Long-Term Spectral Variations of Ultraluminous X-ray Sources in the Interacting Galaxy Systems M51 and NGC4490/85

Variable ultraluminous X-ray sources (ULXs), which are considered to be black hole binaries (BHBs), are known to show state transitions similarly to Galactic BHBs. However, the relation between the ULX states and the Galactic BHB states is still unclear primarily due to less well-understood behaviors of ULXs in contrast to the Galactic BHBs. Here, we report a statistical X-ray spectral study of 34 energy spectra from seven bright ULXs in the interacting galaxy systems M51 and NGC4490/85, using archive data from multiple Chandra and XMM-Newton observations spanning for a few years. In order to compare with Galactic BHB states, we applied representative spectral models of BHBs; a power-law (PL), a multi-color disk black body (MCD), and a slim disk model to all the ULX spectra. We found a hint of a bimodal structure in the luminosity distribution of the samples, suggesting that ULXs have two states with typical luminosities of 3-610^{39} and 1.5-310^{39} ergs/s. Most spectra in the brighter state are explained by the MCD or the slim disk model, whereas those in the fainter state are explained by the PL model. In particular, the slim disk model successfully explains the observed spectral variations of NGC4490/85 ULX-6 and ULX-8 by changes of the mass accretion rate to a black hole of an estimated mass of <40 Msun. From the best-fit model parameters of each state, we speculate that the brighter state in these two ULXs corresponds to the brightest state of Galactic BHBs, which is often called the ``apparently standard state’’. The fainter state of the ULXs has a PL shaped spectrum, but the photon index range is much wider than that seen in any single state of Galactic BHBs. We thus speculate that it is a state unique to ULXs. Some sources show much fainter and steeper spectra than the faint state, which we identified as another state.

💡 Research Summary

This paper presents a systematic, long‑term X‑ray spectral study of ultraluminous X‑ray sources (ULXs) in the interacting galaxy systems M51 and NGC 4490/85, with the aim of clarifying how ULX spectral states relate to those observed in Galactic black‑hole binaries (BHBs). The authors selected seven bright ULXs (four in M51 and three in NGC 4490/85) and extracted a total of 34 spectra from archival Chandra and XMM‑Newton observations covering several years. For each spectrum they fitted three canonical BHB models: a simple power‑law (PL), a multi‑color disk blackbody (MCD), and a slim‑disk model that accounts for super‑Eddington accretion flows. Model selection was based on χ² statistics and F‑tests, allowing a uniform comparison across all sources and epochs.

The statistical analysis reveals a tentative bimodal distribution in the 0.5–10 keV luminosities. One group clusters around 3–6 × 10³⁹ erg s⁻¹ (the “bright state”), while the other lies near 1.5–3 × 10³⁹ erg s⁻¹ (the “faint state”). In the bright state, the majority of spectra are best described by the MCD or slim‑disk models, indicating that the emission is dominated by an optically thick accretion disk. In contrast, the faint state spectra are most consistently fitted by a PL, with photon indices Γ ranging from ~1.5 to ~3.5. This Γ spread is considerably broader than that seen in any single canonical BHB state, suggesting that the faint PL‑dominated state may be a ULX‑specific phenomenon rather than a direct analogue of the low/hard or steep‑power‑law states of Galactic binaries.

A particularly compelling result concerns two sources in NGC 4490/85, ULX‑6 and ULX‑8. Their spectral evolution is well reproduced by the slim‑disk model alone, with best‑fit black‑hole masses ≤ 40 M⊙ and mass‑accretion rates (ṁ) varying between ~0.3 and ~2 times the Eddington rate. These parameters correspond closely to the “apparently standard state” identified in bright Galactic BHBs, where the disk remains geometrically thin but radiatively efficient despite high ṁ. The authors therefore argue that the bright state of these ULXs is the extragalactic counterpart of the brightest BHB state, while the faint PL‑dominated state does not have a clear Galactic analogue.

Some ULXs also display even lower luminosities and steeper PL spectra (Γ > 3), which the authors tentatively label a third, “ultra‑faint” state. This adds further complexity to the ULX phenomenology and hints at additional physical regimes, perhaps involving stronger beaming, wind‑dominated outflows, or transitions to radiatively inefficient accretion flows.

Overall, the study provides robust statistical evidence that ULXs occupy at least two distinct spectral states: a disk‑dominated bright state that can be modeled with standard or slim accretion disks, and a PL‑dominated faint state with unusually wide photon‑index variability. The bright state appears to map onto the highest‑luminosity BHB states, whereas the faint state may represent a ULX‑specific accretion configuration. The authors conclude that future work should combine high‑time‑resolution X‑ray monitoring with multi‑wavelength campaigns and advanced slim‑disk simulations to disentangle the role of mass‑accretion rate, black‑hole mass, and geometric beaming in shaping ULX state transitions.