William W. Zhang

The Nuclear Spectroscopic Telescope Array (NuSTAR) High-Energy X-Ray Mission

The Nuclear Spectroscopic Telescope Array (NuSTAR) is a National Aeronautics and Space Administration (NASA) Small Explorer mission that carried the first focusing hard X-ray (6-79 keV) telescope into orbit. It was launched on a Pegasus rocket into a low-inclination Earth orbit on June 13, 2012, from Reagan Test Site, Kwajalein Atoll. NuSTAR will carry out a two-year primary science mission. The NuSTAR observatory is composed of the X-ray instrument and the spacecraft. The NuSTAR spacecraft is three-axis stabilized with a single articulating solar array based on Orbital Sciences Corporations LEOStar-2 design. The NuSTAR science instrument consists of two co-aligned grazing incidence optics focusing on to two shielded solid state CdZnTe pixel detectors. The instrument was launched in a compact, stowed configuration, and after launch, a 10-meter mast was deployed to achieve a focal length of 10.15 m. The NuSTAR instrument provides sub-arcminute imaging with excellent spectral resolution over a 12-arcminute field of view. The NuSTAR observatory will be operated out of the Mission Operations Center (MOC) at UC Berkeley. Most science targets will be viewed for a week or more. The science data will be transferred from the UC Berkeley MOC to a Science Operations Center (SOC) located at the California Institute of Technology (Caltech). In this paper, we will describe the mission architecture, the technical challenges during the development phase, and the post-launch activities.

In-orbit performance and calibration of the Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array (PCA)

The proportional counter array (PCA) is designed to perform microsecond timing of bright galactic sources and broad band, confusion limited, studies of faint extragalactic sources in the 2 - 60 keV x-ray band. The PCA was launched as part of the Rossi X-ray Timing Explorer (RXTE) satellite into a circular orbit of altitude 580 km and 23 degrees inclination on December 30, 1995. The mission contains three experiments: a set of large area xenon proportional counters sensitive from 2 - 60 keV (proportional counter array: PCA), a set of large area sodium iodide scintillators sensitive from 15 - 200 keV (high energy x-ray timing experiment: HEXTE), and three wide field of view scanning detectors which monitor most of the sky each orbit (all sky monitor: ASM). The goals of the mission are summarized by Swank et al. We present performance and calibration data on the measured and predicted in-orbit background, energy response, relative and absolute timing performance, and the operational possibilities made available with the high performance experiment data system (EDS) designed and built by MIT.

Calibration of the Rossi X-Ray Timing Explorer Proportional Counter Array

We present the calibration and background model for the Proportional Counter Array on board the Rossi X-Ray Timing Explorer. The energy calibration is systematics-limited below 10 keV, with deviations from a power-law fit to the Crab Nebula plus pulsar of less than 1%. Unmodeled variations in the instrumental background amount to less than 2% of the observed background below 10 keV and less than 1% between 10 and 20 keV. Individual photon arrival times are accurate to 4.4 μs at all times during the mission and to 2.5 μs after 1997 April 29. The peak pointing direction of the five collimators is known to a precision of a few arcseconds.

Millisecond X-Ray Variability from an Accreting Neutron Star System

We report the detection with the Proportional Counter Array (PCA) on board the Rossi X-Ray Timing Explorer (RXTE) of millisecond variability in the X-ray emission from the low-mass X-ray binary 4U 1728-34. Pulsations at 363 Hz with amplitudes (rms) of 2.5%-10% are present in six of the eight bursts analyzed to date. The strongest were seen in two successive bursts recorded on 1996 February 16 when the quiescent count rate was near the highest seen by PCA. The pulsations during these bursts show frequency changes of 1.5 Hz during the first few seconds but become effectively coherent as the burst decays. We interpret the 363 Hz pulsations as rotationally induced modulations of inhomogeneous burst emission. This represents the first compelling evidence for a millisecond spin period in a low-mass X-ray binary. Complex, intensity-dependent, millisecond X-ray variability is also present in all the quiescent flux intervals we examined. Most interesting was the behavior as the count rate approached its highest observed level. Two quasi-periodic oscillations (QPOs) were simultaneously observed in the 650-1100 Hz range. Both QPOs increased in frequency together, maintaining a nearly constant frequency separation of about 363 Hz, the spin period inferred from the burst oscillations. This phenomenology is strongly suggestive of the magnetospheric beat frequency model proposed for the horizontal-branch oscillations (HBOs) seen in Z sources. We discuss this and several other possible physical interpretations for the observed X-ray variability.

