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Peer-reviewedCC BY 4.0The Astrophysical Journal

Systematic Effects of Hydrogen and Helium Atmosphere Mismatch on Radius Inference in PSR J0740+6620–like Synthetic NICER Data

  • Isiah M. Holt1,2
  • M. Coleman Miller1
  • Alexander J. Dittmann3
  • Frederick K. Lamb4
4 affiliations
  1. 1University of Maryland
  2. 2NASA Goddard Space Flight Center
  3. 3Institute for Advanced Study
  4. 4University of Illinois at Urbana-Champaign
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Abstract

Constraints on neutron star radii provide insight into the properties of the cold, dense matter in their interiors. Previous studies using synthetic Neutron Star Interior Composition Explorer (NICER) pulse waveform data have demonstrated that radius inferences derived therefrom are robust against several classes of modeling systematics. Here we explore the consequences of assuming the wrong atmospheric composition, using synthetic data based on the ∼2.1 M ⊙ pulsar PSR J0740+6620. We find that the assumption of a hydrogen atmosphere when the synthetic data assumed a helium atmosphere, or vice versa, produces little bias in the inferred radius at the spot-to-background ratio of the actual PSR J0740+6620 data. However, when we increase the spot-to-background ratio by a factor of ∼20 while keeping the total number of counts fixed at the observed ∼5.5 × 10 ⁵ , we find that composition mismatch can produce significantly biased radius estimates while remaining hidden inside a fit with statistically acceptable residuals. Even in these cases, the Bayesian evidence consistently identifies the correct atmospheric model. Our findings reinforce the importance of using the Bayesian evidence for model comparison and goodness-of-fit tests.

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Cite

BibTeXRIS
BibTeX
@article{holt2026systematic,
  title = {{Systematic Effects of Hydrogen and Helium Atmosphere Mismatch on Radius Inference in PSR J0740+6620–like Synthetic NICER Data}},
  author = {Isiah M. Holt and M. Coleman Miller and Alexander J. Dittmann and Frederick K. Lamb},
  journal = {The Astrophysical Journal},
  year = {2026},
  month = oct,
  doi = {10.3847/1538-4357/aeaa30},
  url = {https://iopscience.iop.org/article/10.3847/1538-4357/aeaa30},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - Systematic Effects of Hydrogen and Helium Atmosphere Mismatch on Radius Inference in PSR J0740+6620–like Synthetic NICER Data
AU  - Isiah M. Holt
AU  - M. Coleman Miller
AU  - Alexander J. Dittmann
AU  - Frederick K. Lamb
PY  - 2026
DA  - 2026/10/19
JO  - The Astrophysical Journal
DO  - 10.3847/1538-4357/aeaa30
UR  - https://iopscience.iop.org/article/10.3847/1538-4357/aeaa30
AB  - Constraints on neutron star radii provide insight into the properties of the cold, dense matter in their interiors. Previous studies using synthetic Neutron Star Interior Composition Explorer (NICER) pulse waveform data have demonstrated that radius inferences derived therefrom are robust against several classes of modeling systematics. Here we explore the consequences of assuming the wrong atmospheric composition, using synthetic data based on the ∼2.1 M ⊙ pulsar PSR J0740+6620. We find that the assumption of a hydrogen atmosphere when the synthetic data assumed a helium atmosphere, or vice versa, produces little bias in the inferred radius at the spot-to-background ratio of the actual PSR J0740+6620 data. However, when we increase the spot-to-background ratio by a factor of ∼20 while keeping the total number of counts fixed at the observed ∼5.5 × 10 ⁵ , we find that composition mismatch can produce significantly biased radius estimates while remaining hidden inside a fit with statistically acceptable residuals. Even in these cases, the Bayesian evidence consistently identifies the correct atmospheric model. Our findings reinforce the importance of using the Bayesian evidence for model comparison and goodness-of-fit tests.
SN  - 0004-637X
SN  - 1538-4357
ER  -