Ultraviolet Bump Research Articles

Spectroscopic investigation of interstellar hydrogenated carbon clusters

Context. The assignment of the mid-infrared (mid-IR) emission features and plateaus observed in C-rich and H-rich regions of the interstellar medium (ISM) is still debated. Such mid-IR assignments must also be related to their contribution to the extinction curve in our galaxy and to the ultraviolet (UV) bump. Aims. The aim of this work is to investigate the influence of hydrogenation rate on the mid-IR spectra of populations of carbon clusters in order to constraint the nH/nC ratios in regions of the ISM where carbon is an important component. Their potential contribution to the extinction curve and in particular to the UV bump is also investigated. Methods. The absorption IR and optical spectra of tens of thousands of C24Hn (n = 0, 6, 12, 18, 24) isomers classified into structural families – namely flakes, branched, pretzels, and cages – were computed using the density functional based tight binding electronic structure method and its time-dependent version, respectively. Final spectra were obtained by averaging the spectra of many individual isomers. Results. The shapes and the relative intensities of the bands centered at ~3.25 and 3.40 µm and assigned to the C–H stretch of sp2 and sp3 carbon atoms, respectively, present a clear dependence on the nH/nC ratio. From a comparison with the astronomical spectrum from the Orion bar H2S1, the most interesting emitting candidates would pertain to the flakes population; this is the most energetically favorable family of clusters, possessing a high content of five and six carbon rings and being mostly planar, with no sp1 carbon atoms and with a nH/nC ratio of lower than 0.5. The same conclusion is drawn when comparing the computed IR features in the [4–20 µm] region with the observed plateaus from some C-rich and H-rich planetary nebulae objects of the Small Magellanic Cloud. The contribution of the same family could be considered for the UV bump. When nH/nC increases, only a contribution to the high-energy part of the continuum due to σ → π✶ excitations can reasonably be considered. Conclusions. These results bring some constraints on the structural features and on the nH/nC ratio of the hydrogenated carbon populations emitting in the mid-IR domain in interstellar objects such as protoplanetary and reflection nebulae. The flakes population, with a low nH/nC ratio, is an interesting candidate for the carbon population emitting in these regions, but not for that absorbing in the diffuse ISM. None of the populations studied in the present work can account for the UV bump, but they would contribute to a broad extinction rise in this domain. The computed features reported in this article could be used to interpret future astronomical data provided by the James Webb Space Telescope.

NGC 3147: a prototypical low-luminosity active galactic nucleus with double-peaked optical and ultraviolet lines

ABSTRACT A previous narrow-slit (0.1 arcsec) Hubble Space Telescope observation unveiled a broad relativistic Hα profile in NGC 3147, a low-luminosity (Lbol ∼ 1042 erg s−1), low-Eddington ratio (Lbol/LEdd ∼ 10−4) active galactic nucleus (AGN), formerly believed to be a candidate true type 2 AGN intrinsically lacking the broad-line region. The new observations presented here confirm the double-peaked profile of the Hα line, which further shows variability both in flux and in the inner radius of the emitting disc with respect to the previous epoch. Similar disc line profiles are also found in prominent ultraviolet (UV) lines, in particular Lyα and C iv. The new data also allow us to build a simultaneous subarcsec optical-to-X-ray spectral energy distribution of NGC 3147, which is characterized by the absence of a thermal UV bump, and an emission peak in the X-rays. The resulting very flat αox = −0.82 is typical of low-luminosity AGNs, and is in good agreement with the extrapolation to low luminosities of the well-known trend with luminosity observed in a standard AGN. Indeed, we are possibly observing the accretion disc emission in NGC 3147 in the optical, close to the expected peak. On the other hand, the steep −2 UV power law may be Comptonization of that cold disc by a warm corona, what is instead generally observed as a ‘soft excess’ in a more luminous AGN.

