Accretion disks around neutron stars regularly undergo sudden strong irradiation by Type-I X-ray bursts powered by unstable thermonuclear burning on the stellar surface. We investigate the impact on the disk during one of the first X-ray burst observations with the Neutron Star Interior Composition Explorer (NICER) on the International Space Station. The burst is seen from Aql X-1 during the hard spectral state. In addition to thermal emission from the neutron star, the burst spectrum exhibits an excess of soft X-ray photons below 1 keV, where NICER's sensitivity peaks. We interpret the excess as a combination of reprocessing by the strongly photoionized disk and enhancement of the pre-burst persistent flux, possibly due to Poynting–Robertson drag or coronal reprocessing. This is the first such detection for a short sub-Eddington burst. As these bursts are observed frequently, NICER will be able to study how X-ray bursts affect the disk and corona for a range of accreting neutron star systems and disk states.
Focus on Early Results from the Neutron star Interior Composition Explorer (NICER)
Zaven Arzoumanian & Keith C. Gendreau (NASA Goddard Space Flight Center)
From its perch on the International Space Station (ISS), NASA's NICER Mission of Opportunity aims to improve our understanding of neutron stars while harnessing a unique combination of X-ray timing and spectroscopy capabilities to explore a wealth of additional astrophysics. This Focus Issue brings together investigations accomplished in the initial months of NICER operations.
NICER was launched on 2017 June 3. Over the next 10 days, the payload traveled to the ISS and was robotically installed; it was activated on June 14. Upon completion of commissioning and performance verification activities, the mission's nominal science operations—initially approved for 18 months' duration—began on July 17. Accommodations provided by the ISS include power and telemetry of data to the ground, while NICER's location on a zenith-side attachment point affords a good view of nearly a full hemisphere of sky from Earth orbit at approximately 400 km altitude. The first actively pointed astronomy payload on ISS, NICER typically tracks 3–4 targets during each 90-minute orbit with its X-ray Timing Instrument (XTI), a collection of 56 co-aligned X-ray optics and detectors (of which 52 were functional at launch and remain available on-orbit) mounted on an elevation-over-azimuth gimbal. The XTI collects photons in the 0.2–12 keV band from a single 6 arcmin-diameter "pixel" on the sky, time-stamping each photon with better than 100 nsec GPS-referenced accuracy and estimating the photon's energy with resolution typical of solid-state silicon X-ray detectors (2.5% at 6 keV).
NICER's driving scientific motivation is to probe the extraordinary properties and behaviors of neutron stars—the dense-matter physics of their interiors, the dynamic processes that enliven their crusts and surfaces, the extreme electromagnetic environments of their magnetospheres—with implications for a broad array of physical and astrophysical studies, from bulk quantum phenomena to accretion processes to strong gravity. Principal NICER measurement objectives include precise radii and masses for a handful of neutron stars, definitive assessment of the rotational stabilities of nature's best clocks, and the origins of thermal and nonthermal emissions in a multiwavelength context. Deep exposures and accurate calibration knowledge are critical requirements. While these are built up to enable the mission's key science, some compelling early investigations have borne results, compiled here, for neutron-star systems, black-hole binaries, active stars, and other cosmic X-ray sources.
Highlights of the contributions to this Focus Issue include:
Tracking the full spectral and time evolution of a strong "photospheric radius expansion" Type I X-ray burst, the short-lived flash from thermonuclear fusion of accreted material on the surface of the neutron star in the X-ray binary 4U 1820–30 (Keek et al. 2018).
Confirmation of IGR J17062-6143 as an accreting millisecond pulsar and measurement of its orbital period, 38 minutes, as the shortest known for this class of object (Strohmayer et al. 2018).
Measurement of weakening "kilohertz" quasi-periodic oscillation (QPO) amplitudes in the soft X-ray band, discriminating among models for the origin of such modulations, favoring those in which variations in accretion rate at the inner edge of the accretion disk, likely due to strong-gravity effects related to the neutron star's spin, result in luminosity variations (Bult et al. 2018).
