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Register a new userThe long term X-ray spectral variability of AGN
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We present the results from the spectral analysis of more than 7,500 RXTE spectra of 10 AGN, which have been observed by RXTE regularly over a long period of time ~ 7-11 years. These observations most probably sample most of the flux and spectral variations that these objects exhibit, thus, they are ideal for the study of their long term X-ray spectral variability. We modelled the 3-10 spectrum of each observation in a uniform way using a simple power-law model (with the addition of Gaussian line and/or edge to model the iron Kalpha emission/absorption features, if necessary) to consistently parametrize the shape of the observed X-ray continuum. We found that the average spectral slope does not correlate with source luminosity or black hole mass, while it correlates positively with the average accretion rate. We have also determined the (positive) spectral slope-flux relation for each object, over a larger flux range than before. We found that this correlation is similar in almost all objects. We discuss this global spectral slope-flux trend in the light of current models for spectral variability. We consider (i) intrinsic variability, expected e.g. from Comptonization processes, (ii) variability caused by absorption of X-rays by a single absorber whose ionization parameter varies proportionally to the continuum flux variations, (iii) variability resulting from the superposition of a constant reflection component and an intrinsic power-law which is variable in flux but constant in shape, and, (iv) variability resulting from the superposition of a constant reflection component and an intrinsic power-law which is variable both in flux and shape. Our final conclusion is that scenario (iv) describes better our results.
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We have analysed the soft X-ray emission from the nuclear source of the nearby spiral galaxy M81, using the available data collected with ROSAT, ASCA, BeppoSAX and Chandra. The source flux is highly variable, showing (sometimes dramatic: a factor of 4 in 20 days) variability at different timescales, from 2 days to 4 years, and in particular a steady increase of the flux by a factor of >~ 2 over 4 years, broken by rapid flares. After accounting for the extended component resolved by Chandra, the nuclear soft X-ray spectrum (from ROSAT/PSPC, BeppoSAX/LECS and Chandra data) cannot be fitted well with a single absorbed power-law model. Acceptable fits are obtained adding an extra component, either a multi-color black body (MCBB) or an absorption feature. In the MCBB case the inner accretion disk would be far smaller than the Schwartzchild radius for the 3-60X 10^6 solar masses nucleus requiring a strictly edge-on inclination of the disk, even if the nucleus is a rotating Kerr black hole. The temperature is 0.27 keV, larger than expected from the accretion disk of a Schwartzchild black hole, but consistent with that expected from a Kerr black hole. In the power-law + absorption feature model we have either high velocity (0.3 c) infalling C_v clouds or neutral C_i absorption at rest. In both cases the C:O overabundance is a factor of 10.
Variability at all observed wavelengths is a distinctive property of AGN. Hard X-rays provide us with a view of the innermost regions of AGN, mostly unbiased by absorption along the line of sight. Swift/BAT offers the unique opportunity to follow, on time scales of days to years and with a regular sampling, the 14-195 keV emission of the largest AGN sample available up to date for this kind of investigation. We study the amplitude of the variations, and their dependence on sub-class and on energy, for a sample of 110 radio quiet and radio loud AGN selected from the BAT 58-month survey. About 80% of the AGN in the sample are found to exhibit significant variability on months to years time scales, radio loud sources being the most variable. The amplitude of the variations and their energy dependence are incompatible with variability being driven at hard X-rays by changes of the absorption column density. In general, the variations in the 14-24 and 35-100 keV bands are well correlated, suggesting a common origin of the variability across the BAT energy band. However, radio quiet AGN display on average 10% larger variations at 14-24 keV than at 35-100 keV and a softer-when-brighter behavior for most of the Seyfert galaxies with detectable spectral variability on month time scale. In addition, sources with harder spectra are found to be more variable than softer ones. These properties are generally consistent with a variable power law continuum, in flux and shape, pivoting at energies >~ 50 keV, to which a constant reflection component is superposed. When the same time scales are considered, the timing properties of AGN at hard X-rays are comparable to those at lower energies, with at least some of the differences possibly ascribable to components contributing differently in the two energy domains (e.g., reflection, absorption).
