A mysterious feature in the NICER spectrum of 4U 1820-30: A gravitationally redshifted absorption line?

TL;DR

NICER observations of the ultracompact X-ray binary 4U 1820-30 after a superburst reveal a pronounced broad absorption feature near 3.8 keV that is best described as a gravitationally redshifted iron line originating from the neutron-star photosphere. Modeling with a photoionized absorber yields a redshift of $1+z \approx 1.72$, translating to a compactness of $R/M \approx 4.46 \pm 0.13$ km/M$_\odot$ (or $3.02 \pm 0.09$ in dimensionless units), and enabling constraints on the mass-radius relation under plausible rotation scenarios. The iron abundance appears solar-like, and the line width is consistent with rotational Doppler broadening at $\nu \approx 716$ Hz, supporting a surface-origin interpretation. These results illustrate the potential of gravitationally redshifted absorption lines as direct probes of the neutron-star equation of state, while underscoring the need for higher-throughput, higher-resolution X-ray spectroscopy to refine these constraints.

Abstract

A mysterious absorption feature at approximately 3.8 keV has been identified in the NICER spectrum of the low-mass X-ray binary system 4U 1820-30. We interpret this feature as a gravitationally redshifted iron absorption line. This interpretation is supported by the temporal proximity of the NICER observation to the detection of a carbon superburst by the X-ray monitor MAXI, suggesting that the presence of the line is associated with this rare and extreme event. From the inferred redshift of the absorption line, the compactness of the neutron star can be derived. Using a photoionization absorption model, we measure a gravitational redshift of about 1.72, which corresponds to a compactness R/M of 4.46 \pm 0.13 km per solar mass, or 3.02 \pm 0.09 in dimensionless units. This unique feature highlights the importance of further observations and detailed modelling, offering promising insights into the equation of state of matter under extreme density conditions.

Figures (9)

• Figure 1: Comparative light curves of 4U 1820-30 from NICER (0.5–10 keV, top) and MAXI/GSC (2–10 keV, bottom). In the NICER panel, color indicates hardness ratio (HR) between the [2–5] and [0.5–2] keV bands, ranging from 0.2 to 0.4. A red dashed line marks the observation with peak HR = 0.664. MAXI data show a brief increase in count rate coinciding with the NICER observation window.
• Figure 2: Light curves of the source 4U 1820-30 observed by MAXI in four different energy bands from MJD 59446 to MJD 59453. The vertical lines in red and blue mark the commencement and conclusion of NICER Obs. 105 and 106, respectively. The noticeable surge in the count rate across all selected energy bands correlates with a superburst event, which occurred approximately six hours prior to the initiation of NICER Obs. 105.
• Figure 3: Unfolded spectrum and residuals in unit of $\sigma$ for the averaged spectrum obtained from the NICER Obs. 105, adopting Model 1 (left panel) and Model 4 (right panel). The thermal emission from the accretion disk and the Comptonized component are in red and blue, respectively.
• Figure 4: Data-to-model ratios of the averaged spectrum extracted from Obs. 105, adopting Model 1, Model 2, Model 3, and Model 4 (from top to bottom, respectively). The dashed blue lines indicate a ratio of 1.5%, corresponding to the systematic error associated with the spectrum.
• Figure 5: NICER light curves for Obs. 105 and 106 in the 0.5–2 keV (top) and 2–5 keV (middle) bands. The bottom panel shows the hardness ratio (HR). Green shaded regions mark intervals used for spectral extraction; red and blue lines indicate the end of Obs. 105 and start of Obs. 106, respectively.