arXiv subjects

Paul J. Wiita

Publications and source records attributed to Paul J. Wiita.

At least 145 records · Page 8Linked to original sources

arXiv2006

Testing Models of the Individual and Cosmological Evolutions of Powerful Radio Galaxies

We seek to develop an essentially analytical model for the evolution of Fanaroff-Riley Class II radio galaxies as they age individually and as their numbers vary with cosmological epoch. Such modeling is required in order to probe in more detail the impact of radio galaxies on the growth of structures in the universe, which appears likely to have been quite significant at z > 1. In this first paper of a series we compare three rather sophisticated analytical models for the evolution of linear size and lobe power of FR II radio galaxies, those of Kaiser, Dennett-Thorpe & Alexander (1997), Blundell, Rawlings & Willott (1999), and Manolakou & Kirk (2002). We perform multi-dimensional Monte Carlo simulations in order to compare the predictions of each model for radio powers, sizes, redshifts and spectral indices with data. The observational samples used here are the low frequency radio surveys, 3CRR, 6CE and 7CRS, which are flux-limited and complete. We search for and describe the best parameters for each model, after doing statistical tests on them. We find that no existing model can give acceptable fits to all the properties of the surveys considered, although the Kaiser, Dennett-Thorpe & Alexander (1997) model gives better overall results than do the Manolakou & Kirk (2002) or Blundell, Rawlings & Willott (1999) models for most of the tests we performed. We suggest ways in which these models may be improved.

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arXiv2006

Active Galactic Nuclei: Unification, Blazar Variability and the Radio Galaxy/Cosmology Interface

We first review some basic properties of the most important classes of active galactic nuclei (AGN), including quasars, blazars, Seyfert galaxies and radio galaxies. The most commonly accepted type of scheme designed to individually unify the radio-loud and radio-quiet categories of AGN is based upon three parameters: black hole mass, accretion rate, and our orientation to the accretion flow. Some recent evidence from optical microvariability of several classes of AGN points in favor of a strong unification scheme that unites both radio-loud and radio-quiet categories. An important question concerning the nature of blazars and other AGN whose jet emission appears to dominate their spectral energy distributions involves the velocities of those flows. A variety of apparently contradictory observations can be reconciled if such flows are ultrarelativistic but have an opening angle of a few degrees. Radio galaxies (RGs) were much more numerous at redshifts ~2 than they are today. Combining this fact with the realization that older RGs at such redshifts are very difficult to detect, and with cosmological simulations of the growth of structure in the universe has led us to propose that RG lobes have impacted a significant fraction of the cosmic web of baryons. These impacts may have triggered extensive star formation and perhaps even engendered new galaxies; they also probably played important roles in the spreading of magnetic fields and heavier elements into the intergalactic medium.

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arXiv2006

Bulk Motion of Ultrarelativistic Conical Blazar Jets

Allowing for the conical shape of ultrarelativistic blazar jets with opening angles of a few degrees on parsec-scales we show that their bulk Lorentz factors and viewing angles can be much larger than the values usually inferred by combining their flux variability and proper motion measurements. This is in accord with our earlier finding that such ultrarelativistic (Lorentz factor, Gamma > 30) conical jets can reconcile the relatively slow apparent motions of VLBI knots in TeV blazars with the extremely fast flows implied by their rapid gamma-ray variability. This jet geometry also implies that de-projected jet opening angles will typically be significantly underestimated from VLBI measurements. In addition, de-projected jet lengths will be considerably overestimated if high Lorentz factors and significant opening angles are not taken into account.

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arXiv2005

Multiband optical monitoring of the blazars S5 0716+714 and BL Lacertae

We report results of multiband optical monitoring of two well known blazars, S5 0716+714 and BL Lacertae, carried out in 1996 and 2000-01 with an aim to study optical variations on time scales from minutes to hours and longer.The light curves were derived relative to comparison stars present on the CCD frames. Night to night flux variations of >0.1 mag were observed in S5 0716+714 during a campaign of ~2 weeks in 1996.A good correlation between the lightcurves in different optical bands was found for both inter-night and intra-night observations. Two prominent events of intra-night optical variability were detected in S5 0716+714.Each of these rapidly varying segments of the lightcurves trace an exponential flux profile whose rate of variation is the same in both cases. Our long-term monitoring data of S5 0716+714 showed a distinct flare around JD 2451875 which can be identified in the BVRI bands.This flare coincides with the brightest phase recorded during 1994-2001 in the long-term lightcurves reported by Raiteri et al.(2003). No evidence for the bluer when brighter trend was noticed on inter-night and intra-night time scales. On the other hand, our nearly simultaneous multiband observations of BL Lacertae in October 2001 showed flux variations that were not achromatic. This blazar was found to become bluer when brighter on intra-night time scales and there is a hint of the same trend on inter-night time scales. Based on five nights of observations during a week, BL Lacertae showed a peak night-to-night variability of ~0.6 mag in B. Thus, we found that the present observations of the two blazars, reveal a contrasting behaviour in terms of the dependence of spectral hardening with increasing brightness, at least on intra-night, and possibly also on inter-night, time scales.

