An Evolving Broad Iron Line from the First Galactic Ultraluminous X-Ray Pulsar Swift J0243.6+6124

Abstract

We present a spectral study of the ultraluminous Be/X-ray transient pulsar Swift J0243.6+6124 using Neutron Star Interior Composition Explorer (NICER) observations during the system's 2017-2018 giant outburst. The 1.2-10 keV energy spectrum of the source can be approximated with an absorbed cut-off power law model. We detect strong, luminosity-dependent emission lines in the 6-7 keV energy range. A narrow 6.42 keV line, observed in the sub-Eddington regime, is seen to evolve into a broad Fe-line profile in the super-Eddington regime. Other features are found at 6.67 and 6.97 keV in the Fe-line complex. An asymmetric broad line profile, peaking at 6.67 keV, is possibly due to Doppler effects and gravitational redshift. The 1.2-79 keV broadband spectrum from NuSTAR and NICER observations at the outburst peak is well described by an absorbed cut-off power law plus multiple Gaussian lines and a blackbody component. Physical reflection models are also tested to probe the broad iron line feature. Depending on the mass accretion rate, we found emission sites that are evolving from ∼5000 km to a range closer to the surface of the neutron star. Our findings are discussed in the framework of the accretion disk and its implication on the magnetic field, the presence of optically thick accretion curtain in the magnetosphere, jet emission, and the massive, ultra-fast outflow expected at super-Eddington accretion rates. We do not detect any signatures of a cyclotron absorption line in the NICER or NuSTAR data.

AstroSat observations of the first Galactic ULX pulsar Swift J0243.6+6124

“…Therefore, it may be possible that the thermal emission observed at a temperature of about 1.2 keV during Obs-2 is due to the emission from the accretion column while the origin of the lower temperature (0.38 keV) black-body component is due to the possible presence of optically-thick outflows. However, we note that a recent study Jaisawal et al (2019) suggested that, in the ultra-luminous state, the iron line is complex, and if accurately modelled, then the 2 additional black-body components are not required at extreme luminosity. Mushtukov et al (2017) proposed that during super-Eddington accretion, the presence of an accretion envelope plays a key role in the accretion process at extreme mass accretion rates.…”

“…These sources might therefore act as an important connecting bridge between the classical X-ray pulsars and ULPs. Jaisawal et al (2019) found a narrow 6.42 keV line when the source was in the sub-Eddington regime. The absence of an iron emission feature in the LAXPC spectra (when the source was accreting at a sub-critical level) could be due to the limited energy resolution of the instrument, which is ∼20% at 6.4 keV (see Yadav et al 2017).…”

“…Very recently, Zhang et al (2019) reported results obtained from HXMT monitoring of J0243, finding no evidence for a cyclotron feature up to 150 keV. However, based on the spin evolution study of J0243 performed using HXMT data, they also suggested that the NS's magnetic field is ∼ 10 13 G. However, this estimate contrasts with that proposed by Jaisawal et al (2019), whose broad iron line (peaking at 6.67 keV) in NICER spectra requires a dipolar magnetic field in a narrow range between 10 11 G and 10 12 G if it is to originate in the accretion disc. The presence of a weakly magnetized neutron star is also supported by a sharp state transition of the timing and spectral properties of the source at super-Eddington accretion rates (Doroshenko et al 2020).…”

NICER observations of the black hole candidate MAXI J0637–430 during the 2019–2020 outburst

“…Data below 0.7 keV are not included in our spectral fitting due to observed calibration issues like excess at lower energies. We added a systematic of 1.5% during the spectral analysis, as recommended by the NICER team 3 (Jaisawal et al 2019).…”

Revisiting the spectral and timing properties of 4U 1909+07 with NuSTAR and Astrosat

“…The neutron star in these systems usually revolves in a wide, eccentric orbit and is known to accrete matter from the circumstellar disk. The abrupt mass accretion by the neutron star, at the closest approach (periastron), leads to the occurrence of intense X-ray outbursts with luminosity in the range of 10 36 -10 38 erg s −1 (Naik et al 2013;Reig & Nespoli 2013;Wilson-Hodge et al 2018;Jaisawal et al 2019).…”