Future X-ray Telescopes Research Articles

Thermonuclear burst oscillations and the dense matter equation of state

AbstractMatter in neutron star cores reaches extremely high densities, forming states of matter that cannot be generated in the laboratory. The Equation of State (EOS) of the matter links to macroscopic observables, such as mass M and radius R, via the stellar structure equations. A promising technique for measuring M and R exploits hotspots (burst oscillations) that form on the stellar surface when material accreted from a companion star undergoes a thermonuclear explosion. As the star rotates, the hotspot gives rise to a pulsation, and relativistic effects encode information about M and R into the pulse profile. However the burst oscillation mechanism remains unknown, introducing uncertainty when inferring the EOS. I review the progress that we are making towards cracking this long-standing problem, and establishing burst oscillations as a robust tool for measuring M and R. This is a major goal for future large area X-ray telescopes.

Jetted tidal disruptions of stars as a flag of intermediate mass black holes at high redshifts

Tidal disruption events (TDEs) of stars by single or binary supermassive black holes (SMBHs) brighten galactic nuclei and reveal a population of otherwise dormant black holes. Adopting event rates from the literature, we aim to establish general trends in the redshift evolution of the TDE number counts and their observable signals. We pay particular attention to two types of TDEs which are expected to be observable out to high redshifts, namely (i) jetted TDEs whose luminosity is boosted by relativistic beaming, and (ii) TDEs around binary black holes. We show that the brightest (jetted) TDEs are expected to be produced by massive black hole binaries if the occupancy of intermediate mass black holes (IMBHs) in low mass galaxies is high. The same binary population will also provide gravitational wave sources for eLISA. In addition, we find that the shape of the X-ray luminosity function of TDEs strongly depends on the occupancy of IMBHs and could be used to constrain scenarios of SMBH formation. Finally, we make predictions for the expected number of TDEs observed by future X-ray telescopes as a function of their sensitivity limits. We find that an instrument which is 50 times more sensitive than the Burst Alert Telescope (BAT) on board the Swift satellite is expected to trigger ~10 times more events than BAT with 50% of the events coming from z>2. Because of their long decay times, high-redshift TDEs can be mistaken for fixed point sources in deep field surveys such as the 4Ms survey with the Chandra X-ray Observatory and targeted observations of the same deep field with year-long intervals could reveal TDEs. If the occupation fraction of IMBHs is high, 6-20 TDEs are expected in each deep field observed by a telescope 50 times more sensitive than Chandra.

X-RAY COUNTERPART OF GRAVITATIONAL WAVES DUE TO BINARY NEUTRON STAR MERGERS: LIGHT CURVES, LUMINOSITY FUNCTION, AND EVENT RATE DENSITY

ABSTRACT Zhang proposed a type of GRB-less X-ray transient associated with double neutron star (NS–NS) mergers under the conjecture of a rapidly spinning magnetar merger product with the line of sight off the short gamma-ray burst (GRB) jet. We investigate possible light curves of these transients by considering different observers’ viewing angles. We perform Monte Carlo simulations to calculate the peak luminosity function (LF) and event rate density of these X-ray transients. By considering that a fraction of massive neutron stars may be supra-massive and later collapse into black holes after spinning down, we investigate how the predicted LF depends on the equation of state (EoS) of the central object and the geometry of the system. In general, the LF can be fit by two log-normal distributions peaking around and , corresponding to the trapped and free zones, respectively. For the majority of the EoS models, the current non-detection is consistent with having a free zone solid angle, at most a few times the solid angle of the short GRB jet. The event rate density of these X-ray transients is around a few tens of for luminosity above 1045 . We predict that future X-ray telescopes (such as Einstein Probe) with sensitivity would detect as many as several tens of such transients per year per steradian. Within 200 Mpc, the aLIGO average range for NS–NS mergers, the estimated event rate of these transients is about 1 transient per year all sky.

