High Optical Polarization Research Articles

Polarimetry and spectrophotometry of the QSO IRAS 13349+2438 and the unification of active galaxies

We have made new observations of the bright (V = 14.7) infrared- selected QSO IRAS 13349 + 2438 (Beichman et al.), which provide important clues about the structure of the inner few parsecs of active galactic nuclei (AGNs). The principal observational facts are as follows: 1. There is high linear optical polarization, increasing with decreasing wavelength from 1.4% at 2.2 microns (K-band) to 8% at 0.36 microns (U-band). 2. The position angle of polarization is independent of wavelength in the optical region and is aligned with the direction of elongation of the r-band optical image, presumably the major axis of the host galaxy. This is probably true in the IR as well. 3. The position angle and degree of polarization of the broad Hα emission line is similar to that of the underlying continuum. 4. We find no variability of the polarization or flux density on time scales from days to months. 5. There is strong optical Fe II emission and the broad lines are relatively narrow (2100 km s^-1^). The emission from the narrow-line region is very weak. 6. The hydrogen broad-line ratios, Paα/Hα = 0.15 and Hα/Hβ = 5.9, are among the largest observed for a QSO (or even among Seyfert 1 galaxies.) If this indicates dust extinction, then comparison with these ratios in typical QSOs suggests E(B - V)~0.3. 7. The optical-UV continuum spectral energy distribution falls off very steeply toward the UV. The reddening required to match this continuum to those of typical luminous QSOs also corresponds to E(B-V) ~ 0.3. The high, time-independent polarization, its wavelength dependence, and its similarity in the continue- and broad Hα emission indicate that the polarization is caused by scattering in an anisotropic geometry. The polarized flux density spectrum looks like that of a normal high- luminosity QSO, with a strong UV excess; this is naturally explained if the scattered light has a degree of polarization independent of wavelength and has undergone little reddening The scatterers could be free electrons, or small dust grains with a range of scattering properties. The total observed spectrum is then the sum of this scattered, polarized spectrum and a reddened, transmitted spectrum. The decreased dilution of the scattered by the transmitted spectrum toward short wavelengths causes the rise in degree of polarization toward the UV. We derive a redening of E(B - V)~0.3 for the transmitted (direct) spectrum, depending on the intrinsic polarization of the scattered spectrum. We deduce a bipolar geometry for the inner regions of the QSO: a direct, but attenuated, AGN spectrum reaches the observer through an inclined, thick dusty disk that is parallel to the plane of the host galaxy, and probably between the broad- and narrow-line regions. Light escaping in polar directions is much less attenuated, with E(B - V)<0.05, and is scattered toward the observer and hence polarized parallel to the major axis of the host galaxy. IRAS 13349+2438 is the archetype of luminous highly polarized radio- quiet QSOs, and because of these properties is an important link in schemes that unify the apparently different kinds of AGNs by viewing angle, including the luminous "normal" QSOs, broad absorption line QSOs (BALQs), and (IRAS) ultraluminous AGNs. Specifically, we predict the spectroscopic and polarimetric properties of our model for IRAS 13349+2438 as it would appear at different inclinations and compare these predictions for unobscured luminous AGNs and AGNs suggested to contain "buried" QSO nuclei. A statistical comparison with samples of luminous AGNs chosen by orientation-independent infrared (eg., ~60 micron) flux density suggests that inclination is a key parameter in determining the appearance of these AGNs. Present optically selected samples of QSOs may be seriously incomplete, and investigation of more objects like IRAS 13349 + 2438 may tell us how incomplete they are.

