Orientation-based Unification Scheme Research Articles

Investigating the unification of LOFAR-detected powerful AGN in the Boötes field

Low radio frequency surveys are important for testing unified models of radio-loud quasars and radio galaxies. Intrinsically similar sources that are randomly oriented on the sky will have different projected linear sizes. Measuring the projected linear sizes of these sources provides an indication of their orientation. Steep-spectrum isotropic radio emission allows for orientation-free sample selection at low radio frequencies. We use a new radio survey of the Bo\"otes field at 150 MHz made with the Low Frequency Array (LOFAR) to select a sample of radio sources. We identify 44 radio galaxies and 16 quasars with powers $P>10^{25.5}$ W Hz$^{-1}$ at 150 MHz using cross-matched multi-wavelength information from the AGN and Galaxy Evolution Survey (AGES), which provides spectroscopic redshifts. We find that LOFAR-detected radio sources with steep spectra have projected linear sizes that are on average 4.4$\pm$1.4 larger than those with flat spectra. The projected linear sizes of radio galaxies are on average 3.1$\pm$1.0 larger than those of quasars (2.0$\pm$0.3 after correcting for redshift evolution). Combining these results with three previous surveys, we find that the projected linear sizes of radio galaxies and quasars depend on redshift but not on power. The projected linear size ratio does not correlate with either parameter. The LOFAR data is consistent within the uncertainties with theoretical predictions of the correlation between the quasar fraction and linear size ratio, based on an orientation-based unification scheme.

X-ray spectral properties of Seyfert galaxies and the unification scheme

Aims: The unification scheme of Seyfert galaxies predicts that the observed differences between type 1 and type 2 Seyfert galaxies are solely due to the differing orientations of the toroidal-shaped obscuring material around AGN. The observed X-ray spectra of Seyfert type 2s compared to type 1s are expected to be affected by higher absorbing column density due to the edge-on view of the obscuring torus. We study the 0.5 - 10 keV X-ray spectral properties of Seyfert type 1s and type 2s with the aim to test the predictions of Seyfert unification scheme in the X-ray regime. Methods: We use an optically selected Seyfert sample in which type 1s and type 2s have matched distributions in the orientation independent parameters of AGN and host galaxy. Results: The 0.5 - 10 keV XMM-Newton pn X-ray spectra of Seyfert galaxies are in general best fitted with a model consists of an absorbed power-law, a narrow Gaussian fitted to the Fe K{\alpha} emission line and an often seen soft excess component characterized by either a thermal plasma model with temperature kT \sim 0.1 - 1.0 keV and/or a steep power-law. The 2.0 - 10 keV hard X-ray continuum emission in several Seyfert type 2s is reflection dominated and suggests the Compton-thick obscuration. Results on the statistical comparison of the distributions of the observed X-ray luminosities in the soft (0.5 - 2.0 keV) and hard (2.0 - 10.0 keV) bands, the X-ray absorbing column densities, the equivalent widths of Fe K{\alpha} line and the flux ratios of hard X-ray to [OIII] {\lambda}5007{\AA} for the two Seyfert subtypes are consistent with the obscuration and orientation based unification scheme.

AN INFRARED COMPARISON OF TYPE-1 AND TYPE-2 QUASARS

We model the optical to far-infrared spectral energy distributions (SEDs) of a sample of six type-1 and six type-2 quasars selected in the mid-infrared. The objects in our sample are matched in mid-IR luminosity and selected based on their Spitzer IRAC colors. We obtained new targeted Spitzer Infrared Spectrograph and Multiband Imaging Photometer for Spitzer observations and used archival photometry to examine the optical to far-IR SEDs. We investigate whether the observed differences between samples are consistent with orientation-based unification schemes. The type-1 objects show significant emission at 3 μm. They do not show strong polycyclic aromatic hydrocarbon (PAH) emission and have less far-IR emission on average when compared to the type-2 objects. The SEDs of the type-2 objects show a wide assortment of silicate features, ranging from weak emission to deep silicate absorption. Some also show strong PAH features. In comparison, silicate is only seen in emission in the type-1 objects. This is consistent with some of the type-2's being reddened by a foreground screen of cooler dust, perhaps in the host galaxy itself. We investigate the active galactic nucleus contribution to the far-IR emission and find it to be significant. We also estimate the star formation rate (SFR) for each of the objects by integrating the modeled far-IR flux and compare this with the SFR found from PAH emission. We find that the type-2 quasars have a higher average SFR than the type-1 quasars based on both methods, though this could be due to differences in bolometric luminosities of the objects. While we find pronounced differences between the two types of objects, none of them are inconsistent with orientation-based unification schemes.

