Simultaneous X-ray and ultraviolet spectroscopy of the Seyfert galaxy NGC�5548 (original) (raw)
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Journal of Astrophysics and Astronomy, 2021
We present a detailed timing and spectral study of an extremely variable narrow-line Seyfert 1 galaxy NGC 4748 using observations in the year 2017 and 2014 performed with AstroSat and XMM-Newton, respectively. Both observations show extremely variable soft and hard X-ray emission that are correlated with each other. In the 2014 data set, the source retains its general behaviour of "softer when brighter" while the 2017 observation exhibits a "harder when brighter" nature. Such changing behaviour is rare in AGNs and is usually observed in the black hole binary systems. The "harder when brighter" is confirmed with the anti-correlation between the photon index and the 0.3-10 keV power-law flux. This suggests a possible change in the accretion mode from standard to the advection-dominated flow. Additionally, both the observations show soft X-ray excess below 2 keV over the power-law continuum. This excess was fitted with a single or multiple blackbody component(s). The origin of soft excess during the 2017 observation is likely due to the cool Comptonization as the photon index changes with time. On the other hand, the broad iron line and delayed UV emission during the 2014 observation strongly suggest that X-ray illumination onto the accretion disk and reflection and reprocessing play a significant role in this AGN.
Simultaneous X-ray and UV spectroscopy of the Seyfert galaxy NGC�5548
Astronomy and Astrophysics, 2005
We present the results from a 500 ks Chandra observation of the Seyfert 1 galaxy NGC 5548. We detect broadened (full width half maximum = 8000 km s −1 ) emission lines of O VII and C VI in the spectra, similar to those observed in the optical and UV bands. The source was continuously variable, with a 30 % increase in luminosity in the second half of the observation. The gradual increase in luminosity occurred over a timescale of ∼ 300 ks. No variability in the warm absorber was detected between the spectra from the first 170 ks and the second part of the observation. The longer wavelength range of the LETGS resulted in the detection of absorption lines from a broad range of ions, in particular of C, N, O, Ne, Mg, Si, S and Fe. The velocity structure of the X-ray absorber is consistent with the velocity structure measured simultaneously in the ultraviolet spectra. We find that the highest velocity outflow component, at −1040 km s −1 , becomes increasingly important for higher ionization parameters. This velocity component spans at least three orders of magnitude in ionization parameter, producing both highly ionized X-ray absorption lines (Mg XII, Si XIV) as well as UV absorption lines. A similar conclusion is very probable for the other four velocity components. Based upon our observations, we argue that the warm absorber probably does not manifest itself in the form of photoionized clumps in pressure equilibrium with a surrounding wind. Instead, a model with a continuous distribution of column density versus ionization parameter gives an excellent fit to our data. From the shape of this distribution and the assumption that the mass loss through the wind should be smaller than the accretion rate onto the black hole, we derive upper limits to the solid angle as small as 10 −4 sr. From this we argue that the outflow occurs in density-stratified streamers. The density stratification across the stream then produces the wide range of ionization parameter observed in this source. We determine an upper limit of 0.3 M⊙ yr −1 for the mass loss from the galaxy due to the observed outflows.
