IXPE observation confirms a high spin in the accreting black hole 4U 1957+115
Lorenzo Marra, M. Brigitte, Nicole Rodriguez Cavero, Sohee Chun, James F. Steiner, Michal Dovčiak, Michael A. Nowak, S. Bianchi, Fiamma Capitanio, Adam Ingram, G. Matt, Fabio Muleri, Jakub Podgorný, Juri Poutanen, Jiří Svoboda, Roberto Taverna, F. Ursini, Alexandra Veledina, Alessandra De Rosa, Javier A. García, A. Lutovinov, I. A. Mereminskiy, R. Farinelli, Shuichi Gunji, P. Kaaret, T. R. Kallman, H. Krawczynski, Yukiko Kan, Kun Hu, Andrea Marinucci, Guglielmo Mastroserio, Romana Mikušincová, Maxime Parra, Pierre-Olivier Petrucci, Ajay Ratheesh, P. Soffitta, Francesco Tombesi, Silvia Zane, I. Agudo, L. A. Antonelli, Matteo Bachetti, L. Baldini, W. H. Baumgartner, R. Bellazzini, Stephen D. Bongiorno, R. Bonino, A. Brez, N. Bucciantini, Simone Castellano, E. Cavazzuti, Chien‐Ting Chen, S. Ciprini, E. Costa, E. Del Monte, Laura Di Gesu, Niccolò Di Lalla, Alessandro Di Marco, I. Donnarumma, Victor Doroshenko, Steven R. Ehlert, Teruaki Enoto, Y. Evangelista, Sergio Fabiani, Riccardo Ferrazzoli, K. Hayashida, Jeremy Heyl, W. Iwakiri, S. G. Jorstad, V. Karas, Fabian Kislat, T. Kitaguchi, Jeffery J. Kolodziejczak, Fabio La Monaca, L. Latronico, Ioannis Liodakis, S. Maldera, Alberto Manfreda, Frédéric Marin, Alan P. Marscher, Herman L. Marshall, F. Massaro, I. Mitsuishi, Tsunefumi Mizuno, Michela Negro, C.‐Y. Ng, Stephen L. O’Dell, N. Omodei, C. Oppedisano, Alessandro Papitto, George G. Pavlov, Abel L. Peirson, M. Perri, M. Pesce-Rollins, M. Pilia, Andrea Possenti, Simonetta Puccetti, Brian D. Ramsey, John Rankin, O. J. Roberts, Roger W. Romani
Abstract
We present the results of the first X-ray polarimetric observation of the low-mass X-ray binary 4U 1957+115, performed with the Imaging X-ray Polarimetry Explorer in May 2023. The binary system has been in a high-soft spectral state since its discovery and is thought to host a black hole. The ∼571 ks observation reveals a linear polarisation degree of 1.9%±0.6% and a polarisation angle of −41.°8±7.°9 in the 2–8 keV energy range. Spectral modelling is consistent with the dominant contribution coming from the standard accretion disc, while polarimetric data suggest a significant role of returning radiation: photons that are bent by strong gravity effects and forced to return to the disc surface, where they can be reflected before eventually reaching the observer. In this setting, we find that models with a black hole spin lower than 0.96 and an inclination lower than 50° are disfavoured.