Nucleus Of Seyfert Galaxy NGC Research Articles

Changing Looks of the Nucleus of Seyfert Galaxy NGC 3516 during 2016–2020

The results of spectral observations of NGC 3516 with the 2-m telescope of the Shamakhy Astrophysical Observatory during 2016-2020 are presented. In the first half of 2016, the intensive broad component Hbeta was found, which indicates a spectral type change compared to 2014, when the broad component was almost invisible. In the second half of 2016, the broad component H${\beta}$ again was weakened and was practically not observed, remaining as weak until the end of 2019. At the end of 2019, the broad component Hbeta strengthened again, and in May 2020 reached a typical level for the high state of the object. During 2016-2020 we observed several changing looks of NGC 3516.

Variations of the X-ray INTEGRAL spectrum of the active galactic nucleus of NGC 4945

Results of 11-year-long X-ray INTEGRAL observations of the nucleus of Seyfert galaxy NGC 4945 in the 3–500 keV range were processed. A two-component spectrum model, which includes strong radiation absorption in the Compton-thick torus around the AGN “central engine” and secondary radiation reflected from the torus walls, was used in the analysis. The following primary spectrum parameters were determined based on the data accumulated throughout the entire exposure period: photon index Γ = 1.60 ± 0.07, exponential cutoff energy Ec =157-22+29 keV, and column density of the medium that absorbs primary radiation NH,1 =5.0-0.9+1.0 × 1024 cm–2. The column density of the medium absorbing reflected radiation is two orders of magnitude lower. Both the X-ray flux in the ranges of 20–40, 40–60, and 60–100 keV and the shape of the X-ray spectrum of NGC 4945 vary. The spectrum shape variations may be induced by inhomogeneities of the absorbing medium surrounding the AGN. At the same time, there is some evidence for moderate spectrum variations in the highenergy region, which may be associated with changes in the “central engine.”

Infrared Spectroscopy of NGC 4151: Probing the Obscured Ionizing Active Galactic Nucleus Continuum

The Infrared Space Observatory (ISO) Short-Wavelength Spectrometer (SWS) infrared spectroscopic observations of the nucleus of Seyfert galaxy NGC 4151, which are described in a companion paper, are used in conjunction with a compilation of UV to IR narrow emission line data to determine the spectral shape of the obscured extreme-UV continuum that photoionizes the narrow-line-emitting gas in the active galactic nucleus (AGN). We present a new method for determining the best-fitting photoionizing continuum and emission-line cloud model from a heterogeneous set of emission-line data. For NGC 4151, we find a best-fit model that reproduces the observed line fluxes to within a factor of 2 on average, and that is consistent with the observed geometry of the optical narrow-line region (NLR). Our model consists of a clumpy, optically thick (ionization-bounded) gas distribution, with a hydrogen gas density of ~1000 cm-3 and a volume-filling factor of 6.510-4. Our best-fitting spectral energy distribution (SED) falls sharply beyond the Lyman limit and then rises sharply again toward 100 eV. In particular, it does not display the blue signature of a hot accretion disk. We find that this SED, which best reproduces the NLR line emission on the 100-500 pc scale, does not have enough UV photons to produce the observed broad-line region (BLR) recombination emission from the inner 1 pc. This suggests that the BLR is photoionized by the intrinsic continuum source, which does have a strong UV component (perhaps a big blue bump), but that this UV component is absorbed by material located between the NLR and BLR. Our analysis suggests that the absorber consists of ~51019 cm-2 of neutral hydrogen. Such an absorber has been independently detected by UV absorption lines. Using our new method, we confirm our previous conclusion that a big blue bump is present in the SED of the Seyfert 2 galaxy Circinus.

The gas kinematics in the nucleus of seyfert galaxy NGC 5548

A modeling of the response of the λ 1549 CIV emission line profile in the spectrum of Seyfert galaxy NGC 5548 to a sharp continuum drop which happened in 1989 is carried out on the basis of observational data taken from the literature. A more rapid response of the red wing of CIV profile suggesting the accretion of CIV-emitting clouds in the broad-line region (BLR) is reproduced. It is shown that the accretion velocity near the central object is consistent with a free fall law whereas at large distances from the center it varies approximately asr−2. The best agreement between observed CIV profile changes and modeled ones is obtained when the mass of the central objectM ≃ 1.2 × 108M⊙. The first approximation of the BLR geometry is also found: the inflow takes place only outside two wide cones with half-opening angle ≃ 65° which are disposed like a sand-glass and filled by outflowing material. The angle between the axis of such biconical structure and our line of sight is found to be small (≃ 9°). It is shown that the outer radius of an accretion disk around the central object does not exceed ~ I light-day (i.e., < 100 Schwarzschild radii), and the outer radius of “CIV-emitting” BLR is ≃ 14 light-days. The nature of a broad blue excess in the CIV profile which did not respond to the continuum change is discussed.

A study of emission-line variability in the nucleus of Seyfert galaxy NGC 3516 in 1986-1988

The TV scanner of the 6m telescope was used in March 1986 and in February- March 1988 to observe the nucleus of the Seyfert galaxy NGC 3516. The data obtained were processed making allowance for the contribution of the absorption spectrum of host galaxy which proved to be of the order of 70% in the region of Hβ line. The broad He II 4686 A and Fe II (multiplets 37 and 38) lines have been found. Within two years, the Hβ, Hγ, and He II 4686 A line intensities changed the intensity ratio of the Hγ and Hβ broad components increased by a factor of 2, and the He II 4686 A broad component enhanced. The [O III] 4363 A line was not observed in 1986- 1988.

On the H?, [OIII] lines and continuum variability character in the nucleus of Seyfert galaxy NGC 1275

The radiation fluxes of the NGC 1275 galaxy central region are being observed on the 1.25-m telescope, using a scanning spectrophotometer with the entrance aperture 10″ in three Δλ=80 A spectral regions: Hβ, 4959+5007 A [OIII] and continuum. There were 35 nights of observations during 1982–1987. With the time resolution of half an hour 379 measurements were obtained in each spectral region. The analysis of these results shows: (1) The standard deviations of measurements in each spectral region 2–3 times exceed the errors of observations. (2) The radiation flux distribution resembles to normal one only for Hβ line. (3) Two-humps forms of continuum flux distribution curve is like that of radio emission in 8 mm and 2.6 cm wavelengths. (4) Various forms of fluxes distribution curves of Hβ and [OIII] lines permit us to suppose that the location of these lines emission regions near the sources of excitation are different.

Variability of the Emission Line Spectrum of the Nucleus of Seyfert Galaxy NGC 1275

Recent spectroscopic observations of the nucleus of NGC 1275, together with results from earlier workers, confirm the variability of the emission lines and indicate corresponding variations in the electron temperatures and densities of different zones. These changes may be associated with the microwave outbursts from this source.