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

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 2024 October 17, 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 counts s −1 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 rates 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 a 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 confirm previous results that the disk is close to the NS ( R in = <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>1.0</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.01</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.21</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> R ISCO ). These models also measure a low systemic inclination ( i = 5° ± 1°), 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.