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Register a new userEvidence for periodicity in 43 year-long monitoring of NGC 5548
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We present an analysis of 43 years (1972 to 2015) of spectroscopic observations of the Seyfert 1 galaxy NGC 5548. This includes 12 years of new unpublished observations (2003 to 2015). We compiled about 1600 H$beta$ spectra and analyzed the long-term spectral variations of the 5100 AA continuum and the H$beta$ line. Our analysis is based on standard procedures including the Lomb-Scargle method, which is known to be rather limited to such heterogeneous data sets, and new method developed specifically for this project that is more robust and reveals a $sim$5700 day periodicity in the continuum light curve, the H$beta$ light curve and the radial velocity curve of the red wing of the H$beta$ line. The data are consistent with orbital motion inside the broad emission line region of the source. We discuss several possible mechanisms that can explain this periodicity, including orbiting dusty and dust-free clouds, a binary black hole system, tidal disruption events, and the effect of an orbiting star periodically passing through an accretion disc.
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Supermassive black holes in the nuclei of active galaxies expel large amounts of matter through powerful winds of ionized gas. The archetypal active galaxy NGC 5548 has been studied for decades, and high-resolution X-ray and UV observations have previously shown a persistent ionized outflow. An observing campaign in 2013 with six space observatories shows the nucleus to be obscured by a long-lasting, clumpy stream of ionized gas never seen before. It blocks 90% of the soft X-ray emission and causes simultaneous deep, broad UV absorption troughs. The outflow velocities of this gas are up to five times faster than those in the persistent outflow, and at a distance of only a few light days from the nucleus, it may likely originate from the accretion disk.
NGC 5548 has been intensively monitored by the AGN Space Telescope and Optical Reverberation Mapping collaboration. Approximately after half of the light curves, the correlation between the broad emission lines and the lag-corrected ultraviolet continua becomes weak. This anomalous behavior is accompanied by an increase of soft X-ray emission. We propose a simple model to understand this anomalous behavior, i.e., the corona might fall down, thereby increasing the covering fraction of the inner disk. Therefore, X-ray and extreme ultraviolet emission suffer from spectral variations. The ultraviolet continua variations are driven by both X-ray and extreme ultraviolet variations. Consequently, the spectral variability induced by the falling corona would dilute the correlation between the broad emission lines and the ultraviolet continua. Our model can explain many additional observational facts, including the dependence of the anomalous behavior on velocity and ionization energy. We also show that the time lag and correlation between the X-ray and the ultraviolet variations change as NGC 5548 displays the anomalous behavior. The time lag is dramatically longer than the expectation from disk reprocessing if the anomalous behavior is properly excluded. During the anomalous state, the time lag approaches the light-travel timescale of disk reprocessing albeit with a much weaker correlation. We speculate that the time lag in the normal state is caused by reprocessing of the broad line region gas. As NGC 5548 enters the abnormal state, the contribution of the broad line region gas is smaller; the time lag reflects disk reprocessing. We also discuss alternative scenarios.
An extensive multi-satellite campaign on NGC 5548 has revealed this archetypal Seyfert-1 galaxy to be in an exceptional state of persistent heavy absorption. Our observations taken in 2013-2014 with XMM-Newton, Swift, NuSTAR, INTEGRAL, Chandra, HST and two ground-based observatories have together enabled us to establish that this unexpected phenomenon is caused by an outflowing stream of weakly ionised gas (called the obscurer), extending from the vicinity of the accretion disk to the broad-line region. In this work we present the details of our campaign and the data obtained by all the observatories. We determine the spectral energy distribution of NGC 5548 from near-infrared to hard X-rays by establishing the contribution of various emission and absorption processes taking place along our line of sight towards the central engine. We thus uncover the intrinsic emission and produce a broadband continuum model for both obscured (average summer 2013 data) and unobscured ($<$ 2011) epochs of NGC 5548. Our results suggest that the intrinsic NIR/optical/UV continuum is a single Comptonised component with its higher energy tail creating the soft X-ray excess. This component is compatible with emission from a warm, optically-thick corona as part of the inner accretion disk. We then investigate the effects of the continuum on the ionisation balance and thermal stability of photoionised gas for unobscured and obscured epochs.
Lags measured from correlated X-ray/UV/optical monitoring of AGN allow us to determine whether UV/optical variability is driven by reprocessing of X-rays or X-ray variability is driven by UV/optical seed photon variations. We present the results of the largest study to date of the relationship between the X-ray, UV and optical variability in an AGN with 554 observations, over a 750d period, of the Seyfert 1 galaxy NGC 5548 with Swift. There is a good overall correlation between the X-ray and UV/optical bands, particularly on short timescales (tens of days). These bands lag the X-ray band with lags which are proportional to wavelength to the power 1.23+/-0.31. This power is very close to the power (4/3) expected if short timescale UV/optical variability is driven by reprocessing of X-rays by a surrounding accretion disc. The observed lags, however, are longer than expected from a standard Shakura-Sunyaev accretion disc with X-ray heating, given the currently accepted black hole mass and accretion rate values, but can be explained with a slightly larger mass and accretion rate, and a generally hotter disc. Some long term UV/optical variations are not paralleled exactly in the X-rays, suggesting an additional component to the UV/optical variability arising perhaps from accretion rate perturbations propagating inwards through the disc.
We present our investigation into the long-term variability of the X-ray obscuration and optical-UV-X-ray continuum in the Seyfert 1 galaxy NGC 5548. In 2013 and 2014, the Swift observatory monitored NGC 5548 on average every day or two, with archival observations reaching back to 2005, totalling about 670 ks of observing time. Both broadband spectral modelling and temporal rms variability analysis are applied to the Swift data. We disentangle the variability caused by absorption, due to an obscuring weakly-ionised outflow near the disk, from variability of the intrinsic continuum components (the soft X-ray excess and the power-law) originating from the disk and its associated coronae. The spectral model that we apply to this extensive Swift data is the global model that we derived for NGC 5548 from analysis of the stacked spectra from our multi-satellite campaign of summer 2013 (including XMM-Newton, NuSTAR and HST). The results of our Swift study show that changes in the covering fraction of the obscurer is the primary and dominant cause of variability in the soft X-ray band on timescales of 10 days to ~ 5 months. The obscuring covering fraction of the X-ray source is found to range between 0.7 and nearly 1.0. The contribution of the soft excess component to the X-ray variability is often much less than that of the obscurer, but it becomes comparable when the optical-UV continuum flares up. We find that the soft excess is consistent with being the high-energy tail of the optical-UV continuum and can be explained by warm Comptonisation: up-scattering of the disk seed photons in a warm, optically thick corona as part of the inner disk. To this date, the Swift monitoring of NGC 5548 shows that the obscurer has been continuously present in our line of sight for at least 4 years (since at least February 2012).
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