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arXiv · 2511.21182

Broad-band temporal and spectral study of TeV blazar TXS 0518+211

Avik Kumar Das·Pankaj Kushwaha·Veeresh Singh·Sandeep Kumar Mondal·Goldy Ahuja·Deekshya R. Sarkar
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Abstract

We present a long-term broad-band temporal and spectral study of a TeV BL Lac source TXS 0518+211 by analyzing nearly 16 years (MJD 54682 -- 60670) of simultaneous optical, UV and X-ray light curves from \textit{Swift}-XRT/UVOT and gamma-ray light curves from \textit{Fermi}-LAT. Based on the availability of simultaneous multi-wavelength data and considering flux level as the depiction of AGN-jet activity we identified 11 epochs (named as Epoch-A to Epoch-K) and investigated temporal as well as spectral variability during these epochs to understand the emission properties in this source. The fractional variability analysis reveals that, in all epochs, X-ray light curve exhibits relatively high degree of variability in compared to the optical, UV and gamma-ray light curves. The flux-flux plots among different bands, in general, show weak to moderate correlation with Spearman correlation coefficient ranging from 0.29 to 0.58. Notably, during Epoch-I, we detect a possible orphan flare exhibiting increase in the X-ray flux level ($\sim$ 2.4 times of the total average flux) but with no corresponding counterpart seen in the optical, UV bands. In contrast, during Epoch-K, we detect a significant decrease in the X-ray flux but no corresponding decrease in optical, UV and gamma-ray bands. Overall, our study reveals several changes in the flux states and complex nature of jet dominated emission processes. We tested one-zone and two-zone leptonic scenarios and for most of the epochs, the latter one provides a better description of the broad-band emission in this TeV BL Lac source.

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Avik Kumar Das, Pankaj Kushwaha, Veeresh Singh, Sandeep Kumar Mondal, Goldy Ahuja, Deekshya R. Sarkar. 2026-03-05. Broad-band temporal and spectral study of TeV blazar TXS 0518+211. https://arxiv.org/abs/2511.21182

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