We investigate the long-term spectral variability in the ultra-luminous X-ray source Holmberg IX X--1. By analyzing the data from eight {it Suzaku} and 13 {it XMM-Newton} observations conducted between 2001 and 2015, we perform a detailed spectral modeling for all spectra with simple models and complex physical models. We find that the spectra can be well explained by a disc plus thermal Comptonization model. Applying this model, we unveil correlations between the X-ray luminosity ($L_{rm X}$) and the spectral parameters. Among the correlations, a particular one is the statistically significant positive correlation between $L_{rm X}$ and the photon index ($Gamma$), while at the high luminosities of $> 2times10^{40},{rm~erg s}^{-1}$, the source becomes marginally hard and that results a change in the slope of the $Gamma - L_{rm X}$ correlation. Similar variability behavior is observed in the optical depth of the source around $L_{rm X} sim 2times10^{40},{rm~erg s}^{-1}$ as the source becomes more optically thick. We consider the scenario that a corona covers the inner part of the disc, and the correlations can be explained as to be driven by the variability of seed photons from the disc input into the corona. On the basis of the disc-corona model, we discuss the physical processes that are possibly indicated by the variability of the spectral parameters. Our analysis reveals the complex variability behavior of Holmberg IX X--1 and the variability mechanism is likely related to the geometry of the X-ray emitting regions.
We present the results from a detailed X-ray variability analysis of 66 AGN in the Lockman Hole, which have optical spectroscopic identifications. We compare, quantitatively, their variability properties with the properties of local AGN, and we study the variability-luminosity relation as a function of redshift, and the variability-redshift relation in two luminosity bins. We use archival data from the last 10 XMM observations of the Lockman Hole field to extract light curves in the rest frame, 2-10 keV band. We use the normalized excess variance to quantify the variability amplitude. Using the latest results regarding the AGN power spectral shape and its dependence on black hole mass and accretion rate, we are able to compute model variability-luminosity curves, which we compare with the relations we observe. When we consider all the sources in our sample, we find that their variability amplitude decreases with increasing redshift and luminosity. These global anti-correlations are less pronounced when we split the objects in various luminosity and redshift bins. We do not find a significant correlation between variability amplitude and spectral slope. The variability-luminosity relation that we detect has a larger amplitude when compared to that of local AGN. We also find that, at a given luminosity, the variability amplitude increases with redshift up to z~1, and then stays roughly constant. Our results imply that the AGN X-ray mechanism operates in the same way at all redshifts. Among objects with the same luminosity in our sample, the black hole mass decreases and the accretion rate increases with larger redshift.
We present new X-ray spectral data for the Seyfert 1 nucleus in NGC 4151 observed with Chandra for 200 ks. A significant ACIS pileup is present, resulting in a non-linear count rate variation during the observation. With pileup corrected spectral fitting, we are able to recover the spectral parameters and find consistency with those derived from unpiled events in the ACIS readout streak and outer region from the bright nucleus. The absorption corrected 2-10 keV flux of the nucleus varied between 6E-11 and 1E-10 erg s^{-1} cm^{-2}. Similar to earlier Chandra studies of NGC 4151 at a historical low state, the photon indices derived from the same absorbed power-law model are Gamma~0.7-0.9. However, we show that Gamma is highly dependent on the adopted spectral models. Fitting the power-law continuum with a Compton reflection component gives Gamma~1.1. By including passage of non-uniform X-ray obscuring clouds, we can reproduce the apparent flat spectral states with Gamma~1.7, typical for Seyfert 1 AGNs. The same model also fits the hard spectra from previous ASCA long look observation of NGC 4151 in the lowest flux state. The spectral variability during our observation can be interpreted as variations in intrinsic soft continuum flux relative to a Compton reflection component that is from distant cold material and constant on short time scale, or variations of partially covering absorber in the line of sight towards the nucleus. An ionized absorber model with ionization parameter logxi ~ 0.8-1.1 can also fit the low-resolution ACIS spectra. If the partial covering model is correct, adopting a black hole mass M_{BH} ~ 4.6E+7 Msun we constrain the distance of the obscuring cloud from the central black hole to be r<~9 light-days, consistent with the size of broad emission line region of NGC 4151 from optical reverberation mapping.
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