Development of low-noise high-speed analog ASIC for X-ray CCD cameras and wide-band X-ray imaging sensors

We report on the development and performance evaluation of the mixed-signal Application Specific Integrated Circuit (ASIC) developed for the signal processing of onboard X-ray CCD cameras and various types of X-ray imaging sensors in astrophysics. The quick and low-noise readout is essential for the pile-up free imaging spectroscopy with a future X-ray telescope. Our goal is the readout noise of 5e−r.m.s. at the pixel rate of 1Mpix/s that is about 10times faster than those of the currently working detectors. We successfully developed a low-noise ASIC as the front-end electronics of the Soft X-ray Imager onboard Hitomi that was launched on February 17, 2016. However, it has two analog-to-digital converters per chain due to the limited processing speed and hence we need to correct the difference of gain to obtain the X-ray spectra. Furthermore, its input equivalent noise performance is not satisfactory (>100μV) at the pixel rate higher than 500kpix/s. Then we upgrade the design of the ASIC with the fourth-order ΔΣ modulators to enhance its inherent noise-shaping performance. Its performance is measured using pseudo CCD signals with variable processing speed. Although its input equivalent noise is comparable with the conventional one, the integrated non-linearity (0.1%) improves to about the half of that of the conventional one. The radiation tolerance is also measured with regard to the total ionizing dose effect and the single event latch-up using protons and Xenon, respectively. The former experiment shows that all of the performances does not change after imposing the dose corresponding to 590years in a low earth orbit. We also put the upper limit on the frequency of the latch-up to be once per 48years.

Development of mirrors made of chemically tempered glass foils for future X-ray telescopes

Thin slumped glass foils are considered good candidates for the realization of future X-ray telescopes with large effective area and high spatial resolution. However, the hot slumping process affects the glass strength, and this can be an issue during the launch of the satellite because of the high kinematical and static loads occurring during that phase. In the present work we have investigated the possible use of Gorilla glass (produced by Corning), a chemical tempered glass that, thanks to its strength characteristics, would be ideal. The un-tempered glass foils were curved by means of an innovative hot slumping technique and subsequently chemically tempered. In this paper we show that the chemical tempering process applied to Gorilla glass foils does not affect the surface micro-roughness of the mirrors. On the other end, the stress introduced by the tempering process causes a reduction in the amplitude of the longitudinal profile errors with a lateral size close to the mirror length. The effect of the overall shape changes in the final resolution performance of the glass mirrors was studied by simulating the glass foils integration with our innovative approach based on glass reinforcing ribs. The preliminary tests performed so far suggest that this approach has the potential to be applied to the X-ray telescopes of the next generation.

Synthesis of Crack-Free PZT thin Films by Sol-gel Processing on Glass Substrate

PZT thin films deposited on glass substrates have been proposed for many applications such as adjustable optics in future X-ray telescopes. Sol-gel method is widely utilized to prepare PZT thin films due to its low reaction temperature, uniform mixing of reactants, and precise control of component. Crack-free transparent PZT thin films were prepared by dip-coating the solutions of complex alkoxides. PZT thin films were calcined at various temperatures and characterized by X-ray diffraction, field emission scanning electron microscopy to examine the surface morphology, and FT-IR spectroscopy. The XRD analysis confirmed the formation of perovskite phase at 600°C. FE-SEM results showed nucleation of PZT perovskite phase. Furthermore, the average grain size was measured in the range of 55 to 190nm.