Relations between Mid-infrared Dust Emission and UV Extinction

Abstract We analyze low-resolution Spitzer infrared (IR) 5−14 μm spectra of the diffuse emission toward a carefully selected sample of stars. The sample is composed of sight lines toward stars that have well-determined ultraviolet (UV) extinction curves and that are shown to lie beyond effectively all of the extinguishing and emitting dust along their lines of sight. Our sample includes sight lines whose UV curve extinction curves exhibit a wide range of curve morphology and that sample a variety of interstellar environments. As a result, this unique sample enabled us to study the connection between the extinction and emission properties of the same grains, and to examine their response to different physical environments. We quantify the emission features in terms of the polycyclic aromatic hydrocarbon (PAH) model given by Draine & Li and a set on additional features not known to be related to PAH emission. We compare the intensities of the different features in the Spitzer mid-infrared spectra with the Fitzpatrick & Massa parameters that describe the shapes of UV to near-infrared extinction curves. Our primary result is that there is a strong correlation between the area of the 2175 Å UV bump in the extinction curves of the program stars and the strengths of the major PAH emission features in the mid-infrared spectra for the same lines of sight.

Improved Fe ii Emission-line Models for AGNs Using New Atomic Data Sets

Abstract Understanding the Fe II emission from active galactic nuclei (AGNs) has been a grand challenge for many decades. The rewards from understanding the AGN spectra would be immense, involving both quasar classification schemes such as “Eigenvector 1” and tracing the chemical evolution of the cosmos. Recently, three large Fe II atomic data sets with radiative and electron collisional rates have become available. We have incorporated these into the spectral synthesis code Cloudy and examined predictions using a new generation of AGN spectral energy distribution (SED), which indicates that the ultraviolet (UV) emission can be quite different depending on the data set utilized. The Smyth et al. data set better reproduces the observed Fe II template of the I ZW 1 Seyfert galaxy in the UV and optical regions, and we adopt these data. We consider both thermal and microturbulent clouds and show that a microturbulence of ≈100 km s−1 reproduces the observed shape and strength of the so-called Fe II “UV bump.” Comparing our predictions to the observed Fe II template, we derive a typical cloud density of 1011 cm−3 and photon flux of 1020 cm−2 s−1, and show that these largely reproduce the observed Fe II emission in the UV and optical. We calculate the I(Fe II)/I(Mg II) emission-line intensity ratio using our best-fitting model and obtain log(I(Fe II)/I(Mg II)) ∼ 0.7, suggesting many AGNs have a roughly solar Fe/Mg abundance ratio. Finally, we vary the Eddington ratio and SED shape as a step in understanding the Eigenvector 1 correlation.

Revealing the dust attenuation properties on resolved scales in NGC 628 with SWIFT UVOT data

ABSTRACT Understanding how dust attenuation laws vary between and within galaxies is a key question if we want to reliably measure the physical properties of galaxies at both global and local scales. To shed new light on this question, we present a detailed study of the slope and bump strength of the attenuation law in the nearby spiral galaxy NGC 628 at the resolved spatial scale of 325 pc. To do so, we have modelled a broad multiwavelength data set from the ultraviolet (UV) to the infrared (IR) with the state-of-the-art SED fitting code cigale, including SWIFT UVOT data for which we have developed a new optimized reduction pipeline. We find that the median dust attenuation curve of NGC 628 is fairly steep, but not as steep as the SMC curve, and has a sub-MW-type UV bump. We observe intriguing variations within the galaxy, with regions of high AV exhibiting a shallower attenuation curve. We argue that the flattening of the curve is due to a dominance of absorption over scattering events at higher AV. No trend between the bump strength and the IRAC 8.0 $\mu$m emission was found. However, this does not necessarily rule out polycyclic aromatic hydrocarbons as the main contributors to the UV bump.

A Theory for the Variation of Dust Attenuation Laws in Galaxies

Abstract In this paper, we provide a physical model for the origin of variations in the shapes and bump strengths of dust attenuation laws in galaxies by combining a large suite of cosmological “zoom-in” galaxy formation simulations with 3D Monte Carlo dust radiative transfer calculations. We model galaxies over three orders of magnitude in stellar mass, ranging from Milky Way–like systems to massive galaxies at high redshift. Critically, for these calculations, we employ a constant underlying dust extinction law in all cases and examine how the role of geometry and radiative transfer effects impacts the resultant attenuation curves. Our main results follow. Despite our usage of a constant dust extinction curve, we find dramatic variations in the derived attenuation laws. The slopes of normalized attenuation laws depend primarily on the complexities of star-to-dust geometry. Increasing fractions of unobscured young stars flatten normalized curves, while increasing fractions of unobscured old stars steepen curves. Similar to the slopes of our model attenuation laws, we find dramatic variation in the 2175 Å ultraviolet bump strength, including a subset of curves with little to no bump. These bump strengths are primarily influenced by the fraction of unobscured O and B stars in our model, with the impact of scattered light having only a secondary effect. Taken together, these results lead to a natural relationship between the attenuation curve slope and 2175 Å bump strength. Finally, we apply these results to a 25 Mpc h −1 box cosmological hydrodynamic simulation in order to model the expected dispersion in attenuation laws at integer redshifts from z = 0 to 6. A significant dispersion is expected at low redshifts and decreases toward z = 6. We provide tabulated results for the best-fit median attenuation curve at all redshifts.

