Relativistic Beaming Effects Research Articles

A possible EGRET source with a two-sided radio jet structure on a parsec scale - 3EG J0808+5114

We report on the European VLBI Network observations of the radio source 0803+5126 in the field of the Energetic Gamma Ray Experiment Telescope (EGRET) source 3EG J0808+5114. High-resolution images of 0803+5126 at 6 cm showed a core emission with an extremely low brightness temperature plus an asymmetric two-sided jet structure on both parsec and tens of parsec scales, suggesting a relatively weak beaming effect in radio emission. The radio structures on different scales exhibit quite different orientations of extension, indicating the occurrence of jet bending. Based on the positional uncertainty of 3EG J0808+5114, its separations to various possible radio counterparts, and very low gamma-ray variability, it is reasonable to believe that 0803+5126 is the most probable radio counterpart of 3EG J0808+5114, making it the first EGRET-detected active galactic nucleus to be associated with a two-sided jet structure on a parsec scale. The central engine is estimated to have a mass of similar to 5.0 x 10(8) M-circle dot by using Mg II emission line features, for which the Eddington limit is approximately two times lower than the integrated gamma-ray luminosity. The minimum Doppler factor in gamma-ray emission is similar to 2.5, but the true one should not be much larger than this value on account of its quiet stable gamma-ray emission. This is consistent with our very long baseline interferometry results of the two-sided radio jet structure. The relativistic beaming effect and downstream jet bending may play an important role in the paradox between the large gamma-ray luminosity far above its Eddington limit and the observed asymmetric two-sided radio structure.

The Black Hole Fundamental Plane from a Uniform Sample of Radio and X‐Ray‐emitting Broad‐Line AGNs

We derived the black hole fundamental plane relationship among the 1.4GHz radio luminosity (L_r), 0.1-2.4keV X-ray luminosity (L_X), and black hole mass (M) from a uniform broad line SDSS AGN sample including both radio loud and radio quiet X-ray emitting sources. We found in our sample that the fundamental plane relation has a very weak dependence on the black hole mass, and a tight correlation also exists between the Eddington luminosity scaled X-ray and radio luminosities for the radio quiet subsample. Additionally, we noticed that the radio quiet and radio loud AGNs have different power-law slopes in the radio--X-ray non-linear relationship. The radio loud sample displays a slope of 1.39, which seems consistent with the jet dominated X-ray model. However, it may also be partly due to the relativistic beaming effect. For radio quiet sample the slope of the radio--X-ray relationship is about 0.85, which is possibly consistent with the theoretical prediction from the accretion flow dominated X-ray model. We briefly discuss the reason why our derived relationship is different from some previous works and expect the future spectral studies in radio and X-ray bands on individual sources in our sample to confirm our result.

The Jet Power, Radio Loudness, and Black Hole Mass in Radio‐loud Active Galactic Nuclei

Jet formation is thought to be closely connected with the mass of the central supermassive black hole in active galactic nuclei. The radio luminosity commonly used in investigating this issue is merely an indirect measure of the energy transported through the jets from the central engine and is severely Doppler boosted in core-dominated radio quasars. In this work, we investigate the relationship between the jet power and the black hole mass, by estimating the jet power using extrapolated extended 151 MHz flux density from the VLA 5 GHz extended radio emission, for a sample of 146 radio-loud quasars compiled from the literature. After removing the effect of relativistic beaming in the radio and optical emission, we find a significant intrinsic correlation between the jet power and the black hole mass. It strongly implies that the jet power, like jet formation, is closely connected with the black hole mass. To eliminate the beaming effect in the conventional radio loudness, we define a new radio loudness as the ratio of the radio extended luminosity to the optical luminosity estimated from the broad-line luminosity. In a tentatively combined sample of radio-quiet with our radio-loud quasars, the apparent gap around the conventional radio loudness R = 10 is not prominent for the new-defined radio loudness. In this combined sample, we find a significant correlation between the black hole mass and new-defined radio loudness.