5.9-keV Mn K-shell X-ray luminosity from the decay of 55Fe in Type Ia supernova models

We show that the X-ray line flux of the Mn Kalpha line at 5.9 keV from the decay of 55Fe is a promising diagnostic to distinguish between Type Ia supernova (SN Ia) explosion models. Using radiation transport calculations, we compute the line flux for two 3D explosion models: a near-Chandrasekhar mass delayed detonation and a violent merger of two white dwarfs. Both models are based on solar metallicity zero-age main sequence progenitors. Due to explosive nuclear burning at higher density, the delayed-detonation model synthesises 3.5 times more radioactive 55Fe than the merger model. As a result, we find that the peak Mn Kalpha line flux of the delayed-detonation model exceeds that of the merger model by a factor of 4.5. Since in both models the 5.9 keV X-ray flux peaks five to six years after the explosion, a single measurement of the X-ray line emission at this time can place a constraint on the explosion physics that is complementary to those derived from earlier phase optical spectra or light curves. We perform detector simulations of current and future X-ray telescopes to investigate the possibilities of detecting the X-ray line at 5.9 keV. For the delayed-detonation scenario, a line detection is feasible with Chandra up to 3 Mpc for an exposure time of 10^6 s. We find that it should be possible with currently existing X-ray instruments (with exposure times 5x10^5 s) to detect both of our models at sufficiently high S/N to distinguish between them for hypothetical events within the Local Group. The prospects for detection will be better with future missions. For example, the proposed Athena/X-IFU instrument could detect our delayed-detonation model out to a distance of 5 Mpc. This would make it possible to study future events occurring during its operational life at distances comparable to those of the recent supernovae SN 2011fe (6.4 Mpc) and SN 2014J (3.5 Mpc).

A model of the steep power-law spectra and high-frequency quasi-periodic oscillations in luminous black hole X-ray binaries

We propose a new model of the steep power law state of luminous black hole X-ray binaries. The model uses the fact that at high luminosities, the inner radii of radiation pressure dominated accretion discs are expected to i) become effectively optically thin and ii) produce significant luminosities. The gas temperature therefore rises sharply inwards, producing local saturated Compton spectra with rapidly increasing peak energy. These spectra sum together to form a steep power law tail to the spectrum. A given photon energy on this tail corresponds to a narrow range in radius, so that local vertical oscillations of the disc naturally produce high quality high-frequency quasi-periodic oscillations (HFQPOs) in the hard X-ray band. The two lowest order modes have a robust frequency ratio of ~1.53. This model explains the appearance of steep power law spectra and HFQPOs at high luminosity, the 3:2 HFQPO frequency ratios, and their association with the power law spectral component. We predict an increase in QPO quality factor when the power spectrum is restricted to a narrower photon energy band, and an increase in HFQPO frequency at higher X-ray energies or lower luminosities. Future X-ray telescopes could detect additional HFQPOs from higher order modes. We demonstrate how this model could be used to measure black hole spin from HFQPOs, and qualitatively estimate the spin of GRO J1655-40 as a/M ~ 0.4 - 0.7.

MEASURING SUPERMASSIVE BLACK HOLE SPINS IN AGN

Measuring the spins of supermassive black holes (SMBHs) in active galactic nuclei (AGN) can inform us about the relative role of gas accretion vs. mergers in recent epochs of the life of the host galaxy and its AGN. Recent theoretical and observation advances have enabled spin measurements for ten SMBHs thus far, but this science is still very much in its infancy. Herein, I discuss how we measure black hole spin in AGN, using recent results from a long Suzaku campaign on NGC 3783 to illustrate this process and its caveats. I then present our current knowledge of the distribution of SMBH spins in the local universe. I also address prospects for improving the accuracy, precision and quantity of these spin constraints in the next decade and beyond with instruments such as NuSTAR, Astro-H and future large-area X-ray telescopes.

