First detection of X-ray polarization from the accreting neutron star 4U 1820-303

• Published in: arXiv.org 2023-08
• Main Authors: Alessandro Di Marco, Fabio La Monaca, Russell, Thomas D, Alessio Anitra, Farinelli, Ruben, Mastroserio, Guglielmo, Muleri, Fabio, Bachetti, Matteo, Burderi, Luciano, Carotenuto, Francesco, Tiziana Di Salvo, Gnarini, Andrea, Iaria, Rosario, Kajava, Jari J E, Liu, Kuan, Middei, Riccardo, O'Dell, Stephen L, Pilia, Maura, Rankin, John, Sanna, Andrea, Jakob van den Eijnden, Weisskopf, Martin C, Bobrikova, Anna, Capitanio, Fiamma, Kaaret, Philip, Marino, Alessio, Soffitta, Paolo, Ursini, Francesco, Ambrosino, Filippo, Cocchi, Massimo, Fabiani, Sergio, Marshall, Herman L, Giorgio, Matt, Motta, Sara Elisa, Papitto, Alessandro, Stella, Luigi, Tarana, Antonella, Zane, Silvia, Antonelli, Lucio A, Baumgartner, Wayne H, Bellazzini, Ronaldo, Bianchi, Stefano, Bongiorno, Stephen D, Bonino, Raffaella, Brez, Alessandro, Bucciantini, Niccolo, Castellano, Simone, Cavazzuti, Elisabetta, Chien-Ting, Chen, Ciprini, Stefano, Laura Di Gesu, Donnarumma, Immacolata, Ehlert, Steven R, Evangelista, Yuri, Ferrazzoli, Riccardo, Garcia, Javier A, Gunji, Shuichi, Hayashida, Kiyoshi, Heyl, Jeremy, Iwakiri, Wataru, Jorstad, Svetlana G, Karas, Vladimir, Kislat, Fabian, Kitaguchi, Takao, Kolodziejczak, Jeffery J, Krawczynski, Henric, Latronico, Luca, Liodakis, Ioannis, Maldera, Simone, Manfreda, Alberto, Marin, Frederic, Marinucci, Andrea, Massaro, Francesco, Mitsuishi, Ikuyuki, Mizuno, Tsunefumi, Negro, Michela, Omodei, Nicola, Oppedisano, Chiara, Pavlov, George G, Peirson, Abel L, Perri, Matteo, Pesce-Rollins, Melissa, Petrucci, Pierre-Olivier, Possenti, Andrea, Puccetti, Simonetta, Ramsey, Brian D, Ratheesh, Ajay, Romani, Roger W, Sgrò, Carmelo, Swartz, Douglas A, Tamagawa, Toru, Tavecchio, Fabrizio, Taverna, Roberto, Tawara, Yuzuru, Tennant, Allyn F, Trois, Alessio, Tsygankov, Sergey S, Turolla, Roberto, Vink, Jacco, Wu, Kinwah
• Format: Article
• Language: English
• Subjects: Accretion disks; Confidence intervals; Linear polarization; Neutron stars; Polarimetry; X ray imagery; X ray spectra; X-rays
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2306.08476

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.