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.
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.
A frame transfer charge-coupled device (CCD) designed for X-ray detection on board the SUZAKU spacecraft includes an input serial register and a charge injection structure which allows a very uniform ...injection of extremely small charge packets into the imaging section of the device. A variation of the fill-and-spill method was implemented to inject charge into the CCD. Very small charge packets (down to just a few electrons) can be reproducibly injected with noise as low as five-electron rms. The operation of the structure is described, and the results of the measurements are compared with the simulations. We have measured electron ldquoevaporationrdquo over potential barrier as a function of time, results being in excellent agreement with our model. By fitting a model to the data, it is possible to determine the internal capacitance of the input node. Charge injection noise as a function of signal charge was measured, and the results are also in agreement with theory. The designed structure can be used as a tool for studying and mitigating radiation damage effects in CCDs.
We have developed a back-illuminated active pixel sensor (APS) which includes an SOI readout circuit and a silicon diode detector array implemented in a separate high-resistivity wafer. Both are ...connected together using a per-pixel 3-D integration technique developed at Lincoln Laboratory. The device was fabricated as part of a program to develop a photon-counting APS for imaging spectroscopy in the soft X-ray (0.3-10-keV) spectral band. Here, we report single-pixel X-ray response with spectral resolution of 181-eV full-width at half-maximum at 5.9 keV. The X-ray data allow us to characterize the responsivity and input-referred noise properties of the device. We measured interpixel crosstalk and found large left-right asymmetry explained by coupling of the sense node to the source follower output. We have measured noise parameters of the SOI transistors and determined factors which limit the device performance.
The front-illuminated charge-coupled device (CCD) detectors in the Chandra X-ray Observatory's ACIS instrument suffered radiation damage from soft protons focused by the telescope mirror early in the ...mission. In the course of assessing this damage, the focal plane was temporarily warmed from its normal operating temperature (then -100/spl deg/C) to +30/spl deg/C. Following this "bakeout", the radiation-damaged CCDs exhibited significantly greater charge transfer inefficiency (CTI). We performed a laboratory experiment with a sibling of the flight detectors in an attempt to reproduce and better understand this phenomenon. The test CCD was cooled to -100/spl deg/C, irradiated by 120 keV protons and then warmed to +30/spl deg/C for 8 hours. As expected, after the initial irradiation, but before detector warmup, a substantial CTI increase was observed. The subsequent warmup itself then produced an additional factor /spl ap/2.5 increase in CTI. Following smaller subsequent irradiations with the detector cold, a "bakeout" for 8 hours at -60/spl deg/C produced no observable increase in CTI. However, a subsequent bakeout to +30/spl deg/C for another 8 hours resulted in an additional increase in CTI of roughly 15%. The CTI changes produced by the room temperature bakeout are accompanied by dramatic changes in the de-trapping times of electron traps responsible for the CTI. The distributions of signal amplitudes in the pixels trailing X-ray events indicate that annealing at room temperature can cause large changes of the trap emission times, from which we infer that conversion of trapping defects takes place. The observed phenomena can be explained by the previously suggested mechanism of carbon-related defect transformation. Specifically, the room-temperature annealing may allow carbon interstitials to form metastable complexes with phosphorus and/or carbon substitutional atoms.
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