Broadband X-ray spectral variability of the pulsing ULX NGC 1313 X-2

¹ Università degli Studi di Palermo, Dipartimento di Fisica e Chimica, via Archirafi 36, 90123 Palermo, Italy
² INAF/IASF Palermo, via Ugo La Malfa 153, 90146 Palermo, Italy
e-mail: alessandra.robba@inaf.it
³ Institute of Astronomy, Madingley Road, CB3 0HA Cambridge, UK
⁴ College of Astronomy and Space Sciences, University of the Chinese Academy of Sciences, Beijing 100049, PR China
⁵ Sydney Institute for Astronomy, School of Physics A28, The University of Sydney, Sydney, NSW 2006, Australia
⁶ Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
⁷ Department of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK
⁸ Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
⁹ Science Operations Department, European Space Astronomy Centre (ESA/ESAC), Villanueva de la Canada, 28692 Madrid, Spain
¹⁰ University of Crete, Department of Physics, 71003 Heraklion, Greece
¹¹ Institute of Astrophysics, FORTH, 71110 Heraklion, Greece
¹² Institut de Ciències de l’Espai, Carrer de Can Magrans, 08193 Cerdanyola del Vallès, Barcelona, Spain
¹³ MIT Kavli Institute for Astrophysics and Space Research, Cambridge, MA 02139, USA

Received: 25 March 2021
Accepted: 7 June 2021

Abstract

Context. It is thought that ultraluminous X-ray sources (ULXs) are mainly powered by super-Eddington accreting neutron stars or black holes as shown by the recent discovery of X-ray pulsations and relativistic winds.

Aims. This work presents a follow-up study of the spectral evolution over two decades of the pulsing ULX NGC 1313 X-2 in order to understand the structure of the accretion disc. The primary objective is to determine the shape and nature of the dominant spectral components by investigating their variability with the changes in the source luminosity.

Methods. We performed a spectral analysis over the canonical 0.3–10.0 keV energy band of all the high signal-to-noise XMM-Newton observations (96% of the available data), and we tested a number of different spectral models, which should approximate super-Eddington accretion discs. The baseline model consists of two thermal blackbody components with different temperatures plus an exponential cutoff powerlaw.

Results. The baseline model provides a good description of the X-ray spectra. In particular, the hotter and brighter (L_X ∼ 6–9 × 10³⁹ erg s⁻¹) thermal component describes the emission from the super-Eddington inner disc and the cutoff powerlaw describes the contribution from the accretion column of the neutron star. Instead, the cooler component describes the emission from the outer region of the disc close to the spherisation radius and the wind. The luminosity-temperature relation for the cool component follows a negative trend, which is not consistent with L ∝ T⁴, as is expected from a sub-Eddington thin disc of Shakura-Sunayev. This is not consistent with L ∝ T² either, as is expected for an advection-dominated disc. However, this would rather agree with a wind-dominated X-ray emitting region. Instead, the (L_x, T_disk) relation for the hotter component is somewhere in between the first two theoretical scenarios.

Conclusions. Our findings agree with the super-Eddington scenario and provide further detail on the disc structure. The source spectral evolution is qualitatively similar to that seen in NGC 1313 X-1 and Holmberg IX X-1, indicating a common structure and evolution among archetypal ULXs.