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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

TL;DR

This work presents a 16-year, multi-wavelength temporal-spectral study of the TeV-detected BL Lac TXS 0518+211 using Swift-XRT/UVOT and Fermi-LAT data to identify distinct activity epochs and characterize variability across optical, UV, X-ray, and gamma-ray bands. The authors perform thorough timing analyses, finding pronounced X-ray variability relative to other bands, modest cross-band correlations, and an orphan X-ray flare in Epoch-I, along with an unusual X-ray drop in Epoch-K. Broadband SED modeling with leptonic scenarios shows that a one-zone SSC model cannot fully account for the data, while a two-zone SSC framework provides a better, cohesive fit across epochs, with inner and outer zones contributing differently to the high-energy and optical–UV emission. The study highlights the complex jet structure in TXS 0518+211 and underscores the need for coordinated, panchromatic observations (including NuSTAR) to better constrain the particle distributions and emission regions, informing our understanding of jet physics in IBLs.

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 Swift-XRT/UVOT and gamma-ray light curves from 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. In all the epochs, two-zone leptonic model provides a better description of the broad-band emission in this TeV BL Lac source.

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

TL;DR

This work presents a 16-year, multi-wavelength temporal-spectral study of the TeV-detected BL Lac TXS 0518+211 using Swift-XRT/UVOT and Fermi-LAT data to identify distinct activity epochs and characterize variability across optical, UV, X-ray, and gamma-ray bands. The authors perform thorough timing analyses, finding pronounced X-ray variability relative to other bands, modest cross-band correlations, and an orphan X-ray flare in Epoch-I, along with an unusual X-ray drop in Epoch-K. Broadband SED modeling with leptonic scenarios shows that a one-zone SSC model cannot fully account for the data, while a two-zone SSC framework provides a better, cohesive fit across epochs, with inner and outer zones contributing differently to the high-energy and optical–UV emission. The study highlights the complex jet structure in TXS 0518+211 and underscores the need for coordinated, panchromatic observations (including NuSTAR) to better constrain the particle distributions and emission regions, informing our understanding of jet physics in IBLs.

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 Swift-XRT/UVOT and gamma-ray light curves from 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 ( 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. In all the epochs, two-zone leptonic model provides a better description of the broad-band emission in this TeV BL Lac source.

Paper Structure

This paper contains 20 sections, 12 equations, 8 figures, 5 tables.

Table of Contents

  1. Introduction
  2. MULTI-WAVELENGTH OBSERVATION AND DATA ANALYSIS
  3. Simultaneous optical, UV, and X-ray observations from Swift observatory
  4. The Swift-UVOT data analysis
  5. The Swift-XRT data analysis
  6. Fermi-Large Area Telescope (LAT)
  7. TEMPORAL STUDY
  8. Optical, UV, X-ray, Gamma-ray light curves and spectral indices
  9. Fractional variability
  10. Variability timescales
  11. The correlation between fluxes in different bands
  12. Intra-Night Optical Variability (INOV)
  13. BROADBAND SPECTRAL STUDY
  14. Broad-band SED modeling
  15. One-zone SSC model:
  16. ...and 5 more sections

Figures (8)

  • Figure 1: Multi-wavelength light curve of TXS 0518+211 during MJD 54682 $-$ 60148. The eleven different epochs (Epoch-A to Epoch-K) identified based on the availability of simultaneous data across all bands are shown by grey shaded vertical strips. The first four panels from top to bottom represent optical ($V$, $B$, $U$ band fluxes plotted with different colours in the same panel), UV ($UVW1$, $UVM2$, $UVW2$ band fluxes plotted with different colours in the same panel), X-ray, and gamma-ray light curves. The last three panels from top to bottom represent optical-UV photon index (${\Gamma}_{\rm UVOT}$), X-ray photon (${\Gamma}_{\rm X}$), and gamma-ray photon index (${\Gamma}_{\gamma}$). The optical/UV, X-ray, and gamma-ray fluxes are in the unit of 10$^{-11}$ erg cm$^{-2}$ s$^{-1}$, 10$^{-11}$ erg cm$^{-2}$ s$^{-1}$, and 10$^{-6}$ ph cm$^{-2}$ s$^{-1}$, respectively
  • Figure 2: Flux-flux correlation plots.
  • Figure 3: Photon index/spectral index versus flux plots.
  • Figure 4: The differential light curves (DLCs) of TXS 0518+211 in $R$-band for three intra-night monitoring sessions. The date, time duration, and variability status are mentioned in the top of each DLC plot. In each plot, the uppermost panel represents the DLC of two comparison sources (C1 and C2), while the other panels show DLCs of target w.r.t. to two comparison sources (T-C1, T-C2). The lowermost panel shows seeing (PSF) variations across the monitoring session.
  • Figure 5: Multi-wavelength SED for nine different epochs (including Epoch-K).
  • ...and 3 more figures