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arXiv · 2311.03667

X-ray Polarization Reveals the Precessions of the Neutron Star in Hercules X-1

Jeremy Heyl·Victor Doroshenko·Denis González-Caniulef·Ilaria Caiazzo·Juri Poutanen·Alexander Mushtukov·Sergey S. Tsygankov·Demet Kirmizibayrak·Matteo Bachetti·George G. Pavlov·Sofia V. Forsblom·Christian Malacaria·Valery F. Suleimanov·Iván Agudo·Lucio Angelo Antonelli·Luca Baldini·Wayne H. Baumgartner·Ronaldo Bellazzini·Stefano Bianchi·Stephen D. Bongiorno·Raffaella Bonino·Alessandro Brez·Niccolò Bucciantini·Fiamma Capitanio·Simone Castellano·Elisabetta Cavazzuti·Chien-Ting Chen·Stefano Ciprini·Enrico Costa·Alessandra De Rosa·Ettore Del Monte·Laura Di Gesu·Niccolò Di Lalla·Alessandro Di Marco·Immacolata Donnarumma·Michal Dovčiak·Steven R. Ehlert·Teruaki Enoto·Yuri Evangelista·Sergio Fabiani·Riccardo Ferrazzoli·Javier A. Garcia·Shuichi Gunji·Kiyoshi Hayashida·Wataru Iwakiri·Svetlana G. Jorstad·Philip Kaaret·Vladimir Karas·Fabian Kislat·Takao Kitaguchi·Jeffery J. Kolodziejczak·Henric Krawczynski·Fabio La Monaca·Luca Latronico·Ioannis Liodakis·Simone Maldera·Alberto Manfreda·Frédéric Marin·Andrea Marinucci·Alan P. Marscher·Herman L. Marshall·Francesco Massaro·Giorgio Matt·Ikuyuki Mitsuishi·Tsunefumi Mizuno·Fabio Muleri·Michela Negro·C. -Y. Ng·Stephen L. O'Dell·Nicola Omodei·Chiara Oppedisano·Alessandro Papitto·Abel Lawrence Peirson·Matteo Perri·Melissa Pesce-Rollins·Pierre-Olivier Petrucci·Maura Pilia·Andrea Possenti·Simonetta Puccetti·Brian D. Ramsey·John Rankin·Ajay Ratheesh·Oliver J. Roberts·Roger W. Romani·Carmelo Sgrò·Patrick Slane·Paolo Soffitta·Gloria Spandre·Douglas A. Swartz·Toru Tamagawa·Fabrizio Tavecchio·Roberto Taverna·Yuzuru Tawara·Allyn F. Tennant·Nicholas E. Thomas·Francesco Tombesi·Alessio Trois·Roberto Turolla·Jacco Vink·Martin C. Weisskopf
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Abstract

In an accreting X-ray pulsar, a neutron star accretes matter from a stellar companion through an accretion disk. The high magnetic field of the rotating neutron star disrupts the inner edge of the disc, funneling the gas to flow onto the magnetic poles on its surface. Hercules X-1 is in many ways the prototypical X-ray pulsar; it shows persistent X-ray emission and it resides with its companion HZ Her, a two-solar-mass star, at about 7~kpc from Earth. Its emission varies on three distinct timescales: the neutron star rotates every 1.2~seconds, it is eclipsed by its companion each 1.7~days, and the system exhibits a superorbital period of 35~days which has remained remarkably stable since its discovery. Several lines of evidence point to the source of this variation as the precession of the accretion disc, the precession of the neutron star or both. Despite the many hints over the past fifty years, the precession of the neutron star itself has yet not been confirmed or refuted. We here present X-ray polarization measurements with the Imaging X-ray Polarimetry Explorer (IXPE) which probe the spin geometry of the neutron star. These observations provide direct evidence that the 35-day-period is set by the free precession of the neutron star crust, which has the important implication that its crust is somewhat asymmetric fractionally by a few parts per ten million. Furthermore, we find indications that the basic spin geometry of the neutron star is altered by torques on timescale of a few hundred days.

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Jeremy Heyl, Victor Doroshenko, Denis González-Caniulef, Ilaria Caiazzo, Juri Poutanen, Alexander Mushtukov, Sergey S. Tsygankov, Demet Kirmizibayrak, Matteo Bachetti, George G. Pavlov, Sofia V. Forsblom, Christian Malacaria, Valery F. Suleimanov, Iván Agudo, Lucio Angelo Antonelli, Luca Baldini, Wayne H. Baumgartner, Ronaldo Bellazzini, Stefano Bianchi, Stephen D. Bongiorno, Raffaella Bonino, Alessandro Brez, Niccolò Bucciantini, Fiamma Capitanio, Simone Castellano, Elisabetta Cavazzuti, Chien-Ting Chen, Stefano Ciprini, Enrico Costa, Alessandra De Rosa, Ettore Del Monte, Laura Di Gesu, Niccolò Di Lalla, Alessandro Di Marco, Immacolata Donnarumma, Michal Dovčiak, Steven R. Ehlert, Teruaki Enoto, Yuri Evangelista, Sergio Fabiani, Riccardo Ferrazzoli, Javier A. Garcia, Shuichi Gunji, Kiyoshi Hayashida, Wataru Iwakiri, Svetlana G. Jorstad, Philip Kaaret, Vladimir Karas, Fabian Kislat, Takao Kitaguchi, Jeffery J. Kolodziejczak, Henric Krawczynski, Fabio La Monaca, Luca Latronico, Ioannis Liodakis, Simone Maldera, Alberto Manfreda, Frédéric Marin, Andrea Marinucci, Alan P. Marscher, Herman L. Marshall, Francesco Massaro, Giorgio Matt, Ikuyuki Mitsuishi, Tsunefumi Mizuno, Fabio Muleri, Michela Negro, C. -Y. Ng, Stephen L. O'Dell, Nicola Omodei, Chiara Oppedisano, Alessandro Papitto, Abel Lawrence Peirson, Matteo Perri, Melissa Pesce-Rollins, Pierre-Olivier Petrucci, Maura Pilia, Andrea Possenti, Simonetta Puccetti, Brian D. Ramsey, John Rankin, Ajay Ratheesh, Oliver J. Roberts, Roger W. Romani, Carmelo Sgrò, Patrick Slane, Paolo Soffitta, Gloria Spandre, Douglas A. Swartz, Toru Tamagawa, Fabrizio Tavecchio, Roberto Taverna, Yuzuru Tawara, Allyn F. Tennant, Nicholas E. Thomas, Francesco Tombesi, Alessio Trois, Roberto Turolla, Jacco Vink, Martin C. Weisskopf. 2023-11-07. X-ray Polarization Reveals the Precessions of the Neutron Star in Hercules X-1. https://arxiv.org/abs/2311.03667

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