Investigation of dust attenuation and star formation activity in galaxies hosting GRBs

Context. The gamma-ray bursts hosts (GRBHs) are excellent targets to study the extinction properties of dust and its effects on the global emission of distant galaxies. The dust extinction curve is measured along the GRB afterglow line of sight and the analysis of the spectral energy distribution (SED) of the host galaxy gives access to the global dust attenuation of the stellar light. Aims. In this pilot study we gather information on dust extinction in GRBHs to compare the properties of the extinction curve to those of the dust obscuration affecting the total stellar light of the host galaxy. Assuming the extinction curve to be representative of the dust properties, we aim to investigate which dust-stars geometries and local dust distribution in the inter stellar medium (ISM) can reproduce the observed attenuation curve. Methods. We selected a sample of 30 GRBs for which the extinction curve along the GRB afterglow line-of-sight (l.o.s.) is measured in the rest-frame ultraviolet (UV) up to optical and we analysed the properties of the extinction curve as a function of the host galaxy properties. From these 30 GRBs, we selected seven GRBHs with a good rest-frame UV to near-infrared (NIR) spectral coverage for the host. The attenuation curve was derived by fitting the SEDs of the GRBH sample with the CIGALE SED fitting code. Different star formation histories (SFH) were studied to recover the star formation rates (SFR) derived using Hα luminosities. Implications for the dust-stars geometries in the ISM are inferred by a comparison with radiative transfer simulations. Results. The most extinguished GRBs are preferentially found in the more massive hosts and the UV bump is preferentially found in the most extinguished GRB l.o.s. Five out of seven hosts are best fitted with a recent burst of star formation, leading to lower stellar mass estimates than previously found. The average attenuation in the host galaxies is about 70% of the amount of extinction along the GRB l.o.s. We find a great variety in the derived attenuation curves of GRBHs, the UV slope can be similar, flatter or even steeper than the extinction curve slope. Half of the attenuation curves are consistent with the Calzetti attenuation law and there is evidence of a UV bump in only one GRBH. We find that the flatter (steeper) attenuation curves are found in galaxies with the highest (lowest) SFR and stellar masses. The comparison of our results with radiative transfer simulations leads to a uniform distribution of dust and stars in a very clumpy ISM for half the GRBHs and various dust-stars geometries for the second half of the sample.

Interstellar extinction in Orion: variation of the strength of the ultraviolet bump across the complex

There is growing observational evidence of dust coagulation in the dense filaments within molecular clouds. Infrared observations show that the dust grains size distribution gets shallower and the relative fraction of small to large dust grains decreases as the local density increases. Ultraviolet (UV) observations show that the strength of the 2175 {\AA} feature, the so-called UV bump, also decreases with cloud density. In this work, we apply the technique developed for the Taurus study to the Orion molecular cloud and confirm that the UV bump decreases over the densest cores of the cloud as well as in the heavily UV irradiated {\lambda} Orionis shell. The study has been extended to the Rosette cloud with uncertain results given the distance (1.3 kpc).

