The method of surrogates is one of the key concepts of nonlinear data analysis. Here, we demonstrate that commonly used algorithms for generating surrogates often fail to generate truly linear time series. Rather, they create surrogate realizations w
ith Fourier phase correlations leading to non-detections of nonlinearities. We argue that reliable surrogates can only be generated, if one tests separately for static and dynamic nonlinearities.
The vast majority of optically identified active galactic nuclei (AGNs) in the local Universe reside in host galaxies with prominent bulges, supporting the hypothesis that black hole formation and growth is fundamentally connected to the build-up of
galaxy bulges. However, recent mid-infrared spectroscopic studies with Spitzer of a sample of optically normal late-type galaxies reveal remarkably the presence of high-ionization [NeV] lines in several sources, providing strong evidence for AGNs in these galaxies. We present follow-up X-ray observations recently obtained with XMM-Newton of two such sources, the late-type optically normal galaxies NGC 3367 and NGC 4536. Both sources are detected in our observations. Detailed spectral analysis reveals that for both galaxies, the 2-10 keV emission is dominated by a power law with an X-ray luminosity in the 10^39 - 10^40 ergs s^-1 range, consistent with low luminosity AGNs. While there is a possibility that X-ray binaries account for some fraction of the observed X-ray luminosity, we argue that this fraction is negligible. These observations therefore add to the growing evidence that the fraction of late-type galaxies hosting AGNs is significantly underestimated using optical observations alone. A comparison of the mid-infrared [NeV] luminosity and the X-ray luminosities suggests the presence of an additional highly absorbed X-ray source in both galaxies, and that the black hole masses are in the range of 10^5 - 10^7 M_solar for NGC 3367 and 10^4 - 10^6 M_solar for NGC 4536.
PKS 0558-504 is the brightest radio-loud Narrow-Line Seyfert 1 galaxy at X-ray energies. Here we present results from the radio, optical, UV, and X-ray bands obtained with Swift, XMM, and ATCA during a 10-day monitoring campaign in September 2008. Th
e simultaneous coverage at several wavelengths makes it possible to investigate in detail the broadband spectral energy distribution (SED) and the energetic of this source. The main results can be summarized as follows. The ATCA reveals the presence of an extended radio emission in PKS 0558-504 with two lobe-like structures at ~7 from the bright central source. The extended radio structure and the low value of the radio-loudness similar to radio-quiet Seyfert galaxies coupled with constraints from higher energy bands argue against a jet-dominated emission. The study of the SED, which is dominated by a nearly constant optical-UV emission, supports the conclusion that PKS 0558-504 is accreting at super-Eddington rate. This conclusion was reached assuming M_BH=2.5e8 M_sun, which was obtained with a new scaling method based on X-ray spectral variability results. A comparison between the accretion luminosity and the kinetic power associated with the jet suggests that in this source the accretion power dominates in agreement with the results obtained from Radiation-MHD simulations of Galactic black holes (GBHs) accreting at the Eddington rate. The combined findings from this panchromatic investigation strongly suggest that PKS 0558-504 is a large-scale analog of GBHs in their highly accreting intermediate state. Importantly, PKS 0558-504 may also be the prototype of the parent population of the very radio-loud NLS1s recently detected at gamma-ray energies.
We investigate the possible nonlinear variability properties of the black hole X-ray nova 4U1543-47 to complement the temporal studies based on linear techniques, and to search for signs of nonlinearity in Galactic black hole (GBH) light curves. Firs
t, we apply the weighted scaling index method (WSIM) to characterize the X-ray variability properties of 4U1543-47 in different spectral states during the 2002 outburst. Second, we use surrogate data to investigate whether the variability is nonlinear in any of the different spectral states. The main findings can be summarized as follows. The mean weighted scaling index appears to be able to parametrize uniquely the temporal variability properties of this GBH: the 3 different spectral states of the 2002 outburst of 4U1543-47 are characterized by different and well constrained values. The search for nonlinearity reveals that the variability is linear in all light curves with the notable exception of the very high state. Our results imply that we can use the WSIM to assign a single number, namely the mean weighted scaling index, to a light curve, and in this way discriminate among the different spectral states of a source. The detection of nonlinearity in the VHS, that is characterized by the presence of most prominent QPOs, suggests that intrinsically linear models which have been proposed to account for the low frequency QPOs in GBHs may be ruled out (abridged).
We present results from the spectral analysis of a long XMM-Newton observation of the radio-loud NLS1 galaxy PKS0558-504. The source is highly variable, on all sampled time scales. We did not observe any absorption features in either the soft or hard
X-ray band. We found weak evidence for the presence of an iron line at ~6.8 keV, which is indicative of emission from highly ionized iron. The 2-10 keV band spectrum is well fitted by a simple power law model, whose slope steepens with increasing flux, similar to what is observed in other Seyferts as well. The soft excess is variable both in flux and shape, and it can be well described by a low-temperature Comptonisation model, whose slope flattens with increasing flux. The soft excess flux variations are moderately correlated with the hard band variations, and we found weak evidence that they are leading them by ~20 ksec. Our results rule out a jet origin for the bulk of the X-ray emission in this object. The observed hard band spectral variations suggest intrinsic continuum slope variations, caused by changes in the heating/cooling ratio of the hot corona. The low-temperature Comptonising medium, responsible for the soft excess emission, could be a hot layer in the inner disc of the source, which appears due to the fact that the source is accreting at a super-Eddington rate. The soft excess flux and spectral variations could be caused by random variations of the accretion rate.
