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, Chandra data, and long-term Swift/XRT monitoring. The source episodically enters the hyperluminous regime with X-ray luminosities $L_X>10^{41}$ erg s${}^{-1}$ and drops by a factor of $>45$ from its peak into a deep low state. We detect a clear spectral cutoff at $\sim$5-6 keV in the broadband spectra, which are well modeled by a soft thermal component combined with optically thick thermal Comptonization or an inner advection-dominated disk. In the XMM-Newton spectra, we detect a statistically significant ($\gtrsim 3.9\sigma$) absorption line centered at $E_{\mathrm{line}}\approx 1.97\pm 0.04$ keV with a width of ${\sigma }_{\mathrm{line}}\approx 74\pm 40$ eV. We primarily interpret the line as a candidate proton Cyclotron Resonance Scattering Feature (CRSF), implying a local magnetic field strength of $B\sim 4\times 10^{14}$ 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.

20 pages, 9 figures. Revised version submitted to The Astrophysical Journal