Weak Radiation Research Articles

The cosmological distribution of compact object mergers from dynamical interactions with SMBH binaries

ABSTRACT We combine sophisticated high-precision scattering experiments, together with results from the Millenium-II simulation, to compute the cosmic merger rate of bound compact object (CO) binaries dynamically interacting with supermassive black hole binaries (SMBHBs). We consider binaries composed of white dwarfs (WDs), neutron stars (NSs), and black holes (BHs). The overall merger rates for WD–WD, NS–NS, BH–BH, BH–NS binaries, and EBBH (eccentric binaries of BHs) from redshift ∼5 are found to be $4.32\times 10^3\, \mathrm{yr}^{-1}(5.93\times 10^2\, \mathrm{yr}^{-1}$ for Type Ia SNe), $82.7\, \mathrm{yr}^{-1}$, $96.3\, \mathrm{yr}^{-1}$, $13.1\, \mathrm{yr}^{-1}$, and $148\, \mathrm{yr}^{-1}$, respectively, for a nominal CO binary fraction in the Galactic centre of 0.1. We calculate the distance (R) distribution of the merger sites with respect to the host galaxies of the binaries. The distribution shows a wide range of distances up to ∼Mpc; this tail is produced by escaped hypervelocity CO binaries. Due to the differences in the matter density of the surrounding environment, merger events with different R are expected to display significantly different signatures in their EM counterparts. In particular, merger events (and especially NS–NS) producing a relativistic jet but occurring in the intergalactic medium will have very weak afterglow radiation relative to their prompt emission. These events, which we call ‘off-centre’, can only be produced from a close encounter between CO binaries and SMBHBs; hence the detection of such merger events would indicate the existence of nearby SMBHBs, and in particular with high mass ratio, produced in the aftermath of a major galaxy merger.

The Interannual Calibration and Global Nighttime Light Fluctuation Assessment Based on Pixel-Level Linear Regression Analysis

The Operational Linescan System (OLS) carried by the National Defense Meteorological Satellite Program (DMSP) can capture the weak visible radiation emitted from earth at night and produce a series of annual cloudless nighttime light (NTL) images, effectively supporting multi-scale, long-term human activities and urbanization process research. However, the interannual instability and sensor bias of NTL time series products greatly limit further studies of lighting data in time series with OLS. Several calibration models for OLS have been proposed to implement interannual corrections to improve the continuity and consistency of time series NTL products; however, due to the subjective factors intervention and insufficient automation in the calibration process, the interannual correction study of NTL time series images is still worth being developed further. Therefore, to avoid the involvement of subjective factors and to optimize the Pseudo-Invariant Features (PIF) identification, an interannual calibration model Pixel-based PIF (PBPIF) is proposed, which identifies PIF by pixel fluctuation characteristics. Results show that a PBPIF-based model can reduce subjective interference and improve the degree of automation during the NTL interannual calibration process. The calibration performance evaluation based on Total Sum of Lights (TSOL) and Sum of the Normalized Difference Index (SNDI) shows that compared to the traditional PIF-based (tPIF-based) and Ridgeline Sampling Regression based (RSR-based) models, the PBPIF-based one achieves better performance in reducing NTL interannual turbulence and minimizing the deviation between sensors. In addition, based on the corrected NTL time series products, pixel-level linear regression analysis is implemented to maximize the potential of the NTL resolution to produce global Light Intensity Change Coefficient (LICC). The results of global LICC can be widely applied to the detailed study of the characteristics of economic development and urbanization.

Far-Field Radiation of Three-Dimensional Plasmonic Gold Tapers near Apexes.

Three-dimensional plasmonic gold tapers are widely used structures in nano-optics for achieving imaging at the nanometer scale, enhanced spectroscopy, confined light sources, and ultrafast photoelectron emission. To understand their radiation properties further, especially in the proximity of the apex at the nanoscale, we employ cathodoluminescence spectroscopy with high spatial and energy resolution. The plasmon-induced radiation in the visible spectral range from three-dimensional gold tapers with opening angles of 13° and 47° is investigated under local electron excitation. We observe a much weaker radiation from the apex of the 13° taper than from that of the 47° taper. By means of finite-difference time-domain simulations we show that for small opening angles plasmon modes that are created at the apex are efficiently guided along the taper shaft. In contrast for tapers with larger opening angles, generated plasmon polaritons experience larger radiation damping. Interestingly, we find for both tapers that the most intense radiation comes from locations a few hundreds of nanometers behind the apexes, instead of exactly at the apexes. Our findings provide useful details for the design of plasmonic gold tapers as confined light sources or light absorbers.

