Statistical analyses of long-term variability of AGN at high radio frequencies

We present a study of variability time scales in a large sample of Active Galactic Nuclei at several frequencies between 4.8 and 230 GHz. We investigate the differences of various AGN types and frequencies and correlate the measured time scales with physical parameters such as the luminosity and the Lorentz factor. Our sample consists of both high and low polarization quasars, BL Lacertae objects and radio galaxies. The basis of this work is the 22 GHz, 37 GHz and 87 GHz monitoring data from the Metsahovi Radio Observatory spanning over 25 years. In addition,we used higher 90 GHz and 230 GHz frequency data obtained with the SEST-telescope between 1987 and 2003. Further lower frequency data at 4.8 GHz, 8 GHz and 14.5 GHz from the University of Michigan monitoring programme have been used. We have applied three different statistical methods to study the time scales: The structure function, the discrete correlation function and the Lomb-Scargle periodogram. We discuss also the differences and relative merits of these three methods. Our study reveals that smaller flux density variations occur in these sources on short time scales of 1-2 years, but larger outbursts happen quite rarely, on the average only once in every 6 years. We do not find any significant differences in the time scales between the source classes. The time scales are also only weakly related to the luminosity suggesting that the shock formation is caused by jet instabilities rather than the central black hole.Comment: 19 pages, 12 figures, Accepted for publication in A&

Two classes of radio flares in the blazar PKS 0420−-014

The two 5GHz VLBI (Very Long Baseline Interferometry) observations (1996 June and 1997 November) presented in this paper, combined with several formerb VLBI observations at 8.4GHz and 5GHz, suggest that the radio flares of the blazar PKS 0420$-$014 can be divided into two classes according to their geometric origins in 5 or 8.4GHz VLBI maps and the properties of light curves. One class of flares, which we call {\it core flares}, originate from the core. Core flares have large lags between the light curves at different frequencies, and will probably lead to the ejection of new jet components. The other class of flares, which we call {\it jet flares}, come from jet components. Jet flares vary simultaneously at different wavelengths, and may due to the Doppler boosting effect of rotating knots moving along a helical jet. The radio flare in 1991, accompanied by a simultaneous gamma-ray flare, was identified as a core flare.Comment: 9 pages, 3 figure

Wavelet analysis of a large sample of AGN at high radio frequencies

We have studied the characteristic timescales of 80 AGNs at 22, 37 and 90 GHz examining the properties of the wavelet method and comparing them to traditional Fourier-based methods commonly used in astronomy. We used the continuous wavelet transform with the Morlet wavelet to study the characteristic timescales. We also gain information when the timescale is present in the flux curve and if it is persistent or not. Our results show that the sources are not periodic and changes in the timescales over a long time are common. The property of wavelets to be able to distinguish when the timescale has been present is superior to the Fourier-based methods. Therefore we consider it appropriate to use wavelets when the quasi-periodicities in AGNs are studied.Comment: 14 pages, 10 figures, A&A in pres

Long-term radio variability of AGN: flare characteristics

We have studied the flare characteristics of 55 AGN at 8 different frequency bands between 4.8 and 230 GHz. Our extensive database enables us to study the various observational properties of flares in these sources and compare our results with theoretical models. We visually extracted 159 individual flares from the flux density curves and calculated different parameters, such as the peak flux density and duration, in all the frequency bands. The selection of flares is based on the 22 and 37 GHz data from Mets\"ahovi Radio Observatory and 90 and 230 GHz data from the SEST telescope. Additional lower frequency 4.8, 8, and 14.5 GHz data are from the University of Michigan Radio Observatory. We also calculated variability indices and compared them with earlier studies. The observations seem to adhere well to the shock model, but there is still large scatter in the data. Especially the time delays between different frequency bands are difficult to study due to the incomplete sampling of the higher frequencies. The average duration of the flares is 2.5 years at 22 and 37 GHz, which shows that long-term monitoring is essential for understanding the typical behaviour in these sources. It also seems that the energy release in a flare is independent of the duration of the flare.Comment: 11 pages, 9 figures, 2 tables, accepted for publication in A&