The extremely asymmetric radio structure of the z = 3.1 radio galaxy B3 J2330 + 3927

Abstract We report on 1.7- and 5.0-GHz observations of the z= 3.087 radio galaxy B3 J2330 + 3927, using the Very Long Baseline Array (VLBA), and archival 1.4- and 8.4-GHz Very Large Array (VLA) data. Our VLBA data identify a compact, flat-spectrum (α5 GHz1.7 GHz=−0.2 ± 0.1; Sν∝να) radio component as the core. The VLA images show that the fraction of core emission is very large (fc≈ 0.5 at 8.4 GHz), and reveal a previously undetected, very faint counterjet, implying a radio lobe flux density ratio R≳ 11 and a radio lobe distance ratio Q≈ 1.9. Those values are much more common in quasars than in radio galaxies, but the optical/near-infrared spectra show a clear type II active galactic nucleus for B3 J2330 + 3927, confirming that it is indeed a radio galaxy. Unlike all other radio galaxies, the bright Lyα-emitting gas is located towards the furthest radio arm. We argue against environmental and relativistic beaming effects being the cause of the observed asymmetry, and suggest this source has intrinsically asymmetric radio jets. If this is the case, B3 J2330 + 3927 is the first example of such a source at high redshift, and seems to be difficult to reconcile with the unified model, which explains the differences between quasars and radio galaxies as being due to orientation effects.

Synchrotron nebulae created by anisotropic magnetized pulsar winds

In this paper, we give a detailed description of the first attempt to study the properties of the flow produced by a magnetized pulsar wind within a plerionic nebula via fully relativistic magnetohydrodynamic (MHD) simulations. Following the current theoretical models of pulsar winds, we assume that in the equatorial direction the magnetization of the wind drops to zero but its energy flux reaches a maximum. The results of our 2D axisymmetric simulations reveal complex dynamics of the post-shock flow, very different from the steady quasi-radial outflow assumed in earlier analytical models for plerions. The termination shock has the shape of a distorted torus and most of the downstream flow is initially confined to the equatorial plane. Provided the wind magnetization is higher than a certain value, the magnetic hoop stress stops the outflow in the surface layers of the equatorial disc and redirects it into magnetically confined polar jets. The outflow in the inner layers of the equatorial disc continues until it reaches the slowly expanding outer shell and then turns back and forms the vortex flow filling the nebular volume at intermediate latitudes. We simulated the synchrotron images of the nebula taking into account the relativistic beaming effect and the particle energy losses. These images are strikingly similar to the well-known images of the Crab and other pulsar wind nebulae obtained by Chandra and the Hubble Space Telescope. They exhibit both a system of rings, which makes an impression of an equatorial disc-like or even a toroidal structure, and well-collimated polar jets, which appear to originate from the pulsar. A number of fine details of the inner Crab nebula find natural explanation including the bright knot discovered by Hester et al. in 1995 very close to the Crab pulsar.

Identifying Compact Symmetric Objects in the Southern Sky

We present results of multifrequency polarimetric VLBA observations of 20 compact radio sources. The observations represent the northern and southern extensions of a large survey undertaken to identify Compact Symmetric Objects (CSOs) Observed in the Northern Sky (COINS). CSOs are young radio galaxies whose jet axes lie close to the plane of the sky, and whose appearance is therefore not dominated by relativistic beaming effects. The small linear sizes of CSOs make them valuable for studies of both the evolution of radio galaxies and testing unified schemes for active galactic nuclei (AGN). In this paper we report on observations made of 20 new CSO candidates discovered in the northern and southern extremities of the VLBA Calibrator Survey. We identify 4 new CSOs, and discard 12 core-jet sources. The remaining 4 sources remain candidates pending further investigation. We present continuum images at 5 GHz and 15 GHz and, where relevant, images of the polarized flux density and spectral index distributions for the 8 new CSOs and CSO candidates.