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arXiv2005

Accretion Disks, Jets and Blazar Variability

Although blazar variability is probably dominated by emission from relativistic jets, accretion disks should be present in all blazars. These disks produce emission over most of the electromagnetic spectrum; various unstable processes operate in those disks which lead to variable emission. Here I summarize some of the most relevant disk mechanisms for AGN variability. I also discuss some aspects of jet variability, focusing on the possibility that ultrarelativisitic jets of modest opening angle can reconcile TeV blazar emission with the many subluminal VLBI knots seen in those sources. Finally, I present recently illuminated characteristics of optical microvariability of different classes of AGN which have important implications for the dominant processes involved.

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arXiv2004

Jet propagation and the asymmetries of CSS radio sources

As Compact Steep Spectrum radio sources have been shown to be more asymmetrical than larger sources of similar powers, there is a high probability that they interact with an asymmetric medium in the central regions of the host elliptical galaxy. We consider a simple analytical model of the propagation of radio jets through a reasonable asymmetric environment and show that they can yield the range of arm-length and luminosity asymmetries that have been observed. We then generalize this to allow for the effects of orientation, and quantify the substantial enhancements of the asymmetries that can be produced in this fashion. We present two-dimensional and three-dimensional simulations of jets propagating through multi-phase media and note that the results from the simulations are also broadly consistent with the observations.

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arXiv2004

Expansion of radio galaxies in a cosmologically evolving medium: Possible implications for the cosmic star-formation history

We compare earlier estimates of the volumes filled by lobes of radio galaxies during the quasar era based upon non-evolving ambient media with new ones assuming a strong cosmological evolution of the ambient medium. If the sources remain active for over 10^8 years the volumes filled by them are found to be comparable for the two scenarios. This strengthens our earlier inference that much of the cosmic web of gaseous filaments, the site of galaxy formation, was probably permeated by radio lobes during the quasar era and this could have triggered extensive star formation and made large contributions to the spread of magnetic fields and metals through the universe by z ~ 2.

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arXiv2004

Asymmetries in Powerful Extragalactic Radio Sources

This review provides the background and current perspective of some of the striking asymmetries exhibited by extragalactic double-lobed radio sources, paying special attention to the reported interrelationships among some of them. We particularly aim to highlight some less discussed aspects pertaining to the radio lobes and hot-spots for which any relativistic beaming effects are milder than those imposed on the jets. Various attributes of these features, such as their geometry, intensity, spectrum, and polarization provide important underpinnings to models of radio galaxies and quasars, as well as to theories that attempt to unify them. These contraints are sharpened by exploiting the asymmetry properties of the extended optical emission-line regions associated with powerful radio sources, including the giant halos of Lyman-alpha emission associated with them. We suggest that highly extended disks of dusty gaseous material, which could also be parts of cosmic filaments, around the host galaxies of radio sources may hold the key to understanding the correlated radio--optical asymmetry and perhaps also the depolarization asymmetry of the twin radio lobes. We also discuss the nature and possible implications of the extreme type of asymmetry witnessed in the enigmatic quasar 3C273, whose entire radio emission is observed on just one side of the nucleus. In this context, theoretical scenarios for the production of one-sided jets are also briefly recounted. Taking a clue from theobserved one-sidedness of the extended radio jets even in `weak-headed-quasars', we also propose a scenario for the late evolutionary stage of double radio sources. We end with a list of key questions concerning radio source asymmetries which still require final answers. (Abridged)

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arXiv2004

Do the mildly superluminal VLBI knots exclude ultrarelativistic blazar jets?

We compute the effective values of apparent transverse velocity and flux boosting factors for the VLBI radio knots of blazar jets, by integrating over the angular distributions of these quantities across the widths of jets with finite opening angles but constant velocities. For high bulk Lorentz factors (Gamma > 10) variations across the jet can be quite large if the opening angle, omega, is even a few degrees on sub-parsec scales. The resulting apparent speeds are often much lower than those obtained from the usual analyses that ignore the finite jet opening angles. We can thus reconcile the usually observed subluminal or mildly superluminal speeds with the very high (>~ 20) Gamma factors, required by the inverse Compton origin and rapid variability of TeV fluxes, as well as by intraday radio variability. Thus it is possible to associate the VLBI radio knots directly with shocks in the ultra-relativistic main jet flow, without invoking very rapid jet deceleration on parsec scales, or extremely unlikely viewing angles.