A survey for high optical polarization in quasars with core-dominant radio structure - Is there a beamed optical continuum?

view Abstract Citations (216) References (84) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Survey for High Optical Polarization in Quasars with Core-dominant Radio Structure: Is There a Beamed Optical Continuum? Wills, Beverley J. ; Wills, D. ; Breger, Michel ; Antonucci, R. R. J. ; Barvainis, Richard Abstract We present 129 new optical linear polarization measurements of 84 quasars (defined as starlike on Sky Survey prints). We have discovered 23 new highly polarized objects (p> 3%), of which 14 are known to have broad emission lines. This represents a significant increase in the numbers of these objects that were known. With the addition of data from the literature, our measurements constitute well-defined samples of flat-spectrum core-dominant radio quasars brighter than about 18 mag, one selected from the list published by Perley (1982) and another from that by Kuhr et al. (1981). The results of this survey of 123 quasars and BL Lac objects are the following: 1. The fraction of objects that have been found to be highly polarized on at least one measurement is 45% +/- 5%. This is a lower limit to the total fraction of highly polarized quasars because the well-observed highly polarized objects of this class have p <3% some of the time. 2. There is a highly significant correlation between percentage polarization and the dominance of a compact radio core as measured by R, the ratio of radio core flux density to flux density in the extended lobes. In order to include low-R objects and to increase the sample size, we have added data from the literature. We find that all 13 quasars (including 10 BL Lac objects) with log R> 2 are highly polarized while for log R < 0 this fraction is only 5%. If relativistic beaming is the explanation for the strong synchrotron emission of the radio core, then the strength of the polarized component of the optical continuum also has a strong angle dependence, suggesting that the optical flux is beamed as well. 3. The fraction of quasars with p> 3% in a fixed observed passband apparently decreases with increasing redshift. Using the maximum observed values of p for each object, we find that for z < 1, 59% +/- 6% of the sample have p > 3%, compared with 24% +/- 6% for z > 1. A likely interpretation is that many of these highly polarized quasars show decreasing percentage polarization at shorter rest wavelengths where a very low polarization flat-spectrum "Big Blue Bump" component dominates over a highly polarized- steep-spectrum "synchrotron" component. Shorter rest wavelengths shift through our observing window with increasing quasar redshift. We present a simple spectrum model consisting of a steep polarized optical synchrotron component related to the radio core, and a flat, unpolarized Big Blue Bump, which explains results 1,2, and 3. The combined R and redshift dependence of polarization (or of fraction of highly polarized objects) results in a very high incidence of highly polarized objects, at least 74% +/- 8%, among those 36 objects with log R > 1.25 and z < 1. This has two immediate consequences. High optical polarization quasars and flat-spectrum core-dominant quasars are essentially the same objects. Therefore, the blazar unified scheme advocated by Blandford & Rees, and by Antonucci & Ulvestad, and the generalized unified scheme of Blandford & Konigl, Orr & Browne, and Wills & Browne, are approximately equivalent. Also, the apparent similarity in the radio and optical "beaming cones" suggests that the corresponding emission regions are closely related. The optical synchrotron radiation may originate in the radio core and in unresolved superluminal radio components producing the radio emission. There is a significantly lower incidence of high polarization among the intrinsically small compact steep-spectrum (CSS) and gigahertz peaked spectrum (GPS) sources - only two out of 23. Either their structure cannot be unified by relativistic beaming geometry, the core spectra are too steep for significant optical synchrotron emission, or the selection criteria for these sources eliminate the highly beamed examples. Finally, fewer than about 2% of the 3CR population of all FR II and compact sources are beamed in our direction, as indicated by the upper limit to the fraction of highly polarized quasars and core-dominant objects among those sources with S_ext_ > 10 Jy. If this fraction represents the radiation angle of a simple relativistic jet, the Lorentz factor, γ >= 5, is consistent with that deduced from other methods. Publication: The Astrophysical Journal Pub Date: October 1992 DOI: 10.1086/171869 Bibcode: 1992ApJ...398..454W Keywords: Bl Lacertae Objects; Galactic Structure; Optical Polarization; Quasars; Sky Surveys (Astronomy); Astronomical Models; Astronomical Spectroscopy; Radio Emission; Red Shift; Spectral Line Width; Very Large Array (Vla); Astrophysics; GALAXIES: QUASARS: GENERAL; POLARIZATION; GALAXIES: BL LACERTAE OBJECTS: GENERAL full text sources ADS | data products NED (268) SIMBAD (267)

