XRISM Resolves Relativistic Effects from the Innermost Accretion Disk in Serpens X-1

TL;DR

The study investigates whether the Fe K line in Serpens X-1 arises from relativistic reflection off an inner accretion disk around a neutron-star LMXB, using XRISM/Resolve high-resolution spectroscopy to resolve the line shape. It applies the relxillNS reflection model to time-averaged and flux-selected spectra and tests alternative line-formation scenarios, finding that relativistic reflection provides the best description. The main result is a near-ISCO inner radius and a very low inclination, with Fe abundance enhanced relative to solar, indicating reflection-dominated line formation. This demonstrates Resolve's capability to distinguish reflection from absorption or emission alternatives and has implications for constraining neutron-star geometry and, in future work, spin and equation-of-state studies.

Abstract

We present the first XRISM/Resolve observation of the persistently accreting neutron star (NS) low-mass X-ray binary Serpens X-1. The source was observed on October 17th, 2024, for approximately 350 ks of elapsed time, resulting in 171 ks of exposure. The source exhibited 22% variability with respect to the average count rate of 73.1 count/s during the observation, but remained in a spectrally soft state throughout. The time averaged spectrum was analyzed in conjunction with spectra extracted from periods of different count rate to check for variations in spectral components. The unprecedented energy resolution of 4.5 eV at 6 keV of XRISM/Resolve provides a detailed look at the shape and structure of the Fe emission line within the data, which shows a dual-peaked structure with an extended red-wing, and steep decline in the blue-wing of the line profile. Fits with the reflection model relxillNS are able to describe the structure in the Fe line region, and confirms previous results that the disk is close to the NS ($R_{\rm in}$ = $1.02_{-0.01}^{+0.21}\ R_{\rm ISCO}$). These models also measure a low systemic inclination ($i=5^{\circ}\pm1^{\circ}$), confirming prior X-ray and optical studies. Alternative models were explored to describe the structure of the Fe line profile, however, relativistic reflection provides the simplest and statistically best explanation of the data.

Figures (4)

• Figure 1: The XRISM/Resolve (a) light curve and (b) color-color diagram for Serpens X-1. The average count rate during the observation was 73.1 counts/s with variability of 22% from the average. We designate the high flux state as count rates in excess of 78.2 counts/s (upper 15% of the source intensity) and the low flux state as count rates below 67.9 counts/s. The vertical dotted lines indicate the time of the bursts that were removed from the data analyzed herein. The color-color diagram shows that the source remains in the soft banana state throughout the observation. The designated low and high flux states correspond to the lower banana (LB) and upper banana (UB) states, respectively. The XRISM data were binned to 500 s.
• Figure 2: The ratio of the XRISM/Resolve data to the continuum model for the time-averaged ($t_{\rm avg}$), lower banana (LB) state, and upper banana (UB) state, respectively. Data were rebinned for clarity.
• Figure 3: The XRISM/Resolve counts spectrum and model components for (a) the time-averaged ($t_{\rm avg}$), (b) lower banana (LB), and (c) upper banana (UB) states, respectively. The overall model is indicated by the dot-dot-dot-dashed line, the power-law by the dotted line, the multi-temperature disk blackbody by the dot-dashed line, and the reflection model by the solid line. The panels (d)--(e) directly below the counts spectra show the ratio of the data to the overall model for each spectrum. Panels (g)--(i) show the Fe line profile from Figure \ref{['fig:fe']} with the reflection model line predicted profile overlaid based upon fitting the counts spectra. Data were rebinned for clarity.
• Figure 4: Predicted Fe line profile from the use of (a) absorption, (b) photoionized emission models, and (c) different blurring kernals. Time averaged data are shown in light grey and were rebinned for plotting purposes. The reflection ( relxillNS) model predicted line profile is shown as a solid orange line in panels (a) and (b). In panel (a) the green dashed line indicates the model predicted line profile when a Gaussian absorber is required, whereas the blue dot-dashed line shows the line profile when warmabs is applied (the profile of which overlaps with the reflection only model prediction). In panel (b) the purple dot-dot-dot-dashed line shows the line profile with the addition of a photoionized emitting plasma in the system. The Fe$\;$ line is stronger and broader in comparison to the reflection model predicted line profile, but the Fe$\;$ line is less pronounced. The red long dashed line in panel (b) denotes the line predicted model profile after removing the reflection model and allowing relativistically blurred photoionized emission to fit the line complex. The model appears to under-predict the Fe$\;$ emission. In panel (c) we show the photoionized emission component convolved with relativistic effect (red dashed line) versus a Gaussian blurring kernal (pink dotted line); the latter of which is unable to adequately describe the red-wing of the line profile.