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arXiv2004

Dependence of General Relativistic Accretion on Black Hole Spin

We propose a novel approach to the investigation of how black hole spin influences the behavior of astrophysical accretion. We examine the terminal behavior of general relativistic, multi-transonic, hydrodynamic advective accretion in the Kerr metric, by analyzing various dynamical as well as thermodynamic properties of accreting matter extremely close to the event horizon, as a function of Kerr parameter $a$. The examination of the transonic properties of such flows can be useful in self-consistently discriminating between prograde and retrograde relativistic accretion, and in studying the spectral signatures of black hole rotation.

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arXiv2004

Brightness suppression of relativistic radio jets of quasars: The role of the lower electron energy cut-off

Although the possibility of a low energy cut-off (LEC) of the relativistic electron population (E_{min} ~ 0.1 GeV) in the different components of radio galaxies has been discussed in the literature, from both theoretical and observational viewpoints, its possible role in causing a distinct spectral flattening, and thus reducing the apparent brightness temperature of the radio nucleus, has thus far not been explicitly discussed. Here we point out that such an effect may in fact be quite significant for the parsec-scale, superluminal emission regions associated with the inner radio jets. This is because the observed frequency of the spectral turnover arising from the LEC increases linearly with the bulk Doppler factor (δ) of the jet flow, whereas the frequency of synchrotron self-absorption (the well known cause of spectral flattening and turnover at low frequencies) shows a mild decrease as δrises. For an observationally relevant range of source parameters, we investigate here the role of the LEC. We argue that some statistical trends revealed by the recent VLBI surveys can in fact be understood in terms of effects arising from LEC, including the faster superluminal motion found in the VLBI surveys at higher frequencies, and the apparent inability of most radio cores to even attain the brightness temperatures consistent with the equipartition condition. We also point out some possible implications of the LEC for X-ray observations of the kiloparsec scale relativistic jets of quasars.

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arXiv2003

Intranight optical variability of Blazars

We present results of a multi-epoch intra-night optical monitoring of eleven blazars consisting of six BL Lac objects and five radio core-dominated quasars (CDQs). These densely sampled and sensitive R-band CCD observations, carried out from November 1998 through May 2002 during a total of 47 nights with an average of 6.5 hours/night, have enabled us to detect variability amplitudes as low as $\sim 1%$ on intra-night time scales. A distinction is found for the first time between the intra-night optical variability (INOV) properties of the these two classes of relativistically beamed radio-loud AGNs. BL Lacs are found to show a duty cycle (DC) of INOV of $\sim$60%, in contrast to CDQs, which show a much smaller INOV DC of $\sim$20%, the difference being attributable mainly to the weakly polarized CDQs. On longer time scales (i.e., between a week to a few years) variability is seen from all the CDQs and BL Lacs in our sample. The results reported here form part of our long-term programme to understand the intra-night optical variability characteristics of the four main classes of luminous AGNs, i.e., radio-quiet quasars (RQQs) and radio lobe-dominated quasars (LDQs), as well as CDQs and BL Lac objects.

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arXiv2003

Radio emission and the optical isophotal twist of radio-loud ellipticals

Using the surface photometric analysis data for a well defined sample of 79 nearby radio galaxies (z < 0.12), it is shown that the level of radio emission associated with an elliptical galaxy during the radio-loud phase is related to the presence of large amounts of isophotal twist in its optical image. In particular, radio galaxies with P_408 > 3 x 10^{25} W/Hz appear to show a preference to be associated with elliptical hosts exhibiting an isophotal twist in excess of ~20 degrees.

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arXiv2003

The Origin of X-shaped Radio Galaxies: Clues from the Z-symmetric Secondary Lobes

Existing radio images of a few X-shaped radio galaxies reveal Z-symmetric morphologies in their weaker secondary lobes which cannot be naturally explained by either the galactic merger or radio-lobe backflow scenarios, the two dominant models for these X-shaped radio sources. We show that the merger picture can explain these morphologies provided one takes into account that, prior to the coalescence of their supermassive black holes, the smaller galaxy releases significant amounts of gas into the ISM of the dominant active galaxy. This rotating gas, whose angular momentum axis will typically not be aligned with the original jets, is likely to provide sufficient ram pressure at a distance ~10 kpc from the nucleus to bend the extant jets emerging from the central engine, thus producing a Z-symmetry in the pair of radio lobes. Once the two black holes have coalesced some 10^7 yr later, a rapid reorientation of the jets along a direction close to that of the orbital angular momentum of the swallowed galaxy relative to the primary galaxy would create the younger primary lobes of the X-shaped radio galaxy. This picture naturally explains why such sources typically have powers close to the FR I/II break. We suggest that purely Z-symmetric radio sources are often en route to coalescence and the concomitant emission of substantial gravitational radiation, while X-shaped ones have already merged and radiated.

