Investigating ULX accretion flows and cyclotron resonance in NGC 300 ULX1

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 phaseaveraged 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) vs 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 been recently proposed for the emission of the optically thick accretion envelope expected to form in ultraluminous (L X > 10 39 erg/s), highly magnetized accreting neutron stars. We combine 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 modelled 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 truncated at a large distance from the central object, as has been recently suggested for a large fraction of both pulsating and nonpulsating ULXs. The hard, pulsed emission is not described by a smooth spectral continuum. If modelled by a broad Gaussian absorption line, the fit residuals can be interpreted as a cyclotron scattering feature (CRSF) compatible with a ∼10 12 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 yields physically meaningful parameter values. Therefore strong doubts are cast on the presence of a CRSF in NGC 300 ULX1.

Long-term X-ray spectral evolution of ultraluminous X-ray sources: implications on the accretion flow geometry and the nature of the accretor

“…luminosity, radius, temperature, etc.). As the latest studies revealed that the 0.3−10 keV band can be modelled by two thermal components (e.g., Mukherjee et al 2015;Koliopanos et al 2017Koliopanos et al , 2019Walton et al 2020), which can reproduce the curvature seen at high-energies, we first considered a phenomenological model based on two multi-colour blackbody disks (diskbb in xspec; Mitsuda et al 1984) to fit the data in this band, taking into account interstellar absorption by neutral hydrogen with two absorbing components tbabs in xspec. One was frozen at the Galactic value along the source line of sight (see Table 1), and the other one was left free to vary to take into account possible absorption from the host galaxy and the system itself.…”

Chandra reveals a possible ultrafast outflow in the super-Eddington Be/X-ray binary Swift J0243.6+6124

“…These extragalactic X-ray emitters have X-ray luminosities (greatly) exceeding ∼ 10 39 erg/s, or the Eddington luminosity of a 10M black hole. Recently, a handful of ULXs has been identified as accreting neutron stars through the detection of pulsations (Bachetti et al 2014;Fürst et al 2016;Israel et al 2017a,b;Carpano et al 2018), with several additional candidates found through possible cyclotron resonance scattering features (Brightman et al 2018;Koliopanos et al 2019;Walton et al 2018c). This confirms the super-Eddington nature of at least a fraction of ULXs.…”

From SN 2010da to NGC 300 ULX-1: Ten Years of Observations of an Unusual High Mass X-Ray Binary in NGC 300

“…More detailed and physically-motivated studies of the broadband X-ray continuum were performed by Walton et al [42] and Koliopanos et al [43]. In Walton et al [42], the pulsed spectrum of NGC 300 ULX-1 was constructed by extracting the data from the brightest and faintest quarters of the pulsed cycle and removing the later from the former ("pulse-on"-"pulse-off"; the pulse profile is nearly sinusoidal, [28]).…”

“…In the accretion column model, both the flux and the slope of the high-energy power-law component are expected to change. This could be due to a change in the inclination angle at which the observer views the column during the "pulse-on" and "pulse-off" phases; during the "pulse-on" phase, the accretion column is closer to face-on, which increases the scattering optical depth and hardens the power-law slope [35,43]. This behavior is not observed in NGC 300 ULX-1, where the fitted "pulse-on" power-law slope is marginally softer than in the "pulse-off" spectrum [43].…”