Science - Research Interests
X-ray Astronomy — Astrophysical Objects & Methods
One of my main research focuses are X-ray binaries, where matter is transferred from a companion star to a compact object. If the compact object is a neutron star, pulsations may be seen in X-rays, and the object is called an accreting X-ray pulsar (XRP). The data we use to study such systems typically consists of information of the detected photon’s (1) arrival time, (2) energy, and (3) spatial position (in some cases, this list may also include (4) polarization information). These XRPs can exhibit variability on various timescales, such as the neutron star’s spin period, the binary period or a super-orbital period.
Under certain circumstances, an accretion disk can form around the neutron star, and in addition to matter, angular momentum can be “accreted”, causing the neutron star to spin-up. Sometimes sudden spin-reversals are observed, and the neutron star starts to spin-down instead. These reversals are not well understood and in my bachelor’s thesis I applied a model that accounts for the misalignment of magnetic and spin axes using data from Fermi/ GBM and Swift/BAT. More on this here.
Another aspect of XRPs is super-orbital modulation, which is observed in only a few systems, including Her X-1. It is generally believed that the modulation in flux every ~35 days is caused by a warped accretion disk surrounding the pulsar. Interestingly, the pulse profile (associated with the neutron star’s rotation period of 1.24 s) also changes over the course of the 35 days. Since both the NuSTAR and HXMT X-ray telescopes have observed Her X-1 extensively, part of my master’s thesis was to analyze these large data sets to investigate the relationship between the 35-day pulse profile evolution. More information can be found here.
The pulse profiles themselves, however, are also a mystery. The modulation of intensity on the timescale of their rotation period is caused and modified by a number of factors, such as the magnetic field configuration, the shape of the emitting regions, gravitational light bending, or the geometric configuration. Since we do not know the contributions of the each pole to the total pulse profile, in part because we can even see light from both poles simultaneously, it is very difficult to understand more about the physical processes and geometry of the emission region.
My PhD project addressed this very problem and aims to solve it using a novel method called “phase correlated variability analysis”. The basis of this method is the blind source separation problem, most commonly explained in terms of the cocktail party problem: At a crowded party, a person can easily follow a conversation even in the presence of heavy chatter and background noise.
Although this is a complicated problem in digital signal processing, several algorithms have been developed to solve it. In the context of XRPs, I developed a method that aimed to “hear” the individual accretion “voices” of the two poles of the neutron star. To develop this, I used not only simulations but also extracted data from RXTE of Cen X-3 to test and apply the method and compare the results with previously published decompositions. Find more informatin on this topic in this post or this paper.
My research interests can therefore be summarized in two areas: X-ray astronomy and the methods used to study astrophysical objects and phenomena.