TITLE: GCN CIRCULAR NUMBER: 40483 SUBJECT: EP-WXT trigger (ID: 01709176312): a new X-ray outburst of HMXB RX J0111.2-7317 DATE: 25/05/17 15:40:03 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

Q. C. Liu (THU), X.-Y. Zhou (PRIC), Y.-H. I. Yin (NJU) and C. C. Jin (NAO, CAS) report on behalf of the Einstein Probe team:

We report on the detection of a new X-ray outburst of the High Mass X-ray Binary (HMXB) RX J0111.2-7317 by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The burst triggered EP-WXT (ID: 01709176312) at 2025-05-16T19:26:03 (UTC). The emission lasted for around 100 min. The WXT position of the source, inferred from the onground analysis, is R.A. = 17.759 deg, DEC = -73.279 deg (J2000) with an uncertainty of 2.5 arcmin in radius (90% C.L. statistical and systematic). The averaged WXT spectrum in 0.5-4 keV can be fitted by an absorbed power law model a Galactic hydrogen column density of 4.9 x 10^21 cm^-2 and a photon index of 3.10 (+/-0.43). The average unabsorbed 0.5-4 keV flux is 1.3 (-2.0, +2.0) x 10^-10 erg/cm^2/s.

Following the WXT trigger, a follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. The FXT observation started at 2025-05-16T19:26:35 (UTC), around 30 seconds after the trigger. The analysis of the data, with an exposure of 2 ks, reveals a detection of an X-ray source at RA = 17.7851 deg, Dec = -73.2799 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). This position is consistent with the known HMXB RX J0111.2-7317, confirming the WXT detection as the outburst of this X-ray source. The averaged FXT spectrum in 0.5-10 keV can be roughly fitted by an absorbed power law model a Galactic hydrogen column density of 4.9 x 10^21 cm^-2 and a photon index of 1.86 (+/-0.01). The average unabsorbed 0.5-10 keV flux is 1.48 (-0.02, +0.02) x 10^-10 erg/cm^2/s, which is consistent with the flux detected by the WXT.

The X-ray outburst was previously detected by WXT at 2025-05-13T14:15:40 (UTC). The observed flux is around 3.26 x 10^-11 erg/cm^2/s, consistent with the following WXT and FXT observations. More follow-up observations are encouraged.

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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