Pulsating ultraluminous X-ray sources: Modeling the thermal emission and polarization properties

¹ Dipartimento di Fisica e Astronomia “G. Galilei”, Università degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
² INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ Dipartimento di Fisica e Astronomia “G.Galilei”, Università degli Studi di Padova, Via Marzolo 8, 35121 Padova, Italy
⁴ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Surrey RH5 6NT UK
⁵ INAF/IASF Palermo, Via Ugo la Malfa 153, 90146 Palermo, Italy
⁶ University of Hertfordshire Centre for Astrophysics Research, Hatfield, UK
⁷ INAF-Osservatorio Astronomico di Roma, Via Frascati 33, I-00078 Monte Porzio Catone, Italy

^⋆ Corresponding author; silvia.conforti@phd.unipd.it

Received: 1 March 2024
Accepted: 10 November 2024

Abstract

Context. Ultraluminous X-ray sources (ULXs) are enigmatic sources first discovered in the 1980s in external galaxies. They are characterized by their extraordinarily high X-ray luminosity, which often exceeds 10⁴⁰ erg s⁻¹.

Aims. Our study aims to obtain more information about pulsating ULXs (PULXs), first of all, their viewing geometry, since it affects almost all the observables, such as the flux, the pulsed fraction, the polarization degree (PD), and polarization angle (PA).

Methods. We present a simplified model, which primarily describes the thermal emission from an accreting, highly magnetized neutron star, simulating the contributions of an accretion disk and an accretion envelope surrounding the star magnetosphere, both described by a multicolor blackbody. Numerical calculations are used to determine the flux, PD, and PA of the emitted radiation, considering various viewing geometries. The model predictions are then compared to the observed spectra of two PULXs, M51 ULX-7 and NGC 7793 P13.

Results. We identified the best fitting geometries for these sources, obtaining values of the pulsed fraction and the temperature at the inner radius of the disk compatible with those obtained from previous works. We also found that measuring the polarization observables can give considerable additional information on the source.