Do X-ray binary spectral state transition luminosities vary?

We tabulate the luminosities of the soft-to-hard state transitions of all X-ray binaries for which there exist good X-ray flux measurements at the time of the transition, good distance estimates, and good mass estimates for the compact star. We show that the state transition luminosities are at about 1–4% of the Eddington rate, markedly smaller than those typically quoted in the literature, with a mean value of 2%. Only the black hole candidate GRO J 1655-40 and the neutron star systems Aql X-1 and 4U 1728-34 have measured state transition luminosities inconsistent with this value at the 1σ level. GRO J 1655-40, in particular, shows a state transition luminosity below the mean value for the other sources at the level. This result, combined with the known inner disk inclination angle (the disk is nearly parallel to the line of sight) from GRO J 1655-40's relativistic jets suggest that the hard X-ray emitting region in GRO J 1655-40 can have a velocity of no more than about , with a most likely value of about , and a minimum speed of , assuming that the variations in state transition luminosities are solely due to relativistic beaming effects. The variance in the state transition luminosities suggests an emission region with a velocity of ~0.2c. The results are discussed in terms of different emission models for the low/hard state. We also discuss the implications for measuring the dimensionless viscosity parameter α. We also find that if its state transitions occur at typical luminosities, then GX 339-4 is likely to be at a distance of at least 7.6 kpc, much further than typically quoted estimates.

A multi-wavelength test of the FR I–BL Lac unifying model

We collect multi-wavelength measurements of the nuclear emission of 20 low luminosity FR I radio-galaxies to test the viability of the FR I - BL Lac unifying model. Although poorly sampled, the Spectral Energy Distributions (SED) of FR Is are consistent with the double peaked shape characteristic of BL Lacs. Furthermore while the distribution of the FR Is in the broad-band spectral index planes shows essentially no overlap with the regions where HBL and LBL are located, this can be simply due to the effects of relativistic beaming. More quantitatively, deriving the beaming Doppler factor of a given radio-galaxy from its X-ray luminosity ratio with respect to BL Lacs with similar extended radio luminosity, we find that i) the luminosity in all bands, ii) the value of the spectral indices, iii) the slope of the X-ray spectrum, iv) the overall SED shape, may be all simultaneously reproduced. However, the corresponding jet bulk Lorentz factors are significantly smaller than those derived for BL Lacs from other observational and theoretical considerations. This suggests to consider a simple variant of the unification scheme that allows for the presence of a velocity structure in the jet.

An Ultraluminous X-Ray Object with a 2 Hour Period in M51

Ultraluminous X-ray objects (ULXs) are off-nucleus point sources with LX = 1039-1041 ergs s-1, but the nature of such systems is largely unidentified. Here we report a 2.1 hr period observed in a Chandra ACIS observation for ULX M51 X-7, which is located on the edge of a young star cluster in the star-forming region in a spiral arm. In two ACIS observations separated by 1 yr, the ULX changed from a high-hard to a low-soft spectral state, in contrast to most Galactic low-mass X-ray binaries. On the basis of its period and spectral behaviors, we suggest that this ULX is a low-mass X-ray binary system, with a dwarf companion of 0.2-0.3 M☉ and a compact accretor, either a neutron star or a black hole, whose mass is not well constrained. Relativistic beaming effects are likely involved to produce the observed high X-ray luminosities, given its low accretion rate as inferred from a sustainable accretion scenario via Roche lobe overflow.