Sputter deposition of PZT piezoelectric films on thin glass substrates for adjustable x-ray optics

Piezoelectric PbZr(0.52)Ti(0.48)O(3) (PZT) thin films deposited on thin glass substrates have been proposed for adjustable optics in future x-ray telescopes. The light weight of these x-ray optics enables large collecting areas, while the capability to correct mirror figure errors with the PZT thin film will allow much higher imaging resolution than possible with conventional lightweight optics. However, the low strain temperature and flexible nature of the thin glass complicate the use of chemical-solution deposition due to warping of the substrate at typical crystallization temperatures for the PZT. RF magnetron sputtering enabled preparation of PZT films with thicknesses up to 3 μm on Schott D263 glass substrates with much less deformation. X-ray diffraction analysis indicated that the films crystallized with the perovskite phase and showed no indication of secondary phases. Films with 1 cm(2) electrodes exhibited relative permittivity values near 1100 and loss tangents below 0.05. In addition, the remanent polarization was 26 μC/cm(2) with coercive fields of 33 kV/cm. The transverse piezoelectric coefficient was as high as -6.1±0.6 C/m(2). To assess influence functions for the x-ray optics application, the piezoelectrically induced deflection of individual cells was measured and compared with finite-element-analysis calculations. The good agreement between the results suggests that actuation of PZT thin films can control mirror figure errors to a precision of about 5 nm, allowing sub-arcsecond imaging.

Super-smooth finishing of diamond turned hard X-ray molding dies by combined fluid jet and bonnet polishing

Fabricating super-smooth aspheric optics for future hard X-ray telescopes will require a new process chain. Whilst diamond turning of electroless nickel plating can generate ~100nm P-V aspheric molding dies, associated turning marks must be removed before replication. An innovative two-step freeform finishing method is presented, that combines fluid jet and precessed bonnet polishing on a common 7-axis CNC platform. The removal rate and surface texture relationship of fluid jet with abrasive type and pressure is documented, and form correction demonstrated down to 27nm P-V. A novel bonnet tool-pathing method called “continuous precessing” is then applied, which delivers super-smooth anisotropic surface texture of 0.28nm rms.

Positron annihilation as a cosmic ray probe

ABSTRACT Recently, the gamma-ray telescopes AGILE and Fermi observed several middle-aged supernova remnants (SNRs) interacting with molecular clouds. A plausible emission mechanism of the gamma-rays is the decay of neutral pions produced by cosmic ray (CR) nuclei (hadronic processes). However, observations do not rule out contributions from bremsstrahlung emission due to CR electrons. TeV gamma-ray telescopes also observed many SNRs and discovered many unidentified sources. It is still unclear whether the TeV gamma-ray emission is produced via leptonic processes or hadronic processes. In this Letter, we propose that annihilation emission of secondary positrons produced by CR nuclei is a diagnostic tool of the hadronic processes. We investigate MeV emissions from secondary positrons and electrons produced by CR protons in molecular clouds. The annihilation emission of the secondary positrons from SNRs can be robustly estimated from the observed gamma-ray flux. The expected flux of the annihilation line from SNRs observed by AGILE and Fermi is sufficient for the future Advanced Compton Telescope to detect. Moreover, synchrotron emission from secondary positrons and electrons and bremsstrahlung emission from CR protons can be also observed by the future X-ray telescope NuSTAR and Astro-H.

Enabling High Performance Instruments for Astronomy and Space Exploration with ALD

Future UV, X-ray, and sub-millimeter telescopes and spectrometers have the potential to revolutionize our understanding of the formation and habitability of the modern universe.[1-4] Star formation, dark energy, and the composition of the intergalactic medium are only some of the key scientific topics that can be addressed by UV astronomy and astrophysics. Sub-millimeter astronomy can probe the fine structure of the cosmic microwave background, giving glimpses into the early universe immediately following the Big Bang.[5] Various surface engineering techniques including molecular beam epitaxy and atomic layer deposition have been utilized here to significantly boost the performance of key components of instruments for astronomy and astrophysics investigation, taking advantage of the atomic level precision that these techniques provide.

