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NICER / ISS Science Nugget
for August 4, 2022



Eclipses reveal more than they conceal

Discovered in 2018, the binary system known as Swift J1858.6-0814 consists of a neutron star and a low-mass companion star in a 21.3 hour orbit. Initially seen in a flaring state that suggested an accreting black hole might lurk within the system, NICER observations during a phase of steady emission in 2020 established that the accreting object in Swift J1858 is in fact a neutron star, and also showed that the system's X-ray brightness drops to essentially zero periodically, revealing the orbital period. Because the X-ray emission originates from both the neutron star's surface and a compact disk of hot accreting gas, the brightness minima lasting nearly 1.25 hours were naturally interpreted as eclipses, the passage of the companion star through our line of sight, obscuring our view of the accretion region.

In all, NICER observed 5 eclipse entrances (ingress) and 7 exits (egress) before Swift J1858 faded to quiescence. These data clearly showed that ingress and egress were unequal in duration - approximately 100 vs. 200 seconds in duration, and that these durations varied with the energy of the measured X-ray photons (Figure 1). This raised the possibility that modeling of the eclipses could reveal information about the nature and structure of the companion star. A paper recently published in the UK journal Monthly Notices of the Royal Astronomical Society, led by Amy Knight of the University of Oxford, now accomplishes this modeling. The authors infer both the ratio of the masses of the two orbiting stars as well as the inclination (or "tilt") of the orbit relative to our line of sight. This information in turn constrains the companion stars radius to be in the range 1.02-1.29 times our Sun's radius, and its mass to be in the range 0.183-0.372 times the mass of our Sun. The results also imply that radiation from the neutron star is ablating material from the companion, with this material responsible for the longer eclipse egress. Future measurements of Doppler shifted spectral lines from the companion should be able to provide a precise mass measurement for the neutron star as well.


Averaged and normalized eclipse profiles of Swift J1858.6-0814 shown for seven energy bands; 0.4-1.0 keV (red), 1.0-2.0 keV (orange), 2.0-4.0 keV (yellow), 4.0-6.0 keV (green), 6.0-8.0 keV (blue), 8.0-10.0 keV (magenta) and 0.4-10.0 keV (grey). From Knight et al. (2022). Probability distributions for the mass ratio, q(top), and binary inclination, i (right). These are shown for both the Gaussian (blue) and exponential (red) eclipse models of Knight et al. (2022). Blue and red dashed lines show 1-sigma confidence intervals. The center plot shows a 2D projection of these distributions for both density profile models, plotted with the theoretical q-i relation (black). Dark, medium and light shades of blue and red highlight 1, 2 and 3-sigma contours in this 2D parameter space.

Figure: Left: Averaged and normalized eclipse profiles of Swift J1858.6-0814 shown for seven energy bands; 0.4-1.0 keV (red), 1.0-2.0 keV (orange), 2.0-4.0 keV (yellow), 4.0-6.0 keV (green), 6.0-8.0 keV (blue), 8.0-10.0 keV (magenta) and 0.4-10.0 keV (grey). From Knight et al. (2022). Right: Probability distributions for the mass ratio, q(top), and binary inclination, i (right). These are shown for both the Gaussian (blue) and exponential (red) eclipse models of Knight et al. (2022). Blue and red dashed lines show 1-sigma confidence intervals. The center plot shows a 2D projection of these distributions for both density profile models, plotted with the theoretical q-i relation (black). Dark, medium and light shades of blue and red highlight 1, 2 and 3-sigma contours in this 2D parameter space.



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