William W. Craig

The NuSTAR spectrum of Mrk 335: extreme relativistic effects within two gravitational radii of the event horizon?

We present 3–50keV NuSTAR observations of the active galactic nuclei Mrk 335 in a very low flux state. The spectrum is dominated by very strong features at the energies of the iron line at 5–7keV and Compton hump from 10–30keV. The source is variable during the observation, withthevariabilityconcentratedatlowenergies,whichsuggestingeitherarelativisticreflection oravariableabsorptionscenario.Inthiswork,wefocusonthereflectioninterpretation,making use of new relativistic reflection models that self consistently calculate the reflection fraction, relativistic blurring and angle-dependent reflection spectrum for different coronal heights to model the spectra. We find that the spectra can be well fitted with relativistic reflection, and that the lowest flux state spectrum is described by reflection alone, suggesting the effects of extreme light-bending occurring within ∼2 gravitational radii (RG) of the event horizon. The reflection fraction decreases sharply with increasing flux, consistent with a point source moving up to above 10 RG as the source brightens. We constrain the spin parameter to greater than 0.9 at the 3σ confidence level. By adding a spin-dependent upper limit on the reflection fraction to our models, we demonstrate that this can be a powerful way of constraining the spin parameter, particularly in reflection dominated states. We also calculate a detailed emissivity profile for the iron line, and find that it closely matches theoretical predictions for a compact source within a few RG of the black hole.

THE BROADBAND SPECTRAL VARIABILITY OF MCG–6-30-15 OBSERVED BY NUSTAR AND XMM-NEWTON

MCG–6-30-15, at a distance of 37 Mpc (z = 0.008), is the archetypical Seyfert 1 galaxy showing very broad Fe Kα emission. We present results from a joint NuSTAR and XMM-Newton observational campaign that, for the first time, allows a sensitive, time-resolved spectral analysis from 0.35 keV up to 80 keV. The strong variability of the source is best explained in terms of intrinsic X-ray flux variations and in the context of the light-bending model: the primary, variable emission is reprocessed by the accretion disk, which produces secondary, less variable, reflected emission. The broad Fe Kα profile is, as usual for this source, well explained by relativistic effects occurring in the innermost regions of the accretion disk around a rapidly rotating black hole. We also discuss the alternative model in which the broadening of the Fe Kα is due to the complex nature of the circumnuclear absorbing structure. Even if this model cannot be ruled out, it is disfavored on statistical grounds. We also detected an occultation event likely caused by broad-line region clouds crossing the line of sight.

Development of the High-Energy Focusing Telescope (HEFT) balloon experiment

The High Energy Focusing Telescope (HEFT) is a balloon-borne experiment employing focusing optics in the hard X-ray/soft gamma-ray band (20 - 100 keV) for sensitive observations of astrophysical sources. The primary scientific objectives include imaging and spectroscopy of 44Ti emission in young supernova remnants, sensitive hard X-ray observations of obscured Active Galactic Nuclei, and spectroscopic observations of accreting high-magnetic field pulsars. Over the last four years, we have developed grazing-incidence depth-graded multilayer optics and high spectral resolution solid stat Cadmium Zinc Telluride pixel detectors in order to assemble a balloon-borne experiment with sensitivity and imaging capability superior to previous satellite missions operating in this band. In this paper, we describe the instrument design, and present recent laboratory demonstrations of the optics and detector technologies.

W/SiC x-ray multilayers optimized for use above 100 keV.

We have developed a new depth-graded multilayer system comprising W and SiC layers, suitable for use as hard x-ray reflective coatings operating in the energy range 100-200 keV. Grazing-incidence x-ray reflectance at E = 8 keV was used to characterize the interface widths, as well as the temporal and thermal stability in both periodic and depth-graded W/SiC structures, whereas synchrotron radiation was used to measure the hard x-ray reflectance of a depth-graded multilayer designed specifically for use in the range E approximately 150-170 keV. We have modeled the hard x-ray reflectance using newly derived optical constants, which we determined from reflectance versus incidence angle measurements also made using synchrotron radiation, in the range E = 120-180 keV. We describe our experimental investigation in detail compare the new W/SiC multilayers with both W/Si and W/B4C films that have been studied previously, and discuss the significance of these results with regard to the eventual development of a hard x-ray nuclear line telescope.

