[en] In this paper, we present studies of the I-V characteristics of CdZnTe (CZT) detectors with Pt contacts fabricated from high-resistivity single crystals grown by the high-pressure Bridgman process. We have analyzed the experimental I-V curves using a model that approximates the CZT detector as a system consisting of a reversed Schottky contact, in series with the bulk resistance. Least-square fit to the experimental data yields 0.78-0.79 eV for the Pt-CZT Schottky barrier height, and <20 V for the voltage required to deplete a 2 mm thick CZT detector. We demonstrate that, at high bias, the thermionic current over the Schottky barrier, the height of which is reduced due to an interfacial layer between the contact and CZT material, controls the leakage current of the detectors. In many cases, the dark current is not determined by the resistivity of the bulk material, but rather the properties of the contacts; namely, by the interfacial layer between the contact and CZT material

[en] Future astronomical X-ray telescopes, including the balloon-borne High-Energy Focusing Telescope (HEFT) and the Constellation-X Hard X-ray Telescope (Con-X HXT) plan to incorporate depth-graded multilayer coatings in order to extend sensitivity into the hard X-ray (10 < or approx. E < or approx. 80 keV) band. In this paper, we present measurements of the reflectance in the 18-170 keV energy range of a cylindrical prototype nested optic taken at the European Synchrotron Radiation Facility (ESRF). The mirror segments, mounted in a single bounce stack, are coated with depth-graded W/Si multilayers optimized for broadband performance up to 69.5 keV (WK-edge). These designs are ideal for both the HEFT and Con-X HXT applications. We compare the measurements to model calculations to demonstrate that the reflectivity can be well described by the intended power law distribution of the bilayer thicknesses, and that the coatings are uniform at the 5% level over the mirror surface. Finally, we apply the measurements to predict effective areas achievable for HEFT and Con-X HXT using these W/Si designs

[en] We report on X-ray properties of the gamma-ray binary 1FGL J1018.6–5856 using observations obtained with the Swift X-ray telescope. Using 54 observations made between MJD 55575 and 55984, we find that the X-ray flux is modulated at a period of 16.57 ± 0.11 days, which is consistent with previous reports based on gamma-ray data. We find that the X-ray maximum at phase 0 previously reported may not be a persistent feature of the source: the dramatic increases at phase 0 were only detected for ∼100 days and were not detected thereafter. Rather, the persistent sinusoidal maximum seems to be at phase 0.3-0.4, and is misaligned with the gamma-ray (GeV) peak. We also find evidence that the source's X-ray flux is correlated with the spectral hardness in the 0.5-10 keV band. Such a correlation has also been reported in the gamma-ray binaries LS 5039 and LS I +61°303 and can help us to understand the X-ray emission mechanisms of the sources

[en] We present optical spectroscopic measurements of the eclipsing high-mass X-ray binary (HMXB) XMMU J013236.7+303228 in M 33. Based on spectra taken at multiple epochs of the 1.73 day binary orbital period we determine physical as well as orbital parameters for the donor star. We find the donor to be a B1.5IV subgiant with effective temperature T = 22, 000-23, 000 K. From the luminosity, temperature, and known distance to M 33 we derive a radius of R 8.9 ± 0.5 R_☉. From the radial-velocity measurements, we determine a velocity semi-amplitude of K_opt = 63 ± 12 km s^–1. Using the physical properties of the B star determined from the optical spectrum, we estimate the star's mass to be M_opt = 11 ± 1 M_☉. Based on the X-ray spectrum, the compact companion is likely a neutron star, although no pulsations have yet been detected. Using the spectroscopically derived B star mass we find the neutron star companion mass to be M_X = 2.0 ± 0.4 M_☉, consistent with the neutron star mass in the HMXB Vela X-1, but heavier than the canonical value of 1.4 M_☉ found for many millisecond pulsars. We attempt to use as an additional constraint that the B star radius inferred from temperature, flux, and distance should equate to the Roche radius, since the system accretes by Roche lobe overflow. This leads to substantially larger masses, but by trying to apply the technique to known systems, we find that the masses are consistently overestimated. Attempting to account for that in our uncertainties, we derive M_X = 2.2^+0.8_–0.6 M_☉ and M_opt = 13 ± 4 M_☉. We conclude that precise constraints require detailed modeling of the shape of the Roche surface.

[en] We present results from a Nuclear Spectroscopic Telescope Array (NuSTAR) observation of the Crab made at a large off-axis angle of 1.°5. At these angles, X-rays do not pass through the optics and instead illuminate the detectors directly, due to incomplete baffling. Due to the simplicity of the instrument response in this configuration and the good absolute calibration of the detectors, we are able to measure the absolute intrinsic flux of the Crab to better than 4%. We find the spectral parameters of the power law to be $\mathrm{\Gamma <!--\; ??\; -->}=2.106\pm <!--\; ??\; -->0.006$, $N=9.71\pm <!--\; ??\; -->0.16$, in agreement with the values measured 42 years ago by Toor and Seward. This suggests that the observed variability of the Crab is not part of a long-term trend, but instead results from fluctuations around a steady mean. The NuSTAR observation also enabled improved measurement of the detector absorption parameters without the added complications of the mirror response.

