We report the discovery with the Neutron Star Interior Composition Explorer ( NICER ) of narrow emission and absorption lines during photospheric radius expansion ( PRE ) X-ray bursts from the ultracompact binary 4U 1820 - 30 . NICER observed 4U 1820 - 30 in 2017 August during a low flux , hard spectral state , accumulating about 60 ks of exposure .
Five thermonuclear X-ray bursts were detected of which four showed clear signs of PRE .
We extracted spectra during the PRE phases and fit each to a model that includes a comptonized component to describe the accretion-driven emission , and a black body for the burst thermal radiation .
The temperature and spherical emitting radius of the fitted black body are used to assess the strength of PRE in each burst .
The two strongest PRE bursts ( burst pair 1 ) had black body temperatures of \approx { 0.6 } keV and emitting radii of \approx 100 km ( at a distance of 8.4 kpc ) .
The other two bursts ( burst pair 2 ) had higher temperatures ( \approx 0.67 keV ) and smaller radii ( \approx 75 km ) .
All of the PRE bursts show evidence of narrow line emission near 1 keV .
By co-adding the PRE phase spectra of burst pairs 1 and , separately , 2 we find , in both co-added spectra , significant , narrow , spectral features near 1.0 ( emission ) , 1.7 and 3.0 keV ( both in absorption ) .
Remarkably , all the fitted line centroids in the co-added spectrum of burst pair 1 appear systematically blue-shifted by a factor of 1.046 \pm 0.006 compared to the centroids of pair 2 , strongly indicative of a gravitational shift , a wind-induced blue-shift , or more likely some combination of both effects .
The observed shifts are consistent with this scenario in that the stronger PRE bursts in pair 1 reach larger photospheric radii , and thus have weaker gravitational redshifts , and they generate faster outflows , yielding higher blue-shifts .
We discuss possible elemental identifications for the observed features in the context of recent burst-driven wind models .