Repeating fast radio bursts with WSRT/Apertif

Repeating fast radio bursts (FRBs) present excellent opportunities to identify FRB progenitors and host environments, as well as decipher the underlying emission mechanism. Detailed studies of repeating FRBs might also hold clues to the origin of FRBs as a population. We aim to detect the first two repeating FRBs: FRB 121102 (R1) and FRB 180814.J0422+73 (R2), and characterise their repeat statistics. We also want to significantly improve the sky localisation of R2. We use the Westerbork Synthesis Radio Telescope to conduct extensive follow-up of these two repeating FRBs. The new phased-array feed system, Apertif, allows covering the entire sky position uncertainty of R2 with fine spatial resolution in one pointing. We characterise the energy distribution and the clustering of detected R1 bursts. We detected 30 bursts from R1. Our measurements indicate a dispersion measure of 563.5(2) pc cm$^{-3}$, suggesting a significant increase in DM over the past few years. We place an upper limit of 8% on the linear polarisation fraction of the brightest burst. We did not detect any bursts from R2. A single power-law might not fit the R1 burst energy distribution across the full energy range or widely separated detections. Our observations provide improved constraints on the clustering of R1 bursts. Our stringent upper limits on the linear polarisation fraction imply a significant depolarisation, either intrinsic to the emission mechanism or caused by the intervening medium, at 1400 MHz that is not observed at higher frequencies. The non-detection of any bursts from R2 implies either a highly clustered nature of the bursts, a steep spectral index, or a combination of both. Alternatively, R2 has turned off completely, either permanently or for an extended period of time.

The bolometric focal plane array of the Polarbear CMB experiment

The Polarbear Cosmic Microwave Background (CMB) polarization experiment is currently observing from the Atacama Desert in Northern Chile. It will characterize the expected B-mode polarization due to gravitational lensing of the CMB, and search for the possible B-mode signature of inflationary gravitational waves. Its 250 mK focal plane detector array consists of 1,274 polarization-sensitive antenna-coupled bolometers, each with an associated lithographed band-defining filter. Each detectors planar antenna structure is coupled to the telescopes optical system through a contacting dielectric lenslet, an architecture unique in current CMB experiments. We present the initial characterization of this focal plane.

Spectral Characterization of a Microbolometer Focal Plane Array at Terahertz Frequencies

We have developed a method to characterize the spectral response of an uncooled microbolometer focal plane array at a broad range of terahertz (THz) frequencies (4~50 THz). This is achieved by using a spectrum-tailored blackbody radiator as a broadband THz source and measuring its spectral power with a Fourier transform infrared (FTIR) interferometer. With an additional measurement with a pyroelectric detector as a reference, the spectral response of the microbolometer relative to the pyroelectric reference is obtained with a signal-to-noise ratio of 100 over a >50 THz bandwidth.

WSPEC: A waveguide filter-bank focal plane array spectrometer for millimeter wave astronomy and cosmology

Imaging and spectroscopy at (sub-)millimeter wavelengths are key frontiers in astronomy and cosmology. Large area spectral surveys with moderate spectral resolution (R=50-200) will be used to characterize large scale structure and star formation through intensity mapping surveys in emission lines such as the CO rotational transitions. Such surveys will also be used to study the SZ effect, and will detect the emission lines and continuum spectrum of individual objects. WSPEC is an instrument proposed to target these science goals. It is a channelizing spectrometer realized in rectangular waveguide, fabricated using conventional high-precision metal machining. Each spectrometer is coupled to free space with a machined feed horn, and the devices are tiled into a 2D array to fill the focal plane of the telescope. The detectors will be aluminum Lumped-Element Kinetic Inductance Detectors (LEKIDs). To target the CO lines and SZ effect, we will have bands at 135-175 GHz and 190-250 GHz, each Nyquist-sampled at R~200 resolution. Here we discuss the instrument concept and design, and successful initial testing of a WR10 (i.e. 90 GHz) prototype spectrometer. We recently tested a WR5 (180 GHz) prototype to verify that the concept works at higher frequencies, and also designed a resonant backshort structure that may further increase the optical efficiency. We are making progress towards integrating a spectrometer with a LEKID array and deploying a prototype device to a telescope for first light.

Substrate-free THz focal plane metamaterial array with high absorption ratio

Microelectromechanical system (MEMS) focal plane array (FPA) with optical readout offers exciting opportunities for real-time terahertz (THz) imaging. However, conventional FPA suffers from a low THz absorption ratio, which further decreases the performance of THz imaging. Here, we present a simple and scalable approach for the realization of THz focal plane metamaterial array with a relatively high absorption ratio. The key idea is to combine the advantages of substrate-free structures with metamaterial. A 100 x100 THz FPA with a 150 x 150 {mu}m pixel is designed, fabricated, and characterized. The dependence of the THz absorption ratio on the thickness of SiNx dielectric substrate film is investigated. The fabricated FPA exhibits a 90.6% resonant absorption at 1.36 THz, agreeing considerably with the theoretical simulation results. Our results imply that such a substrate-free THz focal plane metamaterial array enables the realization of THz imaging.