X-ray, UV, and optical time delays in the bright Seyfert galaxy Ark 120 with co-ordinated Swift and ground-based observations

Abstract

We report on the results of a multiwavelength monitoring campaign of the bright, nearby Seyfert galaxy, Ark 120 using a ∼50-day observing programme with Swift and a ∼4-month co-ordinated ground-based observing campaign, predominantly using the Skynet Robotic Telescope Network. We find Ark 120 to be variable at all optical, UV, and X-ray wavelengths, with the variability observed to be well-correlated between wavelength bands on short timescales. We perform cross-correlation analysis across all available wavelength bands, detecting time delays between emission in the X-ray band and the Swift V, B and UVW1 bands. In each case, we find that the longer-wavelength emission is delayed with respect to the shorterwavelength emission. Within our measurement uncertainties, the time delays are consistent with the τ ∼ λ 4/3 relation, as predicted by a disc reprocessing scenario. The measured lag centroids are τ cent = 11.90 ± 7.33, 10.80 ± 4.08, and 10.60 ± 2.87 days between the X-ray and V, B, and UVW1 bands, respectively. These time delays are longer than those expected from standard accretion theory and, as such, Ark 120 may be another example of an active galaxy whose accretion disc appears to exist on a larger scale than predicted by the standard thin-disc model. Additionally, we detect further inter-band time delays: most notably between the ground-based I and B bands (τ cent = 3.46 ± 0.86 days), and between both the Swift XRT and UVW1 bands and the I band (τ cent = 12.34 ± 4.83 and 2.69 ± 2.05 days, respectively), highlighting the importance of co-ordinated ground-based optical observations.

Multiwavelength power-spectrum analysis of NGC 5548

“…For instance, Porquet et al (2018Porquet et al ( , 2019 used broad-band spectral modelling and found strong indications for the presence of a warm, opticallythick corona, which may be extended, in Ark 120. This may have relevance to the observed time lags, which were reported by Lobban et al (2020) to be larger than expected from standard disc theory (while remaining consistent with the τ ∝ λ 4/3 relation, similar to NGC 5548). If such a warm corona, with a large optical depth exists, then all the UV/optical photons emitted by the underlying disc will be scattered to soft X-rays.…”

Long term X-Ray Observations of Seyfert 1 Galaxy Ark 120: On the origin of soft-excess

“…We find that in XMM1 observation, there is no delay between the low and high energy band, while a positive delay of 4.71 ± 1 ks is detected in XMM2 observation and a negative delay of 4.15 ± 1 ks is seen in XMM3 observation. A correlated variability among the optical, UV, and X-ray bands have already been reported Lobban et al (2020). Also, (Dutta & Chakrabarti 2016;Chatterjee et al 2017b) reported in a different context that the X-ray lag has a strong dependency on the geometric structure of the Comptonization region and orientation of the Keplerian disc.…”

Characterizing Long-term Optical Variability Properties of $γ$-ray Bright Blazars

“…The diagram for lag against ZDCF value of the sample blazars are shown in Figure 4. The lag between the emissions was estimated by computing the centroid of the ZDCF peak, i. e., τ c = i τ i ZDCF i / i ZDCF i , where τ c is calculated using the ZDCFs around the DCF peak that are greater than 0.8 times the ZDCF peak value (see e. g. Lobban et al 2020;Buisson et al 2017). A positive lag implies γ-ray variability features preceding those in the optical observations.…”