On the relativistic beaming and orientation effects in core-dominated quasars

In this paper, we investigate the relativistic beaming effects in a well-defined sample of core-dominated quasars using the correlation between the relative prominence of the core with respect to the extended emission (defined as the ratio of core-to lobe-flux density measured in the rest frame of the source) and the projected linear size as an indicator of relativistic beaming and source orientation. Based on the orientation-dependent relativistic beaming and unification paradigm for high luminosity sources in which the Fanaroff-Riley class-II radio galaxies form the unbeamed parent population of both the lobe- and core-dominated quasars which are expected to lie at successively smaller angles to the line of sight, we find that the flows in the cores of these core-dominated quasars are highly relativistic, with optimum bulk Lorentz factor,γ opt∼ 6–16, and also highly anisotropic, with an average viewing angle, ∼ 9°–16°. Furthermore, the largest boosting occurs within a critical cone angle of ≈ 4°–10°.

블랙흘 쌍성계의 비대칭 광도 곡선과 DOPPLER BEANING 효과

블랙홀 쌍성계는 10 <TEX>$M_{\odot}$</TEX> 정도의 블랙홀과 K또는 분광형을 갖는 만기형 항성의 쌍성계로 10여 개가 보고되어 있다. 블랙홀의 동반성은 Roche lobe를 태우기 때문에, 궤도 위상에 따라서 이들의 밝기가 달라져서, 타원체형 광도 곡선을 나타내는 것으로 알려져 있다. 이들 쌍성계의 광도 곡선에서 최대 밝기 위상에서 수퍼센트 정도의 비대칭성이 보고되었으며, 이 현상은 흔히 항성 흑점 모형 또는 hot impact point모형을 사용하여 설명되고 있다 이 논문에서는 블랙홀 쌍성계의 공전 속도가 대략 400 km <TEX>$s^{-1}$</TEX>일 때에 Doppler 효과에 의 한 흑체의 온도 차이는 <TEX>$DeltaT/T$</TEX> ~ 1/400에 이르며, 이에 의한 광도의 차이가 관측된 비대칭성에 충분히 크게 영향을 줄 수 있음을 지적한다. 따라서, 광도 곡선의 해석에서 특수 상대론적 효과가 고려될 때에, 단순히 계산된 항성 흑점 혹은 hot impact point의 온도와 규모에 중대한 수정이 필요할 것으로 예상된다. Black hole binary candidates are known to be composed of a black hole with 10 <TEX>$M_{\odot}$</TEX> and a K or M type companion. Because the companion is believed to fill the Roche lobe that is very aspherical, the light curves of black hole binaries are characterized by an ellipsoidal variation. It has been known that the ellipsoidal light curves exhibit asymmetric maximum brightness at the orbital phases 0.25 and 0.75, which has been attributed to star spots or the hot impact points of the accretion flow on to the accretion disk around the black hole. In this paper, it is pointed out that the special relativistic beaming effect contributes to the asymmetry of several percent often observed in the light curves. The typical orbital velocity 400 km <TEX>$s^{-1}$</TEX> observed in black hole binaries may induce the temperature difference <TEX>$\DeltaT/T$</TEX> ~ 1/400 of the late type companion star in the observer`s rest frame, because of the special relativistic Doppler beaming effect. This difference in temperature can result in several per cent of brightness sensitively dependent on the wavelength band, which is comparable to what has been observed in most black hole binary candidates. Considering the significant contribution of the special relativistic Doppler beaming effect, we conclude that the estimation of the sizes and temperatures of the star spots or the hot impact point needs serious revision.

Parsec-Scale Radio Structure and Broad Optical Emission Lines in a Complete Sample of 3CR Lobe-dominated Quasars

