Research on accreting neutron stars in low-mass X-ray binaries

The research field of accreting NS in low-mass X-ray binaries (LMXBs) has changed dramatically during the last 15 years with the launch of the Rossi X-ray Timing Explorer (RXTE). In 1996, the first evidence of millisecond periods in NS systems came with the detection of highly coherent oscillation in the 200–600 Hz range during X-ray bursts. By now about 20 NS show such oscillations. These are called nuclear-powered millisecond pulsars since the emission during the bursts is produced by thermonuclear burning at the NS surface. In 1998 RXTE also discovered coherent ms pulsations in the persistent emission of SAX J1808.4–3658. Now there are 15 such sources showing oscillations in the frequency range 180–600 Hz. These are called accreting millisecond pulsars (AMPs, see Fig.1). In addition, a few dozen NS show kHz quasi-periodic oscillations (QPOs).

AMPs are exceptional physical laboratories for studying extreme gravity effects as well as plasma physics of the interaction between the NS magnetosphere and the accretion disc. Coherent ms pulsations in the persistent emission allow us to produce highly accurate pulse profiles by folding the data over a long observational period (days or even weeks). By detailed modelling the folded pulse profiles, one is able to determine the physical parameters such as the NS mass and radius, inclination of the system, position of the emitting region (hotspot) at the NS surface as well as the emission pattern from the hotspot responsible for the observed radiation. We have developed a theory of formation of pulse profiles from rapidly spinning NS including all relativistic effects, and proposed to use pulse profiles to obtain constraints on the NS radius as a function of its mass (Poutanen & Gierlinski 2003). Sometimes, pulse profiles of AMPs show significant deviations from a sine wave showing double-peak structure, which can be interpreted as a signature of the secondary magnetic pole (Ibragimov & Poutanen 2009, Poutanen et al. 2009). Studying the evolution of the profile with the accretion rate then gives us a possibility to geometrically constrain the inner disc radius as a function of accretion rate. The results will allow us to determine the energy dissipation profile in the region between the disc and the magnetosphere and thus use AMPs as laboratories to study the complicated plasma physics of the disc-magnetosphere interaction (Kajava et al. 2011).

Among the projects that we are pursuing now, one can mention detailed modelling of the pulse profiles accounting for the non-sphericity of the NS also accounting for deviations of the metric from the Schwarzschild one. Determining the NS parameters such as their masses and radii from the detailed models of the AMPs’ pulse profiles is one of the primary goals of the Large Observatory For X-ray Timing (LOFT) satellite – a candidate for the ESA M4 mission.

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). Thus understanding the BL/SL physics is of foremost importance. 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). In the near future, our goal is to make a significant step forward in developing the SL model by solving the time-dependent radiation-hydrodynamics equations. We start from a 1D problem and proceed to 2D models where it is possible to produce stable, non-axisymmetric surface features, which we believe are the origin of the QPOs.

Figure 1: Magneto-hydrodynamic simulations of the accretion onto a NS with the inclined dipole magnetic field (Romanova et al. 2004).

Selected publications:

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.