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Correlated X-ray/ultraviolet/optical variability in the very low mass AGN NGC 4395
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Cameron, D. T., McHardy, Ian M., Dwelly, T., Breedt, E., Uttley, P., Lira, Paulina and Arévalo, P. (2012) Correlated X-ray/ultraviolet/optical variability in the very low mass AGN NGC 4395. Monthly Notices of the Royal Astronomical Society, Vol.422 (No.1). pp. 902-912. doi:10.1111/j.1365-2966.2012.20677.x ISSN 0035-8711.
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Official URL: http://dx.doi.org/10.1111/j.1365-2966.2012.20677.x
Abstract
We report the results of a 1-yr Swift X-ray/ultraviolet (UV)/optical programme monitoring the dwarf Seyfert nucleus in NGC4395 in 2008-2009. The UV/optical flux from the nucleus was found to vary dramatically over the monitoring period, with a similar pattern of variation in each of the observed UV/optical bands (spanning 1900-5500 Å). In particular, the luminosity of NGC4395 in the 1900 Å band changed by more than a factor of 8 over the monitoring period. The fractional variability was smaller in the UV/optical bands than that seen in the X-rays, with the X-ray/optical ratio increasing with increasing flux. Pseudo-instantaneous flux measurements in the X-ray and each UV/optical band were well correlated, with cross-correlation coefficients of ≥0.7, significant at 99.9per cent confidence. Archival Swift observations from 2006 sample the intra-day X-ray/optical variability on NGC4395. These archival data show a very strong correlation between the X-ray and b bands, with a cross-correlation coefficient of 0.84 (significant at >99per cent confidence). The peak in the cross-correlation function is marginally resolved and asymmetric, suggesting that X-rays lead the b band, but by ≤1h. In response to recent (2011 August) very high X-ray flux levels from NGC4395 we triggered Swift target of opportunity observations, which sample the intra-hour X-ray/UV variability. These observations indicate, albeit with large uncertainties, a lag of the 1900 Å band behind the X-ray flux of ∼400s. The tight correlation between the X-ray and UV/optical lightcurves, together with the constraints we place on the lag time-scale, is consistent with the UV/optical variability of NGC4395 being primarily due to reprocessing of X-ray photons by the accretion disc.
Item Type: | Journal Article | ||||
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Subjects: | Q Science > QB Astronomy | ||||
Divisions: | Faculty of Science, Engineering and Medicine > Science > Physics | ||||
Library of Congress Subject Headings (LCSH): | Active galactic nuclei | ||||
Journal or Publication Title: | Monthly Notices of the Royal Astronomical Society | ||||
Publisher: | Wiley | ||||
ISSN: | 0035-8711 | ||||
Official Date: | 2012 | ||||
Dates: |
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Volume: | Vol.422 | ||||
Number: | No.1 | ||||
Page Range: | pp. 902-912 | ||||
DOI: | 10.1111/j.1365-2966.2012.20677.x | ||||
Status: | Peer Reviewed | ||||
Publication Status: | Published | ||||
Access rights to Published version: | Restricted or Subscription Access | ||||
Date of first compliant deposit: | 20 December 2015 | ||||
Date of first compliant Open Access: | 20 December 2015 | ||||
Funder: | University of Southampton. School of Physics and Astronomy, Science and Technology Facilities Council (Great Britain) (STFC), Fondo Nacional de Desarrollo Científico y Tecnológico (Chile) (FONDECYT) | ||||
Grant number: | 1110049 (FONDECYT) |
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