AGN torus emission for a homogeneous sample of bright flat-spectrum radio quasars

We have selected a complete sample of 80 flat-spectrum radio quasars (FSRQs) from the WMAP 7-yr catalog within the SDSS area, all with measured redshift, and have inspected their SEDs looking for evidence of an AGN torus emission. A SED fitting algorithm has found such evidence for seven objects and an uncertain indication for one more. These 8 FSRQs belong to the sub-sample of 55 sources showing the optical--ultraviolet bump interpreted as thermal emission from a standard accretion disc. Torus luminosities have been estimated for the eight objects whose torus was identified by the fitting algorithm. For the other 47 FSRQs in the sub-sample we have derived upper limits to the torus luminosity. Our analysis shows that the torus can show up clearly only under quite special conditions: low luminosity and preferentially low peak frequency of the beamed synchrotron emission from the jet; high torus luminosity, close to that of the accretion disc. This implies that the inferred ratios of torus to disc luminosity are biased high. The median value, considering upper limits as detections, is $L_{\rm torus}/L_{\rm disc}\sim 1$ while studies of radio quiet quasars yield average ratios $\langle L_{\rm torus}/L_{\rm disc}\rangle \simeq 1/3-1/2$. Our results are compatible with the FSRQ tori having the same properties of those of radio quiet quasars. At variance with Plotkin et al. (2012), who investigated a sample of optically selected BL Lacs, we find that the Wide-field Infrared Survey Explorer (WISE) infrared colors do not allow us to draw any conclusion on the presence or absence of tori associated with WMAP selected blazars. With the latter selection blazars of all types (FSRQs with and without evidence of torus, BL Lacs, blazars of unknown type) occupy the same region of the WISE color-color plane, and their region overlaps that of SDSS quasars with point-like morphology.

Dust attenuation in galaxies up to redshift ≃ 2

We want to study dust attenuation at ultraviolet (UV) wavelengths at high redshift, where the UV is redshifted to the observed visible light wavelength range. In particular, we search for a bump at 2175 A. We use photometric data in the Chandra Deep Field South (CDFS), obtained in intermediate and broad band filters by the MUSYC project, to sample the UV rest-frame of 751 galaxies with 0.95 < z < 2.2. When available, infrared (IR) Herschel/PACS* data from the GOODS-Herschel project, coupled with Spitzer/MIPS measurements, are used to estimate the dust emission and to constrain dust attenuation. The spectral energy distribution of each source is fit using the CIGALE code†. The average attenuation curve found for our sample galaxies exhibits a UV bump whose amplitude is similar to that observed in the extinction curve of the LMC super-shell region. The slope of the average attenuation curve at UV wavelength is found steeper than that for local starburst galaxies. The amount of dust attenuation at UV wavelengths is found to increase with stellar mass and to decrease as UV luminosity increases.

Obscured star formation atz= 0.84 with HiZELS: the relationship between star formation rate and Hα or ultraviolet dust extinction

[Abridged] We compare H-alpha, ultraviolet (UV) and infrared (IR) indicators of star formation rate (SFR) for a sample of z = 0.84 galaxies from the High-z Emission Line Survey (HiZELS). Using multi-wavelength diagnostics, we estimate that 5 - 11 per cent of H-alpha emitters at this redshift are active galactic nuclei. We detect 35 per cent of the H-alpha emitters individually at 24 microns, and stack the star-forming emitters on deep 24-micron images in order to calculate the typical SFRs of our galaxies. These are compared to the observed H-alpha line fluxes in order to estimate the extinction at z = 0.84, and we find a significant increase in dust extinction for galaxies with higher SFRs. We demonstrate that the local relationship between SFR and extinction is suitable for our sample, and attribute the overall increase in dust extinction for z = 0.84 galaxies to an increase in the average SFR, rather than to a change in dust properties. We calculate the UV extinction, and find a similar dependence on SFR, but no evidence for a 2175 angstrom UV bump in the dust attenuation law. We calculate the conversion between the dust attenuation of nebular and stellar radiation, gamma, and show that gamma = 0.50 +/- 0.14. The extinction / SFR relationship is shown to be applicable to galaxies with a range of morphologies and bulge-to-disk ratios, to both merging and non-merging galaxies, and to galaxies within high- and low-density environments, implying that it is a fundamental property of star-forming regions. In order to allow future studies to correct for a SFR-dependent amount of dust extinction, we present an equation to predict the extinction of a galaxy, based solely upon its observed H-alpha luminosity, and use this to recalculate the H-alpha luminosity function and star formation rate density at z = 0.84.

