118624677
X-RAY OBSERVATIONAL SIGNATURE OF A BLACK HOLE ACCRETION DISK IN AN ACTIVE GALACTIC NUCLEUS RX J1633+4718
We report the discovery of a luminous ultra-soft X-ray excess in a radio-loud narrow-line Seyfert 1 galaxy, RX J1633+4718, from archival ROSAT observations. The thermal temperature of this emission, when fitted with a blackbody, is as low as 32.5(-6.0)(+8.0) eV. This is in remarkable contrast to the canonical temperatures of similar to 0.1-0.2 keV found hitherto for the soft X-ray excess in active galactic nuclei (AGNs) and is interestingly close to the maximum temperature predicted for a postulated accretion disk in this object. If this emission is indeed blackbody in nature, the derived luminosity (3.5(-1.5)(+3.3) x 10(44) erg s(-1)) infers a compact emitting area with a size (similar to 5 x 10(12) cm or 0.33 AU in radius) that is comparable to several times the Schwarzschild radius of a black hole (BH) at the mass estimated for this AGN (similar to 3 x 10(6) M(circle dot)). In fact, this ultra-steep X-ray emission can be well fitted as the (Compton scattered) Wien tail of the multi-temperature blackbody emission from an optically thick accretion disk, whose inferred parameters (BH mass and accretion rate) are in good agreement with independent estimates using the optical emission-line spectrum. We thus consider this feature as a signature of the long-sought X-ray radiation directly from a disk around a supermassive BH, presenting observational evidence for a BH accretion disk in the AGN. Future observations with better data quality, together with improved independent measurements of the BH mass, may constrain the spin of the BH.
5521201
Iron line profiles and self‐shadowing from relativistic thick accretion discs
We present Fe Ka line profiles from and images of relativistic discs with finite thickness around a rotating black hole using a novel code. The line is thought to be produced by iron fluorescence of a relatively cold X-ray-illuminated material in the innermost parts of the accretion disc and provides an excellent diagnostic of accretion flows in the vicinity of black holes. Previous studies have concentrated on the case of a thin, Keplerian accretion disc. This disc must become thicker and sub-Keplerian with increasing accretion rates. These can affect the line profiles and in turn can influence the estimation of the accretion disc and black hole parameters from the observed line profiles. We here embark on, for the first time, a fully relativistic computation which offers key insights into the effects of geometrical thickness and the sub-Keplerian orbital velocity on the line profiles. We include all relativistic effects such as frame-dragging, Doppler boost, time dilation, gravitational redshift and light bending. We find that the separation and the relative height between the blue and red peaks of the line profile diminish as the thickness of the disc increases. This code is also well suited to produce accretion disc images. We calculate the redshift and flux images of the accretion disc and find that the observed image of the disc strongly depends on the inclination angle. The self-shadowing effect appears remarkable for a high inclination angle, and leads to the black hole shadow being in this case, completely hidden by the disc itself.
18240204
POSSIBLE EVIDENCE FOR A TRUNCATED THIN DISK IN SEYFERT 1 GALAXY NGC 4593
We compile the multiband spectral energy distribution of Seyfert 1 galaxy NGC 4593 and investigate its optical-to-ultraviolet (OUV) continuum and iron Kα line profile. No strong "big blue bump" is found in the OUV band. The iron Kα line profile is not as broad as expected from a disk that extends down to the last stable orbit. Both of them can be modeled by a truncated thin disk, and they support the existence of a truncated thin disk in the system. It is of much interest that NGC 4593 may be a typical object with an accretion rate = /MEdd ~ 0.01-0.1 and harbors a two-component accretion flow: an outer thin disk and an inner hot flow—possibly an advection-dominated accretion flow, analogous to the low and intermediate state of low-mass X-ray binaries.
199684732
THE BLUESHIFTING AND BALDWIN EFFECTS FOR THE [O III] λ5007 EMISSION LINE IN TYPE 1 ACTIVE GALACTIC NUCLEI
We use homogeneous samples of radio-quiet Seyfert 1 galaxies and QSOs selected from the Sloan Digital Sky Survey to investigate the connection between the velocity shift and the equivalent width (EW) of the [O III] λ5007 emission line, and their correlations with physical parameters of active galactic nuclei (AGNs). We find a significant and negative correlation between the EW of the core component, EW(core), and the blueshift of either the core (the peak), the wing, or the total profile of [O III] emission; it is fairly strong for the blueshift of the total profile in particular. However, both quantities (EW and velocity shift) generally have only weak, if any, correlations with fundamental AGN parameters such as the nuclear continuum luminosity at 5100 A (L 5100), black hole mass (M BH), and the Eddington ratio (L/L Edd); these correlations include the classical Baldwin effect of EW(core), an inverse Baldwin effect of EW(wing), and the relationship between velocity shifts and L/L Edd. Our findings suggest that both the large object-to-object variation in the strength of [O III] emission and the blueshift-EW(core) connection are not governed primarily by fundamental AGN parameters such as L 5100, M BH, and L/L Edd. We propose that the interstellar medium conditions of the host galaxies play a major role instead in the diversity of the [O III] properties in active galaxies. This suggests that the use of [O III] λ5007 luminosity as a proxy of AGN luminosity does not depend strongly on the above-mentioned fundamental AGN parameters.
