We expect this to result in an improved stability of the 1 mm instrumental polarization, probably enabling the first detections of circular polarization at 1mm.
#Pdf2id installed but not appearing in program upgrade#
The third component of the planned upgrade concerns an improvement of the telescope surface and its thermal behavior. Although our sources are all point–like, the off–axis pixels of the array will give an improved control on instrumental polarization and atmospheric disturbances.
A further planned upgrade at the IRAM 30 m Telescope is the installation of an array of receivers for wide field of view imaging. Modern digital backends can handle such large bandwidths, e.g.,, which may result with XPOL in even a factor 3 gain in sensitivity. The currently used backend VESPA limits the bandwidth used in practice to 500 MHz, much less than the bandwidth of 4 GHz of the receivers.
The most important and decisive component is the backend which performs the cross–correlation between the orthogonally linearly polarized receivers and thus giving Stokes V. This appears quite possible when some of the upgrades under discussion of the IRAM 30 m telescope take place. The circular polarization histogram with all sources combined ( Figure 3, top frame) indeed suggests that the bulk of the sources may become detectable in circular polarization much more often if the sensitivity would be increased by a factor 2. We have pointed out above that the circular polarization observations of POLAMI are severely sensitivity limited. Also, for the phase of the POLAMI program since mid–2021, the source sample has been partly modified to help addressing new scientific challenges in the field of high energy astrophysics, e.g., the origin of high energy extragalactic neutrinos detected by current TeV particle detectors, and the composition of extragalactic relativistic jets and their energetics. Two such events have been recorded during the first 7 years, and a few more since then. The key benefit of the exceptionally long duration is, apart from obtaining a precise statistical overview of the polarization properties of the blazar population, the chance to witness special events which throw new light on jet physics. Recently, three more years of monitoring time have been granted at the IRAM 30 m Telescope, at the end of which we expect a total of more than 6200 valid observations. Additional 2428 more observations have been made since then up to mid-May 2021. The POLAMI program has accumulated 2728 observations during its first 7 years up to the first evaluation date in august 2014. The peculiar behavior of circular polarization in 3C 66A, which we consider a hallmark of circular polarization generation by Faraday conversion in helical fields, is discussed. The widespread presence of circular polarization in the POLAMI sample is likely due to Faraday conversion of the linearly polarized synchrotron radiation in the helical magnetic field of the jets. The process generating circular polarization must not be strongly wavelength-dependent. The circular polarization detection rate and the maximum degree of circular polarization found are comparable with previous surveys at much longer wavelengths, thus, opening a new window for circular polarization and jet composition studies in the mm range. Moreover, the POLAMI program has shown statistical evidence that, for most of the monitored sources, circular polarization emission is displayed the majority of the time. They also further confirm that the short-wavelength (inner) emitting regions have better ordered magnetic fields than the long-wavelength ones (further downstream).
These results are fully compatible with recent multi-zone turbulent jet models, and they definitively rule out the popular single-zone models for blazars. The linear polarization angle is highly variable in all sources with excursions > 180° and for the case of the polarization angle, also the 1 mm variations appear to be faster than those at 3 mm. We found that the variability of the degree of linear polarization is faster and of higher amplitude at 1 mm than at 3 mm and that the linear polarization is also more variable than the total flux. We review results from the POLAMI program, which monitors the polarization properties of 36 blazars at the IRAM 30 m telescope.
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