Kiloparsec Jet Speeds in Classical Double Radio Sources

On the basis of an intrinsically symmetric relativistic jet model, analytical equations between the mean jet speed and the mean jet flux asymmetry, and between their variances are derived in the context of an orientation-based unification scheme. The mean jet speeds of classical double radio sources are estimated on kiloparsec scales by using the asymmetry of jet-counterjet flux densities taken from the 3CRR and B2 samples. For FRI radio sources the mean jet speed is ∼ (0.54 ± 0.03)c, while for FRII low-redshift radio galaxies and intermediate-redshift quasars the values found are ∼ (0.4 ± 0.06)c and ≳ 0.6c respectively.

An asymmetric relativistic model for classical double radio sources

There is substantial observational evidence against the symmetric relativistic model of FR II radio sources. An asymmetric relativistic model is proposed which takes account of both relativistic effects and intrinsic/environmental asymmetries to explain the structural asymmetries of their radio lobes. A key parameter of the model is the jet-side of the double sources, which is estimated for 80 per cent of the FR II sources in the 3CRR complete sample. Statistical analyses of the properties of these sources show that the asymmetric model is in agreement with a wide range of observational data, and that the relativistic and intrinsic asymmetry effects are of comparable importance. Intrinsic/environmental asymmetry effects are more important at high radio luminosities and small physical scales. The mean translational speed of the lobes is found to be consistent with the speeds found from spectral ageing arguments. According to a Gaussian model, the standard deviation of the distribution of vlobe is σvl=0.04c. The results are in agreement with an orientation-based unification scheme in which the critical angle separating the radio galaxies from quasars is about 45°.

