arXiv subjects

M. A. Hashad

Publications and source records attributed to M. A. Hashad.

2 recordsLinked to original sources

arXiv2026

Potential periodic signals in blazars: significance, forecasting and deep learning

Blazars exhibit variable emission on diverse timescales. Some light curves show signs of quasiperiodic oscillations (QPOs), which may encode clues regarding the physical processes behind the emission or point to supermassive binary black holes. We analyzed five blazars with previously reported high significance year-long QPOs, applying the Lomb-Scargle periodogram and Weighted Wavelet Z-transform methods to Fermi-LAT data up to early 2025. We furthermore examined an additional source (PKS 0139-09), where nascent QPO may be present. As the light curves showed longer term trends, we detrended the data using an STL decomposition, which often revealed a large seasonal component. We find that detrending generally leads to an increase in the strength of the QPO signal. However, except for PG 1553+113, where a clearly persistent QPO signal is present, we detect transience on a timescale of $\lesssim$4000 days. We then forecast the light curves over the following four years, using a traditional statistical method as well as a Transformer-based deep learning model. Applied to a test set, the latter showed significant success in predicting behavior that seems unexpected from simple inspection of the past data. Analyzing the extended time series suggests a markedly weaker QPO signals over the coming years in cases where the transient behavior appears near the end of the observational data. In contrast, in the nascent candidate QPO source (PKS 0139-09) the signal is expected to strengthen significantly. These predictions, which may reflect the physical origin of the QPOs, can be tested against future data.

astro-ph.HE↗

arXiv2024

Quasi-periodic $γ$-ray modulations in the blazars PKS 2155-83 and PKS 2255-282

While there has been an increase in interest in the possibility of quasi-periodic oscillations (QPOs) in blazars, the search has hitherto been restricted to sources with well-sampled light curves. Objects with light curves that include gaps have been, to our knowledge, overlooked. Here, we study two such curves, which have the interesting feature of pertaining to relatively high redshift blazars -- FSRQs, PKS 2155-83 and PKS 2255-282 -- observed by Fermi-LAT. Their redshifts border the 'cosmic noon' era of galaxy formation and merging, and their light curves exhibit a distinctive pattern of repetitive high and low (gap dominant) states for $15.6$ years. To accommodate for the gaps in the curves, data is integrated over extended time intervals of 1 month and 2 months. The resulting curves were also examined using methods suitable for sparsely sampled data. This investigation of PKS 2155-83 and PKS 2255-282 suggests QPOs with periods of $4.69\pm0.79$ yr ($3σ$) and $6.82\pm2.25$ yr ($2.8σ$), respectively. The flux PDFs of the blazars, along with the correlation between their flux and spectral index, were also analyzed. Given the epochs the objects are observed, the plausibility of a binary black hole scenario as an origin of the apparent periodicity was examined. We estimated the prospective parameters of such a system using a simple geometric model. The total masses were estimated, and found to be consistent, in principle, with independent (dynamical) measurements of the central black hole masses in the two host galaxies.

astro-ph.HE↗