Fork shaped with inverted L-stub resonator UWB antenna for WiMAX/WLAN rejection band

A compact beveled fed inverted L-stub dual-band rejection, ultra-wideband (UWB) radiating antenna is proposed. The antenna consists of the dual fork-shaped radiating elements and inverted L-shaped stub resonators. The fork-shaped radiating elements together with stub resonator occupies a size of 20 × 14 mm2. This antenna accepts an impedance bandwidth (VSWR ≤ 2) of 9 GHz and rejects two narrow interfering WiMAX (3.1 GHz)/WLAN (5.1 GHz) bands. The rejecting frequencies are attained by using an inverted L-shaped stub resonator. The two rejecting frequencies of WiMAX/WLAN bands have an acceptable gain reduction and weak radiation in UWB communication. Therefore, the design is appropriate for WiMAX/WLAN dense environments. Meanwhile, the measured results of the fabricated prototype exhibit a good matching with the simulated results.

Gas inflow and star formation near supermassive black holes: the role of nuclear activity

ABSTRACT Numerical models of gas inflow towards a supermassive black hole (SMBH) show that star formation may occur in such an environment through the growth of a gravitationally unstable gas disc. We consider the effect of nuclear activity on such a scenario. We present the first three-dimensional grid-based radiative hydrodynamic simulations of direct collisions between infalling gas streams and a 4 × 106 M⊙ SMBH, using ray-tracing to incorporate radiation consistent with an active galactic nucleus (AGN). We assume inflow masses of ≈105 M⊙ and explore radiation fields of 10 per cent and 100 per cent of the Eddington luminosity (Ledd). We follow our models to the point of central gas disc formation preceding star formation and use the Toomre Q parameter (QT) to test for gravitational instability. We find that radiation pressure from UV photons inhibits inflow. Yet, for weak radiation fields, a central disc forms on time-scales similar to that of models without feedback. Average densities of >108 cm−3 limit photoheating to the disc surface allowing for QT ≈ 1. For strong radiation fields, the disc forms more gradually resulting in lower surface densities and larger QT values. Mass accretion rates in our models are consistent with 1–60 per cent of the Eddington limit, thus we conclude that it is unlikely that radiative feedback from AGN activity would inhibit circumnuclear star formation arising from a massive inflow event.

Nobeyama 45 m mapping observations toward Orion A. II. Classification of cloud structures and variation of the 13CO/C18O abundance ratio due to far-UV radiation

Abstract We present results of the classification of cloud structures toward the Orion A Giant Molecular Cloud based on wide-field 12CO (J = 1–0), 13CO (J = 1–0), and C18O (J = 1–0) observations using the Nobeyama 45 m radio telescope. We identified 78 clouds toward Orion A by applying Spectral Clustering for Interstellar Molecular Emission Segmentation (SCIMES) to the data cube of the column density of 13CO. Well-known subregions such as OMC-1, OMC-2/3, OMC-4, OMC-5, NGC 1977, L1641-N, and the dark lane south filament (DLSF) are naturally identified as distinct structures in Orion A. These clouds can also be classified into three groups: the integral-shaped filament, the southern regions of Orion A, and the other filamentary structures in the outer parts of Orion A and the DLSF. These groups show differences in scaling relations between the physical properties of the clouds. We derived the abundance ratio between 13CO and C18O, $X_{^{13}\mathrm{CO}}/X_{\mathrm{C}^{18}\mathrm{O}}$, which ranges from 5.6 to 17.4 on median over the individual clouds. The significant variation of $X_{^{13}\mathrm{CO}}/X_{\mathrm{C}^{18}\mathrm{O}}$ is also seen within a cloud in both the spatial and velocity directions and the ratio tends to be high at the edge of the cloud. The values of $X_{^{13}\mathrm{CO}}/X_{\mathrm{C}^{18}\mathrm{O}}$ decrease from 17 to 10 with the median of the column densities of the clouds at the column density of $N_{\mathrm{C^{18}O}} \gtrsim 1 \times 10^{15}\:$cm−2 or visual extinction of AV ≳ 3 mag under the strong far-ultraviolet (FUV) environment of G0 > 103, whereas it is almost independent of the column density in the weak FUV radiation field. These results are explained if the selective photodissociation of C18O is enhanced under a strong FUV environment and it is suppressed in the dense part of the clouds.

