A candidate cyclotron line at 1.89 keV in the ultraluminous X-ray source NGC 4861 X--2

In this Letter, we report the detection of an absorption-like feature at ∼ 1.89 keV in /ACIS spectra of the ultraluminous X-ray source NGC,4861 X--2, based on the deepest observation (ObsID 20992; sim59 ks). The feature is consistently recovered across independent continuum models and significantly improves the fit statistics. Monte Carlo simulations yield a detection significance of sim3.5--4.1σ, depending on the adopted continuum, and a blind line scan reveals a single, localized peak at the same energy. The observed properties are consistent with a proton cyclotron resonant scattering feature (CRSF), implying a magnetic field strength of B∼(3--4) ∼ 0.8 keV or a strongly curved continuum with a low cutoff ( ) energy (E_ Chandra G. The spectrum is well described by a multicolor disk blackbody ( ) with kT_ diskbb ̊m in cutoffpl ̊m cut keV). The source shows variability confined to the soft X-ray band in the two Chandra observations in which the absorption-like feature is detected. In these observations, a candidate periodic signal at P s is also detected, with a global significance of sim2.5σ.