Quasars, Blazars, and Gamma Rays

Before the launch of the Compton Gamma Ray Observatory (CGRO), the only source of >100-megaelectron volt (MeV) gamma radiation known outside our galaxy was the quasar 3C 273. After less than a year of observing, 13 other extragalactic sources have been discovered with the Energetic Gamma Ray Experiment Telescope (EGRET) on CGRO, and it is expected that many more will be found before the full sky survey is complete. All 14 sources show evidence of blazar properties at other wavelengths; these properties include high optical polarization, extreme optical variability, flat-spectrum radio emission associated with a compact core, and apparent superluminal motion. Such properties are thought to be produced by those few, rare extragalactic radio galaxies and quasars that are favorably aligned to permit us to look almost directly down a relativistically outflowing jet of matter expelled from a supermassive black hole. Although the origin of the gamma rays from radio jets is a subject of much controversy, the gamma-ray window probed by CGRO is providing a wealth of knowledge about the central engines of active galactic nuclei and the most energetic processes occurring in nature.

A high spatial resolution polarization map of GL2591

A high spatial resolution optical polarization map of the nebulosity close to the IR source GL2591 is presented which shows that the pattern of vectors is significantly affects by dichroic extinction within the molecular cloud in which the system is located. After correction for these effects it is found that the dominant illuminator of the nebulosity is the IR source GL2591, but there is also a significant localized contribution from a visible star which may be embedded within the cloud

Optical polarization of a complete sample of radio sources

The radio and optical properties of a complete 5 GHz sample of strong radio sources are studied. High polarization is strongly correlated with the fraction of the total 5 GHz flux density found in a milliarcsec core. The detection rate of radio galaxies containing optical cores with high polarization is consistent with the hypothesis that all radio sources contain optically polarized cores, with strength proportional to the core radio flux density. High optical polarization, optical power-law fraction, line-to-continuum ratio, emission lines of small equivalent widths, and large amplitude flux variability are all strongly correlated with the fraction of the 5 GHz radio flux density that is unresolved on VLBI scales. The distributions of these properties are well matched by a model where radio and optical radiation have the same beaming geometry and with Doppler boosting of a factor of 10-100 in the optical. A striking alignment of the position angle of the VLBI structure axis and the position angle of optical polarization in highly polarized sources is confirmed.

Scattering-induced optical polarization in thick accretion disks

view Abstract Citations (12) References (62) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Scattering-induced Optical Polarization in Thick Accretion Disks Kartje, John F. ; Konigl, Arieh Abstract We present a general formalism for calculating the linear polarization induced by scattering within the central funnel of a thick accretion disk and show that multiple photon reflections off the funnel walls can produce polarization values of up to ~10%, with the polarization position angle aligned parallel to the disk symmetry axis. We suggest that this process is responsible for the observed optical polarization levels in X- ray-selected BL Lac objects (XBLs), which generally show linear polarization percentages P <~ 10%. According to this interpretation, XBLs with high optical polarization are viewed at an angle of <~60^deg^ to the funnel axis and their projected polarization vectors should be preferentially aligned with the associated radio jets. The possible relevance of this model to Seyfert 1 galaxies and quasars is also discussed. Publication: The Astrophysical Journal Pub Date: July 1991 DOI: 10.1086/170169 Bibcode: 1991ApJ...375...69K Keywords: Accretion Disks; Bl Lacertae Objects; Light Scattering; Optical Polarization; Linear Polarization; Polarimetry; Seyfert Galaxies; X Ray Astronomy; Astrophysics; BL LACERTAE OBJECTS; POLARIZATION full text sources ADS |