Weak Transient Electromagnetic Radiation Signal Detection Method Considering the New Watershed Image Segmentation Algorithm

In order to study the detection methods of weak transient electromagnetic radiation signals, a detection algorithm integrating generalized cross-correlation and chaotic sequence prediction is proposed in this paper. Based on the dual-antenna test and cross-correlation information estimation method, the detection of aperiodic weak discharge signals under low signal-to-noise ratio is transformed into the estimation of periodic delay parameters, and the noise is reduced at the same time. The feasibility of this method is verified by simulation and experimental analysis. The results show that under the condition of low signal-to-noise ratio, the integrated method can effectively suppress the influence of 10 noise disturbances. It has a high detection probability for weak transient electromagnetic radiation signals, and needs fewer pulse accumulation times, which improves the detection efficiency and is more suitable for long-distance detection of weak electromagnetic radiation sources.

Ground observation result of an OH radical hyper-resolution spectrometer for the middle and upper atmosphere.

OH radicals in the upper and middle atmosphere are important oxidants and play an important role in atmospheric photochemistry. A hyper-resolution spectrometer based on 308nm glow was developed for obtaining OH radical concentration data in the upper and middle atmosphere. In order to verify the performance of the OH radical hyper-resolution spectrometer, several comprehensive ground experiments were carried out in this paper. The spectrometer observes OH radicals produced by a photochemistry reactor chamber to verify the detection ability of the instrument for characteristic signals. A solar observation experiment is used to evaluate the hyper-resolution spectroscopic ability of the spectrometer and the on-orbit field-of-slice-view function. In order to evaluate the detection ability of weak atmospheric background radiation, the experimental study of solar scattering light observation was carried out. The experimental results show that the spectrometer has the characteristics of ultrahigh spectral resolution (0.0086nm), high sensitivity, and high signal-to-noise ratio. The ground observation results are consistent with the theoretical simulation values.

Ultrasensitive tunable terahertz sensor based on five-band perfect absorber with Dirac semimetal.

For non-invasive detection, terahertz (THz) sensing shows more promising performance compared to visible and infrared regions. But so far, figure of merit (FOM) of THz sensor has been exceeding low due to weak radiation and absorption loss. Here, we propose an easily implemented THz sensor based on bulk Dirac semimetal (BDS). The presented structure not only achieves narrowband absorption and dynamic tunability at five perfect absorption bands, but also exhibits excellent sensing performance with a FOM of 813. These fascinating properties can be explained by the combination of the classical magnetic resonance induced by the anti-parallel current, the electric resonance of adjacent unit cells resulting from the air slots at both ends of the absorber, and Mie resonance supported by coating, respectively. Our work can provide a new avenue for the design of multi-band photodetectors and sensors in the future.

Studies of transverse single-bunch instabilities in booster synchrotrons

Abstract The capability of providing full-charge bunches has become an important requirement for quite a few boosters used in ultralow-emittance storage rings based synchrotron light sources. The transverse single-bunch instabilities may limit the single-bunch charge in these type of boosters. The relatively weak synchrotron radiation damping makes the bunch more unstable at the injection energy than at the extraction energy. Therefore, studies of the transverse single-bunch instabilities are carried out first at the injection energy, with the chromaticity varied, to understand how the threshold changes accordingly. Our studies show that the single-bunch threshold current tends to drop when the chromaticity becomes nonzero at the injection energy of the used booster lattice. The corresponding simulation results are presented and analyzed. Furthermore, the transverse single-bunch instabilities during the energy ramping process are also studied.

The role of the polar vortex strength during winter in Arctic ozone depletion from late winter to spring

The polar ozone destruction occurs as a result of the chemical processes and catalytic cycles with the participation of polar stratospheric clouds (PSCs) in the presence of weak solar radiation inside the polar vortex from late winter to spring. However, in some years, ozone depletion was not observed under strong polar vortex conditions (in the presence of PSCs). Based on the ERA-Interim reanalysis data and the NASA Goddard Space Flight Center (GSFC) satellite data, we show that the polar vortex strength in early winter plays an important role in polar ozone depletion. If the short-term weakening or splitting of the polar vortex, occurring with PSC melting, happened during the winter, then, in most cases, ozone destruction during the subsequent period from late winter to spring was not observed even under conditions of the strong polar vortex containing PSCs.