An X‐Ray Spectral Survey of Radio‐loud Active Galactic Nuclei withASCA

We present a uniform and systematic analysis of the 0.6-10 keV X-ray spectra of radio-loud active galactic nuclei (AGNs) observed by ASCA. The sample, which is not statistically complete, includes 10 broad-line radio galaxies (BLRGs), five radio-loud quasars (QSRs), nine narrow-line radio galaxies (NLRGs), and 10 radio galaxies (RGs) of mixed FR I and FR II types. For several sources the ASCA data are presented here for the first time. The exposure times of the observations and the fluxes of the objects vary over a wide range; as a result, so does the signal-to-noise ratio of the individual X-ray spectra. At soft X-rays, about 50% of NLRGs and 100% of RGs exhibit thermal plasma emission components, with bimodal distributions of temperatures and luminosities. This indicates that the emission in such an object arises in hot gas either in a surrounding cluster or loose group or in a hot corona, consistent with previous ROSAT and optical results. At energies above 2 keV, a hard power-law component (photon index Γ ~ 1.7-1.8) is detected in 90% of cases. The power-law photon indices and luminosities in BLRGs, QSRs, and NLRGs are similar. This is consistent with simple orientation-based unification schemes for lobe-dominated radio-loud sources in which BLRGs, QSRs, and NLRGs harbor the same type of central engine. Moreover, excess cold absorption in the range 1021-1024 cm-2 is detected in most (but not all) NLRGs, consistent with absorption by obscuring tori, as postulated by unification scenarios. The ASCA data provide initial evidence that the immediate gaseous environment of the X-ray source of BLRGs may be different than in Seyfert 1 galaxies: absorption edges of ionized oxygen, common in the latter, are detected in only one BLRG. Instead we detect large columns of cold gas in a fraction (~44%-60%) of BLRGs and QSRs, comparable to the columns detected in NLRGs, which is puzzling. This difference hints at different physical and/or geometrical properties of the medium around the X-ray source in radio-loud AGNs compared to their radio-quiet counterparts, properties that can be explored further with future X-ray observations. For the full sample, the nuclear X-ray luminosity is correlated with the luminosity of the [O III] emission line, the FIR emission at 12 μm, and the lobe radio power at 5 GHz. The Fe Kα line is detected in 50% of BLRGs and in one QSR, with a large range of intrinsic widths and equivalent widths. In the handful of NLRGs where it is detected, the line is generally unresolved. Comparing the average power-law photon indices of the various classes of radio-loud AGNs to their radio-quiet counterparts from the literature, we find only a weak indication that the ASCA 2-10 keV spectra of BLRGs are flatter than those of Seyfert 1 galaxies of comparable X-ray luminosity. This result is at odds with evidence from samples studied by other authors suggesting that radio-loud AGNs have flatter spectra than radio-quiet ones. Rather, it supports the idea that a beamed synchrotron self-Compton component related to the radio source (jet) is responsible for the flatter slopes in those radio-loud AGNs. We argue that, because of the way those samples were constructed, beamed X-ray emission from the radio jets probably contributed to the observed X-ray spectra. The sample studied here includes six weak-line radio galaxies (WLRGs), powerful radio galaxies characterized by [O III] 4569 and 5007 A of unusually low luminosity and by unusually high [O II]/[O III] line ratios. The ASCA spectra of WLRGs can be generally decomposed into a soft thermal component with kT ~ 1 keV, plus a hard component, described either by a flat (Γ = 1.5) absorbed power law or by a very hot (kT ~ 100 keV) thermal bremsstrahlung model. Their intrinsic luminosities are in the range L2-10 keV ~ 1040-1042 ergs s-1, 2 orders of magnitude lower than in other sources in our sample. If the hard X-ray emission is attributed to a low-luminosity AGN, an interesting possibility is that WLRGs represent an extreme population of radio galaxies in which the central black hole is accreting at a rate well below the Eddington rate.

The Linear Sizes of Quasars and Radio Galaxies in the Unified Scheme

A key argument in favor of orientation-based unification schemes is the finding that among the most powerful 3CRR radio sources the (apparent) median linear size of quasars is smaller than that of radio galaxies, which supports the idea that quasars are a subset of radio galaxies, distinguished by being viewed at smaller angles to the line of sight. Recent measurements of radio sizes for a few other low-frequency samples are, however, not in accord with this trend, leading to the claim that orientation may not be the main difference between radio galaxies and quasars. We point out that this inconsistency can be removed by making allowance for the temporal evolution of sources in both size and luminosity, as inferred from independent observations. This approach can also readily explain the other claimed major discrepancy with the unified scheme, namely, the difference between the radio luminosity-size correlations for quasars and radio galaxies.

Reconciling the magnetic field structures seen in variable active galactic nuclei with the unified scheme

IN the orientation-based unification scheme for active galaxies, the most variable active galactic nuclei—the BL Lacertae objects and highly polarized quasars (HPQs)—correspond to radio galaxies with their relativistic jets oriented close to our line of sight1–7. This otherwise successful scheme has recently been chal-lenged by radio polarimetric very-long-baseline interferometry8,9, which reveals different magnetic field structures in the knots of synchrotron emission identified with shocks in these jets. For BL Lacs the field is generally perpendicular to the jet, whereas no such trend is apparent for HPQs. This dichotomy has led to the claim that luminous BL Lacs and HPQs are inherently different types of object10. Here we propose that this difference can be readily explained within the unified scheme by the effect of relativistic aberration: at small viewing angles, the observed knot emission arises primarily from the freshly excited plasma of the shock front, whereas at larger angles, Doppler-boosted emission from the slower, post-shock plasma is the dominant contribution. We identify the former case with BL Lacs and the latter with HPQs.