LEIA detected a bright X-ray flare from 1RXS J054317.4-760222 on May 3rd 2023. The position of the LEIA source is R.A. = 85.761 deg, DEC = -76.01 deg with an uncertainty of 2.2 arcmin (radius, 90%C.L. statistical and systematic), which is 118.7 arcseconds away from 1RXS J054317.4-760222. The source was observed twice by LEIA at 2023-05-03T07:11:44 and 2023-05-03T08:46:05, respectively, and it brightened within 2 hours.

1RXS J054317.4-760222 was detected by ROSAT with a flux of 8.4e-13 erg/cm^2/s in 0.1-2.4 keV, whose nature remains unclear. It was also covered by XMM-Newton Slew Survey with the highest flux of (2.8 +/- 1.3) e-12 erg/cm^2/s in 0.2-12 keV. As a comparison, the detected 0.5 - 4.0 keV flux by LEIA reached ~5e-11 erg/cm^2/s. Thus LEIA has detected an X-ray flare of this source brighter than all of its previous observations by more than one order of magnitude.

After the observation of LEIA, a follow-up ToO observation with Swift/XRT (obs ID: 00016010001, 900s exposure) at 2023/05/03T19:59:04 also detected this source, confirming its position to be consistent with 1RXS J054317.4-760222 (with an angular separation of 2 arcsec). The X-ray spectra can be fitted by a powerlaw with a photon index of 2.2 +/- 0.2 or an apec model with a plasma temperature of 1.3 +/- 0.1 keV (both modified by Galactic absorption), and its 0.5 - 4.0 keV flux was ~1.6e-12 erg/cm^2/s.

In the second ToO of Swift/XRT (obs ID: 00016010002, 3 ks exposure) taken at 2023/05/04T21:23:05, the X-ray flux of the source was found to have decayed by a factor of ~3 with respect to the first ToO observation. The X-ray spectrum can be fitted by a powerlaw with a photon index of 2.5 +/- 0.2 or an apec model with a plasma temperature of 2.3 +/- 0.6 (both modified by Galactic absorption), and its 0.5 - 4.0 keV flux was ~5e-13 erg/cm^2/s. A decline in its optical/UV flux was also indicated by UVOT in these two ToO observations.

Follow-up observations are encouraged to investigate the nature of 1RXS J054317.4-76022 in detail.

We thank the Neil Gehrels Swift Observatory team for making the X-ray observations possible.

The preliminary results have been posted at Astronomer's Telegram as ATel 16029. Click here for more EP/LEIA Astronomer's Telegrams.