NICER Discovers Spectral Lines During Photospheric Radius Expansion Bursts from 4U 1820-30: Evidence for Burst-driven Winds

Abstract

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 the source in 2017 August 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 approximately 0.6 keV and emitting radii of 100 km (at a distance of 8.4 kpc). The other two bursts (burst pair 2) had higher temperatures (~0.67 keV) and smaller radii (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 red-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 red-shifts, 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.

24 pages, 9 figures, accepted for publication in The Astrophysical Journal Letters

References in corpus (8)

• The Mass and Radius of the Neutron Star in 4U 1820-30
• Neutron star mass and radius measurements from atmospheric model fits to X-ray burst cooling tail spectra
• Absorption features in the spectra of X-ray bursting neutron stars
• X-Raying an Accretion Disk in Realtime: the Evolution of Ionized Reflection during a Superburst from 4U 1636-536
• Characterizing the Evolving X-Ray Spectral Features During a Superburst from 4U 1636-536
• NuSTAR observation of a Type I X-ray burst from GRS 1741.9-2853
• Detection of burning ashes from thermonuclear X-ray bursts
• The Incompatibility of Rapid Rotation with Narrow Photospheric X-ray Lines in EXO 0748-676

Cited by in corpus (13)

• NICER views moderate, strong, and extreme photospheric expansion bursts from the ultracompact X-ray binary 4U 1820$-$30
• A Comprehensive Study of Thermonuclear X-ray Bursts from 4U 1820-30 with NICER: Accretion Disk Interactions and a Candidate Burst Oscillation
• The MAVERIC Survey: Variable Jet-Accretion Coupling in Luminous Accreting Neutron Stars in Galactic Globular Clusters
• Expanded atmospheres and winds in Type I X-ray bursts from accreting neutron stars
• Quenching and recovery of persistent X-ray emission during a superburst in 4U 1820$-$30
• Discovery of a long thermonuclear X-ray burst from the ultra-compact binary 4U 1850$-$087
• The imprint of convection on Type I X-ray bursts: Pauses in photospheric radius expansion lightcurves
• Hydrodynamical simulations of proton ingestion flashes in Type I X-ray Bursts
• Emission and Absorption Lines in Photospheric Radius Expansion Bursts of 4U 1820$-$30
• On the impact of an intermediate duration X-ray burst on the accretion environment in IGR J17062-6143
• A transition from mixed-fuel to pure-helium thermonuclear bursts in Terzan 5 X-3/Swift J174805.3-244637
• Neutron star atmospheres composed of fusion ashes
• An unusual type-I X-ray burst from the neutron star X-ray binary IGR J17591-2342: a double-photospheric-radius-expansion burst?