NICER / ISS Science Nugget
for September 26, 2019

An evolving broad iron line from the first Galactic ultraluminous X-ray pulsar, Swift J0243.6+6124

Discovered in October 2017, Swift J0243.6+6124 (J0243) is an X-ray binary pulsar with a 9.8 second pulsation period. During the course of its five month long giant outburst, the source was so intense that it outshined the Crab Nebula by an order of magnitude in the X-ray band, establishing J0243 as an ultraluminous X-ray pulsar (ULXP), the first detected in our own Galaxy. The neutron star in this system accretes from a Be-star companion in a binary, emitting high-energy radiation when the accreted material strikes the neutron star surface at the magnetic poles, under the guidance of its magnetic field.

With NICER, the pulsar's activity was extensively monitored across the different accretion states of the original outburst. Our first results on the evolution of X-ray emission from the neutron star surface were published in Wilson-Hodge et al. 2018 (The Astrophysical Journal, vol. 863, 9). This study inferred a magnetic field value for the pulsar at 1013 Gauss. A detailed spectral analysis was carried out more recently.

The X-ray energy spectrum of a pulsar can be described by standard empirical models such as, typically, a power-law modified by a cutoff at the highest energies. However, this standard expression was unable to explain the spectral shape of J0243 because of a strong emission feature in the 6-7 keV band, corresponding to ionized atoms of iron. At lower luminosity, only the 6.4 keV line appeared in the spectrum; it was found to evolve into a broadened line profile at higher luminosities (see Figure), above the so-called Eddington limit of the neutron star (the point at which pressure from light emitted at the surface is strong enough to push back accreting material). Broad iron lines are known from accreting (Galactic or super-massive) black holes and neutron stars with low magnetic fields, less than 1011 Gauss, in which the line-emitting region reaches close to the compact object. As a result, the emission line profile gets skewed by Doppler effects and gravitational red-shift near the surface of the accreting object.

NICER spectra around 6 keV for Swift J0243.6+6124 in order of increasing luminosity

Figure: Ratio of NICER data to the continuum model (scaled arbitrarily) obtained by fitting 1.2-10 keV energy spectra of Swift J0243.6+6124. These are arranged in order of increasing luminosity; "L37" stands for unabsorbed luminosity in units of 1037 erg/s. Three emission lines in the 6-7 keV range are detected, at 6.4, 6.67, and 6.98 keV energies (vertical dash-dot lines).

Swift J0243 represents the first time a pulsar or ULXP has exhibited a broadened iron line profile in this way. Our detailed analysis suggests that the neutron star may not be as strongly magnetized as earlier believed. This work, led by G. Jaisawal (Technical University of Denmark), was accepted for peer-reviewed publication this week in The Astrophysical Journal.

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