We report the detection of a weak X-ray point source coincident with the nucleus of the bulgeless disk galaxy NGC 3621, recently discovered by Spitzer to display high ionization mid-infrared lines typically associated with AGN. These Chandra observat
ions provide confirmation for the presence of an AGN in this galaxy, adding to the growing evidence that black holes do form and grow in isolated bulgeless disk galaxies. Although the low signal-to-noise ratio of the X-ray spectrum prevents us from carrying out a detailed spectral analysis of the nuclear source, the X-ray results, combined with the IR and optical spectroscopic results, suggests that NGC 3621 harbors a heavily absorbed AGN, with a supermassive black hole of relatively small mass accreting at a high rate. Chandra also reveals the presence of two bright sources straddling the nucleus located almost symmetrically at 20 from the center. Both sources have X-ray spectra that are well-fitted by an absorbed power-law model. Assuming they are at the distance of NGC 3621, these two sources have luminosities of the order of 1.e39 erg/s, which make them ULXs and suggest that they are black hole systems. Estimates of the black hole mass based on the X-ray spectral analysis and scaling laws of black hole systems suggest that the 2 bright sources might be intermediate mass black holes with M_BH of the order of a few thousand solar masses. However, higher quality X-ray data combined with multi-wavelength observations are necessary to confirm these conclusions.
We present the first X-ray monitoring observations of the X-ray bright FRI radio galaxy NGC6251 observed with RXTE for 1 year. The primary goal of this study is to shed light on the origin of the X-rays, by investigating the spectral variability with
model-independent methods coupled with time-resolved and flux-selected spectroscopy. The main results can be summarized as follows: 1) Throughout the monitoring campaign, NGC6251 was in relatively high-flux state. 2) The flux persistently changed with fluctuations of the order of ~2 on time scales of 20-30 days. 3) When the hardness ratio is plotted against the average count rate, there is evidence for a spectral hardening as the source brightens; this finding is confirmed by a flux-selected spectral analysis. 4) The fractional variability appears to be more pronounced in the hard energy band (5-12 keV) than in the soft one (2.5-5 keV). 5) 2-month averaged and flux-limited energy spectra are adequately fitted by a power law. A Fe Kalpha line is never statistically required, although the presence of a strong iron line cannot be ruled out, due to the high upper limits on the line equivalent width. The inconsistency of the spectral variability behavior of NGC6251 with the typical trend observed in Seyfert galaxies and the similarity with blazars lead support to a jet-dominated scenario during the RXTE monitoring campaign. However, a possible contribution from a disk-corona system cannot be ruled out.
Aims: We investigate the origin of X-rays and the nature of accretion flow in 4 LINERs hosted by radio galaxies, namely NGC1692, PKS0625-35, 3C88, 3C444, recently observed with XMM. Methods: We combine the results from the time-averaged spectral anal
ysis with model-independent information from X-ray temporal and spectral variability analyses, and with additional broadband information (UV and radio). Results: The values of the Eddington ratios of our sample span 2 orders of magnitude. The 4 AGN are adequately fitted by the same continuum model that comprises at least one thermal component and a partially absorbed power law, whose relative contribution and photon index vary substantially from source to source. NGC1692 and PKS0625-35 have fairly steep power-law components, perhaps indicative of synchrotron emission from a jet. Conversely, the flat photon index derived for 3C88 may be indicative of a heavily absorbed object. Finally, the time-averaged spectral properties of 3C444 (Gamma~1.9 and an apparent line-like excess around 6.7 keV) are more in line with Seyfert galaxies. The temporal analysis reveals that PKS0625-35 and 3C88 are significantly variable in the soft energy band. PKS0625-35 also shows suggestive evidence of spectral variability on timescales of months. The main findings from the broadband analysis can be summarized as follows: 1) 3C444, PKS0625-35, and NGC1692 have alpha_OX values consistent with the well known alpha_OX -l_UV correlation. 2) No positive correlation is found between L_X and the inclination angle, suggesting that the X-ray emission is not beamed. 3) The values of the radio-loudness are inversely proportional to the Eddington ratio and locate our objects in between the ``radio-loud and ``radio-quiet branches in the R- l_UV plane.
Blazars are known to display strong and erratic variability at almost all the wavelengths of electromagnetic spectrum. Presently, variability studies at high-energies (hard X-rays, gamma-rays) are hampered by low sensitivity of the instruments. Never
theless, the latest generation of satellites (INTEGRAL, Swift) have given suggestions not yet fully confirmed of variability on intraday timescales. Some specific cases recently observed are presented and physical implications are discussed (e.g. NRAO 530). The contribution that SIMBOL-X could give in this research topic is also outlined.
We present the results from simultaneous chandra and rxte observations of the X-ray bright Broad-Line Radio Galaxy (BLRG) 3C 382. The long (120 ks) exposure with chandra HETG allows a detailed study of the soft X-ray continuum and of the narrow compo
nent of the Fe Kalpha line. The rxte PCA data are used to put an upper limit on the broad line component and constrain the hard X-ray continuum. A strong soft excess below 1 keV is observed in the time-averaged HETG spectrum, which can be parameterized with a steep power law or a thermal model. The flux variability at low energies indicates that the origin of the soft excess cannot be entirely ascribed to the circumnuclear diffuse emission, detected by chandra on scales of 20-30 arcsec (22-33 kpc). A narrow (sigma<90 eV) Fe Kalpha line (with EW< 100 eV) is observed by the chandra HEG. Similar values for the line parameters are measured by the rxte PCA, suggesting that the contribution from a broad line component is negligible. The fact that the exposure is split into two observations taken three days apart allows us to investigate the spectral and temporal evolution of the source on different timescales. Significant flux variability associated with spectral changes is observed on timescales of hours and days. The spectral variability is similar to that observed in radio-quiet AGN ruling out a jet-dominated origin of the X-rays.
