---
title: 'relxillNS: Neutron-Star X-ray Reflection Model'
url: https://www.emergentmind.com/topics/relxillns
type: topic
---

# relxillNS: Neutron-Star X-ray Reflection Model

Searching arXiv for relxillNS and recent applications to ground the article in current literature.
relxillNS is a relativistic X-ray reflection model for accreting neutron stars in which the primary illuminating continuum is a single-temperature blackbody emitted either at the surface of the star or at the boundary layer, rather than the power-law continuum usually assumed for accreting black holes [2111.12838]. It is the relativistic counterpart of xillverNS and is distributed as a neutron-star-specific flavor within the relxill suite [2111.12838]. In practice, relxillNS has become a standard tool for modeling Fe K emission, Compton humps, burst-driven disk reprocessing, and inner-disk geometry in neutron-star low-mass X-ray binaries, including Aql X-1, 4U 1636–536, Cygnus X-2, Serpens X-1, Sco X-1, GX 13+1, GX 340+0, Swift J17480-2446, and 4U 1820–30 [2503.17653].

## 1. Definition and lineage

relxillNS was introduced together with xillverNS as a set of reflection models “specifically tailored to model the X-ray radiation reprocessed in accretion disks around neutron stars,” under the assumption that the irradiating spectrum is blackbody-like and originates in the neutron-star surface or boundary layer [2111.12838]. This point is central: the model is not merely a reparameterized black-hole reflection model, but a different illumination prescription designed for neutron-star accretion physics.

The model inherits the broader relxill strategy of combining a local reflection calculation with relativistic transfer, but its local spectrum is computed for blackbody illumination rather than for a power-law or cutoff power-law [2111.12838]. Earlier blackbody-illuminated reflection calculations existed, including BBrefl and reflionx_BB, but relxillNS was presented as an updated, publicly available implementation with relativistic blurring and parameter coverage appropriate to neutron-star disks [2111.12838].

The Kerr metric is used for the relativistic transfer. For neutron stars, the stated justification is that for low spin the Kerr approximation remains adequate, with discrepancies of order \(\le 10\%\) at \(a_*<0.3\) [2111.12838]. This makes relxillNS most naturally suited to slowly rotating or moderately rotating neutron-star systems, while preserving the standard relativistic observables used in disk-reflection work: inclination, inner radius, emissivity profile, ionization state, and elemental abundance.

## 2. Physical assumptions and parameterization

The underlying geometry is a geometrically thin, optically thick accretion disk modeled locally as a plane-parallel slab of constant density, illuminated by a single-temperature blackbody [2111.12838]. The ionization state is parameterized by
\[
\xi = \frac{4\pi F_{\rm x}}{n_e},
\]
where \(F_{\rm x}\) is the incident flux and \(n_e\) is the electron density [2111.12838]. Radial illumination is typically described by an emissivity law of the form \(\epsilon(r)\propto r^{-q}\), either as a single power law or a broken power law depending on the application [2111.12838].

The model space reported for relxillNS includes the following principal ranges [2111.12838]:

| Parameter | Meaning | Range |
|---|---|---|
| \(kT_{\rm bb}\) | Blackbody temperature | \(0.5\)–\(10\) keV |
| \(\log \xi\) | Ionization parameter | \(1\)–\(4\) |
| \(\log n_e\) | Electron density | \(15\)–\(19\) |
| \(A_{\rm Fe}\) | Iron abundance | \(0.5\)–\(10\) |
| \(a_*\) | Spin parameter | \(-0.998\) to \(+0.998\) |
| \(i\) | Inclination | \(3^\circ\)–\(87^\circ\) |
| \(R_{\rm in}\) | Inner disk radius | \(1\)–\(1000\) ISCO |
| \(R_{\rm out}\) | Outer disk radius | \(1\)–\(1000\,R_g\) |

In many applied fits, several of these parameters are fixed because burst or time-resolved

Source: https://www.emergentmind.com/topics/relxillns