I. E. Papadakis

Optical and radio behaviour of the BL Lacertae object 0716+714

Eight optical and four radio observatories have been intensively monitoring the BL Lac object 0716+714 in the last years: 4854 data points have been collected in the UBVRI bands since 1994, while radio light curves extend back to 1978. Many of these data, which all together constitute the widest optical and radio database available on this object, are presented here for the first time. Four major optical outbursts were observed at the beginning of 1995, in late 1997, at the end of 2000, and in fall 2001. In particular, an exceptional brightening of 2.3 mag in 9 days was detected in the R band just before the BeppoSAX pointing of October 30, 2000. A big radio outburst lasted from early 1998 to the end of 1999. The long-term trend shown by the optical light curves seems to vary with a characteristic time scale of about 3.3 years, while a longer period of 5.5–6 years seems to characterize the radio long-term variations. In general, optical colour indices are only weakly correlated with brightness; a clear spectral steepening trend was observed during at least one long-lasting dimming phase. Moreover, the optical spectrum became steeper after

X-ray variability of active galactic nuclei : a universal power spectrum with luminosity-dependent amplitude

\rm JD \sim 2\,451\,000

Revisiting algorithms for generating surrogate time series

, the change occurring in the decaying phase of the late-1997 outburst. The radio flux behaviour at different frequencies is similar, but the flux variation amplitude decreases with increasing wavelength. The radio spectral index varies with brightness (harder when brighter), but the radio fluxes seem to be the sum of two different-spectrum contributions: a steady base level and a harder-spectrum variable component. Once the base level is removed, the radio variations appear as essentially achromatic, similarly to the optical behaviour. Flux variations at the higher radio frequencies lead the lower-frequency ones with week–month time scales. The behaviour of the optical and radio light curves is quite different, the broad radio outbursts not corresponding in time to the faster optical ones and the cross-correlation analysis indicating only weak correlation with long time lags. However, minor radio flux enhancements simultaneous with the major optical flares can be recognized, which may imply that the mechanism producing the strong flux increases in the optical band also marginally affects the radio one. On the contrary, the process responsible for the big radio outbursts does not seem to affect the optical emission.

Temporal and spectral variability of AGN with RXTE

We present power-law model fits to the power spectra of 12 high-quality long look observations of AGNs from the EXOSAT data base. The AGNs concerned differ in X-ray luminosity L x over a range 10 4 . The results are consistent with all objects having a power spectrum index equal to the mean value α=1.55. There is no correlation with L x , suggesting that the power spectrum has little curvature over many decades of frequency. The mean slope is inconsistent with both standard shot-noise processes and traditional 1/f noise but close to the prediction of simple rotating hot spot models. The power spectrum amplitude shows large object-to-object scatter but varies systematically as L x -0.5 . This is inconsistent with any fixed-shape shot-noise model

The NGC 6426 RR Lyrae Variables and Horizontal-Branch Morphology

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 with Fourier phase correlations leading to nondetections of nonlinearities. We argue that reliable surrogates can only be generated, if one tests separately for static and dynamic nonlinearities.

The Age of NGC 6426, a Metal-poor Globular Cluster in the Galactic Halo

We present RXTE PCA observations of temporal and spectral variability of the four X-ray bright Seyfert galaxies NGC 4051, NGC 5506, NGC 5548 and MGC-6-30-15 covering a period of 18 months from February 1996. We find evidence of a knee in the long timescale power spectrum of NGC 4051 at a frequency of 5×10−5 Hz. There is possible evidence of a knee at a slightly lower frequency in the higher luminosity source MCG-6-30-15 but no knees have been found in the still higher luminosity sources NGC 5506 and NGC 5548. If the knee frequency scales inversely with the central black hole mass then, by comparison with galactic black hole X-ray binaries, we deduce a mass of approximately 105M⊙ in NGC 4051. Strong continuum reflection components and iron fluorescent lines are seen in all four galaxies and we find evidence of strong X-ray spectral variability in all four galaxies. Here we discuss the spectra of NGC 5506 and MCG-6-30-15 as NGC 4051 is discussed elsewhere in these proceedings [1] and a possibly confusing BL Lac lies within the PCA field of view of NGC 5548. In both NGC 5506 and MCG-6-30-15 we find strong evidence for a steeping of the continuum spectral slope as the intensity increases. In MCG-6-30-15 the iron line flux increases only weakly as the continuum flux rises and, indeed, the iron line flux is consistent with being constant. In NGC 5506, however, there is an approximately linear relationship between the iron line flux and continuum flux with only weak evidence for an additional steady component. One explanation, although by no means confirmed, of the spectral index variability and relative quiescence of the iron line flux is that, in addition to rapidly varying steep spectrum continuum emission, there is also a harder reflected component from the surrounding torus. The torus component would not vary on the timescales of our observations and would dominates at very low flux levels. The difference between the variability of NGC 5506 and MCG-6-30-15 may be a lesser contribution from the torus in NGC 5506.