On the use of X-ray and γ-ray telescopes for identifying the origin of electrons and positrons observed by ATIC, Fermi, and PAMELA

X-ray and γ-ray observations can help understand the origin of the electron and positron signals reported by ATIC, PAMELA, PPB-BETS, and Fermi. It remains unclear whether the observed high-energy electrons and positrons are produced by relic particles, or by some astrophysical sources. To distinguish between the two possibilities, one can compare the electron population in the local neighborhood with that in the dwarf spheroidal galaxies, which are not expected to host as many pulsars and other astrophysical sources. This can be accomplished using X-ray and γ-ray observations of dwarf spheroidal galaxies. Assuming the signal detected by Fermi and ATIC comes from dark matter and using the inferred dark matter profile of the Draco dwarf spheroidal galaxy as an example, we calculate the photon spectrum produced by electrons via inverse Compton scattering. Since little is known about the magnetic fields in dwarf spheroidal galaxies, we consider the propagation of charged particles with and without diffusion. Extending the analysis of Fermi collaboration for Draco, we find that for a halo mass ∼10 9 M ⊙, even in the absence of diffusion, the γ-ray signal would be above the upper limits. This conclusion is subject to uncertainties associated with the halo mass. If dwarf spheroidal galaxies host local magnetic fields, the diffusion of the electrons can result in a signal detectable by future X-ray telescopes.

Study of thick CZT detectors for X-ray and Gamma-ray astronomy

CdZnTe (CZT) is a wide bandgap II–VI semiconductor developed for the spectroscopic detection of X-rays and γ-rays at room temperature. The Swift Burst Alert Telescope is using an 5240 cm 2 array of 2 mm thick CZT detectors for the detection of 15–150 keV X-rays from Gamma-ray Bursts. We report on the systematic tests of thicker (⩾0.5 cm) CZT detectors with volumes between 2 cm 3 and 4 cm 3 which are potential detector choices for a number of future X-ray telescopes that operate in the 10 keV to a few MeV energy range. The detectors contacted in our laboratory achieve Full Width Half Maximum energy resolutions of 2.7 keV (4.5%) at 59 keV, 3 keV (2.5%) at 122 keV and 4 keV (0.6%) at 662 keV. The 59 keV and 122 keV energy resolutions are among the world-best results for ⩾0.5 cm thick CZT detectors. We use the data set to study trends of how the energy resolution depends on the detector thickness and on the pixel pitch. Unfortunately, we do not find clear trends, indicating that even for the extremely good energy resolutions reported here, the achievable energy resolutions are largely determined by the properties of individual crystals. Somewhat surprisingly, we achieve the reported results without applying a correction of the anode signals for the depth of the interaction. Measuring the interaction depths thus does not seem to be a pre-requisite for achieving sub-1% energy resolutions at 662 keV.

Cosmology versus standard model

I will discuss a proposal for a unified solution of the problems of neutrino masses, dark matter, baryon asymmetry of the Universe and inflation, which does not require introduction of any new energy scale besides already known, namely the electroweak and the Planck scales. This point of view, supplemented by a requirement of simplicity, has a number of experimental predictions which can be tested, at least partially, with the use of existing accelerators and the LHC, with current and future X-ray telescopes, and with the Planck mission.

The impact of feedback on the low-redshift intergalactic medium

We analyse the evolution of the properties of the low-redshift Intergalactic Medium (IGM) using high-resolution hydrodynamic simulations that include a detailed chemical evolution model. We focus on the effects that two different forms of energy feedback, strong galactic winds driven by supernova explosion and Active Galactic Nuclei (AGN) powered by gas accretion onto super-massive black holes (BHs), have on the thermo- and chemo-dynamical properties of of the low redshift IGM. We find that feedback associated to winds (W) and BHs leave distinct signatures in both the chemical and thermal history of the baryons, especially at redshift z<3 [..] We present results for the enrichment in terms of mass and metallicity distributions for the WHIM phase, both as a function of density and temperature. Finally, we compute the evolution of the relative abundances between different heavy elements, namely Oxygen, Carbon and Iron. While both C/O and O/Fe evolve differently at high redshifts for different feedback models, their values are similar at z=0 [..]. The sensitivity of WHIM properties on the implemented feedback scheme could be important both for discriminating between different feedback physics and for detecting the WHIM with future far-UV and X-ray telescopes.

