NICER / ISS Science Nugget for December 20, 2018

Brightening in Circinus X-1 reveals outflowing wind

Many X-ray telescopes have observed a peculiar source discovered in 1969 by a sounding rocket, Circinus X-1, which consists of a neutron star and a low-mass star in an eccentric 16.9 day orbit viewed edge-on. Recently, Cir X-1 was revealed to be one of the youngest known X-ray binary systems.

This past July, NICER triggered observations of Cir X-1 after the MAXI instrument on the ISS reported a re-brightening of the source. NICER detected a remarkable spectrum (see figure) that clearly shows absorption line features.

Figure: The NICER X-ray spectrum of Cir X-1, showing brightness as a function of photon energy (a). The red curve is a model made up of summing multi-temperature accretion disk emission and a single-temperature blackbody representing the neutron star (dotted lines). The black points (with error bars) are the NICER data in comparison to the model. The inset (c) zooms in on the region of the spectrum with absorption lines from iron atoms. (d) and (b) are plots of residuals of the data compared to the model and bring out the absorption features.

This suggests that the source of X-rays – the innermost part of the accretion disk or the neutron star itself – is shining through an outflowing wind leaving the vicinity of the neutron star. By contrast, past observations obtained during quiescent, non-flaring states of Cir X-1 showed spectra with emission lines, which are apparently suppressed in the neutron star's flaring state. These spectral features are a useful probe of early-stage mass accretion processes onto young, potentially highly magnetized, neutron stars.

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