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Register a new userXMM-Newton study of the complex and variable spectrum of NGC 4051
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Authors
G. Ponti
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We study the X-ray spectral variability of the Narrow Line Seyfert 1 galaxy NGC 4051 as observed during two XMM-Newton observations. The data show evidence for a neutral and constant reflection component and for constant emission from photoionized gas, which are included in all spectral models. The nuclear emission can be modelled both in terms of a ``standard model (pivoting power law plus a black body component for the soft excess) and of a two--component one (power law plus ionized reflection from the accretion disc). The standard model results indicate that the soft excess does not follow the standard black body law. Moreover, although the spectral slope is correlated with flux, which is consistent with spectral pivoting, the hardest photon indexes are so flat as to require rather unusual scenarios. These problems can be solved in terms of the two-component model in which the soft excess is not thermal, but due to the ionized reflection component. The variability of the reflection component from the inner disc closely follows the predictions of the light bending model, suggesting that most of the primary nuclear emission is produced in the very innermost regions, only a few gravitational radii from the central black hole. (abridged)
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Archival XMM-Newton data on the nearby Seyfert galaxy NGC 4051, taken in relatively high and low flux states, offer a unique opportunity to explore the complexity of its X-ray spectrum. We find the hard X-ray band to be significantly affected by reflection from cold matter, which can also explain a non-varying, narrow Fe K fluorescent line. We interpret major differences between the high and low flux hard X-ray spectra in terms of the varying ionisation (opacity) of a substantial column of outflowing gas. An emission line spectrum in the low flux state indicates an extended region of photoionised gas. A high velocity, highly ionised outflow seen in the high state spectrum can replenish the gas in the extended emission region over ~10^3 years, while having sufficient kinetic energy to contribute significantly to the hard X-ray continuum.
We present a detailed analysis of the hard X-ray (>2.5 keV) EPIC spectra from the first observations of NGC 4151 made by XMM-Newton. We fit the spectra with a model consisting of a power-law continuum modified by line-of-sight absorption (arising in both partially photoionized and neutral gas) plus additional iron-K emission and absorption features. This model provides an excellent overall fit to the EPIC spectra. The iron K-alpha line is well modelled as a narrow Gaussian component. In contrast to several earlier studies based on data from ASCA, a relativistically broadened iron K-alpha emission feature is not required by the XMM-Newton data. The upper limit on the flux contained in any additional broad line is ~8% of that in the narrow line. The measured intrinsic line width (sigma =32+_7 eV) may be ascribed to (i) the doublet nature of the iron K-alpha line and (ii) emission from low ionization states of iron, ranging from neutral up to ~FeXVII. The additional iron absorption edge arises in cool material and implies factor ~2 overabundance of iron in this component.
We present timing and spectral analysis of the data collected by the Extreme Ultraviolet Explorer Satellite (EUVE) for the Seyfert 1 galaxy NGC 4051 during 1996. NGC 4051 was observed twice in May 1996 and again in December 1996 for a total of more than 200 ks. The observations were always simultaneous with hard X-ray observations conducted with the Rossi X-Ray Timing Explorer (RXTE). The EUVE light curves are extremely variable during each observation, with the maximum variability of more than a factor of 15 from peak to minimum. We detected signal in the EUVE spectrograph in the 75-100 A range which is well fitted by absorbed power law models. We illustrate the results of our spectral and detailed power spectrum analysis performed on EUVE data and the comparison with RXTE lightcurves and discuss the constraint we can place on the mass of the central object and on the size of the emitting region.
We report on the nuclear X-ray properties of the radio galaxy NGC 6251 observed with XMM-Newton. NGC 6251 is a well-known radio galaxy with intermediate FRI/II radio properties. It is optically classified as a Seyfert 2 and hosts a supermassive black hole with mass~6e8 solar masses. The 0.4-10 keV EPIC pn continuum is best fitted by two thermal components (kT~0.5 and 1.4 keV, respectively), plus a power law with photon index ~1.9 absorbed by a column density NH~5e20 cm-2. We confirm the previous ASCA detection of a strong iron line. The line, resolved in the EPIC pn spectrum, is adequately fitted with a broad (sigma~0.6 keV) Gaussian at rest-frame energy 6.4 keV with EW 220 eV. We also detect, for the first time, short-term, low-amplitude variability of the nuclear flux on a timescale of a few ks. The spectral properties argue in favor of the presence of a standard accretion disk, ruling out the base of the jet as the sole origin of the X-rays. The moderate X-ray luminosity and lack of strong intrinsic absorption suggest that NGC 6251 is a ``pure type 2 AGN which lacks a broad-line region.
We present the analysis of multi-wavelength XMM-Newton data from the Seyfert galaxy NGC 3783, including UV imaging, X-ray and UV lightcurves, the 0.2-10 keV X-ray continuum, the iron K-alpha emission line, and high-resolution spectroscopy and modelling of the soft X-ray warm absorber. The 0.2-10 keV spectral continuum can be well reproduced by a power-law at higher energies; we detect a prominent Fe K-alpha emission line, with both broad and narrow components, and a weaker emission line at 6.9 keV which is probably a combination of Fe K-beta and Fe XXVI. We interpret the significant deficit of counts in the soft X-ray region as being due to absorption by ionised gas in the line of sight. This is demonstrated by the large number of narrow absorption lines in the RGS spectrum from iron, oxygen, nitrogen, carbon, neon, argon, magnesium, silicon and sulphur. The wide range of iron states present in the spectrum enables us to deduce the ionisation structure of the absorbing medium. We find that our spectrum contains evidence of absorption by at least two phases of gas: a hotter phase containing plasma with a log ionisation parameter xi (where xi is in erg cm/s) of 2.4 and greater, and a cooler phase with log xi centred around 0.3. The gas in both phases is outflowing at speeds of around 800 km/s. The main spectral signature of the cold phase is the Unresolved Transition Array (UTA) of M-shell iron, which is the deepest yet observed; its depth requires either that the abundance of iron, in the cold phase, is several times that of oxygen, with respect to solar abundances, or that the absorption lines associated with this phase are highly saturated. The cold phase is associated with ionisation states that would also absorb in the UV.
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