Synchrotron radiation from NGC 470 HLX1 - a hidden hyperluminous accreting neutron star?

TL;DR

This work tests a high-latitude synchrotron emission model for the hyperluminous X-ray source NGC 470 HLX1, examining whether an accreting magnetized neutron star can account for its broadband spectrum. Using joint XMM-Newton and NuSTAR data, the authors fit a soft blackbody component plus a non-thermal synchrotron spectrum, exploring a range of magnetic-field strengths $B$ and emission latitudes $\\theta$, with a fixed electron distribution slope $p=2.2$ and $\\gamma_{max}=1000\\gamma_{min}$. The results show good fits across epochs, with $\\gamma_{min}$ spanning from roughly $10^2$ to $10^6$ depending on $B$ and $\\theta$, and a hard X-ray turnover that NuSTAR helps constrain; a purely pulsar-like timing signal is not detected. The analysis supports a neutron-star-powered interpretation for at least some HLXs, highlights the role of latitude-dependent synchrotron emission in producing the observed cutoff near ~5 keV, and motivates broader surveys to reassess the prevalence of NS accretors among HLXs. The work also discusses the physical implications for particle acceleration, magnetospheric structure, and the origin of the soft disk/outflow component, suggesting avenues for future observations to distinguish latitude effects and refine magnetic-field estimates.

Abstract

We present the first broadband spectral analysis of NGC 470 HLX1, a hyperluminous X-ray source that exhibits significant flux variability over different epochs. We investigate the feasibility of synchrotron radiation with varying latitude from a magnetized neutron star to explain the source's spectra. We also shed light on the particle acceleration mechanisms and maximum Lorentz factor of electrons within the neutron star magnetospheric plasma under super-Eddington accretion conditions. In our broadband spectral modeling, the detection of a blackbody-like component suggests the presence of a disk near the corotation radius or an outflow ejected from the disk. The viability of synchrotron emission in an HLX system offers new insights about the nature of these sources, motivating further sample studies to assess whether most of these sources are powered by accreting neutron stars.

Figures (4)

• Figure 1: The relation between cutoff energy ($\hbar n_c \omega_B$) with $\gamma$ for different $\theta$ and $B$. Clearly, for a fixed cutoff energy, $\gamma$ and $B$ have anti-correlation for $\theta = 0^{\circ}$, whereas for higher latitudes (here, i.e., $1^{\circ}$), they are correlated after a certain $\gamma$ (See text for detailed explanation).
• Figure 2: The unfolded spectra are plotted using powerlaw model of 0 index and arbitrary normalization. For visual clarity, only XMM-Newton pn and NuSTAR FPMA spectra are shown. The data has been rebinned for visual purposes.
• Figure 3: Left: The spectra, model components, and residual for bb+synchrotron fit for broadband data (XM2009 and XM2023+Nu2023). Right: Same for the XM2004 data with only synchrotron model.
• Figure 4: Electron maximum Lorentz factor varying with distance from the NS surface, according to Hillas criterion and after correcting for synchrotron cooling for different surface magnetic field strengths $B_0$ (see text for details). The vertical dotted lines show the corresponding $R_M$ for each $B_0$ case.