Weakly-magnetized neutron stars
Majority of old NS have weak magnetic fields and the accretion disc extends to their surface forming a boundary/spreading layer (BL/SL), where the rapidly rotating gas decelerates down to the stellar angular velocity. The X-rays from these NS show quasi-periodic oscillations (QPOs) at kHz frequencies. The mechanisms responsible for QPOs are unknown (most of the proposed models are purely kinematical). The observations univocally argue in favour of the BL/SL as a source of QPOs (Gilfanov et al. 2003). Recently, using the data from an transient XTE J1701−462, we showed that the spectral energy distribution of the boundary layer stays virtually the same over factor of 20 variations of the source luminosity (Revnitsev, Suleimanov, Poutanen 2014, see Fig.1). The stability of its temperature (around 2.5 keV) and its closeness to the maximum observed colour temperature in the photospheric radius expansion X-ray bursts, argues in favour of emission at the Eddington limited flux from the neutron star surface. Understanding the BL/SL physics is of foremost importance.
Figure 1: Spectra of XTE J1701−462 collected at luminosities that differ by a factor of 20. From Revnitsev, Suleimanov, Poutanen (2014).
To describe the structure of the BL/SL, Inogamov & Sunyaev (1999) solved a 1D problem in shallow-water approximation assuming that the layer has velocity close to the Keplerian at the equator. We later included the GR effects, considered different chemical composition of the accreting material and computed the spectra. Comparison to the observed X-ray spectra from the BL gave constraints on the NS masses and radii (Suleimanov & Poutanen 2006). Recently, we have extended the model for the SL by doing 2D shallow-water time-dependent radiation-hydrodynamics simulations (see Fig. 2 and Abolmasov, Nättilä, Poutanen 2020). We have also developed a simple 1D mechanical model for the SL which describes successfully the parallel tracks phenomenon of kHz QPOs (see Fig.3 and Abolmasov & Poutanen 2021).
Figure 2: Time-dependent radiation-hydrodynamics simulations of spreading of matter on a neutron star surface.
Figure 3: Dependence of rotation frequency of the speading layer on luminosity for three different ratios of the depletion to friction timescales. From Abolmasov & Poutanen (2021).
Selected publications:
- Bobrikova A., Forsblom S.V., Di Marco A., La Monaca F., Poutanen J., et al. (incl. Loktev V., Kajava J.J.E., Veledina A., Nitindala A.P., Tsygankov S.S.; IXPE collaboration),
Discovery of a strong rotation of the X-ray polarization angle in the galactic burster GX 13+1,
2024, A&A, submitted - Rankin J., Kravtsov V., Muleri F., Poutanen J.,et al.,
X-ray Polarimetry as a Tool to Constrain Orbital Parameters in X-Ray Binaries,
2024, ApJ, in press - Fabiani S., Capitanio F., Ilaria R., Poutanen J., …, Bobrikova A., …, Tsygankov S.S., et al. (IXPE collaboration),
Discovery of variable X-ray polarization from the accreting neutron star GX 5−1,
2024, A&A, in press - Rankin J., La Monaca F., Di Marco A., Poutanen J., Bobrikova A., Kravtsov V., Muleri F., Pilia M., Veledina A., …, Tsygankov S.S., et al. (IXPE collaboration),
X-Ray Polarized View on the Accretion Geometry in the X-Ray Binary Circinus X-1,
2024, ApJ Letters, 961, L8 - La Monaca F., Di Marco A., Poutanen J., …, Bobrikova A., …, Veledina A., …, Tsygankov S., et al. (IXPE collaboration),
Highly Significant Detection of X-Ray Polarization from the Brightest Accreting Neutron Star Sco X-1,
2024, ApJ Letters, 960, L11 - Di Marco A., La Monaca F., Poutanen J.,…, Kajava J.J.E.,…, Bobrikova A. et al. (IXPE collaboration, incl. Tsygankov S.S.),
First detection of X-ray polarization from the accreting neutron star 4U 1820−303,
2023, ApJ Letters, 953, L22 - Ursini F., Farinelli R., Gnarini A., Poutanen J.,…, Kajava J.J.E. et al. (IXPE collaboration, incl. Tsygankov S.S.),
X-ray polarimetry and spectroscopy of the neutron star low-mass X-ray binary GX 9+9: an in-depth study with IXPE and NuSTAR,
2023, A&A, 676, A20 - Cocchi M., Gnarini A., Fabiani S., Ursini F., Poutanen J.,.., Bobrikova A.,…, Veledina A.,…, Kajava J.J.E., et al. (IXPE collaboration, incl. Tsygankov S.S.),
Discovery of strongly variable X-ray polarization in the neutron star low-mass X-ray binary transient XTE J1701−462,
2023, A&A Letters, 674, L10 - Capitanio F., et al. (IXPE collaboration, incl. Poutanen J., Kajava J.J.E., Tsygankov S.S.),
Polarization properties of the weakly magnetized neutron star X-ray binary GS 1826-238 in the high soft state,
2023, ApJ, 943, 129 - Farinelli R., Fabiani S., Poutanen J., Cocchi M., Mikusincova R., Farinelli R., Bianchi S., Kajava J.J.E. et al. (incl. Tsygankov S.S.),
Accretion geometry of the neutron star low mass X-ray binary Cyg X-2 from X-ray polarization measurements,
2023, MNRAS, 519, 3681–3690 - Abolmasov P., Poutanen J., 2021,
Mechanical model of a boundary layer for the parallel tracks of kilohertz quasi-periodic oscillations in accreting neutron stars,
A&A, 647, A45 - Abolmasov P., Nättilä J., Poutanen J., 2020,
Kilohertz quasi-periodic oscillations from neutron star spreading layers,
A&A, 638, A142 - Kajava J.J.E., Koljonen K.I.I., Nättilä J., Suleimanov V.F., Poutanen J., 2017,
Variable spreading layer in 4U 1608−52 during thermonuclear X-ray bursts in the soft state,
MNRAS, 472, 78-89 - Revnivtsev M.G., Suleimanov V.F., Poutanen J., 2013, On the spreading layer emission in luminous accreting neutron stars,
MNRAS, 434, 2355-2361 - Babkovskaia N., Brandenburg A., Poutanen J., 2008, Boundary layer on the surface of a neutron star,
MNRAS, 386, 1038-1044 - Suleimanov V., Poutanen J., 2006, Spectra of the spreading layers on the neutron star surface and constraints on the neutron star equation of state,
MNRAS, 369, 2036-2048