The geometry of the magnetic field in the central molecular zone measured by PILOT

International audienceWe present the first far infrared (FIR) dust emission polarization map covering the full extent of Milky Way’s central molecular zone (CMZ). The data, obtained with the PILOT balloon-borne experiment, covers the Galactic center region − 2° < ℓ < 2°, − 4° < b < 3° at a wavelength of 240 μm and an angular resolution of 2.2′. From our measured dust polarization angles, we infer a magnetic field orientation projected onto the plane of the sky (POS) that is remarkably ordered over the full extent of the CMZ, with an average tilt angle of ≃22° clockwise with respect to the Galactic plane. Our results confirm previous claims that the field traced by dust polarized emission is oriented nearly orthogonally to the field traced by GHz radio synchrotron emission in the Galactic center region. The observed field structure is globally compatible with the latest Planck polarization data at 353 and 217 GHz. Upon subtraction of the extended emission in our data, the mean field orientation that we obtain shows good agreement with the mean field orientation measured at higher angular resolution by the JCMT within the 20 and 50 km s−1 molecular clouds. We find no evidence that the magnetic field orientation is related to the 100 pc twisted ring structure within the CMZ. The low polarization fraction in the Galactic center region measured with Planck at 353 GHz combined with a highly ordered projected field orientation is unusual. This feature actually extends to the whole inner Galactic plane. We propose that it could be caused by the increased number of turbulent cells for the long lines of sight towards the inner Galactic plane or to dust properties specific to the inner regions of the Galaxy. Assuming equipartition between magnetic pressure and ram pressure, we obtain magnetic field strength estimates of the order of 1 mG for several CMZ molecular clouds

Inflight performance of the PILOT balloon-borne experiment

International audienceThe Polarized Instrument for Long-wavelength Observation of the Tenuous interstellar medium (PILOT) is a balloon-borne experiment that aims to measure the polarized emission of thermal dust at a wavelength of 240 µm (1.2 THz). A first PILOT flight of the experiment took place from Timmins, Ontario, Canada, in September 2015 and a second flight took place from Alice Springs, Australia in April 2017. In this paper, we present the inflight performance of the instrument. Here we concentrate on the instrument performance as measured during the second flight, but refer to the performance observed during the first flight, if it was significantly different. We present a short description of the instrument and the flights. We measure the time constants of the detectors using the decay of the observed signal during flight following high energy particle impacts (glitches) and switching off the instrument's internal calibration source. We use these time constants to deconvolve the timelines and analyze the optical quality of the instrument as measured on planets. We then analyze the structure and polarization of the instrumental background. We measure the detector response flat field and its time variations using the signal from the residual atmosphere and from the internal calibration source. Finally, we analyze the spectral and temporal properties of the detector noise. The inflight performance is found to be satisfactory and globally in line with expectations from ground calibrations. We conclude by assessing the expected inflight sensitivity of the instrument in light of the measured inflight performance

The PILOT optical alignment for its first flight

PILOT is a balloon-borne astronomy experiment designed to study the polarization of dust emission in the diffuse interstellar medium in our Galaxy at wavelengths 240 and 550 µm with an angular resolution of about two arc-min. PILOT optics is composed of an off-axis Gregorian telescope and a refractive re-imager system. All these optical elements, except the primary mirror, are in a cryostat cooled to 3K. We used optical and 3D measurements combined with thermo-elastic modeling to perform the optical alignment. This paper describes the system analysis, the alignment procedure, and finally the performances obtained during the first flight in September 201