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

TL;DR

This study analyzes 16 years of multi-wavelength data from the TeV blazar TXS 0518+211, revealing complex variability patterns and favoring a two-zone leptonic emission model over a one-zone model.

Contribution

It provides the first detailed long-term multi-wavelength variability analysis of TXS 0518+211 and compares emission models, highlighting the complex jet emission processes.

Findings

X-ray variability is higher than optical, UV, and gamma-ray bands.

Detected an orphan X-ray flare with no optical counterpart.

Two-zone leptonic model better explains the broad-band emission.

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.