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Register a new userThe X-ray variability of the Narrow-Line Seyfert 1 galaxy IRAS 13224-3809 from an XMM-Newton observation
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We report on the XMM-Newton timing properties of the most X-ray variable, radio-quiet, NLS1 IRAS 13224-3809. IRAS 13224-3809 continues to display the extremely variable behavior that was previously observed with ROSAT and ASCA; however, no giant, rapid flaring events are observed. We detect variations by a factor as high as ~8 during the 64 ks observation, and the variability is persistent throughout the light curve. Dividing the light curve into 9 minute segments we found almost all of the segments to be variable at > 3 sigma. When the time-averaged cross-correlation function is calculated for the 0.3-0.8 keV band with the 3-10 keV band, the cross-correlation profile is skewed indicating a possible smearing of the signal to longer times (soft band leading the hard). A correlation between count rate and hardness ratio is detected in four energy bands. In three cases the correlation is consistent with spectral hardening at lower count rates which can be explained in terms of a partial-covering model. The other band displays the reverse effect, showing spectral hardening at higher count rates. We can explain this trend as a more variable power-law component compared to the soft component. We also detect a delay between the 0.3-1.5 keV count rate and the 0.8-1.5 keV to 0.3-0.8 keV hardness ratio, implying flux induced spectral variability. Such delays and asymmetries in the cross correlation functions could be suggesting reprocessing of soft and hard photons. In general, much of the timing behavior can be attributed to erratic eclipsing behavior associated with the partial covering phenomenon, in addition to intrinsic variability in the source. The variability behavior of IRAS 13224-3809 suggests a complicated combination of effects which we have started to disentangle with this present analysis.
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We study the soft excess variability of the narrow line Seyfert 1 galaxy IRAS 13224-3809. We considered all five archival XMM-Newton observations, and we applied the flux-flux plot (FFP) method. We found that the flux-flux plots were highly affected by the choice of the light curves time bin size, most probably because of the fast and large amplitude variations, and the intrinsic non-linear flux--flux relations in this source. Therefore, we recommend that the smallest bin-size should be used in such cases. Hence, We constructed FFPs in 11 energy bands below 1.7 keV, and we considered the 1.7-3 keV band, as being representative of the primary emission. The FFPs are reasonably well fitted by a power-law plus a constant model. We detected significant positive constants in three out of five observations. The best-fit slopes are flatter than unity at energies below $sim 0.9$ keV, where the soft excess is strongest. This suggests the presence of intrinsic spectral variability. A power-law-like primary component, which is variable in flux and spectral slope (as $Gammapropto N_{rm PL}^{0.1}$) and a soft-excess component, which varies with the primary continuum (as $F_{rm excess}propto F_{rm primary}^{0.46}$), can broadly explain the FFPs. In fact, this can create positive `constants, even when a stable spectral component does not exist. Nevertheless, the possibility of a stable, soft--band constant component cannot be ruled out, but its contribution to the observed 0.2-1 keV band flux should be less than $sim 15$ %. The model constants in the FFPs were consistent with zero in one observation, and negative at energies below 1 keV in another. It is hard to explain these results in the context of any spectral variability scenario, but they may signify the presence of a variable, warm absorber in the source.
We present a detailed X-ray timing analysis of the highly variable NLS1 galaxy, IRAS 13224-3809. The source was recently monitored for 1.5 Ms with XMM-Newton which, combined with 500 ks archival data, makes this the best studied NLS1 galaxy in X-rays to date. We apply standard time- and Fourier-domain in order to understand the underlying variability process. The source flux is not distributed lognormally, as would be expected for accreting sources. The first non-linear rms-flux relation for any accreting source in any waveband is found, with $mathrm{rms} propto mathrm{flux}^{2/3}$. The light curves exhibit significant strong non-stationarity, in addition to that caused by the rms-flux relation, and are fractionally more variable at lower source flux. The power spectrum is estimated down to $sim 10^{-7}$ Hz and consists of multiple peaked components: a low-frequency break at $sim 10^{-5}$ Hz, with slope $alpha < 1$ down to low frequencies; an additional component breaking at $sim 10^{-3}$ Hz. Using the high-frequency break we estimate the black hole mass $M_mathrm{BH} = [0.5-2] times 10^{6} M_{odot}$, and mass accretion rate in Eddington units, $dot m_{rm Edd} gtrsim 1$. The non-stationarity is manifest in the PSD with the normalisation of the peaked components increasing with decreasing source flux, as well as the low-frequency peak moving to higher frequencies. We also detect a narrow coherent feature in the soft band PSD at $0.7$ mHz, modelled with a Lorentzian the feature has $Q sim 8$ and an $mathrm{rms} sim 3$ %. We discuss the implication of these results for accretion of matter onto black holes.