We present results from VLBI observations of 24 of the 25 lobe-dominated quasars (LDQs) in the 3CR complete sample and from optical spectrophotometry of 14 of these objects. The VLBI observations were made with a variety of arrays—most recently the Very Long Baseline Array—at frequencies ranging from 5 to 22 GHz during the period 1981–1997. The optical spectra were obtained with the Hale 200 inch (5 m) telescope at Palomar Observatory, using the blue and red CCDs of the Double Spectrograph, between 1984 and 1992. The radio nuclei range in strength over nearly 3 orders of magnitude, from ∼0.9 Jy down to ∼3 mJy, and were imaged at typical resolutions of ∼0.5–1.0 mas and sensitivities of ∼0.1–0.2 mJy beam-1. All 24 LDQs show detectable radio structure in their nuclei. All 19 objects for which VLBI images could be made show one-sided nuclear jets, often several milliarcseconds in length and significantly curved, on the same side of the compact core as the one-sided large-scale jets seen on Very Large Array images. No counterjets were observed; jet-to-counterjet ratios that virtually all exceed ∼10 suggest that these objects are all oriented within ∼70° to the line of sight. For the 10 sources in which parsec-scale jet speeds could be estimated, the well-defined motions range from ≲h-1c to ∼4h-1c; some ambiguous cases might allow for motions as fast as ∼8h-1c, even then they are not indicative of bulk Lorentz factors, γ, in excess of ∼10h-1 (H0 = 100h km s-1 Mpc-1, q0 = 0.5). While the present distribution of jet speeds is consistent with random orientations, it is more easily accomodated by a restricted range of orientations. Evidence for slower jet speeds and larger apparent bends close to the core support the concept of a "transition" zone in the inner few parsecs of these jets. There are significant correlations among the prominences of the cores and jets, relative jet length, maximum jet deflection angles, and variability amplitude, as well as possible trends involving jet speed and strength of compact jet knots; furthermore, these objects tend to exhibit flat-spectrum cores and steep-spectrum jets. These results are all consistent with orientation-dependent relativistic beaming effects and unification of core- and lobe-dominated quasars. We have also defined a new postulated measure of orientation, based on both prominence of the radio nucleus and projected linear size, that correlates very well with properties predicted to have strong orientation dependence on beaming models. Finally, we confirm earlier studies demonstrating an anticorrelation between the prominence of the radio nucleus and the width of broad Mg II λ2798 optical emission lines; we also extend this result to C III] λ1909 and, more generally, to all broad lines by simple scaling and normalization methods. This relationship suggests a restricted range of orientations for LDQs, and that the broad-line clouds may have a flattened distribution associated with the accretion zone surrounding a central supermassive black hole.

The Effects of Relativistic Bulk Motion of X‐Ray Flares in the Corona on the Iron Kα Line in Seyfert 1 Galaxies

It is likely that dramatic magnetic flares in a corona above a black hole accretion disk dominate the X-ray emission in Seyfert 1 galaxies. Such flares are likely to move with mildly relativistic bulk velocity. We study the effects of the bulk motion of X-ray flares on the shape and equivalent width of the iron Kα line from an untruncated cold disk around a Kerr black hole using fully relativistic calculations. The flares are located above a cold accretion disk, either on or off the rotation axis. The upward/outward bulk motion of flares causes a reduction of the iron Kα line width, while the downward/inward bulk motion of flares causes an increase of the iron Kα line width. To a distant observer with a low inclination angle (θo 30°), larger upward/outward bulk velocities decrease the extension of the red wing, with little change in the location of the blue edge. In contrast, an observer at a large inclination angle (e.g., θo = 60°) sees both the red wing and the blue edge change with the bulk velocity. The equivalent width of the iron Kα line decreases rapidly with increasing bulk velocity of flares. However, the narrower line profiles observed in some objects (e.g., IC 4329A and NGC 4593) are difficult to produce using the outflowing magnetic flare model with an appropriate equivalent width unless the X-ray emission is concentrated in an outer region with a of several tens of rg = GM/c2 or more. This suggests that other parameters, such as the truncation radius due to disruption or ionization of the inner disk, may still be needed. An important result is that the iron Kα line intensity is found to be constant even though the continuum flux varies significantly, which is true for outflowing magnetic flares with different bulk velocities but similar intrinsic luminosities when located close to the central black hole. This is caused by the combination of the effects of relativistic beaming and gravitational lensing. We find that fluctuations in the bulk velocities of outflowing low-height flares located at the inner region (r 15rg) can account for a constant iron Kα line and significant continuum variation as observed in MCG -6-30-15 and NGC 5548. This is especially interesting for MCG -6-30-15, whose behavior is difficult to explain using the ionization model.

