Chandra localization of EP J120944.2+585060: likely not associated with FRB 20250316A

ATel #17119; H. Sun, D. Y. Li, C. C. Jin, Y. Liu (NAO, CAS), B. Zhang (UNLV), Y. Li, J. J. Geng, X. F. Wu (PMO, CAS), C. K. Li, L. Tao, H. Feng (IHEP, CAS), A. Rau (MPE), N. Rea (ICE-CSIC), W. Yuan (NAO, CAS) report on behalf of the Einstein Probe team:
on 29 Mar 2025; 13:10 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)

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We observed the X-ray source EP J120944.2+585060, which was suggested to be possibly associated with FRB 20250316A (ATel #17100, ATel #17086, ATel #17081), with the High Resolution Camera (HRC) on board the Chandra X-ray Observatory. The observation started on 2025 March 27 at 21:31:18 UT with an exposure of 10.65 ks under a DDT proposal (PI: Sun). Within the FXT error circle, an X-ray source is detected (at >8 sigma confidence level) at the position:

RA (J2000) = 12:09:43.72

Dec (J2000) = +58:50:51.4

with an uncertainty of 0.74 arcsec (68% confidence level, stat+sys).

The unabsorbed flux of the source measured in the 0.5-10 keV is 1.3(-0.4, +0.6)e-14 erg/cm2/s (uncertainties at the 90% confidence level), which is consistent with the previous EP-FXT measurements, confirming this source as EP J120944.2+585060. However, the position is offset by 7 arcsec from the precise localization of FRB 20250316A provided by the full CHIME/FRB Outriggers array (ATel #17114). This relatively large spatial separation, considering the sub-arcsec uncertainties of both the FRB and Chandra/HRC positions, suggests that EP J120944.2+585060 is likely not physically associated with FRB 20250316A.

At the position of FRB 20250316A, we derive a 90% flux upper limit of 7.6e-15 erg/cm2/s in the 0.5 - 10 keV band, assuming an absorbed power-law spectrum with a photon index of 2 and NH = 1.4e20 cm^-2.

We thank the CXO team for scheduling these very fast observations, which enabled the precise localization of the X-ray source.