GroundBIRD : A CMB polarization experiment with MKID arrays

The cosmic microwave background (CMB) is a isotropic blackbody radiation at the temperature of 2.7255 K WMAP . The CMB has small anisotropies at the $10^{-5}~{}\mathrm{K}$ level in temperature and sub-$\mu\mathrm{K}$ level in polarization. The CMB anisotropies can be characterized by the angular power spectrum, and it provides constraints on numerous cosmological parameters Dodelson .

The E-modes of CMB polarization have been well observed by various experiments WMAP and lensing B-modes have been measured by other experiments, PolarBEAR and BICEP2 Polarbear ; Bicep2 . The B-modes larger than degree scale, generated by primordial gravitational waves predicted by the inflation model, has not been detected yet Guth1981 . The observation of B-modes from the primordial gravitational waves will reveal information about the very early universe hidden behind the background radiations.

GroundBIRD is aiming to measure the B-modes of CMB polarization Tajima2012 . The observation site is Teide observatory. Earlier this year, we transported the GroundBIRD telescope from Japan to Instituto de Astrofísica de Canarias (IAC). In IAC, the telescope is assembled and is now under test. We are preparing for installation of the telescope at the Teide observatory in parallel. Once the assembly and tests are finished at IAC, we will move the telescope to the Teide observatory and continue the commissioning of the telescope with sky signals.

The GroundBIRD telescope is designed for the precise observation of CMB polarization. The target sensitivity is to constrain the tensor-to-scalar ratio $r$ with a precision of $\sigma_{r}\sim 0.01$ with three year observation Oguri2016 . To achieve high enough sensitivity to observe B-modes for large sky coverage, we employ three features: fast rotation scanning, microwave kinetic inductance detector (MKID) and cold optics Tajima2012 .

Aside from the thermal noise, the main source of systematics in CMB observations at large angular scale is $1/f$ noise due to the atmospheric fluctuations. To suppress this, the sky should be measured repeatedly with a scanning rate faster than the typical value of $f_{knee}=0.1$ Hz Tajima2012 . The telescope scans the sky with continuous rotations at 20 rpm (revolutions per minute) and an elevation of 60 degrees. The rotation speed results the scanning rate of 0.33 Hz which means that the telescope scans a ring every 3 seconds. A sky area of $10^{-6}$ sr (steradian) is observed on average 1000 times per day assuming a pixelization which divides the whole sky into 12,582,912 pixels.

The MKID is a photon sensor using superconducting resonators Day . An MKID array for the focal plane is under fabrication in RIKEN (Institute of Physical and Chemical Research). Figure 1(a) shows a full scale design of our MKID. An orthomode transducer (OMT) in each pixel receives photons in two perpendicular directions. The signals from the OMTs are transmitted to the MKIDs through the identical circuits with transmission line converters and filters, and the MKIDs detect the intensities of the photon in two directions. The meander line at the end of the circuit is MKID as shown in Fig. 1(a). The resonant frequencies for our MKIDs are controlled by their length, and are in the range of 4 to 8 GHz. The OMTs will be coupled with a corrugated horn antennae or a hemispherical lenslets for higher optical efficiency. Due to the fast response and short dead time of MKID, our system can have a fast sampling rate of 1 kSps.

The focal plane of the GroundBIRD telescope has 7 modules as shown in Fig. 1(b). GroundBIRD will cover two frequency bands, 145 GHz and 220 GHz, for the separation of dust foreground. The module in the center is for 220 GHz and outer 6 modules are for 145 GHz. Each module has 23 pixels and the total number of pixels are 161 of which 138 for 145 GHz and 23 for 220 GHz. The sensitivity for the sky signal is determined by the integration time for observation per pixel. The expected sky pixel noise level is 21 $\mu$K arcmin by assuming three-year observation, 138 pixels and sky noise equivalent temperature (NET) of 300 $\mu$K. By combining with the large sky coverage, this value guarantees that the GroundBIRD can measure the tensor-to-scalar ratio from B-mode polarizations patterns at large angular scales ($l\sim 6$).

The cryostat of the GroundBIRD telescope is a cylindrical vacuum chamber with diameter of 1.5 m and 1.5 m in height as shown in Fig. 2(a). Within the chamber, there are 40 K and 4 K stages. The temperatures of theses stages are maintained by two-stage pulse tube cooler (PTC). The 4 K stage contains the main part of the telescope including optics and focal plane. It is insulated by 40 K and 4 K shields. The shields are covered with multilayer insulation (MLI) sheets. In addition, these are wrapped in anti-magnetic sheets to reduce the effect of external magnetic fields on the MKID array Kutsuma2018 . The aperture windows of 40 K and 4 K shields are covered by low-pass metal mesh filters Metalmesh and radio transparent multilayer insulator (RT-MLI) Choi2013 .

The crossed-Dragone reflector is the main optics of the GroundBIRD telescope Tajima2012 . The incident radiations are reflected on the primary and secondary mirrors. The two reflectors focus a plane wave to a point on the focal plane. The resulting beam width for a pixel in the sky is about 0.5 degrees by this focusing. The crossed-Dragone reflector is cooled down on the the 4 K stage. By cooling the reflector, thermal noise caused by the surface emission can be suppressed Tajima2012 .

The telescope is rotated by our rotation table. It consists of the rotation stage in the center, supporting legs and the upper plate. The rotation stage rotates the upper structure. A custom-made rotary joint is used for the electricity supply, and helium gas circulation during the rotation Tajima2012 .

The observation site is Teide Observatory in Tenerife, Canary islands, Spain. The altitude and longitude are $28\degree 18^{\prime}$N and $16\degree 30^{\prime}$W and the altitude is 2390 m.

Even for only one day observation, we can make a sky map of a strip centered at declination of $28\degree$ with width of $60\degree$. The resulting coverage is 40% of the full sky and angular resolution is 0.5 degrees. To improve the signal-to-noise ratio, we will stack the maps for year-scale. We expect to obtain the angular power spectrum for the angular scales of $6<l<300$, which enables to observe both recombination ($l\sim 100$) and reionization ($l<10$) bumps in B-mode spectrum Tajima2012 .

In the first half of this year, the GroundBIRD telescope has been transported from Japan to IAC in Tenerife, Spain. We are assembling the telescope and preparing its deployment on the Teide observatory. We shipped our telescope after we confirmed the receiver performances with rotation on the telescope mount. The temperature of focal plane and cold optics during rotation at 20 rpm. For 16 hours after the start of rotation, the cold optics temperature is maintained at 3.5 K and the temperature of the focal plane is maintained between 250 and 270 mK. Since the detector temperature varies much slower than the rotation of the telescope, the drift in detector response correlated with the temperature change is suppressed with the $1/f$ noise removal algorithm during the data analysis. Additionally the resonant frequencies of MKIDs are calibrated every hour to compensate the shift of the resonance peak due to the temperature change.

GroundBIRD aims to observe the CMB B-mode polarization and measure the tensor-to-scalar ratio $r$ with a precision of $\sigma_{r}\sim 0.01$. The GroundBIRD telescope was moved from Japan to Tenerife and installed at the Teide observatory. The commissioning of the telescope is on going and the MKID array for the actual observation is under development aiming to start observation in 2020.