Detection and modeling of relativistically broadened fluorescence lines from the accretion disk in Serpens X-1, including both the known high-ionization (K-shell) iron emission near 6.4 keV and the previously undistinguished lines from L-shell iron and lower-atomic-number elements near 1 keV (Ludlam et al. 2018).
Characterization of deep and regular intensity variations (on timescales of a few minutes) from the black-hole binary GRS 1915+105, demonstrating that the energetic "wind" outflow from its accretion disk is constant in density over several months, and that it effectively switches on and off with the black hole's short-term intensity modulations, within seconds (Neilsen et al. 2018).
The Neutron Star Interior Composition Explorer (NICER) on the International Space Station (ISS) observed strong photospheric expansion of the neutron star in 4U 1820–30 during a Type I X-ray burst. A thermonuclear helium flash in the star's envelope powered a burst that reached the Eddington limit. Radiation pressure pushed the photosphere out to ∼200 km, while the blackbody temperature dropped to 0.45 keV. Previous observations of similar bursts were performed with instruments that are sensitive only above 3 keV, and the burst signal was weak at low temperatures. NICER's 0.2–12 keV passband enables the first complete detailed observation of strong expansion bursts. The strong expansion lasted only 0.6 s, and was followed by moderate expansion with a 20 km apparent radius, before the photosphere finally settled back down at 3 s after the burst onset. In addition to thermal emission from the neutron star, the NICER spectra reveal a second component that is well fit by optically thick Comptonization. During the strong expansion, this component is six times brighter than prior to the burst, and it accounts for 71% of the flux. In the moderate expansion phase, the Comptonization flux drops, while the thermal component brightens, and the total flux remains constant at the Eddington limit. We speculate that the thermal emission is reprocessed in the accretion environment to form the Comptonization component, and that changes in the covering fraction of the star explain the evolution of the relative contributions to the total flux.
We present Neutron Star Interior Composition Explorer (NICER) observations of the neutron star (NS) low-mass X-ray binary Serpens X-1 during the early mission phase in 2017. With the high spectral sensitivity and low-energy X-ray passband of NICER, we are able to detect the Fe L line complex in addition to the signature broad, asymmetric Fe K line. We confirm the presence of these lines by comparing the NICER data to archival observations with XMM-Newton/Reflection Grating Spectrometer (RGS) and NuSTAR. Both features originate close to the innermost stable circular orbit (ISCO). When modeling the lines with the relativistic line model relline, we find that the Fe L blend requires an inner disk radius of 
RISCO and Fe K is at 
RISCO (errors quoted at 90%). This corresponds to a position of 
km and 
km for a canonical NS mass (
) and dimensionless spin value of a = 0. Additionally, we employ a new version of the relxill model tailored for NSs and determine that these features arise from a dense disk and supersolar Fe abundance.
We present results of recent Neutron Star Interior Composition Explorer (NICER) observations of the accreting millisecond X-ray pulsar (AMXP) IGR J17062−6143 that show that it resides in a circular, ultracompact binary with a 38-minute orbital period. NICER observed the source for ≈26 ks over a 5.3-day span in 2017 August, and again for 14 and 11 ks in 2017 October and November, respectively. A power spectral analysis of the August exposure confirms the previous detection of pulsations at 163.656 Hz in Rossi X-ray Timing Explorer (RXTE) data, and reveals phase modulation due to orbital motion of the neutron star. A coherent search for the orbital solution using the Z2 method finds a best-fitting circular orbit with a period of 2278.21 s (37.97 minutes), a projected semimajor axis of 0.00390 lt-s, and a barycentric pulsar frequency of 163.6561105 Hz. This is currently the shortest known orbital period for an AMXP. The mass function is 9.12 × 10−8 M⊙, presently the smallest known for a stellar binary. The minimum donor mass ranges from ≈0.005 to 0.007 M⊙ for a neutron star mass from 1.2 to 2 M⊙. Assuming mass transfer is driven by gravitational radiation, we find donor mass and binary inclination bounds of 0.0175–0.0155 M⊙ and 19° < i < 27
5, where the lower and upper bounds correspond to 1.4 and 2 M⊙ neutron stars, respectively. Folding the data accounting for the orbital modulation reveals a sinusoidal profile with fractional amplitude 2.04 ± 0.11% (0.3–3.2 keV).
