The first IXPE view of the eclipsing ADC source 4U 1822-37

@misc{anitra2026first,
  title = {{The first IXPE view of the eclipsing ADC source 4U 1822-37}},
  author = {A. Anitra and A. Gnarini and T. Di Salvo and R. Iaria and A. Marino and F. Barra and L. Burderi and A. Sanna and L. Marra and S. Bianchi and G. Matt and F. Ursini and C. Miceli and A. Miraval Zanon and W. Leone. and F. Capitanio and S. Fabiani and P. Kaaret and A. Tarana},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2605.19877},
  eprint = {2605.19877},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2605.19877},
}

TY  - UNPB
TI  - The first IXPE view of the eclipsing ADC source 4U 1822-37
AU  - A. Anitra
AU  - A. Gnarini
AU  - T. Di Salvo
AU  - R. Iaria
AU  - A. Marino
AU  - F. Barra
AU  - L. Burderi
AU  - A. Sanna
AU  - L. Marra
AU  - S. Bianchi
AU  - G. Matt
AU  - F. Ursini
AU  - C. Miceli
AU  - A. Miraval Zanon
AU  - W. Leone.
AU  - F. Capitanio
AU  - S. Fabiani
AU  - P. Kaaret
AU  - A. Tarana
PY  - 2026
DA  - 2026/09/24
PB  - arXiv
DO  - 10.48550/arxiv.2605.19877
UR  - https://arxiv.org/abs/2605.19877
AB  - Accretion-disc corona sources are high-inclination low-mass X-ray binaries in which the innermost regions are hidden and the observed X-ray emission is dominated by radiation scattered above the disc. 4U 1822-37 is a key binary system of this class, but its geometry is still debated. X-ray polarimetry offers a direct probe of the scattering structure. We present the first X-ray spectro-polarimetric study of 4U 1822-37 and test whether its spectrum and polarisation can be explained within the accretion-disc-corona scenario. We analysed a coordinated campaign with IXPE, XMM-Newton, NuSTAR, and Swift. We performed broadband spectral modelling and model-independent, energy-resolved, and orbital-phase-resolved polarimetric analyses. The broadband spectrum requires a soft thermal component, a Comptonised continuum, a hard power-law tail, and relativistically blurred reflection. The observed 0.1-100 keV luminosity, L_obs≃6.1×10^36 erg s^−1, is far below the intrinsic luminosity expected from the orbital evolution, supporting a geometry in which only a small fraction of the intrinsic emission is scattered into the line of sight by an extended, optically thin corona. In the 2-8 keV band, IXPE measures PD=7.9±0.6% and PA=−24^∘±2^∘. The PD increases with energy, while the PA remains approximately constant. During eclipse, the PD decreases to PD=5.5±1.7%, with no significant PA variation. This behaviour is consistent with the companion occulting the most polarimetrically efficient part of the extended corona. The high PD, stable PA, energy-dependent polarisation, and eclipse behaviour support a picture in which 4U 1822-37 is observed in an extreme high-inclination, scattering-dominated regime. The extended corona is the main structure shaping both the observed X-ray emission and its polarisation.
ER  -