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arXiv2003

Optical variability properties of high luminosity AGN classes

We present the results of a comparative study of the intra-night optical variability (INOV) characteristics of radio-loud and radio-quiet quasars, which involves a systematic intra-night optical monitoring of seven sets of high luminosity AGNs covering the redshift range {\it z} $\simeq 0.2$ to {\it z} $\simeq 2.2$. The sample, matched in the optical luminosity -- redshift (M$_B$ -- z) plane, consists of seven radio-quiet quasars (RQQs), eight radio lobe-dominated quasars (LDQs), six radio core-dominated quasars (CDQs) and five BL Lac objects (BLs). Systematic CCD observations, aided by a careful data analysis procedure, have allowed us to detect INOV with amplitudes as low as 1%. Present observations cover a total of 113 nights (720 hours) with only a single quasar monitored as continuously as possible on a night. Considering cases of only unambiguous detections of INOV we have estimated duty cycles (DCs) of 17%, 12%, 20% and 72% respectively for RQQs, LDQs, CDQs, and BLs. The low amplitude and low DC of INOV shown by RQQs compared to BLs can be understood in terms of their having optical synchrotron jets which are modestly misdirected from us. From our fairly extensive dataset, no unambiguous general trend of a correlation between the INOV amplitude and the apparent optical brightness of the quasar is noticed.

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arXiv2003

Intranight optical variability of radio-quiet and radio lobe dominated quasars

We present results of a programme of multi-epoch, intra-night optical monitoring of a sample of non-blazar type AGN, which includes seven radio-quiet QSOs (RQQs) and an equal number of radio-loud, lobe-dominated quasars (LDQs), covering a redshift range from about 0.2 to 2.0. These two sets of optically bright and intrinsically luminous QSOs are well matched in the redshift-optical luminosity ($z - M_B$) plane. Our CCD monitoring covered a total of 61 nights with an average of 6.1 hours of densely sampled monitoring of just a single QSO per night, thereby achieving a typical detection threshold of $\sim 1$% variation over the night. Unambiguous detection of intra-night variability(INOV) amplitudes in the range 1$-$3% on day-like or shorter time scales were thus made for both RQQs and LDQs. Based on these clear detections of INOV, we estimate duty cycles of 17% and 9% for RQQs and LDQs, respectively; inclusion of the two cases of probable variations of LDQs would raise the duty cycle to 15% for LDQs. The similarity in the duty cycle and amplitude of INOV for the RQQs and LDQs suggests, firstly, that the radio loudness alone does not guarantee an enhanced INOV in QSOs and,secondly, that as in LDQs, relativistic jets may also be present in RQQs. We argue that, as compared to BL Lacs, the conspicuously milder, rarer and possibly slower INOV of RQQs and LDQs, can in fact be readily understood in terms of their having optical synchrotron jets which are modestly misaligned from us,but are otherwise intrinsically as relativistic and active as the jets in BL Lacs.

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arXiv2003

Clear Evidence for Intranight Optical Variability in Radio-quiet Quasars

We present new clues to the problem of the radio loudness dichotomy arising from an extensive search for intranight optical variability in seven sets of optically luminous radio-quiet quasars and (radio-loud) BL Lacertae objects, which are matched in optical luminosity and redshift. Our monitoring of radio-quiet quasars has for the first time clearly detected such intranight variability, with peak-to-peak amplitudes ~1%, occurring with a duty cycle of ~ 1/6. The matched BL Lacs have both higher variability amplitudes and duty cycles when observed in the same fashion. We show that the much less pronounced intranight variability of the radio-quiet quasars relative to BL Lacs can be understood in terms of a modest misalignment of the jets in radio-quiet quasars from the line-of-sight. We thus infer that relativistic particle jets may well also emerge from radio-quiet quasars, but while traversing the short optical-emitting distances, they could be snuffed out, possibly through inverse Compton losses in the nuclear region.

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arXiv2003

Radio Galaxies and the Magnetization of the IGM

Observed radio galaxies had a much higher comoving density during the `quasar era', at z ~ 2-3, but these sources are only detectable for small fractions of their active lifetimes at such high z due to expansion losses and increased inverse Compton losses against the cosmic microwave background. Using recent models for the evolution of the size and luminosity of powerful double radio sources, as well as LCDM simulations of the cosmic web of baryonic material, we argue that during the quasar era a high volume fraction of this web was occupied by the lobes of double radio sources. They could have seeded the IGM with an average magnetic field approaching 10^{-8} G. Further, these advancing overpressured lobes could compress the denser interstellar gas clouds of the galaxies engulfed by them and thus trigger starbursts. This can probably account for much of the intense star-formation activity witnessed beyond z ~ 1.5. Also, the sweeping up of the ISM of the gas-rich galaxies by the rapidly advancing radio lobes may well be responsible for the widespread metal pollution of the IGM and proto-galaxies at high redshifts.

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