119249755
ANOMALOUSLY STEEP REDDENING LAW IN QUASARS: AN EXCEPTIONAL EXAMPLE OBSERVED IN IRAS 14026+4341
A fraction of the heavily reddened quasars require a reddening curve that is even steeper than that of the Small Magellanic Cloud. In this paper, we thoroughly characterize the anomalously steep reddening law in quasars via an exceptional example observed in IRAS 14026+4341. By comparing the observed spectrum to the quasar composite spectrum, we derive a reddening curve in the rest-frame wavelength range of 1200-10000 angstrom. It has a steep rise at wavelengths shorter than 3000 angstrom, but no significant reddening at longer wavelengths. The absence of dust reddening in the optical continuum is confirmed by the normal broad-line Balmer decrement (the Ha/Ha ratio) in IRAS 14026+4341. The anomalous reddening curve can be satisfactorily reproduced with a dust model containing silicate grains in a power-law size distribution, dn(a)/da.proportional to a(-1.4), truncated at a maximum size of a(max) = 70 nm. The unusual size distribution may be caused by the destruction of large "stardust" grains by quasar activities or a different dust formation mechanism (i.e., the in situ formation of dust grains in quasar outflows). It is also possible that the analogies of the dust grains observed near the Galactic center are responsible for the steep reddening curve. In addition, we find that IRAS 14026+4341 is a weak emission-line quasar (i.e., PHL 1811 analogies) with heavy dust reddening and blueshifted broad absorption lines.
118512944
RAPID INFRARED VARIABILITY OF THREE RADIO-LOUD NARROW-LINE SEYFERT 1 GALAXIES: A VIEW FROM THE WIDE-FIELD INFRARED SURVEY EXPLORER
Using newly released data from the Wide-field Infrared Survey Explorer, we report the discovery of rapid infrared variability in three radio-loud narrow-line Seyfert 1 galaxies (NLS1s) selected from the 23 sources in the sample of Yuan et al. J0849+5108 and J0948+0022 clearly show intraday variability, while J1505+0326 has a longer measurable timescale within 180 days. Their variability amplitudes, corrected for measurement errors, are ~0.1-0.2 mag. The detection of intraday variability restricts the size of the infrared-emitting region to ~10–3 pc, significantly smaller than the scale of the torus but consistent with the base of a jet. The three variable sources are exceptionally radio-loud, have the highest radio brightness temperature among the whole sample, and all show detected γ-ray emission in Fermi/LAT observations. Their spectral energy distributions resemble those of low-energy-peaked blazars, with a synchrotron peak around infrared wavelengths. This result strongly confirms the view that at least some radio-loud NLS1s are blazars with a relativistic jet close to our line of sight. The beamed synchrotron emission from the jet contributes significantly to and probably dominates the spectra in the infrared and even optical bands.
14854632
Three-Dimensional Simulations of the Parker Instability in a Uniformly-Rotating Disk
We investigate the effects of rotation on the evolution of the Parker instability by carrying out three-dimensional numerical simulations with an isothermal magnetohydrodynamic code. These simulations extend our previous work on the nonlinear evolution of the Parker instability by J. Kim and coworkers. The initial equilibrium system is composed of exponentially stratified gas and a field (along the azimuthal direction) in a uniform gravity (along the downward vertical direction). The computational box, placed at the solar neighborhood, is set to rotate uniformly around the Galactic center with a constant angular speed. The instability has been initialized by random velocity perturbations. In the linear stage, the evolution is not much different from that without rotation, and the mixed (undular + interchange) mode regulates the system. The interchange mode induces alternating dense and rarefied regions with small radial wavelengths, while the undular mode bends the magnetic field lines in the plane of the azimuthal and vertical directions. In the nonlinear stage, flow motion overall becomes chaotic, as in the case without rotation. However, as the gas in higher positions slides down along field lines forming supersonic flows, the Coriolis force becomes important. As oppositely directed flows fall into valleys along both sides of the magnetic field lines, they experience the Coriolis force toward opposite directions, which twists the magnetic field lines there. Hence, we suggest that the Coriolis force plays a role in randomizing the magnetic field. The three-dimensional density structure formed by the instability is still sheetlike with the short dimension along the radial direction, as in the case without rotation. However, the long dimension is now slightly tilted with respect to the mean field direction. The shape of high-density regions is a bit rounder. The maximum enhancement factor of the vertical column density relative to its initial value is about 1.5, which is smaller than that in the case without rotation. We conclude that uniform rotation does not change our point of view that the Parker instability alone is not a viable mechanism for the formation of giant molecular clouds.
18202935
Three-dimensional Evolution of the Parker Instability under a Uniform Gravity
Using an isothermal MHD code, we have performed three-dimensional, high-resolution simulations of the Parker instability. The initial equilibrium system is composed of exponentially decreasing isothermal gas and a magnetic field (along the azimuthal direction) under a uniform gravity. The evolution of the instability can be divided into three phases: linear, nonlinear, and relaxed. During the linear phase, the perturbations grow exponentially with a preferred scale along the azimuthal direction but with the smallest possible scale along the radial direction, as predicted from linear analyses. During the nonlinear phase, the growth of the instability is saturated and flow motion becomes chaotic. Magnetic reconnection occurs, which allows the gas to cross field lines. This, in turn, results in the redistribution of the gas and the magnetic field. The system approaches a new equilibrium in the relaxed phase, which is different from the one seen in two-dimensional works. The structures formed during the evolution are sheetlike or filamentary, whose shortest dimension is radial. Their maximum density enhancement factor relative to the initial value is less than 2. Since the radial dimension is too small and the density enhancement is too low, it is difficult to regard the Parker instability alone as a viable mechanism for the formation of giant molecular clouds.