Neutron stars are not only of astrophysical interest, but are also of great interest to nuclear physicists because their attributes can be used to determine the properties of the dense matter in ...their cores. One of the most informative approaches for determining the equation of state (EoS) of this dense matter is to measure both a star's equatorial circumferential radius Re and its gravitational mass M. Here we report estimates of the mass and radius of the isolated 205.53 Hz millisecond pulsar PSR J0030+0451 obtained using a Bayesian inference approach to analyze its energy-dependent thermal X-ray waveform, which was observed using the Neutron Star Interior Composition Explorer (NICER). This approach is thought to be less subject to systematic errors than other approaches for estimating neutron star radii. We explored a variety of emission patterns on the stellar surface. Our best-fit model has three oval, uniform-temperature emitting spots and provides an excellent description of the pulse waveform observed using NICER. The radius and mass estimates given by this model are km and (68%). The independent analysis reported in the companion paper by Riley et al. explores different emitting spot models, but finds spot shapes and locations and estimates of Re and M that are consistent with those found in this work. We show that our measurements of Re and M for PSR J0030+0451 improve the astrophysical constraints on the EoS of cold, catalyzed matter above nuclear saturation density.
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.
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.
Gamma-ray bursts (GRBs) fall into two classes: short-hard and long-soft bursts. The latter are now known to have X-ray and optical afterglows, to occur at cosmological distances in star-forming ...galaxies, and to be associated with the explosion of massive stars. In contrast, the distance scale, the energy scale and the progenitors of the short bursts have remained a mystery. Here we report the discovery of a short-hard burst whose accurate localization has led to follow-up observations that have identified the X-ray afterglow and (for the first time) the optical afterglow of a short-hard burst; this in turn led to the identification of the host galaxy of the burst as a late-type galaxy at z = 0.16 (ref. 10). These results show that at least some short-hard bursts occur at cosmological distances in the outskirts of galaxies, and are likely to be caused by the merging of compact binaries.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We describe and discuss the global properties of 45 gamma-ray bursts (GRBs) observed by HETE-2 during the first 3 years of its mission, focusing on the properties of X-ray flashes (XRFs) and X-ray ...rich GRBs (XRRs). We find that the numbers of XRFs, XRRs, and GRBs are comparable, and that the durations and the sky distributions of XRFs and XRRs are similar to those of GRBs. We also find that the spectral properties of XRFs and XRRs are similar to those of GRBs, except that the values of the peak energy E of the burst spectrum in F, the peak energy flux F sub(peak), and the energy fluence S sub(E) of XRFs are much smaller (and those of XRRs are smaller) than those of GRBs. Finally, we find that the distributions of all three kinds of bursts form a continuum in the S sub(E)(2 30 keV), S sub(E)(30 400) keV plane, the S sub(E)(2 400 keV), E sub(peak) plane, and the F sub(peak)(50 300 keV), E sub(peak) plane. These results provide strong evidence that all three kinds of bursts arise from the same phenomenon.
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.
TESS CCDs are backside illuminated deep depletion devices fabricated on high resistivity p-type silicon substrate and thinned down to a thickness of 100 microns. In order to fully deplete the ...substrate and increase the near-IR efficiency, we apply a bias voltage to the back side of the device. A bias of -20V is sufficient to fully deplete this device, but overdepletion is desirable to minimize lateral diffusion which can blur images and in turn affect the photometric precision by increasing an area (and, hence, readout and background noise) associated with a given object of interest. For this reason much larger negative bias voltages have been applied. We can achieve sharper images with larger negative substrate voltage, but we also discovered an unexpected dramatic increase of the full well capacity beyond a certain threshold voltage. A large full well above 200,000e- is accompanied by poor charge conservation once the device is in the bloomed well condition. Both phenomena can be explained by backside voltage reaching through the substrate volume in the channel stop region, eliminating the barrier for holes, and dragging the floating channel stops to a more negative potential.
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.
Context. As a result of the numerous missions dedicated to the detection of Gamma-ray bursts (GRBs), the observed properties of these events are now well known. However, studying their parameters in ...the source frame is not simple since it requires having measurements of both the bursts' parameters and of their distances. Aims. Taking advantage of the forthcoming Catalog of the High Energy Transient Explorer 2 (HETE-2) mission, the aim of this paper is to evaluate the main properties of HETE-2 GRBs-the peak energy (E_{peak}), the duration (T sub(90)) and the isotropic energy (E_{iso})-in their source frames and to derive their unbiased distribution. Methods. We first construct a complete sample containing all the bursts localized by the Wide-Field X-ray Monitor (WXM) on-board HETE-2, which are selected with a uniform criterion and whose observed parameters can be constrained. We then derive the intrinsic E_{peak}, T sub(90) and E_{iso} distributions using their redshift when it is available, or their pseudo- redshift otherwise. We finally compute the "volume of detectability" V_{max} of each GRB, i.e. the volume of the universe in which the burst is bright enough to be part of our sample, and the corresponding number of GRB within their visibility volume N_{V{max}}, in order to derive a weight for each detected burst accounting both for the detection significance and the star formation history of the universe. Results. We obtain unbiased distributions of three intrinsic properties of HETE-2 GRBs: E_{peak}{intr}, T_{intr} and the isotropic energy of the burst. These distributions clearly show the predominence of X-ray flashes (XRFs) in the global GRB population. We also derive the rate of local GRBs: R_{0}{H2} ga 11 Gpc super(-3) yr super(-1), which is intermediate between the local rate obtained by considering only the "high-luminosity" bursts ( similar to 1 Gpc super(-3) yr super(-1)) and that obtained by including the "low-luminosity" bursts (\ga200 Gpc super(-3) yr super(-1)). Conclusions. This study shows that the XRFs are predominent in the GRB population and are closely linked to the "classical" GRBs. We show that HETE- 2 detected no low-luminosity GRB like GRB 980425 or XRF 060218, due to the small size of its detectors, excluding this type of burst from our statistical analysis. The comparison of the GRB rate derived in this study with the known rate of type Ib/c supernovae clearly shows that the progenitors of SNe Ib/c must have some special characteristics in order to produce a gamma-ray burst or an X-ray flash.
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