Z source

In astrophysics, a Z source is a type of neutron star low-mass X-ray binary (LMXB) distinguishable from the similar atoll source by the characteristic "Z" shape of its X-ray color-color diagram. Z-sources have high rates of accretion, typically more than 50% of the Eddington limit, and relatively strong magnetic fields, although they are not magnetars. Z-sources exhibit three main spectral states known as the horizontal branch, normal branch, and flaring branch.

Properties

Z sources are a type of low-mass X-ray binary containing a weakly-magnetized (non-magnetar) neutron star accreting matter off of a normal star. They have luminosities of approximately 1038 erg/s and magnetic fields greater than 109 G. Their spectra are usually dominated by non-thermal, Comptonized emission, but thermal blackbody radiation does play a role, making up as much as 50% of the Z source's total luminosity. Z sources can experience quasi-periodic oscillations in any of their three states. They also experience relativistic reflection.

Difference from atoll sources

This section is an excerpt from Atoll source § Difference from Z sources.[edit]
A diagram comparing the paths of Z sources and atoll sources on an X-ray color-color diagram, where the x-axis is soft color and the y-axis is hard color. Z sources trace a zig-zagging diagonal path from the bottom left to the top right of the diagram, while atoll sources trace a path shaped like a backwards L in the bottom left corner of the diagram.
A comparison of typical atoll source (red) and Z source (black) tracks on an X-ray color-color diagram

Atoll sources can be distinguished from Z sources by analyzing their X-ray color-color diagrams. These diagrams compare the ratios of emission of two ranges of high-energy X-ray wavelengths ("hard X-rays") and two ranges of low-energy X-ray wavelengths ("soft X-rays"). When plotted on such a diagram, Z sources have three-branched shapes that look similar to the letter "Z", while atoll sources have long, curved branches and isolated clumps that are reminiscent of an atoll. Atoll sources take longer to move all the way through the diagram, approximately 30–100 days, while Z sources can move through the entire diagram in hours to days.

Atoll sources tend to be less luminous than Z sources, accreting mass less quickly, but luminosity ranges overlap. Atoll sources also have much larger variation in X-ray intensity and energy spectrum than Z sources. X-ray bursting, caused by the unstable nuclear fusion of material accreted onto the neutron star, is generally only seen in atoll sources, with Z sources experiencing longer-lasting X-ray flares instead.

Horizontal, normal, and flaring states

A graph showing energy emission by wavelength for different states of Z sources. Each state peaks at lower X-ray wavelengths, declines sharply, flattens out, and then declines again, although the exact slopes vary by state. One exception is the top of the flaring branch, which peaks, declines sharply, increases slowly, and then decreases again. Additionally, the bottom of the flaring branch has the lowest emission at any wavelength and declines much faster than any other state.
Variations of spectra in Z sources in each of the three states

Z-sources exhibit three main X-ray spectral states, representing the three branches of their "Z" shape on a color-color diagram: the horizontal branch (HB), which forms the top of the Z, the normal branch (NB), which forms the middle of the Z, and the flaring branch (FB), which forms the bottom of the Z. The turning point between the horizontal and normal branches is called the hard apex, while the turning point between the normal and flaring branches is known as the soft apex. Some scientists have suggested that movement across the color-color diagram is due to an increasing accretion rate, while others have argued that it may be caused by variation of the inner radius of the accretion disk.

In the horizontal branch, with the most energetic (hardest) X-ray radiation, light from the Z source is more polarized compared to the normal and flaring branches. It has been suggested that this polarization is due to electron scattering in an accretion disk wind. The normal branch represents an increase in mass accretion rate compared to the flaring branch, resulting in a higher blackbody temperature and increased radiation pressure disrupting the accretion disk and creating relativistic jets. The flaring branch is caused by unstable nuclear fusion of accreted material on the surface of the neutron star, leading to X-ray flares. Some Z sources may have constant flares, giving them higher temperatures and nonstop jets.

