A statistical hardness classification of ultraluminous X-ray sources using long-term Swift/XRT monitoring
Abstract
We characterize long-term hardness behavior in 29 ultraluminous X-ray sources (ULXs) using over 3500 archival Swift/XRT observations. We classify observations as soft-band dominated, hard-band dominated, or uncertain from the hardness ratio (HR; 1.5--10/0.3--1.5 keV count rates) and its confidence bounds. We derive fsoft and fhard and define the Crossing Index (CI) as the fraction of consecutive confidently classified observations on opposite sides of HR=1. We analyze these descriptors with rule-based classification and unsupervised hierarchical clustering. The rule-based analysis identifies eight soft-band-dominated, five hard-band-dominated, and 16 mixed-band systems; clustering yields a similar three-group structure. Known or candidate accretor types tend to occupy band-dominated groups, whereas unknown accretors are more common in the mixed-band group. All four sources with reported long-term X-ray modulation discussed in an orbital or super-orbital context are classified as mixed-band. The three-cluster solution is preferred in 25 of 29 leave-one-out trials, while bootstrap tests find the band-dominated groups more stable than the mixed-band group. As the classification uses observed count rates, the groups represent Swift/XRT signatures rather than intrinsic spectral states. Response-folding tests show that modeled HR depends on absorption column density and photon index. Bayesian reanalysis of a representative low-count source reduces CI from 56.3\% to 38.5\% while preserving its mixed-band classification. Random downsampling produces no systematic shift in CI; halving the observations changes median CI by at most ∼10% across four sources. Larger samples with independent absorption constraints can test links among these observational classes, accretor type, long-term periodic modulation, and other X-ray properties.
Versions
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BibTeX
@misc{allak2026statistical,
title = {{A statistical hardness classification of ultraluminous X-ray sources using long-term Swift/XRT monitoring}},
author = {Sinan Allak and Wei Yu and Menglei Zhou and Honghui Liu and Andrea Santangelo},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.31324},
eprint = {2609.31324},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.31324},
}
RIS
TY - UNPB
TI - A statistical hardness classification of ultraluminous X-ray sources using long-term Swift/XRT monitoring
AU - Sinan Allak
AU - Wei Yu
AU - Menglei Zhou
AU - Honghui Liu
AU - Andrea Santangelo
PY - 2026
DA - 2026/09/25
PB - arXiv
DO - 10.48550/arxiv.2609.31324
UR - https://arxiv.org/abs/2609.31324
AB - We characterize long-term hardness behavior in 29 ultraluminous X-ray sources (ULXs) using over 3500 archival Swift/XRT observations. We classify observations as soft-band dominated, hard-band dominated, or uncertain from the hardness ratio (HR; 1.5--10/0.3--1.5 keV count rates) and its confidence bounds. We derive f_soft and f_hard and define the Crossing Index (CI) as the fraction of consecutive confidently classified observations on opposite sides of HR=1. We analyze these descriptors with rule-based classification and unsupervised hierarchical clustering. The rule-based analysis identifies eight soft-band-dominated, five hard-band-dominated, and 16 mixed-band systems; clustering yields a similar three-group structure. Known or candidate accretor types tend to occupy band-dominated groups, whereas unknown accretors are more common in the mixed-band group. All four sources with reported long-term X-ray modulation discussed in an orbital or super-orbital context are classified as mixed-band. The three-cluster solution is preferred in 25 of 29 leave-one-out trials, while bootstrap tests find the band-dominated groups more stable than the mixed-band group. As the classification uses observed count rates, the groups represent Swift/XRT signatures rather than intrinsic spectral states. Response-folding tests show that modeled HR depends on absorption column density and photon index. Bayesian reanalysis of a representative low-count source reduces CI from 56.3\% to 38.5\% while preserving its mixed-band classification. Random downsampling produces no systematic shift in CI; halving the observations changes median CI by at most ∼10% across four sources. Larger samples with independent absorption constraints can test links among these observational classes, accretor type, long-term periodic modulation, and other X-ray properties.
ER -