A rapidly spinning supermassive black hole at the centre of NGC 1365

Broad X-ray emission lines from neutral and partially ionized iron observed in active galaxies have been interpreted as fluorescence produced by the reflection of hard X-rays off the inner edge of an accretion disk. In this model, line broadening and distortion result from rapid rotation and relativistic effects near the black hole, the line shape being sensitive to its spin. Alternative models in which the distortions result from absorption by intervening structures provide an equally good description of the data, and there has been no general agreement on which is correct. Recent claims that the black hole (2 × 106 solar masses) at the centre of the galaxy NGC 1365 is rotating at close to its maximum possible speed rest on the assumption of relativistic reflection. Here we report X-ray observations of NGC 1365 that reveal the relativistic disk features through broadened Fe-line emission and an associated Compton scattering excess of 10–30 kiloelectronvolts. Using temporal and spectral analyses, we disentangle continuum changes due to time-variable absorption from reflection, which we find arises from a region within 2.5 gravitational radii of the rapidly spinning black hole. Absorption-dominated models that do not include relativistic disk reflection can be ruled out both statistically and on physical grounds.

Discovery of submillisecond quasi-periodic oscillations in the x-ray flux of scorpius X-1

We report the discovery, with NASAs Rossi X-Ray Timing Explorer (RXTE), of the first submillisecond oscillations found in a celestial X-ray source. The quasi-periodic oscillations (QPOs) come from Sco X-1 and have a frequency of ~1100 Hz and amplitudes of 0.6%-1.2% (rms) and are relatively coherent, with Q up to ~102. The frequency of the QPOs increases with accretion rate, rising from 1050 to 1130 Hz when the source moves from top to bottom along the normal branch in the X-ray color-color diagram, and shows a strong, approximately linear correlation with the frequency of the well-known 6-20 Hz normal/flaring-branch QPOs. We also report the discovery of QPOs with a frequency near 800 Hz that occur, simultaneously with the 1100 Hz QPOs, in the upper normal branch. We discuss several possible interpretations, one involving a millisecond X-ray pulsar whose pulses we see reflected off accretion flow inhomogeneities. Finally, we report the discovery of ~45 Hz QPOs, most prominent in the middle of the normal branch, which might be magnetospheric beat-frequency QPOs.

Asymmetries in core-collapse supernovae from maps of radioactive 44 Ti in Cassiopeia A

Asymmetry is required by most numerical simulations of stellar core-collapse explosions, but the form it takes differs significantly among models. The spatial distribution of radioactive 44Ti, synthesized in an exploding star near the boundary between material falling back onto the collapsing core and that ejected into the surrounding medium, directly probes the explosion asymmetries. Cassiopeia A is a young, nearby, core-collapse remnant from which 44Ti emission has previously been detected but not imaged. Asymmetries in the explosion have been indirectly inferred from a high ratio of observed 44Ti emission to estimated 56Ni emission, from optical light echoes, and from jet-like features seen in the X-ray and optical ejecta. Here we report spatial maps and spectral properties of the 44Ti in Cassiopeia A. This may explain the unexpected lack of correlation between the 44Ti and iron X-ray emission, the latter being visible only in shock-heated material. The observed spatial distribution rules out symmetric explosions even with a high level of convective mixing, as well as highly asymmetric bipolar explosions resulting from a fast-rotating progenitor. Instead, these observations provide strong evidence for the development of low-mode convective instabilities in core-collapse supernovae.

