The QUIJOTE experiment is being developed by the Instituto de Astrofisica de Canarias (IAC), the Instituto de Fisica de Cantabria (IFCA), and the Universities of Cantabria, Manchester and Cambridge. We thank the staff of the Teide Observatory for invaluable assistance in the commissioning and operation of QUIJOTE. Partial financial support was provided by the Spanish Ministry of Science and Innovation under the projects AYA2007-68058-C03-01, AYA2007- 68058-C03-02, AYA2010-21766-C03-01, AYA2010-21766-C03-02, AYA2014-60438-P, ESP2015-70646-C2-1-R, AYA2017-84185-P, ESP2017-83921-C2-1-R, AYA2017-90675-REDC (co-funded with EU FEDER funds), PGC2018-101814-B-I00, PID2019-110610RBC21, PID2020-120514GB-I00, IACA13-3E-2336, IACA15-BE3707, EQC2018-004918-P, the Severo Ochoa Programs SEV-2015- 0548 and CEX2019-000920-S, the Maria de Maeztu Program MDM2017-0765, and by the Consolider-Ingenio project CSD2010-00064 (EPI: Exploring the Physics of Inflation). We acknowledge support from the ACIISI, Consejeria de Economia, Conocimiento y Empleo del Gobierno de Canarias and the European Regional Development Fund (ERDF) under grant with reference ProID2020010108. This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement number 687312 (RADIOFOREGROUNDS). This research made use of computing time available on the high-performance computing systems at the IAC. We thankfully acknowledge the technical expertise and assistance provided by the Spanish Supercomputing Network (Red Española de Supercomputacion), as well as the computer resources used: the Deimos/Diva Supercomputer, located at the IAC. FG acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 101001897). EdlH acknowledges partial financial support from the Concepción Arenal Programme of the Universidad de Cantabria. FP acknowledges support from the Spanish State Research Agency (AEI) under grant number PID2019-105552RB-C43. BR-G acknowledges ASI-INFN Agreement 2014-037-R.0. DT acknowledges the support from the Chinese Academy of Sciences President’s International Fellowship Initiative, Grant N. 2020PM0042. This work has made use of S-band Polarisation All Sky Survey (S-PASS) data. Some ofthe resultsin this paper have been derived using the HEALPIX (Gorski et al. 2005) and HEALPY (Zonca et al. 2019) packages. We also use NUMPY (Harris et al. 2020), and MATPLOTLIB (Hunter 2007).

The Haze is an excess of microwave intensity emission surrounding the Galactic Centre. It is spatially correlated with the γ -ray Fermi bubbles, and with the S-PASS radio polarization plumes, suggesting a possible common provenance. The models proposed to explain the origin of the Haze, including energetic events at the Galactic Centre and dark matter decay in the Galactic halo, do not yet provide a clear physical interpretation. In this paper, we present a reanalysis of the Haze including new observations from the Multi-Frequency Instrument (MFI) of the Q-U-I Joint TEnerife (QUIJOTE) experiment, at 11 and 13 GHz. We analyse the Haze in intensity and polarization, characterizing its spectrum. We detect an excess of diffuse intensity signal ascribed to the Haze. The spectrum at frequencies 11 GHz ≤ ν ≤ 70 GHz is a power law with spectral index βH = −2.79 ± 0.08, which is flatter than the Galactic synchrotron in the same region (βS = −2.98 ± 0.04), but steeper than that obtained from previous works (βH ∼ −2.5 at 23 GHz ≤ ν ≤ 70 GHz). We also observe an excess of polarized signal in the QUIJOTE-MFI maps in the Haze area. This is a first hint detection of polarized Haze, or a consequence of curvature of the synchrotron spectrum in that area. Finally, we show that the spectrum of polarized structures associated with Galactic Centre activity is steep at low frequencies (β ∼ −3.2 at 2.3 GHz ≤ ν ≤ 23 GHz), and becomes flatter above 11 GHz.