Discovery of a Candidate 2 keV Cyclotron Resonance Scattering Feature in the HLX NGC 3583 X-1

Abstract

We present a broadband X-ray study of the transient hyperluminous X-ray source (HLX), 2SXPS J111416.1+481833, in the galaxy NGC 3583, using archival XMM-Newton, NuSTAR, and Chandra data, and long-term Swift X-ray Telescope monitoring. The source episodically enters the hyperluminous regime with X-ray luminosities of LX > 1041 erg s−1 and drops by a factor >45 from its peak into a deep low state. We detect a clear spectral cutoff at ∼5–6 keV in the broadband spectra, which are well modeled by a soft thermal component combined with optically thick thermal Comptonization or an inner advection-dominated disk. In the XMM-Newton spectra, we detect a statistically significant (≳3.9σ) absorption line centered at Eline ≈ 1.97 ± 0.04 keV with a width of σline ≈ 74 ± 40 eV. We primarily interpret the line as a candidate proton cyclotron resonance scattering feature, implying a local magnetic field strength of B ∼ 4 × 1014 G. Alternative interpretations, such as an origin in an ionized outflow, were explored and found to be less likely. We do not detect coherent X-ray pulsations, placing 90% confidence upper limits on the pulsed fraction of 19.3% in the 0.3–10 keV band and 36.3% in the 3–15 keV band. The combination of extreme luminosity, a hard spectral state, and the detection of a candidate cyclotron line provides strong evidence for a highly magnetized neutron star accretor.