We report on a spectral-timing analysis of the neutron star low-mass X-ray binary (LMXB) Aql X-1 with the Neutron Star Interior Composition Explorer (NICER) on the International Space Station (ISS). Aql X-1 was observed with NICER during a dim outburst in 2017 July, collecting approximately 50 ks of good exposure. The spectral and timing properties of the source correspond to that of a (hard) extreme island state in the atoll classification. We find that the fractional amplitude of the low-frequency (<0.3 Hz) band-limited noise shows a dramatic turnover as a function of energy: it peaks at 0.5 keV with nearly 25% rms, drops to 12% rms at 2 keV, and rises to 15% rms at 10 keV. Through the analysis of covariance spectra, we demonstrate that band-limited noise exists in both the soft thermal emission and the power-law emission. Additionally, we measure hard time lags, indicating the thermal emission at 0.5 keV leads the power-law emission at 10 keV on a timescale of ∼100 ms at 0.3 Hz to ∼10 ms at 3 Hz. Our results demonstrate that the thermal emission in the hard state is intrinsically variable, and is driving the modulation of the higher energy power-law. Interpreting the thermal spectrum as disk emission, we find that our results are consistent with the disk propagation model proposed for accretion onto black holes.
We report on the detection of a kilohertz quasi-periodic oscillation (QPO) with the Neutron Star Interior Composition Explorer (NICER). Analyzing approximately 165 ks of NICER exposure on the X-ray burster 4U 0614+09, we detect multiple instances of a single-peak upper kHz QPO, with centroid frequencies that range from 400 to 750 Hz. We resolve the kHz QPO as a function of energy, and measure, for the first time, the QPO amplitude below 2 keV. We find the fractional amplitude at 1 keV is on the order of 2% rms, and discuss the implications for the QPO emission process in the context of Comptonization models.
The bright, erratic black hole X-ray binary GRS 1915+105 has long been a target for studies of disk instabilities, radio/infrared jets, and accretion disk winds, with implications that often apply to sources that do not exhibit its exotic X-ray variability. With the launch of the Neutron star Interior Composition Explorer (NICER), we have a new opportunity to study the disk wind in GRS 1915+105 and its variability on short and long timescales. Here we present our analysis of 39 NICER observations of GRS 1915+105 collected during five months of the mission data validation and verification phase, focusing on Fe xxv and Fe xxvi absorption. We report the detection of strong Fe xxvi in 32 (>80%) of these observations, with another four marginal detections; Fe xxv is less common, but both likely arise in the well-known disk wind. We explore how the properties of this wind depend on broad characteristics of the X-ray lightcurve: mean count rate, hardness ratio, and fractional rms variability. The trends with count rate and rms are consistent with an average wind column density that is fairly steady between observations but varies rapidly with the source on timescales of seconds. The line dependence on spectral hardness echoes the known behavior of disk winds in outbursts of Galactic black holes; these results clearly indicate that NICER is a powerful tool for studying black hole winds.
We report on a Neutron star Interior Composition Explorer (NICER) observation of the Galactic X-ray binary and stellar-mass black hole candidate, MAXI J1535−571. The source was likely observed in an "intermediate" or "very high" state, with important contributions from both an accretion disk and hard X-ray corona. The 2.3–10 keV spectrum shows clear hallmarks of relativistic disk reflection. Fits with a suitable model strongly indicate a near-maximal spin parameter of 
and a disk that extends close to the innermost stable circular orbit, 
(1σ statistical errors). In addition to the relativistic spectrum from the innermost disk, a relatively narrow Fe K emission line is also required. The resolution of NICER reveals that the narrow line may be asymmetric, indicating a specific range of emission radii. Fits with a relativistic line model suggest an inner radius of 
for the putative second reflection geometry; full reflection models suggest that radii a few times larger are possible. The origin of the narrow line is uncertain, but a warp likely provides the most physically plausible explanation. We discuss our results in terms of the potential for NICER to reveal new features of the inner and intermediate accretion disk around black holes.