X-ray Variability Of NGC Research Articles

The large amplitude X-ray variability in NGC 7589: possible evidence for accretion mode transition

ABSTRACT We report the discovery of large amplitude X-ray variability in the low-luminosity active galactic nucleus NGC 7589, and present possible observational evidence for accretion mode transition in this source. Long-term X-ray flux variations by a factor of more than 50 are found using X-ray data obtained by Swift/X-Ray Telescope and XMM–Newton over 17 yr. Results of long-term monitoring data in the UV, optical, and infrared bands over ∼20 yr are also presented. The Eddington ratio λEdd increased from 10−3 to ∼0.13, suggesting a transition of the accretion flow from an advection dominated accretion flow to a standard thin accretion disc. Further evidence supporting the thin disc in the high-luminosity state is found by the detection of a significant soft X-ray component in the X-ray spectrum. The temperature of this component ($\sim 19^{+15}_{-7}$ eV, fitted with a blackbody model) is in agreement with the predicted temperature of the inner region for a thin disc around a black hole (BH) with mass of ∼107M⊙. These results may indicate that NGC 7589 had experienced accretion mode transition over a time-scale of a few years, suggesting the idea that similar accretion processes are at work for massive BH and BH X-ray binaries.

Continuum Reverberation Mapping of the Accretion Disks in Two Seyfert 1 Galaxies

Abstract We present optical continuum lags for two Seyfert 1 galaxies, MCG+08-11-011 and NGC 2617, using monitoring data from a reverberation mapping campaign carried out in 2014. Our light curves span the ugriz filters over four months, with median cadences of 1.0 and 0.6 days for MCG+08-11-011 and NGC 2617, respectively, combined with roughly daily X-ray and near-UV data from Swift for NGC 2617. We find lags consistent with geometrically thin accretion-disk models that predict a lag-wavelength relation of τ ∝ λ 4/3. However, the observed lags are larger than predictions based on standard thin-disk theory by factors of 3.3 for MCG+08-11-011 and 2.3 for NGC 2617. These differences can be explained if the mass accretion rates are larger than inferred from the optical luminosity by a factor of 4.3 in MCG+08-11-011 and a factor of 1.3 in NGC 2617, although uncertainty in the SMBH masses determines the significance of this result. While the X-ray variability in NGC 2617 precedes the UV/optical variability, the long (2.6 day) lag is problematic for coronal reprocessing models.

Variability and spectral energy distributions of low-luminosity active galactic nuclei: a simultaneous X-ray/UV look withSwift

We have observed four low-luminosity active galactic nuclei classified as Type 1 LINERs with the X-ray Telescope (XRT) and the UltraViolet-Optical Telescope (UVOT) onboard Swift, in an attempt to clarify the main powering mechanism of this class of nearby sources. Among our targets, we detect X-ray variability in NGC 3998 for the first time. The light curves of this object reveal variations of up to 30% amplitude in half a day, with no significant spectral variability on this time scale. We also observe a decrease of ~30% over 9 days, with significant spectral softening. Moreover, the X-ray flux is ~40% lower than observed in previous years. Variability is detected in M 81 as well, at levels comparable to those reported previously: a flux increase in the hard X-rays (1-10 keV) of 30% in ~3 hours and variations by up to a factor of 2 within a few years. This X-ray behaviour is similar to that of higher-luminosity, Seyfert-type, objects. Using previous high-angular-resolution imaging data from the Hubble Space Telescope (HST), we evaluate the diffuse UV emission due to the host galaxy and isolate the nuclear flux in our UVOT observations. All sources are detected in the UV band, at levels similar to those of the previous observations with HST. The XRT (0.2-10 keV) spectra are well described by single power-laws and the UV-to-X-ray flux ratios are again consistent with those of Seyferts and radio-loud AGNs of higher luminosity. The similarity in X-ray variability and broad-band energy distributions suggests the presence of similar accretion and radiation processes in low- and high-luminosity AGNs.

The discovery of non-linear X-ray variability in NGC 4051

We present a power spectral analysis of the 0.1--2 keV X-ray light curves of NGC 4051. Two long ROSAT observations have been analysed and the power spectra are shown to have significantly different shapes, indicating that the source is not statistically stationary on time-scales of ∼ 1 year. There is also strong evidence that the light curve is non-linear at certain epochs, which means that the power spectra do not adequately represent all the information contained in the light curve. The implications of these discoveries are far reaching and fundamental to the study of the X-ray light curves of active galactic nuclei. The two currently popular models for X-ray variability (multi-time-scale shot noise and rotating flares on a disc) are linear, and so cannot fully describe the data: however, models in which the variability events are correlated rather than random may be more successful in describing the observed light curves.

2.13. Optical and X-ray variability in NGC 4395

The spectrum of the dwarf Seyfert 1 nucleus in NGC 4395 was first reported about a decade ago (Filippenko & Sargent 1989), showing high ionization narrow emission lines and broad permitted lines. The later detection of radio and X-ray compact nuclear sources and of a featureless UV continuum gave support to the idea that NGC 4395 is a genuine low luminosity AGN (Sramek 1992; Filippenko, Ho & Sargent 1993). However, it has been claimed that, unlike classic Seyferts, NGC 4395 does not vary at all (Shields & Filippenko 1992).

Hard X-ray variability of NGC 4151

During a balloon flight on July 15, 1987 of the MIFRASO telescope, a region of the sky containing the Seyfert galaxy NGC 4151 was studied over the photon energy range 15-335 keV. A 13-sigma excess, interpreted as originating from the direction of NGC 4151, was detected in this energy band and the corresponding emission spectrum is represented by a single power law with a photon spectral index of alpha = 1.5 + or {minus} 0.1. With respect to the July 1986 observation of this source with the same telescope, the alpha slope is statistically unchanged, while the X-ray flux in the energy range 27-80 keV shows, at the 95-percent confidence level, an increase of about 40 percent. An observation of the Crab Nebula, that proves the stability of the telescope response through the two measurements of NGC 4151, is also described. 48 refs.