Evidence for Precession due to Supercritical Accretion in Ultraluminous X-Ray Sources

Abstract

Most ultraluminous X-ray sources (ULXs) are thought to be supercritical accreting compact objects, where massive outflows are inevitable. Using the long-term monitoring data with the Swift X-ray Telescope, we identified a common feature in bright, hard ULXs: they display a quasi-periodic modulation in their hard X-ray band but not in their soft band. As a result, some sources show a bimodal distribution on the hardness intensity map. We argue that these model-independent results can be well interpreted in a picture that involves supercritical accretion with precession, where the hard X-ray emission from the central funnel is more or less beamed, while the soft X-rays may arise from the photosphere of the massive outflow and be nearly isotropic. It implies that precession may be ubiquitous in supercritical systems, such as the Galactic microquasar SS 433. How the hard X-rays are modulated can be used to constrain the angular distribution of the hard X-ray emission and the geometry of the accretion flow. We also find that two ULX pulsars (NGC 5907 ULX-1 and NGC 7793 P13) show similar behaviors but no bimodal distribution, which may imply that they have a different beaming shape or mechanism.

Long-term X-ray spectral evolution of ultraluminous X-ray sources: implications on the accretion flow geometry and the nature of the accretor

“…Instead, the variability is driven mostly by the hard component (the soft diskbb changes by a factor 2 in luminosity while the hard component luminosity varies by a factor 5 in both sources). Furthermore, the variability of M 81 X-6 is also likely associated with a 115 day quasi-periodicity (Weng & Feng 2018) compatible with the one seen in NGC 1313 X-2. Given the similarity of these two sources, we suggest that M 81 X-6 also harbours a weakly magnetised NS, meaning that the presence of strong outflows along with source precession may account for its spectral variability.…”

“…NGC 1313 X-2: R m < R sph -This source shows a strong bimodal behaviour in the HLD (see Fig. 6 and also Feng & Kaaret 2006;Pintore & Zampieri 2012) that is likely associated with the 158 day quasi-periodicity reported by Weng & Feng (2018) using Swift-XRT data, although an association is not straightforward because several periodicities are found in the periodogram presented by the authors. Our analysis reveals that this bi-modal behaviour is driven by a highly variable hard component while the soft emission is rather stable (the soft diskbb varies by a factor 2 in luminosity while the hard component luminosity varies by a factor 5, see Fig.…”

“…5) in the case of NGC 7793 P13 and 39 days in the case of M 51 ULX-7 (Vasilopoulos et al 2020). Indeed, long-term Swift-XRT monitoring shows no clear bimodal behaviour (Weng & Feng 2018) in a hardness-intensity diagram in the case of NGC 7793 P13. Both sources also show a very hot hard component (T hard ∼ 3 keV), although it is unclear whether the absence of a third, high-energy component may have boosted the inferred temperature.…”

Discovery of two eclipsing X-ray binaries in M 51

“…It is worth noting that the binary pa-rameters of S2, namely q ≈ 20 and a = (1.5 ± 0.1) × 10 12 cm ≈ 8 × 10 6 rg, correspond precisely to the sub-population of NS high-mass XRBs with observed super-orbital periodicities, interpreted as disk precession (Ogilvie & Dubus 2001, their Figure 7). In summary, the relative contribution of direct (hard) X-ray emission and reprocessed (soft) component in the spectra of super-critical NSs is not a simple function of accretion rate and binary viewing angle, but depends also on magnetic field and angle of misalignment, and can have long-term variations due to precession (Weng & Feng 2018).…”

Thick-disc model to explain the spectral state transition in NGC 247

“…ULXs are not stationary systems. On the contrary, these sources suffer significant X-ray variability (e.g., Kubota et al 2001;Feng & Kaaret 2006;Roberts et al 2006;Kaaret & Feng 2009;Weng & Feng 2018). It is possible that a ULX can transit to ULS and vice versa.…”