[en] We report the discovery of 1RXS J173006.4+033813, a polar cataclysmic variable with a period of 120.21 minutes. The white dwarf primary has a magnetic field of B = 42⁺⁶_-5 MG and the secondary is an M3 dwarf. The system shows highly symmetric double-peaked photometric modulation in the active state as well as in quiescence. These arise from a combination of cyclotron beaming and ellipsoidal modulation. The projected orbital velocity of the secondary is K₂ = 390 ± 4 km s^-1. We place an upper limit of 830 ± 65 pc on the distance.

[en] We present a spectral analysis of the NuSTAR and XMM-Newton observations of the Seyfert 1h galaxy/ULIRG IRAS 05189–2524 taken in 2013. We find evidence for relativistic disk reflection in the broadband X-ray spectrum: a highly asymmetric broad Fe Kα emission line extending down to 3 keV and a Compton scattering component above 10 keV. Physical modeling with a self-consistent disk reflection model suggests that the accretion disk is viewed at an intermediate angle with a supersolar iron abundance, and a mild constraint can be put on the high-energy cutoff of the power-law continuum. We test the disk reflection modeling under different absorption scenarios. A rapid black hole spin is favored; however, we cannot place a model-independent tight constraint on the value. The high reflection fraction ($R_{\mathrm{r}\mathrm{e}\mathrm{f}}\simeq <!--\; ???\; -->$ 2.0–3.2) suggests that the coronal illuminating source is compact and close to the black hole (lying within 8.7 $R_{\mathrm{g}}$ above the central black hole), where light-bending effects are important.

[en] The surface impedance of a superconductor changes when energy is absorbed and Cooper pairs are broken to produce single electron (quasiparticle) excitations. This change may be sensitively measured using a thin-film resonant circuit called a microwave kinetic inductance detector (MKID). The practical application of MKIDs for photon detection requires a method of efficiently coupling the photon energy to the MKID. The authors present results on position sensitive x-ray detectors made by using two aluminum MKIDs on either side of a tantalum photon absorber strip. Diffusion constants, recombination times, and energy resolution are reported. MKIDs can easily be scaled into large arrays

[en] The distribution of elements produced in the inner-most layers of a supernova explosion is a key diagnostic for studying the collapse of massive stars. Here we present the results of a 2.4 Ms NuSTAR observing campaign aimed at studying the supernova remnant Cassiopeia A (Cas A). We perform spatially-resolved spectroscopic analyses of the "4"4Ti ejecta which we use to determine the Doppler shift and thus the three-dimensional (3D) velocities of the "4"4Ti ejecta. We nd an initial 44Ti mass of 1.54±0.21x10"-"4 M which has a present day average momentum direction of 340°±15° projected on to the plane of the sky (measured clockwise from Celestial North) and tilted by 58°±20° into the plane of the sky away from the observer, roughly opposite to the inferred direction of motion of the central compact object. We find some "4"4Ti ejecta that are clearly interior to the reverse shock and some that are clearly exterior to the reverse shock. Where we observe "4"4Ti ejecta exterior to the reverse shock we also see shock-heated iron; however, there are regions where we see iron but do not observe "4"4Ti. This suggests that the local conditions of the supernova shock during explosive nucleosynthesis varied enough to suppress the production of "4"4Ti in some regions by at least a factor of two, even in regions that are assumed to be the result of processes like α-rich freezeout that should produce both iron and titanium.

[en] We compare the relative merits of active galactic nuclei (AGNs) selection at X-ray and mid-infrared wavelengths using data from moderately deep fields observed by both Chandra and Spitzer. The X-ray-selected AGN sample and associated photometric and spectroscopic optical follow-up are drawn from a subset of fields studied as part of the Serendipitous Extragalactic X-ray Source Identification (SEXSI) program. Mid-infrared data in these fields are derived from targeted and archival Spitzer imaging, and mid-infrared AGN selection is accomplished primarily through application of the Infrared Array Camera (IRAC) color-color AGN 'wedge' selection technique. Nearly all X-ray sources in these fields which exhibit clear spectroscopic signatures of AGN activity have mid-infrared colors consistent with IRAC AGN selection. These are predominantly the most luminous X-ray sources. X-ray sources that lack high-ionization and/or broad lines in their optical spectra are far less likely to be selected as AGNs by mid-infrared color selection techniques. The fraction of X-ray sources identified as AGNs in the mid-infrared increases monotonically as the X-ray luminosity increases. Conversely, only 22% of mid-infrared-selected AGNs are detected at X-ray energies in the moderately deep ((t_exp) ∼ 100 ks) SEXSI Chandra data. We hypothesize that IRAC sources with AGN colors that lack X-ray detections are predominantly high-luminosity AGNs that are obscured and/or lie at high redshift. A stacking analysis of X-ray-undetected sources shows that objects in the mid-infrared AGN selection wedge have average X-ray fluxes in the 2-8 keV band 3 times higher than sources that fall outside the wedge. Their X-ray spectra are also harder. The hardness ratio of the wedge-selected stack is consistent with moderate intrinsic obscuration, but is not suggestive of a highly obscured, Compton-thick source population. It is evident from this comparative study that in order to create a complete, unbiased census of supermassive black hole growth and evolution, a combination of sensitive infrared, X-ray, and hard X-ray selection is required. We conclude by discussing what samples will be provided by upcoming survey missions such as WISE, eROSITA, and NuSTAR.