Suzaku observation of the Phoenix Galaxy

In recent years, several Seyfert 2 galaxies have been discovered that change state when observed in X-rays a few years apart, switching from Compton-thin to reflection-dominated or viceversa. We observed a member of this class of "Changing-look" sources, the Phoenix Galaxy, with Suzaku, with the aim of better understanding the nature of the variations. The Suzaku spectrum was analyzed, and the results compared with previous ASCA and XMM-Newton observations. The source was caught in a Compton-thin state, as in XMM-Newton, but differently from ASCA. Comparing the Suzaku and XMM-Newton observations, a variation in the column density of the absorber on a time scale of years is discovered. A similar change, but on much shorter time scales (i.e. ks) may also explain the count-rate variations during the Suzaku observations. A soft excess is also present, likely due to continuum and line emission from photoionized circumnuclear matter.Comment: Accepted for publication in Astronomy & Astrophysic

The properties of the absorbing and line emitting material in IGR J16318-4848

We have performed a detailed analysis of the XMM-Newton observation of IGR J16318-4848, to study the properties of the matter responsible for the obscuration and for the emission of Fe and Ni lines. Even if the line of sight material has a column density of about 2x10^24 cm^-2, from the Fe Kalpha line EW and Compton Shoulder we argue that the matter should have an average column density of a few x10^23 cm^-2, along with a covering factor of about 0.1-0.2. The iron Kalpha line varies on time scales as short as 1000 s, implying a size of the emitting region smaller than about 3x10^13 cm. The flux of the line roughly follows the variations of the continuum, but not exactly, suggesting a variation of the geometrical properties of the emitting region on similar time scales.Comment: accepted for publication in MNRAS, pink page

The formerly X-ray reflection-dominated Seyfert~2 galaxy NGC6300

In this paper, a BeppoSAX observation of the bright Seyfert 2 galaxy NGC6300 is presented. The rapidly variable emission from the active nucleus is seen through a Compton-thin (NH ~ 3x10^23 atomoms/cm/cm) absorber. A Compton-reflection component with an unusually high reflection fraction (R ~ 4.2), and the comparison with a reflection-dominated spectrum measured by RXTE two and half years earlier suggest that NGC6300 belongs to the class of "transient" AGN, undergoing long and repeated periods of low-activity. The spectral transition provides support to the idea that Compton-thick and Compton-thin X-ray absorbers in Seyfert 2 galaxies are decoupled, the former being most likely associated with the "torus", whereas the latter is probably located at much larger distances.Comment: 5 Latex pages, 5 figures, To appear in Monthly Notices of the Royal Astronomical Society (Letters

The flat X-ray spectrum of the LINER NGC1052

We report on ROSAT and ASCA observations of the LINER NGC1052, which is the first one where broad optical lines in polarized light have been observed. The 2-10 keV spectrum is very flat, with an observed photon index (Gamma) ~0.1. A model where a nuclear source is - partly or totally - obscured by a screen of matter with column density ~10^23 atom/cm/cm is the most convincing explanation for the observed flatness. This agrees with the hypothesis that the LINERs are a population of low-luminosity AGN, to which the Seyfert unification scenario applies. The intrinsic spectral index is still rather flat (1.0-1.4), as observed in a few type-2 Seyferts so far or predicted if the accretion occurs in an advection-dominated flow.Comment: 5 pages, Latex, 2 Postscript figures, accepted for publication in MNRA

AGN spectral states from simultaneous UV and X-ray observations by XMM-Newton

The supermassive black holes in active galactic nuclei (AGN) and stellar-mass black holes in X-ray binaries (XRBs) are believed to work in a similar way. While XRBs evolve rapidly and several sources have undergone a few complete cycles from quiescence to an outburst and back, most AGN remain in the same state over periods of decades, due to their longer characteristic timescale proportional to their size. However, the study of the AGN spectral states is still possible with a large sample of sources. Multi-wavelength observations are needed for this purpose since the AGN thermal disc emission dominates in the ultraviolet energy range, while the up-scattered hot-corona emission is detected in X-rays. We compared simultaneous UV and X-ray measurements of AGN obtained by the XMM-Newton satellite. The non-thermal flux was constrained from the 2-12 keV X-ray luminosity, while the thermal disc component was estimated from the UV flux at 2900A. The hardness (ratio between the X-ray and UV plus X-ray luminosity) and the total luminosity were used to construct the AGN state diagrams. For sources with reliable mass measurements, the Eddington ratio was used instead of the total luminosity. The state diagrams show that the radio-loud sources have on average higher hardness, due to the lack of the thermal disc emission in the UV band, and have flatter intrinsic X-ray spectra. In contrast, the sources with high luminosity and low hardness are radio-quiet AGN. The hardness-Eddington ratio diagram reveals that the average radio-loudness is stronger for low-accreting sources, while it decreases when the accretion rate is close to the Eddington limit. Our results indicate that the general properties of AGN accretion states are similar to those of X-ray binaries. This suggests that the AGN radio dichotomy of radio-loud and radio-quiet sources can be explained by the evolution of the accretion states.Comment: 13 pages, 12 figures, accepted in A&

Obscuring clouds playing hide-and-seek in the Active Nucleus H0557-385

This paper reports on two XMM-Newton observations of the Seyfert 1 Galaxy H0557-385 obtained in 2006, which show the source at an historical low flux state, more than a factor of 10 lower than a previous XMM-Newton look in 2002. The low flux spectrum presents a strong Fe Kalpha line associated to a Compton reflection continuum. An additional spectral line around 6.6 keV is required to fit Kalpha emission from Fe XXV. The spectral curvature below 6 keV implies obscuration by neutral gas with a column density of 8*10^{23}cm^{-2} partially covering the primary emission, which still contributes for a few percent of the soft X-ray emission. Absorption by ionised material on the line of sight is required to fit the deep trough below 1 keV. The comparison of the two spectral states shows that the flux transition is to be ascribed entirely to intervening line-of-sight clouds with high column density.Comment: 5 pages, accepted for publication on MNRAS Letter