XMM-Newton long-look observation of the narrow line Seyfert 1 galaxy PKS 0558-504
Astronomy and Astrophysics, 2010
Context. PKS 0558-504 has been observed repeatedly by XMM-Newton as a calibration and performance verification (PV) target.In this work, we present results from the spectral analysis of a long XMM-Newton observation of the radio loud Narrow Line Seyfert 1 galaxy PKS 0558-504. Aims. To study the soft excess component in this object, the spectral variations it exhibits in both the hard and soft X-ray bands, and their correlation. Methods. We used mainly the PN data, and we fitted various spectral models to the time average spectra of the individual orbits as well as the spectra from data segments of shorter duration. We also used the RGS data to search for signs of a warm absorber in the source. Results. The source is highly variable, on all sampled time scales. We did not observe any absorption features in either the soft or hard band. We found weak evidence for the presence of an iron line at ∼ 6.8 keV, which is indicative of emission from highly ionized iron. The 2-10 keV band spectrum of the source is well fitted by a simple power law model, whose slope steepens with increasing flux, similar to what is observed in other Seyferts as well. The soft excess is variable both in flux and shape, and it can be well described by a low-temperature Comptonisation model, whose slope flattens with increasing flux. Finally, the soft excess flux variations are moderately correlated with the hard band variations, and we found weak evidence that they are leading them by ∼ 20 ksec. Conclusions. Our results rule out a jet origin for the bulk of the X-ray emission in this object. We found no signals of a warm absorber. The observed hard band spectral variations suggest intrinsic continuum slope variations, caused by changes in the "heating/cooling" ratio of the hot corona. The low-temperature Comptonising medium, responsible for the soft excess emission, could be a hot layer in the inner disc of the source, which appears due to the fact that the source is accreting at a super-Eddington rate. The soft excess flux and spectral variations could be caused by random variations of the accretion rate.
The changing X-ray period of the Seyfert galaxy IRAS18325-5926
1998
We report on two observations of the Seyfert galaxy IRAS18325-5926 made in 1997 December and 1998 February with the Rossi X-ray Timing Explorer (RXTE). We find evidence for periodicities in the resulting X-ray lightcurves which are shorter than the 58~ks period found from data of the source taken in 1997 March with the imaging satellite ASCA. It is therefore likely that IRAS18325-5926 has a quasi-periodic oscillation (QPO) similar to, but at a much longer period than, the QPO seen in some Galactic Black Hole Candidates. The power spectrum of the February data has several peaks, the second highest of which is consistent with a monotonic decrease in the X-ray period. The period change is then consistent with that expected from two massive black holes spiralling together due to the emission of gravitational radiation. This possibility is very unlikely but mentioned because of its potential importance.
Frequency-dependent Time Lags in the X-Ray Emission of the Seyfert Galaxy NGC 7469
The Astrophysical Journal, 2001
We report the discovery of time lags in the cross-spectra of the X-ray light curves of the Seyfert galaxy NGC 7469. This behavior is common in Galactic black hole and neutron star binaries, and is in the sense that harder X-rays are delayed with respect to the soft with a time lag approximately proportional to the Fourier period. At the longest period probed by our observation (∼ 6 days) we find a time lag of approximately 3.5 hours between the 2-4 and 4-10 keV X-rays. A similar lag and period dependence is found comparing the 2-4 and 10-15 keV light curves, albeit with less significance. We find the coherence function of the light curves to be close to 1 in the frequency range between 10 −5.5 − 10 −3.5 Hz. The implications of these results for the X-ray production mechanism in active galactic nuclei (AGN) is discussed.
Modeling time delays in the X-ray spectrum of the Seyfert galaxy MCG-6-30-15
Astronomy and Astrophysics, 2007
We propose a reflection model of the time delays detected during an exceptionally bright, single flare in MCG-6-30-15. We consider a scenario in which the delays of the hard X-rays with respect to the soft X-rays are caused by the presence of the delayed reflection component. Methods. We employ a model of the flare, which is accompanied by reprocessed emission. We consider two geometries/thermal states of the reprocessing medium: a partially ionized accretion disk surface and a distribution of magnetically confined, cold blobs. Results. The reprocessing by cold blobs predicts positive time delays and a saturation in the time delay -energy relation, which is likely present in the data. The model requires a strong reflection component and relies on the apparent pivoting of the combined primary and reflected spectrum. The reflection by the ionized disk surface does not reproduce the observed delays. We discuss the relation between the two reflection scenarios and argue that they are both present in MCG-6-30-15.