Specifying the Environments around GRB, Explaining the Fe Line in the X-Ray Afterglow of GRB 000214

Abstract We present a model for explaining the Fe K$\alpha$ line and the continuum in the afterglow of GRB 000214. In this paper, we pose the importance to seek the physically natural environment around GRB 000214. For reproducing the observation, we need a ring-like remnant around the progenitor, like that of SN 1987A produced by the mass-loss of the progenitor and the fireball spread over all directions. The observation of GRB 000214, in which the continuum power-law spectrum decreased faster than the line, motivated us to consider two independent systems for the line emission and the continuum spectrum. At first, the continuum spectrum can be fitted by the afterglow emission of the fireball pointing toward the observer, which does not collide with the ring because the emission of GRB and the afterglow are highly collimated to the observer by the relativistic beaming effect. Secondly, the line can be fitted by the fluorescence of the Fe atoms in the ring illuminated by the X-ray afterglow. The significance of this study is that our model may strongly constrain the GRB model.

The Pearson‐Readhead Survey of Compact Extragalactic Radio Sources from Space. II. Analysis of Source Properties

We have performed a multidimensional correlation analysis on the observed properties of a statistically complete core-selected sample of compact radio-loud active galactic nuclei based on data from the VLBI Space Observing Programme (Paper I) and previously published studies. Our sample is drawn from the well-studied Pearson-Readhead (PR) survey and is ideally suited for investigating the general effects of relativistic beaming in compact radio sources. In addition to confirming many previously known correlations, we have discovered several new trends that lend additional support to the beaming model. These trends suggest that the most highly beamed sources in core-selected samples tend to have (1) high optical polarizations; (2) large parsec- kiloparsec-scale jet misalignments; (3) prominent VLBI core components; (4) one-sided, core, or halo radio morphology on kiloparsec scales; (5) narrow emission line equivalent widths; and (6) a strong tendency for intraday variability at radio wavelengths. We have used higher resolution space and ground-based VLBI maps to confirm the bimodality of the jet misalignment distribution for the PR survey and find that the sources with aligned parsec- and kiloparsec-scale jets generally have arcsecond-scale radio emission on both sides of the core. The aligned sources also have broader emission line widths. We find evidence that the BL Lacertae objects in the PR survey are all highly beamed and have very similar properties to the high optically polarized quasars, with the exception of smaller redshifts. A cluster analysis on our data shows that after partialing out the effects of redshift, the luminosities of our sample objects in various wave bands are generally well correlated with each other but not with other source properties.

The Doppler Effect and Spectral Energy Distribution of Blazars

The relativistic beaming model is adopted to discuss quantitatively the observational differences between radio-selected BL Lac objects (RBLs) and X-ray-selected BL Lac objects (XBLs), and between BL Lac objects and flat spectrum radio quasars (FSRQs). The main results are the following: (1) In the Doppler corrected color-color (alpha (in)(ru) - alpha (in)(ox)) diagram, XBLs and FSRQs occupy separated regions, while RBLs bridge the gap between them. These properties suggest that similar intrinsic physical processes operate in all the objects under a range of intrinsic physical conditions. (2) Our results are consistent with the results of Sambruna, Maraschi and Urry (1996) from other methods. We show the alpha (xox) introduced by Sambruna to be a good index for describing the energy distribution because it represents the intrinsic energy distribution and includes the Doppler correction. (3) The Doppler effect of relativistic beaming is the main mechanism, and the physical differences (such as magnetic fields, electron energies) are also important complementary factors for understanding the relation between XBLs and RBLs.