Analytical computation of the off-axis effective area of grazing incidence X-ray mirrors

<i>Aims. <i/>Focusing mirrors for X-ray telescopes in grazing incidence, introduced in the 70s, are characterized in terms of their performance by their imaging quality and effective area, which in turn determines their sensitivity. Even though the on-axis effective area is assumed in general to characterize the collecting power of an X-ray optic, the telescope capability of imaging extended X-ray sources is also determined by the variation in its effective area with the off-axis angle. The effective area, in general, decreases as the X-ray source moves off-axis, causing a loss of sensitivity in the peripheral regions of the telescope's field of view.<i>Methods. <i/>The complex task of designing optics for future X-ray telescopes entails detailed computations of both imaging quality and effective area on- and off-axis. Because of their apparent complexity, both aspects have been, so far, treated by using ray-tracing routines aimed at simulating the interaction of X-ray photons with the reflecting surfaces of a given focusing system. Although this approach has been widely exploited and proven to be effective, it would also be attractive to regard the same problem from an analytical viewpoint, to assess an optical design of an X-ray optical module with a simpler calculation than a ray-tracing routine. This would also improve the efficiency of optimization tasks when designing the X-ray optical modules. In this paper, we thereby focused on developing analytical solutions to compute the off-axis effective area of double-reflection X-ray mirrors.<i>Results. <i/>We have developed useful analytical formulae for the off-axis effective area of a double-reflection mirror in the double cone approximation, requiring only an integration and the standard routines to calculate the X-ray coating reflectivity for a given incidence angle. The computation is easily applicable also to Wolter-I mirrors (such as those of NeXT, NuSTAR, HEXIT-SAT, IXO) and the approximation improves as the f-number of the mirror increases. Algebraic expressions are provided for the mirror geometric area, as a function of the off-axis angle. Finally, the results of the analytical computations presented here are validated by comparison with the corresponding predictions of a ray-tracing code.

Shaped silicon wafers obtained by hot plastic deformation: performance evaluation for future astronomical x-ray telescopes

In order to develop lightweight and high angular resolution x-ray mirrors, we have investigated hot plastic deformation of 4 in. silicon (111) wafers. A sample wafer was deformed using hemispherical dies with a curvature radius of 1000 mm. The measured radius of the deformed wafer was 1030 mm, suggesting that further conditioning is indispensable for better shaping. For the first time to our knowledge, x-ray reflection on a deformed wafer was detected at Al K(alpha) 1.49 keV. An estimated surface roughness of <1 nm from the x-ray reflection profile was comparable to that of a bare silicon wafer without deformation. Hence, no significant degradation of the microroughness was seen.

Hard X-ray emission of the Earth's atmosphere: Monte Carlo simulations

We perform Monte Carlo simulations of cosmic ray-induced hard X-ray radiation from the Earth's atmosphere. We find that the shape of the spectrum emergent from the atmosphere in the energy range 25-300 keV is mainly determined by Compton scatterings and photoabsorption, and is almost insensitive to the incident cosmic ray spectrum. We provide a fitting formula for the hard X-ray surface brightness of the atmosphere as would be measured by a satellite-borne instrument, as a function of energy, solar modulation level, geomagnetic cut-off rigidity and zenith angle. A recent measurement by the INTEGRAL observatory of the atmospheric hard X-ray flux during the occultation of the cosmic X-ray background by the Earth agrees with our prediction within 10 per cent. This suggests that Earth observations could be used for in-orbit calibration of future hard X-ray telescopes. We also demonstrate that the hard X-ray spectra generated by cosmic rays in the crusts of the Moon, Mars and Mercury should be significantly different from that emitted by the Earth's atmosphere.