On the action of UV photons on hydrogenated fulleranes C60H36and C60D36

Fulleranes, the hydrogenated fullerenes C 6o H 36 and C 60 D 36 have been synthesized in n-hexane where they show an absorption maximum at 217 nm in their electronic absorption spectra with molar extinction coefficients e 217 = 17140 and 16480L cm ―1 mol ―1 , respectively, which correspond to absorption cross-sections of order 6500 Mbarn, about 10 times higher than for C 60 . We find that the absorption curves measured for these two fulleranes fit well the wavelength, width and shape of the ultraviolet (UV) bump (217 nm) in the extinction curve of the diffuse interstellar medium. If present in the interstellar medium these fulleranes would contribute significantly to the bump extinction, although interstellar carbon budget considerations indicate that C 60 H 36 cannot be the only contributor. The photolysis of fulleranes with monochromatic light at 245 nm causes a shift of the absorption maximum from 217 nm to longer wavelengths. This band shift of the absorption maximum has already been observed in the photolysis or in the thermal processing of hydrogenated carbon dust. The band shift is due to the release of molecular hydrogen, a process which causes the aromatization of the carbon dust. C 6o H 36 and C 60 D 36 are photolyzed at a rate k C60H36 = 2.45 × 10 -3 s ―1 while k C60D36 = 2.27 × 10- 3 s ―1 . This implies an isotope effect so that C 6 oH 36 is photolyzed 1.08 times faster than C 60 D 36 . In the interstellar medium, this implies a deuterium enrichment of the hydrogenated interstellar carbon dust. The presence of a measurable isotope effect suggests that the photolysis of C 6 oH 36 and C 6 oD 36 involves the rupture of the C-H and C-D bond with release of H 2 . C 6 oH 36 releases completely hydrogen when heated to 630 °C under inert atmosphere producing back C 60 as shown by FT-IR spectroscopy. Thus in the interstellar medium an equilibrium should occur between the hydrogenation of C 60 with atomic hydrogen and the emission of molecular hydrogen because of the action of UV photons or thermal processing. Interstellar fullerenes and interstellar carbon dust containing fullerenes and fullerene-like structures should play a key role in the formation of molecular hydrogen starting from atomic hydrogen and as carriers of the UV bump of the interstellar extinction curve.

Presence of dust with a UV bump in massive, star-forming galaxies at 1 &lt; z &lt; 2.5

Aims. Fundamental properties of the extinction curve, like the slope in the rest-frame ultraviolet (UV) and the presence/absence of a broad absorption excess centred at 2175 A (the UV bump), are investigated for a sample of 108 massive, star-forming galaxies at 1 < z < 2.5, selected from the FDF Spectroscopic Survey, the K20 survey, and the GDDS. Methods. These characteristics are constrained from a parametric description of the UV spectral energy distribution (SED) of a galaxy, as enforced by combined stellar population and radiative transfer models for different geometries, dust/stars configurations, and dust properties. Results. In at least one third of the sample, there is a robust evidence for extinction curves with at least a moderate UV bump. The presence of the carriers of the UV bump is more evident in galaxies with UV SEDs suffering from heavy reddening. We interpret these results as follows. The sample objects possess different mixtures of dust grains and molecules producing extinction curves in between the average ones of the Small and Large Magellanic Cloud, where the UV bump is absent or modest, respectively. Most of the dust embeds the UV-emitting stellar populations or is distributed out of the galaxy mid-plane. Alternatively, even dust with a pronounced UV bump, as for the average Milky-Way extinction curve, can be present and distributed in the galaxy mid-plane. In this case, variations of the continuum scattering albedo with wavelength or an age-dependent extinction are not sufficient to explain the previous trend with reddening. Hence, additional extraplanar dust has to be invoked. The data suggest that the carriers of the UV bump are associated with intermediate-age stellar populations, while they survive in the harshest UV-radiation fields owing to dust self-shielding. Conclusions. The existence of different extinction curves implies that different patterns of evolution and reprocessing of dust exist at high redshift. Ignoring this may produce a non-negligible uncertainty on the star-formation rate estimated from the rest-frame UV.