The extragalactic radio-source population at 95 GHz

We have used the Australia Telescope Compact Array (ATCA) at 95GHz to carry out continuum observations of 130 extragalactic radio sources selected from the Australia Telescope 20GHz (AT20G) survey. Over 90% of these sources are detected at 95 GHz, and we use a triple-correlation method to measure simultaneous 20 and 95 GHz flux densities. We show that the ATCA can measure 95GHz flux densities to ~10% accuracy in a few minutes for sources above ~50mJy. The median 20-95GHz spectral index does not vary significantly with flux density for extragalactic sources with S20>150 mJy. This allows us to estimate the extragalactic radio source counts at 95GHz by combining our observed 20-95GHz spectral-index distribution with the accurate 20GHz source counts measured in the AT20G survey. The resulting 95GHz source counts down to 80 mJy are significantly lower than those found by several previous studies. The main reason is that most radio sources with flat or rising spectra in the frequency range 5-20GHz show a spectral turnover between 20 and 95 GHz. As a result, there are fewer 95GHz sources (by almost a factor of two at 0.1 Jy) than would be predicted on the basis of extrapolation from the source populations seen in lower-frequency surveys. We also derive the predicted confusion noise in CMB surveys at 95GHz and find a value 20-30% lower than previous estimates. The 95GHz source population at the flux levels probed by this study is dominated by QSOs with a median redshift z~1. We find a correlation between optical magnitude and 95GHz flux density which suggests that many of the brightest 95 GHz sources are relativistically beamed, with both the optical and millimetre continuum significantly brightened by Doppler boosting.Comment: Replaced with final version (MNRAS, in press), 15 pages plus two landscape data table

Optical and radio behaviour of the BL Lacertae object 0716+714

Eight optical and four radio observatories have been intensively monitoring the BL Lac object 0716+714 in the last years: 4854 data points have been collected in the UBVRI bands since 1994, while radio light curves extend back to 1978. Many of these data are presented here for the first time. The long-term trend shown by the optical light curves seems to vary with a characteristic time scale of about 3.3 years, while a longer period of 5.5-6 years seems to characterize the radio long-term variations. In general, optical colour indices are only weakly correlated with brightness. The radio flux behaviour at different frequencies is similar, but the flux variation amplitude decreases with increasing wavelength. The radio spectral index varies with brightness (harder when brighter), but the radio fluxes seem to be the sum of two different-spectrum contributions: a steady base level and a harder-spectrum variable component. Once the base level is removed, the radio variations appear as essentially achromatic, similarly to the optical behaviour. Flux variations at the higher radio frequencies lead the lower-frequency ones with week-month time scales. The behaviour of the optical and radio light curves is quite different, the broad radio outbursts not corresponding in time to the faster optical ones and the cross-correlation analysis indicating only weak correlation with long time lags. However, minor radio flux enhancements simultaneous with the major optical flares can be recognized, which may imply that the mechanism producing the strong flux increases in the optical band also marginally affects the radio one.Comment: 18 pages, 15 Postscript figures, 5 JPEG figures, accepted for publication in A&

Multiwaveband Observations of Quasars with Flat Radio Spectra and Strong Millimeter Emission

We present multiwaveband observations of a well selected sample of 28 quasars and two radio galaxies with flat radio spectra and strong millimeter wave emission (referred to here as FSRQ's). The observations include multifrequency VLBI measurements, X-ray observations with ROSAT and submillimeter observations with the JCMT. Particularly interesting among many findings is a correlation between the X-ray to millimeter spectral index and fraction of flux density contained in the VLBI core. This tendency toward higher X-ray fluxes from sources with stronger jet emission implies that the knots in the jet are the prominent source of X-rays.Comment: 38 pages, 17 figures, 12 tables, accepted for publication in Ap J Suppl, May 199