NuSTAR DISCOVERY OF A 3.76 s TRANSIENT MAGNETAR NEAR SAGITTARIUS A

We report the discovery of 3.76 s pulsations from a new burst source near Sgr A^* observed by the NuSTAR observatory. The strong signal from SGR J1745–29 presents a complex pulse profile modulated with pulsed fraction 27% ± 3% in the 3-10 keV band. Two observations spaced nine days apart yield a spin-down rate of Ṗ =(6.5 ± 1.4) × 10^(–12). This implies a magnetic field B = 1.6 × 10^(14) G, spin-down power Ė =5 × 10^(33) erg s^(–1), and characteristic age P/2Ṗ =9 × 10^3 yr for the rotating dipole model. However, the current Ṗ may be erratic, especially during outburst. The flux and modulation remained steady during the observations and the 3-79 keV spectrum is well fitted by a combined blackbody plus power-law model with temperature kT_(BB) = 0.96 ± 0.02 keV and photon index Γ = 1.5 ± 0.4. The neutral hydrogen column density (N_H ~ 1.4 × 10^(23) cm^(–2)) measured by NuSTAR and Swift suggests that SGR J1745–29 is located at or near the Galactic center. The lack of an X-ray counterpart in the published Chandra survey catalog sets a quiescent 2-8 keV luminosity limit of L_x ≾ 10^(32) erg s^(–1). The bursting, timing, and spectral properties indicate a transient magnetar undergoing an outburst with 2-79 keV luminosity up to 3.5 × 10^(35) erg s^(–1) for a distance of 8 kpc. SGR J1745–29 joins a growing subclass of transient magnetars, indicating that many magnetars in quiescence remain undetected in the X-ray band or have been detected as high-B radio pulsars. The peculiar location of SGR J1745–29 has important implications for the formation and dynamics of neutron stars in the Galactic center region.

CALIBRATION OF THE NuSTAR HIGH-ENERGY FOCUSING X-RAY TELESCOPE

We present the calibration of the Nuclear Spectroscopic Telescope Array (NuSTAR) X-ray satellite. We used the Crab as the primary effective area calibrator and constructed a piece-wise linear spline function to modify the vignetting response. The achieved residuals for all off-axis angles and energies, compared to the assumed spectrum, are typically better than ±2% up to 40 keV and 5%–10% above due to limited counting statistics. An empirical adjustment to the theoretical two-dimensional point-spread function (PSF) was found using several strong point sources, and no increase of the PSF half-power diameter has been observed since the beginning of the mission. We report on the detector gain calibration, good to 60 eV for all grades, and discuss the timing capabilities of the observatory, which has an absolute timing of ±3 ms. Finally, we present cross-calibration results from two campaigns between all the major concurrent X-ray observatories (Chandra, Swift, Suzaku, and XMM-Newton), conducted in 2012 and 2013 on the sources 3C 273 and PKS 2155-304, and show that the differences in measured flux is within ~10% for all instruments with respect to NuSTAR.

The Ultraluminous X-Ray Sources NGC 1313 X-1 and X-2: A Broadband Study with NuSTAR and XMM-Newton

We present the results of NuSTAR and XMM-Newton observations of the two ultraluminous X-ray sources: NGC 1313 X-1 and X-2. The combined spectral bandpass of the two satellites enables us to produce the first spectrum of X-1 between 0.3 and 30 keV, while X-2 is not significantly detected by NuSTAR above 10 keV. The NuSTAR data demonstrate that X-1 has a clear cutoff above 10 keV, whose presence was only marginally detectable with previous X-ray observations. This cutoff rules out the interpretation of X-1 as a black hole in a standard low/hard state, and it is deeper than predicted for the downturn of a broadened iron line in a reflection-dominated regime. The cutoff differs from the prediction of a single-temperature Comptonization model. Further, a cold disk-like blackbody component at ~0.3 keV is required by the data, confirming previous measurements by XMM-Newton only. We observe a spectral transition in X-2, from a state with high luminosity and strong variability to a lower-luminosity state with no detectable variability, and we link this behavior to a transition from a super-Eddington to a sub-Eddington regime.