The complete sample of 1 Jansky BL Lacertae objects. I - Summary properties

This paper describes the first homogeneous, flux-limited sample of radio-selected BL Lac objects, taken from the 1 Jy survey of Kuhr et al. These 34 BL Lac objects comprise the only well-defined sample of BL Lac objects outside the X-ray band. The selection criteria include flat radio spectra, optical counterparts visible on Sky Survey Plates, and optical emission lines weaker than 5 A equivalent width in the rest frame. With two exceptions, all of the BL Lac objects also have high optical polarization, P_opt_ &gt; 3%. Redshifts from weak emission lines, usually narrow [O II] λ3727, [O III] λ5007, or broad Mg II λ2798, are available for ~ 3/4 of the sample. The nearby (z &lt; 0.2) BL Lac objects, at least, sit in elliptical galaxies and may occur in groups or small clusters of galaxies. Gravitational lensing appears to affect one of the high-redshift (z &gt;~ 0.5) BL Lac objects and could be important in others as well. The radio number counts derived from the 1 Jy BL Lac sample are roughly Euclidean over the range between 1 and ~ 5 Jy, and there is evidence for evolution at the 2 σ level. The local radio luminosity function of BL Lac objects is derived and the total number density for P_5 GHz_ &gt;~ 6 x 10^31^ ergs s^-1^ Hz^-1^ is ~ 40 Gpc^-3^.

High extinction guided-wave optical polarization splitter

Conventional guided-wave optical polarization splitters exhibit poor polarization extinction characteristics because of the necessity of controlling waveguide parameters. In order to improve the characteristics, an electrical control scheme was investigated in an alternating Delta beta directional coupler. Polarization-dependent electrooptic as well as polarization-independent plasma effects were employed to produce high extinction for two orthogonal polarizations independently. As a result, an extinction ratio as high as 32 dB was obtained with a fabricated GaAs-GaAlAs device. This is the highest value ever reported with a channel waveguide device.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High Optical Polarization in Flat-Spectrum Radio Sources

We have made a polarimetric survey of 84 quasi-stellar objects, to supplement published polarization measurements for two samples of quasars identified with flat-spectrum (core-dominant) radio sources in 5 GHz surveys made at Bonn (Kühr 1980) and at the VLA (Perley 1982). The observations were made using the McDonald 2.1m Struve reflector and the polarimeter described by Breger (1979); the typical accuracy is 0.5% for an 18 mag object after half an hour. Earlier surveys, such as those by Stockman, Moore and Angel (1984), included objects of various radio spectral types, and only a small fraction of the objects showed high polarization (&gt; 3%), but our sample of flat-spectrum quasars reveals many more (about half) of the objects to be highly polarized. Some of them are, expectedly, of the BL Lac class, but many of them have strong broad emission lines. There are two striking correlations among the results: (1)The degree of polarization is strongly correlated with the dominance of the radio core - specifically, with the ratio, R, of core to lobe luminosity (Fig. 1). For example, about 75% of the objects with log R &gt; 1.25 and redshift z &lt; 1 have p &gt; 3%. This relation implies that if the radio core radiation is beamed, as seems likely, then so is the optical synchrotron component.(2)The fraction of objects with p &gt; 3% is inversely correlated with redshift (e.g. Fig. 2). The most likely interpretation of this result is that quasars' degree of polarization decreases with decreasing rest wavelength, and the shorter wavelengths are shifted into our wide observational passband at higher redshifts.

