Signature of iron line profile from a Kerr-like wormhole

¹ Tsung-Dao Lee Institute, Shanghai Jiao Tong University, 1 Lisuo Road, Shanghai 201210, China
² School of Physics and Astronomy, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
³ Key Laboratory for Particle Astrophysics and Cosmology (MOE) and Shanghai Key Laboratory for Particle Physics and Cosmology, Shanghai Jiao Tong University, Shanghai 200240, China
⁴ Institute for Theoretical Physics & Cosmology, Zhejiang University of Technology, Hangzhou 310023, China

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Received: 29 August 2025
Accepted: 17 April 2026

Abstract

Context. Broad and skewed iron Kα emission lines in the X-ray spectra of accreting black holes encode key information about space-time geometry in the innermost disk regions. While relativistic reflection models that assume the Kerr metric have yielded spin measurements for dozens of black hole candidates, horizonless alternatives, particularly traversable “Kerr-like” wormholes, can reproduce many observational signatures of black holes and thus challenge the interpretation of these data.

Aims. We aim to develop and apply a new relativistic reflection framework that explicitly incorporates Kerr-like wormhole geometries to predict the iron line distortions introduced by wormhole throat effects and assess the feasibility of distinguishing event horizons from horizonless throats using current and future X-ray observations.

Methods. Building on a custom ray-tracing subroutine for Kerr-like wormhole space-times, we implemented two custom XSPEC modules, kwline for isolated δ-function line profiles and kwconv for full rest-frame reflection spectra, parameterized by spin, throat radius, and shape-function coefficients. We computed a dense grid of line profiles over the wormhole parameter space, embedded them within multicomponent reflection models, and generated synthetic Nuclear Spectroscopic Telescope Array (NuSTAR) spectra that incorporate realistic response matrices and backgrounds. By fitting these simulated data with canonical Kerr black hole based models and examining fit quality and residual structure, we quantified the deviations attributable to wormhole geometries.

Results. We find that Kerr-like wormholes systematically produce narrower Fe Kα lines with suppressed red wings and angle-dependent morphological shifts as the throat parameter, λ, increases. In spectral fitting simulations of a high-flux 50 ks NuSTAR exposure (mocked with λ = 0.9, a_* = 0.998), standard Kerr black hole convolution models (kerrconv) can effectively mimic the wormhole spectrum with high statistical precision. Conversely, fitting with the self-consistent, angle-dependent reflection model relxillCp results in a formal statistical failure with pronounced structured residuals and unphysical parameter pegging, such as the emissivity index q_in reaching its limit of 10. These results indicate that while Kerr-like wormholes and black holes exhibit significant observational degeneracy under simple convolutional modeling, they can be distinguished through more rigid, self-consistent reflection frameworks.

Conclusions. We conclude that large-throat wormholes are potentially detectable in high-quality X-ray spectra, provided the analysis moves beyond post-processing approximations toward fully consistent relativistic reflection models.