We present XMM-Newton observations of Mrk 359, the first Narrow Line Seyfert 1 galaxy discovered. Even among NLS1s, Mrk 359 is an extreme case with extraordinarily narrow optical emission lines. The XMM-Newton data show that Mrk 359 has a significant soft X-ray excess which displays only weak absorption and emission features. The (2-10) keV continuum, including reflection, is flatter than the typical NLS1, with Gamma approximately 1.84. A strong emission line of equivalent width approximately 200 eV is also observed, centred near 6.4 keV. We fit this emission with two line components of approximately equal strength: a broad iron-line from an accretion disc and a narrow, unresolved core. The unresolved line core has an equivalent width of approximately 120 eV and is consistent with fluorescence from neutral iron in distant reprocessing gas, possibly in the form of a `molecular torus. Comparison of the narrow-line strengths in Mrk 359 and other low-moderate luminosity Seyfert 1 galaxies with those in QSOs suggests that the solid angle subtended by the distant reprocessing gas decreases with increasing AGN luminosity.
We have analyzed the timing properties of the Narrow-line Seyfert 1 galaxy Mrk 766 observed with XMM-Newton during the PV phase. The source intensity changes by a factor of 1.3 over the 29,000 second observation. If the soft excess is modeled by a black body component, as indicated by the EPIC pn data, the luminosity of the black body component scales with its temperature according to L ~ T^4. This requires a lower limit black body size` of about 1.3*10^25 cm^2. In addition, we report the detection of a strong periodic signal with 2.4*10^-4 Hz. Simulations of light curves with the observed time sequence and phase randomized for a red noise spectrum clearly indicate that the periodicity peak is intrinsic to the distant AGN. Furthermore, its existence is confirmed by the EPIC MOS and RGS data. The spectral fitting results show that the black body temperature and the absorption by neutral hydrogen remain constant during the periodic oscillations. This observational fact tends to rule out models in which the intensity changes are due to hot spots orbiting the central black hole. Precession according to the Bardeen-Petterson effect or instabilities in the inner accretion disk may provide explanations for the periodic signal.
We present the results from an X-ray variability study of IRAS 13224-3809. This is probably the best source for X-ray reverberation studies since it is X-ray bright, extremely variable, and it has been extensively observed with XMM-Newton. We used all the archival XMM-Newton data from the three EPIC cameras (to increase the signal-to-noise) and, given the many observations of the source, we were able to compute the time-lags spectra in three different flux levels/periods. We fitted the time-lags and energy spectra, simultaneously, using a new X-ray reverberation code which computes the time dependent reflection spectra of the disc as a response to an X-ray flash from a point source located on the axis of the black-hole (BH) accretion disc (lamp-post geometry). To the best of our knowledge, this is the first time for an AGN that both time-lags and energy spectra are fitted by a model simultaneously in different flux periods. The model fits in the case when the BH is rapidly rotating are significantly better than the model fits in the case of a Schwarzschild BH. This result strongly favours the hypothesis of a rotating central BH in this source. We also detect significant variations in the height of the X-ray corona. The X-ray height appears to increase from 3-5 gravitational radii when the X-ray luminosity is of the order of 1.5-3 percent of the Eddington limit, up to 10 gravitational radii, when the luminosity doubles.
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