NICER / ISS Science Nugget for June 13, 2019

NICER X-rays the photosphere of a neutron star's companion

GRS 1747-312 is a bright transient X-ray source in the globular cluster Terzan 6 located about 9.5 kpc from the Earth. It is a binary system consisting of a neutron star and a companion star in an orbit oriented such that the neutron star is eclipsed by its companion every ~12 hours. This week, NICER reacted to a Target of Opportunity (TOO) request to observe the source while it is in outburst. NICER executed 6 observations during times when the neutron star was expected to egress from being eclipsed by its companion.

The figure shows an X-ray light curve measured by NICER of one of these egresses as well as a 2D histogram showing X-ray photon energies vs time during the egress period. NICER measurements of the transition times will be used to update orbit parameters for the system. The 2D histogram shows that higher energy photons make it out of eclipse before the lower energy photons since they can penetrate the atmosphere of the companion star more easily.

The specific timescales for the emergence of the high- vs. low-energy photons reveal the density of the material through which they are traveling; thus, NICER's spectroscopic data over the short duration of the eclipse transition are effectively "X-raying" the companion star's atmosphere.

Lightcurve and spectra of GRS 1747-312 as the neutron star emerges from eclipse

Figure: Left: Lightcurve of GRS 1747-312 as the neutron star emerges from behind its companion. Right: Summed 2-D spectral plot for 5 egresses shwing that higher-energy photons are detected earlier than low-energy photons as the neutron star exits the eclipse.

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