Detection of time delay between UV and X-ray variability in Mrk 1044 using AstroSat observations

TL;DR

This study detects a time delay of roughly $2.3$ days between X-ray and FUV variability in Mrk 1044, with X-rays leading the UV, supporting a disk reprocessing connection but indicating a more complex disk–corona geometry than simple models predict. Using simultaneous AstroSat UVIT and SXT data, the authors quantify flux variability ($F_{ m var}$), estimate the lag via ICCF and JAVELIN (both ~2.3 days), and model the X-ray spectrum with relxillcp, finding a high spin and a relativistic reflection-dominated spectrum. The lag magnitude exceeds model predictions across plausible parameter spaces, suggesting contributions from an extended warm corona or other geometric effects that increase light-travel paths and reprocessing timescales. The results illustrate the time-dependent and potentially multi-component nature of disk–corona coupling in AGN and motivate future, broader multiwavelength monitoring campaigns to disentangle reprocessing and Comptonization processes.

Abstract

Active galactic nuclei are known to exhibit flux variations across the entire electromagnetic spectrum. Among these, correlations between UV/optical and X-ray flux variations serve as a key diagnostics for understanding the physical connection between the accretion disk and the corona. In this work, we present the results of analysis of ultraviolet (UV) and X-ray flux variations in the narrow line Seyfert 1 galaxy Mrk 1044. Simultaneous observations in the far-UV band (FUV: 1300$-$1800 Å) and the X-ray band (0.5$-$7 keV) obtained during 31 August $-$ 8 September 2018 with the Ultraviolet Imaging Telescope and the Soft X-ray Telescope onboard \textit{AstroSat} were used for this study. Significant flux variability was detected in both FUV and X-ray bands. The fractional root mean square variability amplitude ($F_{\rm var}$) was found to be 0.036 $\pm$ 0.001 in the FUV band and 0.384 $\pm$ 0.004 in the X-ray band. To explore potential time lag between the two bands, cross-correlation analysis was performed using both the interpolated cross-correlation function (ICCF) and just another vehicle for estimating lags in nuclei (JAVELIN) methods. Results from both approaches are consistent within 2$σ$ uncertainty, indicating that X-ray variations lead the FUV variations, with measured lags of 2.25$\pm$0.05 days (ICCF) and $2.35_{-0.01}^{+0.02}$ days (JAVELIN). This is the first detection of a time delay between UV and X-ray variations in Mrk 1044. The observed UV lag supports the disk reprocessing scenario, wherein X-ray emission from the corona irradiates the accretion disk, driving the observed UV variability.

Figures (7)

• Figure 1: UVIT field image (2$\times$2 square arcmin) of Mrk 1044 in F154W filter. The circular aperture (of 5 arcsec radius) is marked in red. The colour scale is in units of counts/s.
• Figure 2: SXT field image of Mrk 1044 in the 0.5$-$7 keV energy range. The image is clipped to the 15 arcmin source region of interest with the detector corner regions excluded. The colour scale is in units of counts.
• Figure 3: The light curves of Mrk 1044: X-ray in the 0.5$-$7 keV energy range (top panel) and FUV light curve using 5 arcsec aperture radius (bottom panel).
• Figure 4: The cross-correlation function (solid line) between X-ray and FUV light curves. The histograms in grey are the distribution of the centroids of the cross-correlation function using the ICCF method. The dashed grey lines are the spurious peaks near the window boundaries attributed to aliasing in the light curves.
• Figure 5: Left: X-ray light curve (top panel) and FUV light curve (bottom panel) of Mrk 1044. The solid lines show the JAVELIN-model fitted light curves, with uncertainties indicated by the shaded regions. Right: The histogram represents the lag probability distribution, with the smooth kernel density shown by the red line.