Notable Z sources

References

  1. 1 2 Gierlinski, M.; Done, C. (2002). "The X-ray spectrum of the atoll source 4U 1608--52". Monthly Notices of the Royal Astronomical Society. 337 (4): 1373–1380. arXiv:astro-ph/0208389. doi:10.1046/j.1365-8711.2002.06009.x.
  2. 1 2 3 van der Klis, Michiel (1989). "The Z/Atoll Classification". Proceedings of the 23rd. ESLAB Symposium on Two Topics in X-Ray Astronomy. 1. European Space Agency: 203. Bibcode:1989ESASP.296..203V.
  3. 1 2 3 4 5 Gnarini, Andrea; Ravi, Swati; Kaaret, Philip; Bobrikova, Anna; Poutanen, Juri; Forsblom, Sofia V.; Ursini, Francesco; Baglio, Maria Cristina; Bianchi, Stefano; Capitanio, Fiamma; Cocchi, Massimo; Díaz Teodori, María Alejandra; Fabiani, Sergio; Farinelli, Ruben; Matt, Giorgio; Ng, Mason; Salganik, Alexander; Soffitta, Paolo; Tarana, Antonella; Zane, Silvia (2026). "Discovery of High X-Ray Polarization from the Neutron Star Low-mass X-Ray Binary Cyg X-2 on the Horizontal Branch". The Astrophysical Journal. 997 (2): 299. doi:10.3847/1538-4357/ae2ad0.
  4. ↑ Muno, Michael P.; Remillard, Ronald A.; Chakrabarty, Deepto (2002). "How do Z and Atoll X-Ray Binaries Differ?". The Astrophysical Journal. 568: L35–L39. arXiv:astro-ph/0111370. doi:10.1086/340269.
  5. 1 2 3 Church, M. J.; Gibiec, A.; Bałucińska-Church, M. (2014). "The nature of the island and banana states in atoll sources and a unified model for low-mass X-ray binaries". Monthly Notices of the Royal Astronomical Society. 438 (4): 2784–2797. arXiv:1312.1823. doi:10.1093/mnras/stt2364.
  6. ↑ Motta, S. E.; Rouco-Escorial, A.; Kuulkers, E.; Muñoz-Darias, T.; Sanna, A. (2017). "Links between quasi-periodic oscillations and accretion states in neutron star low-mass X-ray binaries". Monthly Notices of the Royal Astronomical Society. 468 (2): 2311–2324. arXiv:1703.01263. doi:10.1093/mnras/stx570.
  7. 1 2 van der Klis, Michiel (1989). "The Z/Atoll Classification". Proceedings of the 23rd. ESLAB Symposium on Two Topics in X-Ray Astronomy. 1. European Space Agency: 203. Bibcode:1989ESASP.296..203V.
  8. 1 2 Muno, Michael P.; Remillard, Ronald A.; Chakrabarty, Deepto (2002). "How do Z and Atoll X-Ray Binaries Differ?". The Astrophysical Journal. 568: L35–L39. arXiv:astro-ph/0111370. doi:10.1086/340269.
  9. ↑ Church, M. J.; Gibiec, A.; Bałucińska-Church, M. (2014). "The nature of the island and banana states in atoll sources and a unified model for low-mass X-ray binaries". Monthly Notices of the Royal Astronomical Society. 438 (4): 2784–2797. arXiv:1312.1823. doi:10.1093/mnras/stt2364.
  10. ↑ Agrawal, V. K.; Sreekumar, P. (2003). "X-ray spectral evolution of low-mass X-ray binary GX 349+2". Monthly Notices of the Royal Astronomical Society. 346 (3): 933–939. arXiv:astro-ph/0302581. doi:10.1111/j.1365-2966.2003.07147.x.
  11. ↑ Coughenour, Benjamin M.; Cackett, Edward M.; Miller, Jon M.; Ludlam, Renee M. (2018). "A NuSTAR Observation of the Low-mass X-Ray Binary GX 349+2 throughout the Z-track". The Astrophysical Journal. 867: 64. arXiv:1708.01652. doi:10.3847/1538-4357/aae098.
  12. ↑ Church, M. J.; Halai, G. S.; Bałucińska-Church, M. (2006). "An explanation of the Z-track sources". Astronomy & Astrophysics. 460: 233–244. arXiv:astro-ph/0609821. doi:10.1051/0004-6361:20065035.
  13. ↑ La Monaca, Fabio; Di Marco, Alessandro; Coti Zelati, Francesco; Bobrikova, Anna; Ludlam, Renee M.; Poutanen, Juri; Marino, Alessio; Li, Songwei; Xie, Fei; Feng, Hua; Jin, Chichuan; Rea, Nanda; Tao, Lian; Yuan, Weimin (2025). "X-ray spectropolarimetric characterisation of the Z source GX 340+0 in the normal branch". Astronomy & Astrophysics. 702: A101. arXiv:2508.13278. doi:10.1051/0004-6361/202555134.
  14. ↑ Mazzola, S. M.; Iaria, R.; Di Salvo, T.; Sanna, A.; Gambino, A. F.; Marino, A.; Bozzo, E.; Ferrigno, C.; Riggio, A.; Anitra, A.; Burderi, L. (2021). "Fe K α and Fe K β line detection in the NuSTAR spectrum of the ultra-bright Z source Scorpius X–1". Astronomy & Astrophysics. 654: A102. arXiv:2108.00729. doi:10.1051/0004-6361/202039983.
  15. ↑ Shirey, Robert E.; Bradt, Hale V.; Levine, Alan M. (1999). "The Complete "Z" Track of Circinus X-1". The Astrophysical Journal. 517: 472–487. arXiv:astro-ph/9901003. doi:10.1086/307188.
  16. ↑ Agrawal, V. K.; Misra, R. (2009). "X-ray spectral evolution of the extragalactic Z source LMC X-2". Monthly Notices of the Royal Astronomical Society. 398 (3): 1352–1360. arXiv:0905.0584. doi:10.1111/j.1365-2966.2009.15014.x.