X-Ray Reverberation Mapping and Dramatic Variability of Seyfert 1 Galaxy 1H 1934-063
The Astrophysical Journal
A fraction of active galactic nuclei (AGN) exhibit dramatic variability, which is observed on timescales down to minutes in the X-ray band. We introduce the case study of 1H 1934-063 (z = 0.0102), a Narrow-line Seyfert 1 (NLS1) among the brightest and most variable AGN ever observed with XMM-Newton. This work includes spectral and temporal analyses of a concurrent XMM-Newton and NuSTAR 2015 observation lasting 130 kiloseconds, during which the X-ray source exhibited a steep (factor of ∼6) plummet and subsequent full recovery of flux level, accompanied by deviation from a single log-normal flux distribution. We rule out Compton-thin obscuration as the cause for this dramatic variability observed even at NuSTAR energies. In order to constrain coronal geometry, dynamics, and emission/absorption processes, we compare detailed spectral fitting with Fourier-based timing analysis. Similar to other well-studied, highly variable Seyfert 1s, this AGN is X-ray bright and displays strong reflection features. We find a narrower broad iron line component compared to most Seyfert 1s, and constrain black hole spin to be < 0.1, one of the lowest yet discovered for such systems. Combined spectral and timing results are consistent with a dramatic change in the continuum on timescales as short as a few kiloseconds dictating the nature of this variability. We also discover a Fe-K time lag, measuring a delay of 20 seconds between relativistically-blurred reflection off the inner accretion flow and the hard X-ray continuum emission.
X‐Ray and Optical Variability in NGC 4051 and the Nature of Narrow‐Line Seyfert 1 Galaxies
The Astrophysical Journal, 2000
We report on the results of a three-year program of coordinated X-ray and optical monitoring of the narrow-line Seyfert 1 galaxy NGC 4051. The rapid continuum variations observed in the X-ray spectra are not detected in the optical, although the time-averaged X-ray and optical continuum fluxes are well-correlated. Variations in the flux of the broad Hβ line are found to lag behind the optical continuum variations by 6 days (with an uncertainty of 2-3 days), and combining this with the line width yields a virial mass estimate of ∼ 1.1 × 10 6 M ⊙ , at the very low end of the distribution of AGN masses measured by line reverberation. Strong variability of He ii λ4686 is also detected, and the response time measured is similar to that of Hβ, but with a much larger uncertainty. The He ii λ4686 line is almost five times broader than Hβ, and it is strongly blueward asymmetric, as are the high-ionization UV lines recorded in archival spectra of NGC 4051. The data are consistent with the Balmer lines arising in a low to moderate inclination disk-like configuration, and the high-ionization lines arising in an outflowing wind, of which we observe preferentially the near side. Previous observations of the narrow-line region morphology of this source suggest that the system is inclined by ∼ 50 o , and if this is applicable to the broad Hβ-emitting region, a central mass of ∼ 1.4 × 10 6 M ⊙ can be inferred. During the third year of monitoring, both the X-ray continuum and the He ii λ4686 line went into extremely low states, although the optical continuum and the Hβ broad line were both still present and 6 Deceased.
Astrophys J Suppl Ser, 2006
We present a detailed analysis of the intrinsic X-ray absorption in the Seyfert 1 galaxy NGC 4151 using Chandra/HETGS data obtained 2002 May, as part of a program which included simultaneous UV spectra using HST/STIS and FUSE. NGC 4151 was in a relatively low flux state during the observations reported here, although roughly 2.5 times as bright in the 2 --10 keV band as during a Chandra observation in 2000. The soft X-ray band was dominated by emission lines, which show no discernible variation in flux between the two observations. The 2002 data show the presence of a very highly ionized absorber, in the form of H-like and He-like Mg, Si, and S lines, as well as lower ionization gas via the presence of inner-shell absorption lines from lower-ionization species of these elements. The former is too highly ionized to be radiatively accelerated in a sub-Eddington source such as NGC 4151. We find that the lower ionization gas had a column density a factor of ~ 3 higher during the 2000 observation. If due to bulk motion, we estimate that this component must have a velocity of more than 1250 km/sec transverse to our line-of-sight. We suggest that these results are consistent with a magneto-hydrodynamic flow.