Hydrogenated fulleranes and the anomalous microwave emission of the dark cloud LDN 1622

We study the rotation rates and electric dipole emission of hydrogenated icosahedral fullerenes under the physical conditions of the dark cloud (DC) LDN 1622. The abundance of fullerenes is estimated by fitting theoretical photoabsorption spectra to the characteristics of the ultraviolet (UV) bump extinction in DCs. The UV bump appears to be well reproduced by a mixture of fullerenes following a size-distribution power law, which gives progressively lower abundances as the radius of the fullerene increases. We infer abundances of the order of 0.2 x 10 -6 n(H 2 ) for C 60 . A significant fraction of these molecules are expected to be hydrogenated. We compute the electric dipole rotational emission from these fullerene hydrides, taking into account rotational excitation and damping processes. The recent detection of anomalous microwave emission (5-60 GHz) in LDN 1622 by Casassus et al. can be explained as the result of electric dipole radiation from hydrogenated fullerenes. The bulk of the emission (10-30 GHz) appears to be associated with 60-80 carbon atom fulleranes with a degree of hydrogenation of C:H ≈ 3:1. A small contribution (∼10 per cent) of these molecules residing in the surrounding cold neutral medium and/or photodissociation region of the cloud is required to fit the high-frequency tail (40-60 GHz) of the emission.

Spectropolarimetric Evidence for a Bipolar Flow in β Lyrae

We present ultraviolet and visual spectropolarimetry of the interacting binary star β Lyrae, obtained with the Wisconsin Ultraviolet Photo-Polarimeter Experiment and the HPOL spectropolarimeter at Pine Bluff Observatory. Our observations span 3 years and cover the wavelength range from 1400 to 10500 A, with a resolution of 7.5–16 A. Detailed broad- and narrowband spectropolarimetric analysis allows us to begin to decompose the complex spectrum of β Lyr: by examining the polarization behavior of a line or continuum, we can determine which component scatters the light and, ultimately, from which component that light originates. After removing interstellar polarization from our data and rotating the results to the apparent intrinsic position angle of the system, we find that the polarization of the hydrogen Balmer and vacuum ultraviolet bump emission lines, as well as that of the near-UV continuum, remains mostly constant with phase and is oriented at 90° to the visible polarization, indicating that the scattering plane of the light in these three spectral components is perpendicular to the scattering plane of the visible light. We propose that the UV bump, Balmer emission, and near-UV continuum polarization is produced by electron scattering within a bipolar outflow in the β Lyr system. The intrinsic visible polarization of β Lyr shows eclipses that associate it with material near the accretion disk. We find conflicting evidence regarding both the origin of this visible light and the scattering surface that polarizes it: continuum evidence points toward the secondary object as the illuminator and the accretion disk edge as the scatterer, while line analysis suggests that light from the loser scatters off material between it and the disk. The presence of material away from the orbital plane may help resolve this contradiction.

Radiation from magnetized accretion disks in active galactic nuclei

We present a model of AGN based on accretion disks around 10 exp 8 solar mass black holes, which incorporates a strong magnetic field in the disk, with loops above the disk in which B is about 100 G. The magnetic energy in the loops dissipates, accelerating electrons to relativistic energies. The electrons emit synchrotron and inverse-Compton radiation, much of which is reflected or scattered by gas in the disk. We calculate the emitted spectrum from infrared to gamma rays. Of the total emission, 23 percent goes into a nu exp -1 power law and 77 percent into thermal radiation at 24,000 K (due to the disk, which is heated by nonthermal radiation and energetic ions). The model accounts quantitatively for the X-ray and gamma-ray backgrounds, and it accounts qualitatively for certain features observed in individual AGNs: a far-infrared cutoff, a minimum at a few microns, the ultraviolet bump, the Compton reflection bump at about 30 keV, and a gamma-ray tail.

Physical models for the hypothesized F(nu) varies as the inverse of nu infrared to X-ray continuum of quasi-stellar objects