X-ray spectral variability of NGC 4051

We present BVRI CCD photometry for 12 RR Lyrae variables, including three newly discovered ones in the Oosterhoff type II globular cluster NGC 6426. New light curves and ephemerides are presented. The mean periods of the RRab and RRc variables whose light curves are analyzed in this work are Pab = 0.70 ± 0.02 days and Pc = 0.34 ± 0.03 days, respectively. The number ratio of the RRc type variables to the total number of RR Lyrae type variables is n(c)/n(ab + c) = 0.36. The period-amplitude relation for fundamental-mode RR Lyrae variables (RRab) in NGC 6426 supports the recent conclusion of Clement & Shelton that this relation is not a function of metal abundance. Fourier decomposition of the light curves has been used to determine the mass, luminosity, and temperature for the RRc stars. Application of the formula of Jurcsik & Kovacs, which relates Fourier parameters of RRab stars to [Fe/H], yielded the value [Fe/H]= -2.16 ± 0.13 dex, which is compatible (given the errors) with the value -2.33 ± 0.15 dex, derived from the red giant branch color index developed by Hatzidimitriou et al. From star counts along the horizontal branch (HB), we obtain the Lee et al. HB color distribution index, (B-R)/(B+V+R) = 0.58 ± 0.18. The HB of NGC 6426 resembles that of NGC 5053 and M68. We find almost no faint blue stars analogous to the ones constituting the extended HB in M15.

Improved methods for power spectrum modelling of red noise

We present B, V, R, and I photometry of the metal-poor globular cluster in the galactic halo, NGC 6426. The observations were performed using the 1.3 m Telescope at Skinakas Observatory in Crete. We derived the reddening of the cluster to be E(B-V) = 0.39 ± 0.02 [E(V-I) = 0.53 ± 0.03]. The metal abundance of the cluster was estimated from the shape of the red giant branch following the techniques by Sarajedini and by Da Costa & Armandroff. It was found to be [Fe/H] = -2.33 ± 0.15 dex. The mean V magnitude of the RR Lyrae variables found in the cluster is 18.14 ± 0.02 (based on a paper in preparation). The resulting distance modulus of the cluster is 16.41 ± 0.07. Finally, we derived the relative age of NGC 6426. Using the methodology of Harris et al., we found that NGC 6426 is marginally older than M92 (by 0.7 Gyr). Following the Chaboyer, Demarque, & Sarajedini method we found NGC 6426 to have the same age as the mean metal-poor globular cluster of the galactic halo.

A detailed X-ray variability study of the Seyfert galaxy NGC 4051

Abstract We present results from RXTE and EUVE observations of the bright, highly variable Seyfert 1 galaxy, NGC 4051. Spectral analysis of RXTE data obtained from monitoring observations and a ‘long look’ (∼ 75 ksec) shows: a) strong spectral slope variability in the medium energy X-ray band, b) low energy absorption which increases with decreasing source flux, c) an ∼ 6 keV iron line which remains effectively constant despite continuum flux changes in excess of factor 10. Simultaneous RXTE and EUVE observations confirm the slope variability and also imply the presence of a constant ‘soft-excess’. We discuss the implications of these results for models of iron line and soft-excess production in NGC 4051.