAA-CSIC),  Y.-D. Hu (INAF-OAB), C. Perez del Pulgar, A. Castellon, I. Carrasco (Univ. de Malaga), L. Hernandez-Garcia (Univ. de Valparaiso), M. Gritsevich (Univ. of Helsinki), 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 EP241109a by EP-WXT (Li et al. [GCNC 38140](https://gcn.nasa.gov/circulars/38140
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 )), the 0.6m BOOTES-7 robotic telescope at San Pedro de Atacama (Chile) responded to the alert on Nov. 9, 06:10 UT (i.e. 2-min after trigger, and 1-h after the event). Within the reported EP-FXT error circle we find that the star [Gaia DR3 2534635509050352256](https://vizier.cds.unistra.fr/viz-bin/VizieR-5?-ref=VIZ672f2a07ac09c&-out.add=.&-source=I/355/gaiadr3&-c=018.36205003099%20%2b00.01880744277,eq=ICRS,rs=2&-out.orig=o) decreasing 0.8 mag in brightness (clear filter) during a 40 min time interval, confirming EP241109a as due to a stellar flare. Multiband observations are ongoing.

We thank the staff at San Pedro de Atacama Celestial Observations for their excellent support.

WeiKang Zheng, Ryan Chornock, Natalie LeBaron, Xinze Guo, William Wu

Alexei V. Filippenko (UCB), and Xuhui Han, Pinpin Zhang (NAOC) report

on behalf of the KAIT GRB team:


The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at

Lick Observatory, automatically responded to EP241109a (Li et al.,

GCN 38140
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 ) starting at 06:09 UT, 7 minutes after the trigger. A set

of images with 20s exposure time were obtained in B, V, R I and

clear (roughly R) filters. Observations lasted for about 3.5 hours.

We detected the flare star reported by Perez-Garcia et al. (GCN

38141
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 ) in all our images. Photometry of the star show that its

brightness decreased from the start of our observation in all filters,

especially in B band which decreased by nearly 3 mag. The light curves

became flat after about 2.5 hours after trigger in all bands.


A spectrum was also taken with the Kast spectrograph on the 3 m Shane

telescope at Lick Observatory. Observations was performed at about

2.3 hours after the trigger, covering the 3500-10,000 A wavelength

range, with an exposure time of 300s. The spectrum shows a red

continuum flux overlapped with strong Balmer emission lines as well

as Ca II H&K and TiO emission lines, consistent with a typical dMe star

spectrum. The flux below ~4000A show a bit excess, which might be caused

by the flare, compared with the typical dMe star spectrum at quiescent

time.


Based on the above photometry and spectroscopy results, we therefore

confirm that this EP241109a event is truly associated with this stellar

flare.