Akash Garg
In the passing decades, improved multi-wavelength telescopes have made it possible to peek at the most luminous objects in the Universe and ascertain the underlying physical phenomena. One such class of objects is the Galactic black hole X-ray binaries where a black hole owing to its strong gravity accretes the material from a companion star and thereby emit profusely in the X-ray wavelength. The X-ray flux is known to possess a dynamic variability on time scales of msec to hours. If we turn to the Fourier space, the power density spectrum presents a broadband noise and peaked features known as Quasi-periodic oscillations(QPOs). Such features have been modeled usually as the result of instabilities in the accretion disk. Certain energy-dependent temporal properties like fractional r.m.s. and time lags between different energy bands of seen variability can give crucial information about the geometry of the disk. However, at present, there are fewer models developed that can translate the observed variability to physical interpretations. In our work, we present a technique where we use the radiative processes like black-body radiation and inverse comptonization to model the energy spectra obtained from the accretion disk in the system. The different fitting parameters used here like inner disk temperature, spectral index, electron temperature are converted to their physical counterparts like accretion rate, optical depth, the coronal heating rate which are then varied to obtain the energy-dependent properties of the variability. As an instance, we apply the generic technique to a recently discovered black hole X-ray binary- MAXI J1535-571 which has also been observed to have strong low-frequency QPOs by AstroSat.
In the second slide of our poster, we have shown one possibility of underlying mechanism happening in the accretion disk. The accretion disk around blackhole upto ISCO(Inner most Stable Circular Orbit) is mainly divided into two portions- one is outer truncated disk at radius Rin emitting in soft X-rays and another is inner coronal region emitting Hard X-rays being fed by the soft disk photons from the outer disk. The model suggests that there is a perturbation in Rin which further vary the in-going seed photon flux and hence a variation in the coronal temperature to maintain same power output. Then after a time delay, the perturbation travel inwards and changes the coronal heating rate leading to the possibility of the variability seen in the X-rays.
Such a model needs to be validated with observations. In the third slide, we have shown that it is required to get information about radiative mechanism by performing spectral analysis using a software XSPEC. The procedure is to fit the energy spectrum with inbuilt models like diskbb and Thcomp. The diskbb models the outer accretion disk with blackbody emission using parameters like inner disk temperature KTin and Normalisation Ndbb . The Thcomp models the corona through inverse comptonization process using parameters like coronal temperature KTe, spectral index Γ and scattering fraction f. But such parameters are often not physical and needs to be interpreted into their physical counterparts like KTin with accretion rate, Ndbb with inner disk radius, KTe with coronal heating rate and Γ with coronal optical depth. A typical photon spectrum in the energy range 1.0-30.0 keV is shown being fitted with such models to obtain the values of parameters. Alongside this, we present a power density spectrum which is the amplitude squared of the Fourier transform of the light curve in the second slide. Such a spectrum shows a peak feature at a particular frequency of 2.197 Hz, known as Quasi-periodic oscillation and is indicative of the variability seen in the accretion disk.
In the fourth slide, we explain how it is possible to vary the spectrum obtained above with respect to different parameters to obtain temporal information like fractional rms and phase lag related to QPO at a particular frequency. Such theoretical information can then be fitted with the observed behavior. We are using the AstroSat observations of a black hole binary source, MAXI J1535-571. The model considers the variations in the inner disk temperature, disk normalisation and the heating rate with time delays between them. The figures show that the model fits the observed behaviour well. The physical interpretation would be then same as shown in second slide- Variations in accretion rate drives the phenomenon and is followed by variation in the inner disk radius and then after a time delay, there is a variation in the heating rate.