First detection of X-ray polarization from the accreting neutron star 4U 1820-303
Creators: name: Di Marco, Alessandro
name: La Monaca, Fabio
name: Poutanen, Juri
name: Russell, Thomas D.
name: Anitra, Alessio
name: Farinelli, Ruben
name: Mastroserio, Guglielmo
name: Muleri, Fabio
name: Xie, Fei
name: Bachetti, Matteo
name: Burderi, Luciano
name: Carotenuto, Francesco
name: Del Santo, Melania
name: Di Salvo, Tiziana
name: Dovciak, Michal
name: Gnarini, Andrea
name: Iaria, Rosario
name: Kajava, Jari J. E.
name: Liu, Kuan
name: Middei, Riccardo
name: O'Dell, Stephen L.
name: Pilia, Maura
name: Rankin, John
name: Sanna, Andrea
name: Eijnden, Jakob van den
name: Weisskopf, Martin C.
name: Bobrikova, Anna
name: Capitanio, Fiamma
name: Costa, Enrico
name: Kaaret, Philip
name: Marino, Alessio
name: Soffitta, Paolo
name: Ursini, Francesco
name: Ambrosino, Filippo
name: Cocchi, Massimo
name: Fabiani, Sergio
name: Marshall, Herman L.
name: Matt, Giorgio
name: Motta, Sara Elisa
name: Papitto, Alessandro
name: Stella, Luigi
name: Tarana, Antonella
name: Zane, Silvia
name: Agudo, Ivan
name: Antonelli, Lucio A.
name: Baldini, Luca
name: Baumgartner, Wayne H.
name: Bellazzini, Ronaldo
name: Bianchi, Stefano
name: Bongiorno, Stephen D.
name: Bonino, Raffaella
name: Brez, Alessandro
name: Bucciantini, Niccolo
name: Castellano, Simone
name: Cavazzuti, Elisabetta
name: Chen, Chien-Ting
name: Ciprini, Stefano
name: De Rosa, Alessandra
name: Del Monte, Ettore
name: Di Gesu, Laura
name: Di Lalla, Niccolo
name: Donnarumma, Immacolata
name: Doroshenko, Victor
name: Ehlert, Steven R.
name: Enoto, Teruaki
name: Evangelista, Yuri
name: Ferrazzoli, Riccardo
name: Garcia, Javier A.
name: Gunji, Shuichi
name: Hayashida, Kiyoshi
name: Heyl, Jeremy
name: Iwakiri, Wataru
name: Jorstad, Svetlana G.
name: Karas, Vladimir
name: Kislat, Fabian
name: Kitaguchi, Takao
name: Kolodziejczak, Jeffery J.
name: Krawczynski, Henric
name: Latronico, Luca
name: Liodakis, Ioannis
name: Maldera, Simone
name: Manfreda, Alberto
name: Marin, Frederic
name: Marinucci, Andrea
name: Marscher, Alan P.
name: Massaro, Francesco
name: Mitsuishi, Ikuyuki
name: Mizuno, Tsunefumi
name: Negro, Michela
name: Ng, Chi-Yung
name: Omodei, Nicola
name: Oppedisano, Chiara
name: Pavlov, George G.
name: Peirson, Abel L.
name: Perri, Matteo
name: Pesce-Rollins, Melissa
name: Petrucci, Pierre-Olivier
name: Possenti, Andrea
name: Puccetti, Simonetta
name: Ramsey, Brian D.
name: Ratheesh, Ajay
name: Roberts, Oliver J.
name: Romani, Roger W.
name: Sgrò, Carmelo
name: Slane, Patrick
name: Spandre, Gloria
name: Swartz, Douglas A.
name: Tamagawa, Toru
name: Tavecchio, Fabrizio
name: Taverna, Roberto
name: Tawara, Yuzuru
name: Tennant, Allyn F.
name: Thomas, Nicholas E.
name: Tombesi, Francesco
name: Trois, Alessio
name: Tsygankov, Sergey S.
name: Turolla, Roberto
name: Vink, Jacco
name: Wu, Kinwah
Publisher: name: arXiv
Year: 2023
Subjects: subject: High Energy Astrophysical Phenomena (astro-ph.HE)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: 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 (ATCA). The IXPE observations of 4U 1820-303 were coordinated with Swift-XRT, NICER, and NuSTAR 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) deg in the 4-7 keV energy range, and a polarization degree of 10(2)% and a polarization angle of -67(7) deg 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 about 90 deg 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 99.97% upper limit of 50% at 7.25 GHz.