Discovery of a variable energy-dependent X-ray polarization in the accreting neutron star GX 5−1
: Fabiani, S.; Capitanio, F.; Iaria, R.; Poutanen, J.; Gnarini, A.; Ursini, F.; Farinelli, R.; Bobrikova, A.;Steiner, J. F.; Svoboda, J.; Anitra, A.; Baglio, M. C.; Carotenuto, F.; Del Santo, M.; Ferrigno, C.; Lewis, F.; Russell, D. M.; Russell, T. D.; Eijnden, J.; Cocchi, M.; Di Marco, A.; La Monaca, F.; Liu, K.; Rankin, J.; Weisskopf, M. C.; Xie, F.; Bianchi, S.; Burderi, L.; Di Salvo, T.; Egron, E.; Illiano, G.; Kaaret, P.; Matt, G.; Mikušincová, R.; Muleri, F.; Papitto, A.; Agudo, I.; Antonelli, L. A.; Bachetti, M.; Baldini, L.; Baumgartner, W. H.; Bellazzini, R.; Bongiorno, S. D.; Bonino, R.; Brez, A.; Bucciantini, N.; Castellano, S.; Cavazzuti, E.; Chen, C.; Ciprini, S.; Costa, E.; De Rosa, A.; Del Monte, E.; Di Gesu, L.; Di Lalla, N.; Donnarumma, I.; Doroshenko, V.; Dovčiak, M.; Ehlert, S. R.; Enoto, T.; Evangelista, Y.; Ferrazzoli, R.; Garcia, J. A.; Gunji, S.; Hayashida, K.; Heyl, J.; Iwakiri, W.; Jorstad, S. G.; Karas, V.; Kislat, F.; Kitaguchi, T.; Kolodziejczak, J. J.; Krawczynski, H.; Latronico, L.; Liodakis, I.; Maldera, S.; Manfreda, A.; Marin, F.; Marinucci, A.; Marscher, A. P.; Marshall, H. L.; Massaro, F.; Mitsuishi, I.; Mizuno, T.; Negro, M.; Ng, C.; O’Dell, S. L.; Omodei, N.; Oppedisano, C.; Pavlov, G. G.; Peirson, A. L.; Perri, M.; Pesce-Rollins, M.; Petrucci, P.; Pilia, M.; Possenti, A.; Puccetti, S.; Ramsey, B. D.; Ratheesh, A.; Roberts, O. J.; Romani, R. W.; Sgrò, C.; Slane, P.; Soffitta, P.; Spandre, G.; Swartz, D. A.; Tamagawa, T.; Tavecchio, F.; Taverna, R.; Tawara, Y.; Tennant, A. F.; Thomas, N. E.; Tombesi, F.; Trois, A.; Tsygankov, S. S.; Turolla, R.; Vink, J.; Wu, K.; Zane, S.
Publisher: EDP Sciences
: 2024
Astronomy and Astrophysics
: A&A
: A137
: 684
: 1432-0746
DOI: https://doi.org/10.1051/0004-6361/202347374
: https://doi.org/10.1051/0004-6361/202347374
: https://research.utu.fi/converis/portal/detail/Publication/393319405
: https://arxiv.org/abs/2310.06788
We report on the coordinated observations of the neutron star low-mass X-ray binary (NS-LMXB) GX 5−1 in X-rays (IXPE, NICER, NuSTAR, and INTEGRAL), optical (REM and LCO), near-infrared (REM), mid-infrared (VLT VISIR), and radio (ATCA). This Z-source was observed by IXPE twice in March–April 2023 (Obs. 1 and 2). In the radio band the source was detected, but only upper limits to the linear polarization were obtained at a 3σ level of 6.1% at 5.5 GHz and 5.9% at 9 GHz in Obs. 1 and 12.5% at 5.5 GHz and 20% at 9 GHz in Obs. 2. The mid-IR, near-IR, and optical observations suggest the presence of a compact jet that peaks in the mid- or far-IR. The X-ray polarization degree was found to be 3.7%±0.4% (at 90% confidence level) during Obs. 1 when the source was in the horizontal branch of the Z-track and 1.8%±0.4% during Obs. 2 when the source was in the normal-flaring branch. These results confirm the variation in polarization degree as a function of the position of the source in the color-color diagram, as for previously observed Z-track sources (Cyg X-2 and XTE 1701−462). Evidence of a variation in the polarization angle of ∼20° with energy is found in both observations, likely related to the different, nonorthogonal polarization angles of the disk and Comptonization components, which peak at different energies.