Direct Searches for Hidden-Photon Dark Matter with the SHUKET Experiment.

Hidden photons are dark matter candidates motivated by theories beyond the standard model of particle physics. They mix with conventional photons, and they can be detected through the very weak electromagnetic radiation they induce at the interface between a metal and the air. SHUKET [search for U(1) dark matter with an electromagnetic telescope] is a dedicated experiment sensitive to the hidden photon-induced signal. The results from a hidden photon search campaign are reported, with no significant detection of a dark matter signal. Exclusion limits are derived from the observed noise fluctuations in a 5-6.8GHz frequency range, corresponding to a hidden photon mass region ranging from 20.8 to 28.3 μeV. SHUKET is currently the most sensitive instrument in this mass range, and the obtained limits on the kinetic mixing term constrain significantly dark matter models inspired from string theory.

Lethality and mitotic activity of T98G glioblastoma cells under the influence of pulsed magnetic fields and ionizing radiation.

e13511 Background: Development of alternative methods of non-invasive therapy in neurooncology is determined by the need to prevent the continued growth of glioblastoma and radiation-induced complications. Usage of electromagnetic influence for these reasons is determined by the oscillatory nature of intracellular processes and results of previous studies on the effect of weak electromagnetic radiation in tumor growth. The purpose of the study was to reveal the effect of a pulsed magnetic field (PMF) and ionizing radiation (IR) on human T98G glioblastoma cells. Methods: Human T98G glioblastoma cells were cultured in the RPMI-1640 medium (Biolot, Russia) supplemented with 10% fetal bovine serum (Thermo Scientific HyClone, USA). The culture was seeded in the Biofil plates (at least 8x105 cells per 3 mL of the medium), incubated (CB 150 Binder, Germany) and exposed to IR 10 Gy (TheratronEquinox-BestTheratronics) for 11.7 min. and/or PMF 15 mT or 300 mT with a sequence of frequencies 0.3-3.0-9.0 Hz for 7 min. (Neiro-MS/D by Neirosoft, Russia). The efficiency criteria were lethality and the mitotic activity of human T98G glioblastoma cells. Results: The highest cellular lethality was registered 3 hours (18.7 vs. 5.2% in controls, p≤0.05) and a day after IR exposure. PMF 15 or 300 mT alone increased lethality by 2.5-2.8 times compared to controls (p < 0.05). IR plus PMF (15 mT) did not caused increased lethality. At the same time, the most pronounced decrease in the mitotic activity a day after exposure (by 4.7 times, p < 0.05) was registered in PMF alone (15 mT). Conclusions: The inhibitory effect of PMF on the viability of human glioblastoma cells indicates the prospects for its use as an accompanying treatment in neurooncology.

High-Performance UV Enhancer Molecules Coupled with Photosynthetic Proteins for Ultra-Low-Intensity UV Detection

Summary Dual attributes of UV-photo-responsive organic-ionic conductors are exploited in bio-photoelectrochemical cells based on photosynthetic RC-LH1 proteins from Rhodobacter sphaeroides. These UV enhancer molecules (UVEM) can generate small photocurrents in the absence of protein and are also effective electrolytes for photocurrent generation by RC-LH1 complexes in response to near-infrared excitation. Mixing RC-LH1 and UVEM components strongly enhanced UV photocurrents relative to those obtained with protein or UVEM alone, an effect that is attributed to energy transfer from the hetero-anthracene chromophore of the UVEM to the carotenoids of the RC-LH1 complex. RC-LH1/UVEM bio-photoelectrochemical cells were superior to conventional RC-LH1 cells in terms of UV external quantum efficiency, photo-response sensitivity, and photocurrent rise-decay times. These bio-photodetectors could detect weak UV radiation with intensities as low as 2 μW/cm2. This combination of photosynthetic proteins with dual-function electrolytes is the first attempt to construct fully functional bio-photoelectrochemical UV photodetector based on natural components.

