arXiv subjects

Malik Zahoor

Publications and source records attributed to Malik Zahoor.

3 recordsLinked to original sources

arXiv2023

Probing IC/CMB Interpretation for the X-ray knots of AGN through VHE observations

Detection of hard X-ray spectrum (spectral index < 2) from the kilo-parsec scale jet of active galactic nuclei cannot be accounted to the synchrotron emission mechanism from the electron distribution responsible for the radio/optical emission. Alternate explanations are the inverse Compton scattering of cosmic microwave background photons (IC/CMB) or synchrotron emission from a second electron population. When the X-ray emission is interpreted as IC/CMB process, the Compton spectrum often peaks at GeV energy and many sources were predicted to be the Fermi candidate sources. The absence of significant gamma ray flux from some of these galaxies by Fermi disfavored the IC/CMB interpretation of the high energy emission. We extend this study to predict the very high energy (VHE) gamma ray emission due to IC/CMB model which can be investigated by Cherenkov Telescope Array Observatory (CTAO). The model parameters deciding the broadband spectral energy distribution are estimated using analytical approximation of the emissivity functions. The emission model is extrapolated to VHE energy and then compared with the CTAO sensitivity. For this particular study, we have selected 18 knots with harder X-ray spectrum and for which the IC/CMB model for X-ray emission was suggested.

astro-ph.HE↗

arXiv2022

Multi-wavelength study of blazar 4C +01.02 during its long-term flaring activity in 2014-2017

We conducted a detailed long-term spectral and temporal study of flat spectrum radio quasar 4C +01.02, by using the multi-wavelength observations from Fermi-LAT, Swift-XRT, and Swift-UVOT. The $2$-day bin $γ$-ray lightcurve in the 2014-2017 active state displays $14$ peak structures with a maximum integral flux $(\rm E > 100 \ MeV)$ of $\rm (2.5 \pm 0.2) \times 10^{-6}\ ph\ cm^{-2}\ s^{-1}$ at MJD 57579.1, which is approximately $61$ times higher than the base flux of $\rm (4.1 \pm 0.3) \times 10^{-8}\ ph\ cm^{-2}\ s^{-1}$, calculated by averaging the flux points when the source was in quiescent state. The shortest $γ$-ray variability of $0.66 \pm 0.08$ days is observed for the source. The correlation study between $γ$-ray spectral index and flux suggests that the source deviates from the usual trend of harder when brighter feature shown by blazars. To understand the likely physical scenario responsible for the flux variation, we performed a detailed broadband spectral analysis of the source by selecting different flux states from the multi-wavelength lightcurve. A single zone leptonic model was able to reproduce the broadband spectral energy distribution (SED) of each state. The parameters of the model in each flux state are determined using a $χ^2$ fit. We observed that the synchrotron, synchrotron-self-Compton (SSC), and External-Compton (EC) processes produce the broadband SED under varied flux states. The adjoining contribution of the seed photons from the broad-line region (BLR) and the IR torus for the EC process are required to provide adequate fits to the GeV spectrum in all the chosen states.

astro-ph.HE↗

arXiv2021

Model-independent Redshift Estimation of BL Lac Objects through VHE Observations

The very high energy (VHE) gamma-ray spectral indices of blazars show strong correlation with the source redshift. Absence of any such correlation in low energy gamma rays and X-rays indicate the presence of Extragalactic Background Light (EBL) induced absorption of VHE gamma rays. By employing a linear regression analysis, this observational feature of blazars is used to constrain the redshift of BL Lac objects which was unknown/uncertain earlier. Additionally, we also compare the observed VHE spectral index-redshift correlation with the ones predicted from commonly adopted EBL models. Our study highlights the deviation of the EBL model based predictions from the observation especially at high redshifts.

astro-ph.HE↗