Discovery of a Candidate Isolated Neutron Star in a New Supernova Remnant Near CTB 1

We have discovered a neutron star candidate in the region near the supernova remnant CTB 1 (G116.9+0.2). The source is located near the center of a previously unknown shell of soft X-ray emission (G117.7+0.6), which is consistent with a middle-aged supernova remnant. The X-ray emission is periodic at the 99% confidence level with a period of 0.24181 s. The thermal X-ray emission is probably residual heat of formation from the cooling neutron star. We cannot entirely rule out a less likely possibility that the X-rays have an origin in the magnetosphere. A comparison of the neutron star age and temperature with cooling theories is presented.

Development of Thermally Formed Glass Optics for Astronomical Hard X-ray Telescopes

The next major observational advance in hard X-ray/soft gamma-ray astrophysics will come with the implementation of telescopes capable of focusing 10-200 keV radiation. Focusing allows high signal-to-noise imaging and spectroscopic observations of many sources in this band for the first time. The recent development of depth-graded multilayer coatings has made the design of telescopes for this bandpass practical, however the ability to manufacture inexpensive substrates with appropriate surface quality and figure to achieve sub-arcminute performance has remained an elusive goal. In this paper, we report on new, thermally-formed glass micro-sheet optics capable of meeting the requirements of the next-generation of astronomical hard X-ray telescopes.

Hard x-ray optics for the HEFT balloon-borne payload: prototype design and status

We report on the current status and performance of prototype hard x-ray optics we are producing for use on the high energy focusing telescope (HEFT) experiment. The baseline substrates are thermally formed glass mirrors that are overcoated with multilayers to provide good performance throughout the 20-80 keV bandpass. Progress made in the thermal forming process as well as in the multilayer performance has allowed production of optics that meet or exceed all HEFT requirements. We present metrology on the substrates and result from x-ray characterization. A novel mounting scheme for the individual telescope shells is currently being tested. If successful the mounting technique will produce a monolithic, extremely stiff and robust optic.

Substrates and mounting techniques for the High-Energy Focusing Telescope (HEFT)

The high energy focusing telescope (HEFT) is a balloon-borne system for obtaining arcminute imagery in the 20 - 100 keV energy band. The hard x-ray optics are baselined to use thin epoxy-replicated aluminum foil substrates coated with graded-d multilayers, and we show some results on x-ray performance of prototype foil substrates. We also propose an extremely promising alternative substrate -- thermally formed glass. The advantages of thermally formed glass substrates, their fabrication and preliminary metrology on sample pieces are discussed. If ultimately feasible, the thermally formed glass is a better substrate due to its superior hard x-ray reflectivity and scattering properties in comparison to similarly coated epoxy-replicated aluminum foil. We also discuss some preliminary work on the HEFT mirror mounting concept and the associated angular resolution error budget.

NuSTAR Discovery of a Cyclotron Line in the Be/X-Ray Binary RX J0520.5–6932 during Outburst

We present spectral and timing analysis of NuSTAR observations of RX J0520.5–6932 in the 3-79 keV band collected during its outburst in 2014 January. The target was observed on two epochs and we report the detection of a cyclotron resonant scattering feature with central energies of E_(CRSF) = 31.3_(-0.7)^(+0.8) keV and 31.5_(-0.6)^(+0.7) keV during the two observations, respectively, corresponding to a magnetic field of B ≈ 2 × 10^(12) G. The 3-79 keV luminosity of the system during the two epochs, assuming a nominal distance of 50 kpc, was 3.667 ± 0.007 × 10^(38) erg s^(–1) and 3.983 ± 0.007 × 10^(38) erg s^(–1). Both values are much higher than the critical luminosity of ≈1.5 × 10^(37) erg s^(–1), above which a radiation-dominated shock front may be expected. This adds a new object to the sparse set of three systems that have a cyclotron line observed at luminosities in excess of 10^(38) erg s^(–1). A broad (σ ≈ 0.45 keV) Fe emission line is observed in the spectrum at a central energy of 6.58_(-0.05)^(+0.05) keV in both epochs. The pulse profile of the pulsar was observed to be highly asymmetric with a sharply rising and slowly falling profile of the primary peak. We also observed minor variations in the cyclotron line energy and width as a function of the rotation phase.

Technology development for the Constellation-X hard-x-ray telescope

In addition to high resolving power in the traditional x-ray band, the Constellation X-ray scientific goals require broad bandpass, with response extending to E >= 40 keV. To achieve this objective, Constellation-X will incorporate a hard x-ray telescope (HXT) based on depth graded multilayer- coated grazing incidence optics and position-sensitive solid state detectors. This paper describes the HXT performance requires, provides an overview of the HXT optics and detector technology development efforts, and present example designs.