Gray Radiation Hydrodynamics with the FLASH Code for Astrophysical Applications

Abstract We present the newly incorporated gray radiation hydrodynamics capabilities of the FLASH code based on a radiation flux-limiter-aware hydrodynamics numerical implementation designed specifically for applications in astrophysical problems. The implemented numerical methods consist of changes in the unsplit hydrodynamics solver and adjustments in the flux-limited radiation diffusion unit. Our approach can handle problems in both the strong and weak radiation–matter coupling limits, as well as transitions between the two regimes. Appropriate extensions in the “Helmholtz” equation of state are implemented to treat two-temperature astrophysical plasmas involving the interaction between radiation and matter and the addition of a new opacity unit based on the OPAL opacity database, commonly used for astrophysical fluids. A set of radiation-hydrodynamics test problems is presented aiming to showcase the new capabilities of FLASH and to provide direct comparison to other similar software instruments available in the literature. To illustrate the capacity of FLASH to simulate phenomena occurring in stellar explosions, such as shock breakout, radiative precursors, and supernova ejecta heating due to the decays of radioactive 56Ni and 56Co, we also present 1D supernova simulations and compare the computed light curves to those of the SNEC code. The latest public release of FLASH with these enhanced capabilities is available for download and use by the broader astrophysics community.

Influence of Optical Parameters on Magnetohydrodynamic Natural Convection in a Horizontal Cylindrical Annulus

The coupled phenomena of radiative–magnetohyrodynamic (MHD) natural convection in a horizontal cylindrical annulus are numerically investigated. The buoyant flow is driven by the temperature difference between the inner and outer cylinder walls, while a circumferential magnetic field induced by a constant electric current is imposed. The hybrid approach of finite volume and discrete ordinates methods (FV-DOM) is developed to solve the nonlinear integro-differential governing equations in polar coordinate system, and accordingly, the influences of Hartmann number, radiation–convection parameter, and optical properties of fluid and wall on thermal and hydrodynamic behaviors of the “downward flow,” originally occurring without consideration of radiation and magnetic field, are mainly discussed. The results indicate that both the circulating flow and heat transfer are weakened by the magnetic field, but its suppression effect on the latter is rather small. Under the influence of magnetic field, the “downward flow” pattern has not been obtained from zero initial condition even for the case of weak radiation of NR = 0.1. Besides, the variation of radiative heat transfer rate with angular positions diminishes for the fluid with strong scattering or weak absorption.

Up-conversion photoluminescence properties and energy transfer process of Ho3+,Yb3+ Co-doped BaY2F8 fine fibers

An intermediate product was obtained by sintering PVP/[Ba(CH3COO)2+Y(NO3)3+Ho(NO3)3+Yb(NO3)3] fine fibers prepared by electrospinning. The Ho3+,Yb3+:BaY2F8 fine fibers were obtained by sintering the above products using a double-crucible reduction atmosphere fluorination technology. X-ray diffraction data analysis showed that pure monoclinic phase of Ho3+,Yb3+:BaY2F8 fine fibers was C2/m space group. Scanning electron microscopy (SEM) was used to test the morphology, indicating the size of the fine fibers is near 269 nm. By analyzing the up-conversion fluorescence spectrum, strong green and weak red up-conversion radiation at 536 nm and 658 nm were obtained with the optimal doping concentrations of Ho3+ and Yb3+ ions of 1% and 8%, respectively. The up-conversion luminescence mechanism of Ho3+,Yb3+:BaY2F8 fine fibers and the fluorescence lifetime at 536 nm have also been studied in detail.

Lasers and fiber optics for astrophysics

Optical spectroscopy has been and remains an important feature of astrophysical research. Huge telescopes are being built to collect radiation from the most distant stars in the Universe for subsequent study with unique astronomical spectrographs. The fundamental problem to be faced here is that of transferring the extremely weak radiation focused by a moving telescope to the entrance of a stationary spectrograph. The solution is to connect a telescope to a spectrograph through an optical fiber, and research and development of the system’s necessary components is currently a major task in the field of fiber optics. A key problem in astronomical spectroscopy is the precision measurement of Doppler line shifts in the spectra of stars to determine the velocity of a star movement along the observation line (ray velocity, RV). A remarkable feature of Doppler spectroscopy is that a precision measurement of quite small RV variations (in fact, accelerations) can last for long periods of time. Such star RV variations can be due, for example, to a planet orbiting the star; the action of the planet causes periodical changes in the motion of the star, thus leading to a Doppler shift of the star’s spectrum. The precise measurement of this shift has provided an indirect method for searching for and discovering planets outside the Solar System (exoplanets). The important particular problem of searching for habitable earthlike exoplanets requires a spectral measurement accuracy sufficient to detect RV variations at the level of a few centimeters per second per year. Ten to fifteen years of such measurements would provide a direct estimate of the hypothetical accelerated expansion of the Universe. However, the accuracy required for this is more than conventional spectroscopy techniques (iodine cell and spectral lamps) are capable of. This paper reviews approaches to radically improving Doppler spectroscopy techniques to achieve the required shift measurement accuracy. These approaches include the development of fiber-optic systems for connecting the telescope with the astronomical spectrograph and the development of precision calibrators of astronomical spectrographs based on the advances in laser physics and fiber optics.

