Accretion geometry of the neutron star low mass X-ray binary Cyg X-2 from X-ray polarization measurements

Farinelli R; Fabiani S; Poutanen J; Ursini F; Ferrigno C; Bianchi S; Cocchi M; Capitanio F; De Rosa A; Gnarini A; Kislat F; Matt G; Mikusincova R; Muleri F; Agudo I; Antonelli LA; Bachetti M; Baldini L; Baumgartner WH; Bellazzini R; Bongiorno SD; Bonino R; Brez A; Bucciantini N; Castellano S; Cavazzuti E; Ciprini S; Costa E; Del Monte E; Di Gesu L; Di Lalla N; Di Marco A; Donnarumma I; Doroshenko V; Dovciak M; Ehlert SR; Enoto T; Evangelista Y; Ferrazzoli R; Garcia JA; Gunji S; Hayashida K; Heyl J; Iwakiri W; Jorstad SG; Karas V; Kitaguchi T; Kolodziejczak JJ; Krawczynski H; La Monaca F; Latronico L; Liodakis I; Maldera S; Manfreda A; Marin F; Marscher AP; Marshall HL; Mitsuishi I; Mizuno T; Ng CY; O'Dell SL; Omodei N; Oppedisano C; Papitto A; Pavlov GG; Peirson AL; Perri M; Pesce-Rollins M; Petrucci PO; Pilia M; Possenti A; Puccetti S; Ramsey BD; Rankin J; Ratheesh A; Romani RW; Sgro C; Slane P; Soffitta P; Spandre G; Tamagawa T; Tavecchio F; Taverna R; Tawara Y; Tennant AF; Thomas NE; Tombesi F; Trois A; Tsygankov SS; Turolla R; Vink J; Weisskopf MC; Wu K; Xie F; Zane S
Accretion geometry of the neutron star low mass X-ray binary Cyg X-2 from X-ray polarization measurements

Farinelli R
Fabiani S
Poutanen J
Ursini F
Ferrigno C
Bianchi S
Cocchi M
Capitanio F
De Rosa A
Gnarini A
Kislat F
Matt G
Mikusincova R
Muleri F
Agudo I
Antonelli LA
Bachetti M
Baldini L
Baumgartner WH
Bellazzini R
Bongiorno SD
Bonino R
Brez A
Bucciantini N
Castellano S
Cavazzuti E
Ciprini S
Costa E
Del Monte E
Di Gesu L
Di Lalla N
Di Marco A
Donnarumma I
Doroshenko V
Dovciak M
Ehlert SR
Enoto T
Evangelista Y
Ferrazzoli R
Garcia JA
Gunji S
Hayashida K
Heyl J
Iwakiri W
Jorstad SG
Karas V
Kitaguchi T
Kolodziejczak JJ
Krawczynski H
La Monaca F
Latronico L
Liodakis I
Maldera S
Manfreda A
Marin F
Marscher AP
Marshall HL
Mitsuishi I
Mizuno T
Ng CY
O'Dell SL
Omodei N
Oppedisano C
Papitto A
Pavlov GG
Peirson AL
Perri M
Pesce-Rollins M
Petrucci PO
Pilia M
Possenti A
Puccetti S
Ramsey BD
Rankin J
Ratheesh A
Romani RW
Sgro C
Slane P
Soffitta P
Spandre G
Tamagawa T
Tavecchio F
Taverna R
Tawara Y
Tennant AF
Thomas NE
Tombesi F
Trois A
Tsygankov SS
Turolla R
Vink J
Weisskopf MC
Wu K
Xie F
Zane S
Katso/Avaa
Lataukset: 

OXFORD UNIV PRESS
doi:10.1093/mnras/stac3726
URI
https://doi.org/10.1093/mnras/stac3726
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Julkaisun pysyvä osoite on:
https://urn.fi/URN:NBN:fi-fe2025082789711
Tiivistelmä

We report spectro-polarimetric results of an observational campaign of the bright neutron star low-mass X-ray binary Cyg X-2 simultaneously observed by IXPE, NICER, and INTEGRAL. Consistently with previous results, the broad-band spectrum is characterized by a lower-energy component, attributed to the accretion disc with kTin ≈ 1 keV, plus unsaturated Comptonization in thermal plasma with temperature kTe = 3 keV and optical depth τ ≈ 4, assuming a slab geometry. We measure the polarization degree in the 2–8 keV band P = 1.8 ± 0.3 per cent and polarization angle ϕ = 140° ± 4°, consistent with the previous X-ray polarimetric measurements by OSO-8 as well as with the direction of the radio jet which was earlier observed from the source. While polarization of the disc spectral component is poorly constrained with the IXPE data, the Comptonized emission has a polarization degree P = 4.0 ± 0.7 per cent and a polarization angle aligned with the radio jet. Our results strongly favour a spreading layer at the neutron star surface as the main source of the polarization signal. However, we cannot exclude a significant contribution from reflection off the accretion disc, as indicated by the presence of the iron fluorescence line.

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