Revealing the X-ray source in IRAS 13224-3809 through flux-dependent reverberation lags

IRAS 13224-3809 was observed in 2011 for 500 ks with the XMM-Newton observatory. We detect highly significant X-ray lags between soft (0.3 - 1 keV) and hard (1.2 - 5 keV) energies. The hard band lags the soft at low frequencies (i.e. hard lag), while the opposite (i.e. soft lag) is observed at high frequencies. In this paper, we study the lag during flaring and quiescent periods. We find that the frequency and absolute amplitude of the soft lag is different during high-flux and low-flux periods. During the low flux intervals, the soft lag is detected at higher frequencies and with smaller amplitude. Assuming that the soft lag is associated with the light travel time between primary and reprocessed emission, this behaviour suggests that the X-ray source is more compact during low-flux intervals, and irradiates smaller radii of the accretion disc (likely because of light bending effects). We continue with an investigation of the lag dependence on energy, and find that isolating the low-flux periods reveals a strong lag signature at the Fe K line energy, similar to results found using 1.3 Ms of data on another well known Narrow-Line Seyfert I galaxy, 1H0707-495.Comment: 6 pages, 8 figures, accepted for publication in MNRA

X-ray spectral variability of the Seyfert galaxy NGC 4051

We report on the X-ray spectral variability of the Seyfert 1 galaxy NGC 4051 observed with the Rossi X-ray Timing Explorer (RXTE) during a 1000 day period between May 1996 and March 1999. The spectra were obtained as part of monitoring observations and from two long observations using the RXTE Proportional Counter Array (PCA). During the monitoring period the 2-10 keV flux of NGC 4051 varied between 10E-12 and 7x 10E11 (cgs). We re-analysed RXTE PCA observations from a distinct low state in May 1998 using the latest background and detector response models. The RXTE and BeppoSAX observations of NGC 4051 during the low state show a very hard spectrum with a strong unresolved fluorescence line. This emission, probably due to reflection from a molecular torus, is likely to be constant over long time-scales and is therefore assumed as an underlying component at all flux states. By subtracting the torus component we are able to determine the spectral variability of the primary continuum. In the variable component we observe a strong anti-correlation of X-ray flux and spectral hardness in the PCA energy band. We show that the changes in hardness are caused by slope variability of the primary power law spectrum rather than by changing reflection or variable photoelectric absorption. The primary spectral index varies between Gamma=1.6 for the faintest states and Gamma=2.3 during the brightest states, at which level the spectral index approaches an asympotic value. We find that the response of the flux of the 6.4 keV iron fluorescence line to changes in the continuum flux depends on the timescale of the observation. The profile of the line is very broad and indicates an origin in the innermost regions of the accretion disk.Comment: accepted for publication in MNRA

The swan song: the disappearance of the nucleus of NGC 4051 and the echo of its past glory

BeppoSAX observed the low-luminous Seyfert 1 Galaxy NGC4051 in a ultra-dim X-ray state. The 2-10 keV flux (1.26 x 10^{-12} erg/cm^2/s) was about 20 times fainter than its historical average value, and remained steady along the whole observation (~2.3 days). The observed flat spectrum (\Gamma ~ 0.8) and intense iron line (EW ~600 eV) are best explained assuming that the active nucleus has switched off, leaving only a residual reflection component visible.Comment: 5 pages, Latex, 3 Postscript figures, accepted for publication in MNRA

Complex X-ray spectral behaviour of NGC 4051 in the low flux state

The Narrow Line Seyfert 1 galaxy NGC 4051 was observed in one of its prolonged low-lux states by XMM-Newton in November 2002. Here we present the results of an analysis of EPIC-pn data obtained during the observation. Within the low state, the source shows complex spectral variability which cannot easily be explained by any simple model. However, by making a `flux-flux' plot which combines the low state data with data obtained during a normal flux state, we demonstrate that the extremely hard spectrum observed above 2 keV results from a continuation of the spectral variability seen in the normal state, which is caused by spectral pivoting of the power-law continuum. The pivoting power-law appears to be attached to a Comptonised thermal component of variable flux (blackbody temperature kT~0.1 keV, consistent with the small black hole mass in NGC 4051) which dominates the soft X-ray band in the low state, and is probably the source of seed photons for Comptonisation. Additional constant thermal and reflection components, together with absorption by ionised gas, seem to be required to complete the picture and explain the complex X-ray spectral variability seen in the low state of NGC 4051.Comment: Accepted for publication in MNRAS. 13 pages, 10 figs. A higher resolution eps version of Fig. 8 is included in the source file

Variability of X-ray binaries from an oscillating hot corona

The spectral and timing properties of an oscillating hot thermal corona are investigated. This oscillation is assumed to be due to a magneto-acoustic wave propagating within the corona and triggered by an external, non specified, excitation. A cylindrical geometry is adopted and, neglecting the rotation, the wave equation is solved in for different boundary conditions. The resulting X-ray luminosity, through thermal comptonization of embedded soft photons, is then computed, first analytically, assuming linear dependence between the local pressure disturbance and the radiative modulation. These calculations are also compared to Monte-Carlo simulations. The main results of this study are: (1) the corona plays the role of a low band-pass medium, its response to a white noise excitation being a at top noise Power Spectral Density (PSD) at low frequencies and a red noise at high frequency, (2) resonant peaks are present in the PSD. Their powers depend on the boundary conditions chosen and, more specifically, on the impedance adaptation with the external medium at the corona inner boundary. (3) The flat top noise level and break as well as the resonant peak frequencies are inversely proportional to the external radius rj. (4) Computed rms and f-spectra exhibit an overall increase of the variability with energy. Comparison with observed variability features, especially in the hard intermediate states of X-ray binaries are discussed.Comment: 12 pages, 7 figures, accepted for publication in MNRA

Accretion, ejection and reprocessing in supermassive black holes

This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of active galactic nuclei. For a summary, we refer to the paper.Comment: White Paper in Support of the Mission Concept of the Large Observatory for X-ray Timin

1H0707-495 in 2011: An X-ray source within a gravitational radius of the event horizon

The Narrow Line Seyfert 1 Galaxy 1H0707-495 went in to a low state from 2010 December to 2011 February, discovered by a monitoring campaign using the X-Ray Telescope on the Swift satellite. We triggered a 100 ks XMM-Newton observation of the source in 2011 January, revealing the source to have dropped by a factor of ten in the soft band, below 1 keV, and a factor of 2 at 5 keV, compared with a long observation in 2008. The sharp spectral drop in the source usually seen around 7 keV now extends to lower energies, below 6 keV in our frame. The 2011 spectrum is well fit by a relativistically-blurred reflection spectrum similar to that which fits the 2008 data, except that the emission is now concentrated solely to the central part of the accretion disc. The irradiating source must lie within 1 gravitational radius of the event horizon of the black hole, which spins rapidly. Alternative models are briefly considered but none has any simple physical interpretation.Comment: 9 pages, 19 figures, MNRAS in pres