X-ray, UV and optical time delays in the bright Seyfert galaxy Ark 120 with co-ordinated Swift and ground-based observations. (arXiv:2002.12348v1 [astro-ph.HE])

X-ray, UV and optical time delays in the bright Seyfert galaxy Ark 120 with co-ordinated Swift and ground-based observations. (arXiv:2002.12348v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Lobban_A/0/1/0/all/0/1">A. P. Lobban</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zola_S/0/1/0/all/0/1">S. Zola</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pajdosz_Smierciak_U/0/1/0/all/0/1">U. Pajdosz-Smierciak</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Braito_V/0/1/0/all/0/1">V. Braito</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nardini_E/0/1/0/all/0/1">E. Nardini</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bhatta_G/0/1/0/all/0/1">G. Bhatta</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Markowitz_A/0/1/0/all/0/1">A. Markowitz</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bachev_R/0/1/0/all/0/1">R. Bachev</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Carosati_D/0/1/0/all/0/1">D. Carosati</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Caton_D/0/1/0/all/0/1">D. B. Caton</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Damljanovic_G/0/1/0/all/0/1">G. Damljanovic</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Debski_B/0/1/0/all/0/1">B. Debski</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Haislip_J/0/1/0/all/0/1">J. B. Haislip</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hu_S/0/1/0/all/0/1">S. M. Hu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kouprianov_V/0/1/0/all/0/1">V. Kouprianov</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Krzesinski_J/0/1/0/all/0/1">J. Krzesinski</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Porquet_D/0/1/0/all/0/1">D. Porquet</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nunez_F/0/1/0/all/0/1">F. Pozo Nunez</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Reeves_J/0/1/0/all/0/1">J. Reeves</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Reichart_D/0/1/0/all/0/1">D. E. Reichart</a>

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 shorter-wavelength emission. Within our measurement
uncertainties, the time delays are consistent with the tau ~ lambda^{4/3}
relation, as predicted by a disc reprocessing scenario. The measured lag
centroids are tau_{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 (tau_{cent} = 3.46 +/- 0.86 days), and
between both the Swift XRT and UVW1 bands and the I band (tau_{cent} = 12.34
+/- 4.83 and 2.69 +/- 2.05 days, respectively), highlighting the importance of
co-ordinated ground-based optical observations.

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