First Detection of X-Ray Polarization from the Accreting Neutron Star 4U 1820−303
Alessandro Di Marco, Fabio La Monaca, Juri Poutanen, T. D. Russell, A. Anitra, R. Farinelli, Guglielmo Mastroserio, Fabio Muleri, Fei Xie, Matteo Bachetti, L. Burderi, Francesco Carotenuto, M. Del Santo, T. Di Salvo, Michal Dovčiak, Andrea Gnarini, R. Iaria, J. J. E. Kajava, Kuan Liu, R. Middei, Stephen L. O’Dell, M. Pilia, John Rankin, A. Sanna, J. van den Eijnden, Martin C. Weisskopf, Anna Bobrikova, Fiamma Capitanio, E. Costa, P. Kaaret, A. Marino, P. Soffitta, F. Ursini, Filippo Ambrosino, M. Cocchi, Sergio Fabiani, Herman L. Marshall, G. Matt, S. Motta, Alessandro Papitto, L. Stella, A. Tarana, Silvia Zane, I. Agudo, L. A. Antonelli, L. Baldini, W. H. Baumgartner, R. Bellazzini, S. Bianchi, Stephen D. Bongiorno, R. Bonino, A. Brez, N. Bucciantini, Simone Castellano, E. Cavazzuti, Chien‐Ting Chen, S. Ciprini, Alessandra De Rosa, E. Del Monte, Laura Di Gesu, Niccolò Di Lalla, I. Donnarumma, Victor Doroshenko, Steven R. Ehlert, Teruaki Enoto, Y. Evangelista, Riccardo Ferrazzoli, Javier A. García, Shuichi Gunji, Kiyoshi Hayashida, Jeremy Heyl, W. Iwakiri, Svetlana G. Jorstad, V. Karas, Fabian Kislat, Takao Kitaguchi, Jeffery J. Kolodziejczak, H. Krawczynski, L. Latronico, Ioannis Liodakis, S. Maldera, Alberto Manfreda, Frédéric Marin, Andrea Marinucci, Alan P. Marscher, F. Massaro, Ikuyuki Mitsuishi, Tsunefumi Mizuno, Michela Negro, C.‐Y. Ng, N. Omodei, C. Oppedisano, George G. Pavlov, Abel L. Peirson, M. Perri, M. Pesce-Rollins, Pierre-Olivier Petrucci, Andrea Possenti, Simonetta Puccetti, Brian D. Ramsey
Abstract
Abstract This paper reports the first detection of polarization in the X-rays for atoll-source 4U 1820−303, obtained with the Imaging X-ray Polarimetry Explorer (IXPE) at 99.999% confidence level (CL). Simultaneous polarimetric measurements were also performed in the radio with the Australia Telescope Compact Array. The IXPE observations of 4U 1820−303 were coordinated with Swift X-ray Telescope, Neutron Star Interior Composition Explorer, and Nuclear Spectroscopic Telescope Array aiming to obtain an accurate X-ray spectral model covering a broad energy interval. The source shows a significant polarization above 4 keV, with a polarization degree of 2.0% ± 0.5% and a polarization angle of −55° ± 7° in the 4–7 keV energy range, and a polarization degree of 10% ± 2% and a polarization angle of −67° ± 7° in the 7–8 keV energy bin. This polarization also shows a clear energy trend with polarization degree increasing with energy and a hint for a position-angle change of ≃90° at 96% CL around 4 keV. The spectro-polarimetric fit indicates that the accretion disk is polarized orthogonally to the hard spectral component, which is presumably produced in the boundary/spreading layer. We do not detect linear polarization from the radio counterpart, with a 3 σ upper limit of 50% at 7.25 GHz.