Broadband very high frequency localization of lightning radiation sources based on orthogonal propagator method

Broadband very-high frequency (VHF) localization of lightning radiation sources provides an important means for understanding lightning discharge characteristics and the corresponding physical mechanisms. In order to improve the ability to locate weak radiation sources, the orthogonal propagator method (OPM) is proposed to map the space-time evolution process of lightning discharge channels based on the theory of spatial spectrum estimation. In the method, the linear decomposition of the covariance matrix is used to form the orthogonal propagator, and the spatial spectrum is constructed according to orthogonality of subspaces. Then, the location of lightning radiation sources is determined by searching for the maximum of the spatial spectrum. For broadband VHF signals, the non-coherent subspace method is used to average the effective frequency points in bandwidth to reduce noise interference. Based on a multiple-antenna radiation continuous observation system (MARCOS), locating performance of the method is analyzed by numerical simulation. The method is verified by parameters such as locating error, half-peak width of the spatial spectrum, and angular resolution. Compared with the results from the time reversal technique(FDTR), the location error and recognition probability under a low signal to noise ratio (SNR) of the proposed OPM algorithm are similar to those of FDTR algorithm, but the angular resolution for two radiation sources of OPM algorithm is better than that of FDTR algorithm. Finally, the proposed method is used to map the spatial and temporal development of a classical triggered lightning discharge channels in the summer of 2017. The results show that the proposed method can clearly depict the basic structure of lightning discharge channels with high spatial and temporal resolution. For the upward positive leader of the triggered lightning, the OPM algorithm can locate more radiation sources with a better structure than the FDTR algorithm. It implies that the proposed OPM algorithm is better for locating weak radiation sources than the FDTR algorithm. Meanwhile, the OPM algorithm has better performance for resolving two radiation sources in the same window than the FDTR algorithm. As a result, the proposed OPM method is of great significance for improving the application value of broadband VHF arrays in the study of locating weak radiation sources and lightning initiation mechanisms.

Reply to “Comments on Waveform Optimization for Transmit Beamforming With MIMO Radar Antenna Array”

First of all, we want to thank the authors of [1] for their interest in our work [2] . In [2] , the waveform design was carried out in the space domain only (coding the phases for different elements at the same moment) to form a desired transmit beam pattern at a given moment. However, if the MIMO radar waveforms are designed to satisfy the requirements in transmit beamforming (i.e., generating radiation nulls in specific directions) as well as the orthogonality between the waveforms transmitted from different elements, they must be coded simultaneously in space-time domains [3] . It turns out that the space-time coded waveforms cannot be exactly orthogonal in time domain. More specifically, to form a beam pattern with radiation null on a preselected direction, the waveforms can be optimized to be near-orthogonal with their cross correlation levels of about 1/ $N$ with $N $ being the coding length. In [1] , the authors questioned if the radiation energy in the preselected target direction could really be reduced or minimized based on (2). Since the cross correlations of the designed waveforms are nontrivial for a reasonable coding length, (2) does not hold true for the designed waveforms, and thus, cannot refute the conclusions obtained in [2] . In addition, the evaluation of (2), which is similar to the result of matched filtering, requires the full knowledge of the transmitted waveforms and cannot be done by a noncooperative target. Therefore, the designed MIMO radar waveforms in [2] and [3] would generate near-zero or very weak radiation (defocused beam) in the intended direction and are theoretically undetectable by noncooperative targets in the same direction. 1 However, for MIMO radar itself with full knowledge of transmit waveforms, target detection can be performed with the maximum gain through refocusing transmit-receive beam via space-time matched filtering processing at the radar receiver.