Broadband temporal and spectral study of TeV Blazar TXS 0518+211

We present a long-term broadband temporal and spectral study of a TeV BL Lacertae (BL Lac) object source TXS 0518+211 by analyzing nearly 16 yr (MJD 54682–60670) of simultaneous optical, UV, and X-ray light curves from Swift-XRT/UVOT and gamma-ray light curves from Fermi-LAT. Based on the availability of simultaneous multiwavelength data and considering flux level as the depiction of active galactic nucleus–jet activity, we identified 11 epochs (named 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, an X-ray light curve exhibits a relatively high degree of variability 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 0.29–0.58. Notably, during Epoch-I, we detect a possible orphan flare exhibiting increase in the X-ray flux level (∼2.4 times of the total average flux) but with no corresponding counterpart seen in the optical and UV bands. In contrast, during Epoch-K, we detect a significant decrease in the X-ray flux but no corresponding decrease in the 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 provides a better description of the broadband emission in this TeV BL Lac object source.