view Abstract Citations (3) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Physical Models for the Hypothesized F( nu ) proportional to nu -1 Infrared to X-Ray Continuum of Quasi-stellar Objects Stein, Wayne A. Abstract Models for producing the large ultraviolet bump, low-energy X-rays and the hypothesized F(ν) is proportional to ν^-1^ infrared to X-ray continua of QSOs are investigated. These must be considered together to be consistent within models suggested. Thermal Comptonization in a hot corona of an accretion disk appears to offer the best potential. However, under the energy input conditions in QSOs a corona will reach T>~10^8^ K. It must be optically thin so as to not Comptonize the accretion disk ultraviolet emission to an unacceptable extent. However, it then cannot Comptonize a low-frequency source to a F(ν) is proportional to ν ^-1^ continuum extending from the infrared to X-rays. An inner corona, possibly optically thick because of n is proportional to r^-2^ density increase, is required for the F(ν) is proportional to ν^-1^ continuum, but it cannot therefore cover the ultraviolet-emitting accretion disk. However, then a Wien peak associated with this inner volume may be implied at 10 keV, contrary to observations. Publication: The Astrophysical Journal Pub Date: May 1991 DOI: 10.1086/170021 Bibcode: 1991ApJ...373...54S Keywords: Accretion Disks; Continuous Radiation; Infrared Radiation; Quasars; Stellar Coronas; X Ray Astronomy; Astronomical Models; Compton Effect; Synchrotron Radiation; Ultraviolet Radiation; Astrophysics; QUASARS; RADIATION MECHANISMS full text sources ADS | data products SIMBAD (2)

Interpretation of the microlensing event in QSO 2237 + 0305

view Abstract Citations (196) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Interpretation of the Microlensing Event in QSO 2237+0305 Wambsganss, J. ; Paczynski, B. ; Schneider, P. Abstract We present a model of microlensing for image A of the gravitationally lensed QSO 2237+0305 for which Irwin et al. reported in 1989 an increase of the apparent luminosity by about 0.5 mag on a time scale of a few months. Our model, with the Salpeter mass function over the mass range of 0.1-1.0 M_sun_ and the transverse velocity of the lens (or observer) of 600 km s^-1^, can reproduce the reported luminosity variation if the source of the optical continuum has a radius smaller than about 2 x 10^15^ cm. This size is compatible with the accretion disk interpretation of the big ultraviolet bump in quasar spectra. Our model demonstrates a very large diversity of light curves while the source crosses individual microcaustics or clusters of microcaustics. It will take more than 100 yr before the full variety of light curves will be sampled by the observations. Publication: The Astrophysical Journal Pub Date: August 1990 DOI: 10.1086/185773 Bibcode: 1990ApJ...358L..33W Keywords: Gravitational Lenses; Quasars; Ultraviolet Spectra; Accretion Disks; Astronomical Models; Caustics (Optics); Luminosity; Astrophysics; GRAVITATIONAL LENSES; QUASARS; RELATIVITY full text sources ADS | data products SIMBAD (2) NED (1)

X-ray, optical, and infrared luminosity correlations in radio-quiet quasars

view Abstract Citations (61) References (58) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS X-Ray, Optical, and Infrared Luminosity Correlations in Radio-quiet Quasars Kriss, Gerard A. Abstract For a sample of 88 radio-quiet quasars that includes 15 X-ray upper limits, I find that the X-ray and IR luminosities follow a significantly nonlinear relation (L_2keV_ ~ L^0.73^_1 micron_) that is consistent with that found for the X-ray and optical luminosities. The redshift dependence is weak and may be nonexistent. The shape of the optical through infrared continuum varies only slightly with luminosity. The most significant change is the lack of a strong ultraviolet bump in low luminosity objects with L_1 micron_ < 10^30^ ergs s^-1^ Hz^-1^. More luminous quasars have significantly stronger bumps, but above 10^30^ ergs s^-1^ Hz^-1^ the strength of the bump does not correlate with luminosity. The relatively constant shape of the infrared through optical continuum implies that a systematic change in the X-ray continuum of quasars with increasing luminosity produces the nonlinear relation between the X-ray luminosity and the optical and IR luminosities. Either the overall normalization of the X-ray continuum relative to the optical and IR must decrease, or the X-ray continuum must flatten with increasing luminosity, or the strength of a soft X-ray excess must decrease with increasing luminosity. Without detailed knowledge of the X-ray and infrared spectra as a function of luminosity, it is difficult to place significant constraints on emission mechanisms. Publication: The Astrophysical Journal Pub Date: January 1988 DOI: 10.1086/165940 Bibcode: 1988ApJ...324..809K Keywords: Infrared Spectra; Luminosity; Quasars; Visible Spectrum; X Ray Spectra; Accretion Disks; Black Holes (Astronomy); Continuous Spectra; Correlation; Radio Spectra; Red Shift; Astrophysics; INFRARED: SOURCES; QUASARS; RADIATION MECHANISMS; X-RAYS: SOURCES full text sources ADS | data products SIMBAD (85) NED (84)