Radio properties of the highly polarized, quiescent quasar OI 287

view Abstract Citations (13) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radio Properties of the Highly Polarized, Quiescent Quasar OI 287 Ulvestad, James S. ; Antonucci, Robert R. J. Abstract OI 287 has often been considered to be a blazar because of its high optical polarization. However, it is the only high-polarization quasar with constant optical flux and polarization as well as a lobe-dominant radio source. All known blazars have cores with smooth convex spectra from radio to optical wavelengths, which is taken to indicate the presence of a single synchrotron source for this entire range. The high-resolution 6 cm and 2 cm VLA data described in this paper show that this cannot be the case for OI 287, so that it cannot be considered to be a blazar. Previous moderate-resolution radio mapping showed apparent twin jets with parallel magnetic fields in this high-luminosity source, whereas most high-luminosity sources have one-sided jets. The higher resolution data show that the jets are better described as bridges; therefore the morphology is not particularly unusual for high-luminosity, lobe-dominant sources. The radio core polarization of OI 287 is less than 0.25% at 6 cm, low in comparison to blazars. Both the radio polarization and the other radio/infrared properties are normal for lobe-dominant quasars. Therefore, the optical polarization may not arise from synchrotron radiation as it does in blazars. This is consistent with the idea of Miller and Goodrich in which an edge-on geometrically thin disk blocks our direct view of the continuum and broad-line regions. In that picture, the high optical polarization would arise because only scattered photons reach the observer. Alternatively, it is still possible that OI 287 is related to the broad absorption line quasars, several of which are significantly polarized. Publication: The Astrophysical Journal Pub Date: May 1988 DOI: 10.1086/166314 Bibcode: 1988ApJ...328..569U Keywords: Bl Lacertae Objects; Optical Polarization; Quasars; Radio Sources (Astronomy); Synchrotron Radiation; Blazars; Emission Spectra; High Resolution; Line Spectra; Morphology; Stellar Luminosity; Very Large Array (Vla); Astrophysics; BL LACERTAE OBJECTS; POLARIZATION; QUASARS full text sources ADS | data products SIMBAD (5) NED (5)

THE ANOMALOUS ARMS OF THE SPIRAL GALAXY NGC 4258.

ABSTRACT The nature of the optical emission of the southern "anomalous arms" of the spiral galaxy NGC 4258 is investigated. High-resolution spectral scans at Hα have been obtained with a Fabry-Pérot spectrometer. No spectral line was detected, indicating that the emission of these arms is probably mostly continuum. A search for high optical polarization with polarizing filters and a photon-counting camera resulted in an upper polarization limit of 5% (1 σ), eliminating synchrotron radiation as an important contribution to the optical emission of the arms. It remains to be established whether the optical anomalous arms are made of late-type stars or perhaps emitters of nonthermal continuum.

Blazars can have double radio sources

Blazars (BL Lac objects and optically violently variable quasars) are characterized by high optical polarization, and highly variable optical flux, polarization and polarization position angle1. Many have other extraordinary properties that could be explained by a radiating jet undergoing bulk relativistic motion directed towards the observer. Several workers have suggested that the jets seen in ordinary double radio sources could appear as blazars, if they were viewed end-on1; in this case the two lobes would not appear separately, but would appear in projection as a halo around the point source. Generally, blazar maps in the literature are consistent with this notion2–4, with only two unusual (bent or very asymmetrical) double sources—1400 + 162 (refs 3–6) and 3C279 (ref. 7)—having been reported. However, we have now found three clear double radio sources (two being quite symmetrical) around the strong central point sources, including one classical double. A fourth blazar has head–tail morphology. It may be possible to reconcile these observations with the model cited above if we imagine that the jets do not lie precisely alone the line of sight, or that the lobes are misaligned with the jets8.

Optical nuclear polarization studies of exciton dynamics in anthracene/tetracyanobenzene charge transfer crystals

The high optical nuclear polarization found in A/TCNB charge transfer crystals is due to relaxation caused by triplet exciton motion. The corresponding hopping time τ = 3 x 10 −10 s at 300 K as obtained from the ONP results is attributed to fast intrachain exciton mobility. RF-ONP measurements for the low temperature phase of A/TCNB render the intersite hopping time τ AB for exciton motion between inequivalent positions. The ratio τ/τ AB > 100 shows the one-dimensional character of the excitonic motion.