Research on X-ray pulsars

X-ray pulsars (XRPs) are accreting NSs which have magnetic field strength of about B=1012−1013 G. The strong magnetic field channels the gas towards magnetic poles, where most the gravitational energy is released in the form of X-rays. Due to the misalignment of the magnetic field with the rotational axis of the NS, such an object manifests itself as an XRP.

Understanding the physics of accretion onto strongly magnetized NS requires detailed modeling of interaction between radiation and plasma in strong B-field regime, in particular Compton scattering. We have derived the relativistic kinetic equation for Compton scattering with no limitations on photon or electron energies and the B-field strength, accounting for polarization of both photons and electrons as well as for induced scattering and exclusion principle (see Mushtukov et al. 2012, 2015). Using the exact scattering cross section, which has resonances around the cyclotron frequency, we recently demonstrated (Mushtukov et al. 2014) that the so called critical luminosity of the pulsar (i.e. luminosity when the accretion shock starts to rise above the NS surface) is a non-monotonic function of the luminosity. This critical luminosity separates the regimes where the correlation between the observed cyclotron energy and luminosity is replaced by the anti-correlation.

We have recently also proposed a novel “reflection model” describing very special behavior of cyclotron line in a case of bright sources (see Poutanen et al. 2013), which was discovered earlier (see Tsygankov et al. 2006). According to the model, the cyclotron line forms when the radiation emitted by the accretion column is reflected from the neutron star surface (Figure 1). The reflection model was successfully applied to explain the observed variations of the cyclotron line energy in a bright X-ray pulsar V 0332+53 over a wide range of luminosities. Details of the XRP behavior in the subcritical regime of accretion are not understood yet, but a complete picture of radiation and matter interaction in the vicinity of a highly magnetized NS in a wide luminosity range is gradually taking shape.

Our current investigations are focused on modeling of the accretion column as well as determination of the maximum luminosity for magnetized neutron stars in connection with a recent discovery that some ultra-luminous X-ray sources are in fact X-ray pulsars.



Figure 1: Illustration of the “reflection model” that reproduces the anti-correlation between cyclotron line energy and the X-ray luminosity (Poutanen et al. 2013). The larger is the accretion rate, the higher the column, the larger the illuminated fraction of the stellar surface, the weaker the average magnetic field, and the smaller the cyclotron line energy.

Selected publications:



Contacts: sergey.tsygankov AT utu.fi ; alexander.mushtukov AT utu.fi
Working groups

X-ray binaries

Description of the group activity.
Group meetings take place on Wednesdays (first and third of each month), 15.00 in the coffee room.

Stellar explosions

Description of the group activity.
Group meetings take place on Mondays, 15.00 in the coffee room.

AGNs and Very High Energy Astrophysics

Description of the group activity.
Group meetings take place on Wednesday (second and fourth of each month), 15.00 in the coffee room.