Investigating ULX accretion flows and cyclotron resonance in NGC 300 ULX1

¹ CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
e-mail: fkoliopanos@irap.omp.eu
² Université de Toulouse, UPS-OMP, IRAP, Toulouse, France
³ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
⁴ Department of Astronomy, Yale University, PO Box 208101, New Haven, CT 06520-8101, USA
⁵ Pontificia Universidad Católica de Chile, Instituto de Astrofísica, Casilla 306, Santiago 22, Chile

Received: 27 August 2018
Accepted: 19 November 2018

Abstract

Aims. We investigate accretion models for the newly discovered pulsating ultraluminous X-ray source (ULX) NGC 300 ULX1.

Methods. We analyzed broadband XMM-Newton and NuSTAR observations of NGC 300 ULX1, performing phase-averaged and phase-resolved spectroscopy. Using the Bayesian framework, we compared two physically motivated models for the source spectrum: Non-thermal accretion column emission modeled by a power law with a high-energy exponential roll-off (AC model), and multicolor thermal emission from an optically thick accretion envelope plus a hard power-law tail (MCAE model). The AC model is an often used phenomenological model for the emission of X-ray pulsars, while the MCAE model has recently been proposed for the emission of the optically thick accretion envelope that is expected to form in ultraluminous (L_X >  10³⁹ erg s⁻¹), highly magnetized accreting neutron stars. We combined the findings of our Bayesian analysis with qualitative physical considerations to evaluate the suitability of each model.

Results. The low-energy part (< 2 keV) of the source spectrum is dominated by non-pulsating, multicolor thermal emission. The (pulsating) high-energy continuum is more ambiguous. If modeled with the AC model, a residual structure is detected that can be modeled using a broad Gaussian absorption line centered at ∼12 keV. However, the same residuals can be successfully modeled using the MCAE model, without the need for the absorption-like feature. Model comparison using the Bayesian approach strongly indicates that the MCAE model without the absorption line is the preferred model.

Conclusions. The spectro-temporal characteristics of NGC 300 ULX1 are consistent with previously reported traits for X-ray pulsars and (pulsating) ULXs. All models considered strongly indicate the presence of an accretion disk that is truncated at a large distance from the central object, as has recently been suggested for a large portion of both pulsating and non-pulsating ULXs. The hard, pulsed emission is not described by a smooth spectral continuum. If modeled by a broad Gaussian absorption line, the fit residuals can be interpreted as a cyclotron scattering feature (CRSF) compatible with a ∼10¹² G magnetic field. However, the MCAE model can successfully describe the spectral and temporal characteristics of the source emission, without the need for an additional absorption feature, and it yields physically meaningful parameter values. Therefore strong doubts are cast on the presence of a CRSF in NGC 300 ULX1.