Towards a mechanistic understanding of linguistic diversity

Our species displays remarkable linguistic diversity. While the uneven distribution of this diversity demands explanation, the drivers of these patterns have not been conclusively determined. We address this issue in two steps. First, we review previous empirical studies that have suggested environmental, geographical, and socio-cultural drivers of linguistic diversification. However, contradictory results and methodological variation make it difficult to draw general conclusions. Second, we outline a program for future research. We suggest that future analyses should account for interactions among causal factors, lack of spatial and phylogenetic independence of data, and transitory patterns. Recent analytical advances in biogeography and evolutionary biology, such as simulation modeling of diversity patterns, hold promise for testing four key mechanisms of language diversification proposed here: neutral change, population movement, contact, and selection. Future modeling approaches should also evaluate how the outcomes of these processes are influenced by demography, environmental heterogeneity, and time

Near-Infrared Molecular Hydrogen Emission from the Central Regions of Galaxies: Regulated Physical Conditions in the Interstellar Medium

The central regions of many interacting and early-type spiral galaxies are actively forming stars. This process affects the physical and chemical properties of the local interstellar medium as well as the evolution of the galaxies. We observed near-infrared H2 emission lines: v=1-0 S(1), 3-2 S(3), 1-0 S(0), and 2-1 S(1) from the central ~1 kpc regions of the archetypical starburst galaxies, M82 and NGC 253, and the less dramatic but still vigorously star-forming galaxies, NGC 6946 and IC 342. Like the far-infrared continuum luminosity, the near-infrared H2 emission luminosity can directly trace the amount of star formation activity because the H2 emission lines arise from the interaction between hot and young stars and nearby neutral clouds. The observed H2 line ratios show that both thermal and non-thermal excitation are responsible for the emission lines, but that the great majority of the near-infrared H2 line emission in these galaxies arises from energy states excited by ultraviolet fluorescence. The derived physical conditions, e.g., far-ultraviolet radiation field and gas density, from [C II] and [O I] lines and far-infrared continuum observations when used as inputs to photodissociation models, also explain the luminosity of the observed H2 v=1-0 S(1) line. The ratio of the H2 v=1-0 S(1) line to far-IR continuum luminosity is remarkably constant over a broad range of galaxy luminosities; L_H2/L_FIR = about 10^{-5}, in normal late-type galaxies (including the Galactic center), in nearby starburst galaxies, and in luminous IR galaxies (LIRGs: L_FIR > 10^{11} L_sun). Examining this constant ratio in the context of photodissociation region models, we conclude that it implies that the strength of the incident UV field on typical molecular clouds follows the gas density at the cloud surface.Comment: Accepted for ApJ, 24 pages, 17 figures, for complete PDF file, see http://kao.re.kr/~soojong/mypaper/2004_pak_egh2.pd

Structure of the Accretion Flow in Broad-Line Radio Galaxies: The Case of 3C390.3

We present XMM and Suzaku observations of the Broad-Line Radio Galaxy (BLRG) 3C390.3. The Fe Ka line has a width FWHM ~ 8,800 km/s, consistent within a factor two with the width of the double-peaked H_alpha line, suggesting an origin from the Broad Line Region. The data show for the first time a weak, broad bump extending from 5 to 7 keV. When fitted with a Gaussian, its centroid energy is 6.6 keV in the source's rest-frame with FWHM of 43,000 km/s and EW of 50 eV; its most likely interpretation is emission from He-like Fe (Fe XXV), suggesting the presence of an ionized medium in the inner regions of 3C390.3. The broad-band 0.5-100 keV continuum is well described by a single power law with photon index Gamma=1.6 and cutoff energy 157 keV, plus cold reflection with strength R=0.5. In addition, ionized reflection is required to account for the 6.6 keV bump in the broad-band continuum, yielding an ionization parameter xi ~ 2700 ergs cm s^-1; the inner radius of the ionized reflector is constrained to be larger than 20 r_G, although this result depends on the assumed emissivity profile of the disk. If true, we argue that the lack of broad Fe K emission from within 20 r_G indicates that the innermost regions of the disk in 3C390.3 are obscured and/or poorly illuminated. While the SED of 3C390.3 is generally dominated by accretion-related continuum, during accretion low states the jet can significantly contribute in the optical to X-ray bands via synchrotron self-Compton emission. (Abridged)Comment: 7 figures, 5 tables, accepted for publication in Ap