Measuring the Brightness Temperature Distribution of Extragalactic Radio Sources with Space VLBI

We have used VLBI Space Observatory Programme (VSOP) observations to measure the brightness temperature distribution of a well-defined subset of the Pearson-Readhead sample of extragalactic radio sources. VLBI, which is restricted to Earth-diameter baselines, is not generally sensitive to emitting regions with brightness temperatures greater than approximately 1012 K, coincidentally close to theoretical estimates of brightness temperature limits, 1011-1012 K. We find that a significant proportion of our sample have brightness temperatures greater than 1012 K; many have unresolved components on the longest baselines, and some remain completely unresolved. These observations begin to bridge the gap between the extended jets seen with ground-based VLBI and the microarcsecond structures inferred from intraday variability, evidenced here by the discovery of a relationship between intraday variability and VSOP-measured brightness temperature, likely due to the effects of relativistic beaming. Also, lower limits on jet Lorentz factors, estimated from space VLBI observations, are starting to challenge numerical simulations that predict low Lorentz factor jets.

Modeling Jupiter's synchrotron radiation

We have constructed a computer model to simulate synchrotron emission from relativistic electrons trapped in Jupiter's magnetic field. The computer program generates the four Stokes parameters of the synchrotron emission for assumed electron distributions and magnetic field models. The resulting two dimensional Stokes parameter maps can be compared directly with ground based observations. We use magnetic field models derived from spacecraft measurements, and tailor the electron distributions to fit synchrotron observations. The gross features of data from both VLA and single‐dish observations are fit by a longitudinally symmetric particle distribution. We suggest that higher order terms in the magnetic field, coupled with relativistic beaming effects of synchrotron radiation, are primarily responsible for the observed rotational asymmetry.

Relativistic beaming effects in the spectra of cores and hotspots in radio galaxies and quasars

We present the results of multiwavelength observations of cores and hotspots, at L, C, X and U bands with the Very Large Array, of a matched sample of radio galaxies and quasars selected from the Molonglo Reference Catalogue. We use these observations to determine the spectra of cores and hotspots, and test the unified scheme for radio galaxies and quasars. Radio cores have been detected at all wavelengths in all of the quasars in our sample, whereas only ∼50 per cent of the galaxies have cores detected in at least one of the wavelengths. The degree of core prominence in this sample is consistent with the unified scheme for radio galaxies and quasars. A comparison of the distributions of the two-point spectral index of the cores in our sample of lobe-dominated quasars, with the distributions in a matched sample of core-dominated quasars, shows that the distributions for these two classes are significantly different, and this is consistent with the expectations of the unified scheme. The difference in the spectral indices of the two hotspots on opposite sides is also significantly larger for quasars than for radio galaxies, as is expected in the unified scheme. We also investigate the relationship between the spectral index of the hotspots and the redshift or luminosity for our sample of sources.

Redshifts and Neutral Hydrogen Observations of Compact Symmetric Objects in the COINS Sample

Compact symmetric objects (CSOs) are young radio galaxies whose jet axes lie close to the plane of the sky, and whose appearance is therefore not dominated by relativistic beaming effects. The small linear sizes of CSOs make them valuable for studies of both the evolution of radio galaxies and testing unified schemes for active galactic nuclei (AGNs). A parsec-scale region of gas surrounding the central engine is predicted by both accretion and obscuration scenarios. Working surfaces, or hot spots, and the radio jets of CSOs are close enough to the central engines that this circumnuclear gas can be seen in absorption. The CSOs observed in the northern sky (COINS) sample is comprised of 52 CSO candidates identified in three VLBI surveys. Of these, 27 have now been confirmed as CSOs. Optical redshifts are available in the literature for 28 of the CSO candidates, and H I absorption has been detected toward four. We present new optical spectroscopic redshifts for three of the candidates and summarize the current status of optical identifications. We further report on the discovery of H I in absorption toward the CSO J1816+3457 and summarize the results of neutral hydrogen absorption studies of the sources in this sample.