On the magnetic field in M51 ULX-8

Abstract

The reported discovery of a cyclotron resonance scattering feature (CRSF) in the spectrum of M51 ULX-8 may provide an important clue as to the nature of the magnetic field in those ultraluminous X-ray sources hosting neutron stars. In this paper we present the covariance (linearly correlated variability) spectrum of M51 ULX-8 on long (> 2000s) timescales. This allows us to unambiguously decompose the spectrum which requires multiple components in order to explain the broad-band emission. Having a clearer picture of the spectral decomposition leads to various tests for the dipole field strength of the neutron star which can be extended to other ULXs when certain criteria are met. In the case of M51 ULX-8, we rule out a very strong (~10^15 G) dipole solution with either a sub- or super-critical disc. Instead, our tests indicate an upper limit on the dipole field of ~10^12 G, and a classical super-critical inflow, similar to that inferred in other ULXs found to harbour neutron stars, although we do not rule out the presence of an additional, strong (~10^15 G) multipole field falling off steeply with distance from the neutron star.

9 pages, 4 figures, accepted to MNRAS, updated following corrections

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• Hot disk of the Swift J0243.6+6124 revealed by Insight-HXMT
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• On the Magnetic Fields, Beaming Fractions, and Fastness Parameters of Pulsating Ultra-Luminous X-Ray Sources
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• The rare X-ray flaring activity of the Ultraluminous X-ray source NGC 4559 X7
• The Ultraluminous X-ray sources population of the galaxy NGC 7456
• A multi-wavelength view of distinct accretion regimes in the pulsating ultraluminous X-ray source NGC 1313 X-2
• Propeller states in locally super-critical ULXs
• Detection of 125.5-day Optical Periodic Modulation of the Neutron Star M51 ULX-8
• Optical Counterparts of ULXs in NGC 1672
• High-Mass X-Ray Binaries with Be Donors as Ultraluminous X-Ray Sources
• The impact of precession on the observed population of ULXs
• GR-RMHD Simulations of Super-Eddington Accretion Flows onto a Neutron Star with Dipole and Quadrupole Magnetic Fields
• The High Energy X-ray Probe (HEX-P): Studying Extreme Accretion with Ultraluminous X-ray Sources
• Spectral analysis of ultraluminous X-ray pulsars with models of X-ray pulsars
• Long-Term X-Ray/UV Variability in ULXs
• The First Glimpse of ULXs Through the Near-Infrared Images Captured by the James Webb Space Telescope
• On the Spin Period of the Neutron Star in the Ultraluminous X-Ray Source M51 ULX-8
• Studying magnetic fields of ultraluminous X-ray pulsars using different accretion torques