A 15 years monitoring program at 408 MHz

Bondì, M.; Padrielli, L.; Fanti, R.; Ficarra, A.; Gregorini, L.; Mantovani, F.

doi:10.1051/aas:1996276

Cited by 36 publications

(47 citation statements)

References 4 publications

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“…In the minority of the low-frequency variable sources (LFV), the variability is correlated with that at high frequencies, with events started at high frequencies and propagated to low frequencies with decreasing amplitude. This is a signature of intrinsic variability caused by expansion of the emitting region, which has been confirmed from a comparison between flux density variability at 408 MHz and structural changes observed with VLBI observations at 1.67 GHz in several LFV sources (Bondi et al 1996). The majority of sources selected on the basis of LFV, however, showed no correlation between variability at low and high frequencies, which has been interpreted in terms of extrinsic variability due to refractive interstellar scintillation (e.g., Bondi et al 1994, and references therein).…”

Section: Correlations Between Radio Properties Of the Jetsupporting

confidence: 64%

Radio-optical scrutiny of compact AGN: correlations between properties of pc-scale jets and optical nuclear emission

Arshakian¹,

Torrealba

Chavushyan

et al. 2010

A&A

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Aims. We study the correlations between the VLBA (Very Long Baseline Array) radio emission at 15 GHz, extended emission at 151 MHz, and optical nuclear emission at 5100 Å for a complete sample of 135 compact jets. Methods. We use the partial Kendall's tau correlation analysis to check the link between radio properties of parsec-scale jets and optical nuclear luminosities of host active galactic nuclei (AGN). Results. We find a significant positive correlation for 99 quasars between optical nuclear luminosities and total radio (VLBA) luminosities of unresolved cores at 15 GHz originated at milliarcseconds scales. For 18 BL Lacs, the optical continuum emission correlates with the radio emission of the jet at 15 GHz. We suggest that the radio and optical emission are beamed and originate in the innermost part of the sub-parsec-scale jet in quasars. Analysis of the relation between the apparent speed of the jet and the optical nuclear luminosity at 5100 Å supports the relativistic beaming model for the optical emission generated in the jet, and allows the peak values of the intrinsic optical luminosity of the jet and its Lorentz factor to be estimated for the populations of quasars (2 × 10 20 W Hz −1 and γ = 52), BL Lacs (9 × 10 21 W Hz −1 and γ = 20), and radio galaxies (1.5 × 10 21 W Hz −1 and γ = 9). The radio-loudness of quasars (the ratio of 15 GHz flux density and optical nuclear flux at 5100 Å) is found to increase at high redshifts, which is interpreted as progressively higher Doppler factors in radio regime compared to those in optical. A strong positive correlation is found between the intrinsic kinetic power of the jet (measured from the flux density at 151 MHz) and the apparent luminosities of the total and the unresolved core emission of the jet at 15 GHz. This correlation is interpreted in terms of an intrinsically more luminous parsec-scale jet producing more luminous extended structure, which is detectable at low radio frequencies, 151 MHz. A possibility that the low frequency radio emission is relativistically beamed in superluminal sources and therefore correlates with radio luminosity of the jet at 15 GHz cannot be ruled out. Monitoring of superluminal AGN in a wide range of frequencies is required to check the contribution of each effect.

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Section: Correlations Between Radio Properties Of the Jetsupporting

confidence: 64%

Radio-optical scrutiny of compact AGN: correlations between properties of pc-scale jets and optical nuclear emission

Arshakian¹,

Torrealba

Chavushyan

et al. 2010

A&A

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“…VLBI observations at 1667 MHz of PKS 1127-145 (Bondi et al 1996) have shown that it has a core-jet structure, with a total size less than ∼ 20 milli-arcseconds. This small size implies that the covering factor of the absorbing system is likely to be close to unity.…”

Section: Observations and Resultsmentioning

confidence: 99%

Implications of 21-cm observations for damped Lyalpha systems

Chengalur

Kanekar

2000

Monthly Notices RAS

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We present Giant Metrewave Radio Telescope HI 21cm absorption observations, of candidate and confirmed damped Lyman-α systems. The derived spin temperatures T spin are high, in all cases, ∼ 1000 K or higher. We have also collated from the published literature a list of damped absorbers for which 21cm observations exist, and discuss the implications of the observations for the nature of these systems.A cross-comparison of the HI 21cm profiles (which trace the cold gas) with the low ionization metal profiles (which trace all the neutral HI, both cold and warm) shows that in all cases for which both spectra are available, the 21cm absorption coincides in velocity with the deepest metal line feature. This is consistent with models in which the deep metal line features arise from discrete clouds but not with models where the deepest features are the result of velocity crowding.We also find that the typical derived spin temperatures of damped Lyman-α systems are considerably higher than those in the Milky Way or nearby spiral galaxies. The only exceptions are systems which are known to be associated with the disks of spirals; these do, in fact, show low spin temperature. In a multi-phase medium the derived spin temperature is a weighted average of the temperatures of the individual phases. High apparent T spin values are hence to be expected from small, low metallicity objects since these objects should (as per existing theoretical models of the formation of a multi-phase ISM in the Milky Way and high redshift proto-galaxies) have a lower fraction of the cold phase in their ISM as compared to large galaxies. The high T spin is hence consistent with the observed low metallicities of damped Lyman-α absorbers as well as with recent findings that damped absorption is associated with a variety of galaxy types (as opposed to being confined to the disks of large spirals).Finally, although the number of systems for which observations are available is small, we suggest that the following trend may be identified : at low redshift, damped Lyman-α absorption arises from a range of systems, including spiral galaxy disks, while, at high redshift, absorption occurs predominantly in smaller systems.

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“…At higher frequencies, the Metsähovi group (Teräsranta et al 1998) have reported observations at 22, 37 and 87 GHz of 157 extragalactic radio sources, many of which have been monitored for over 20 years. The Bologna group (Bondi et al 1996) have observed at 408 MHz 125 radio sources for 15 years. A multifrequency monitoring of a large sample of compact radio sources has been carried out by Kovalev et al (2002) with the RATAN-600 telescope.…”

Section: Introductionmentioning

confidence: 99%

A multifrequency analysis of radio variability of blazars

Ciaramella

Bongardo

Aller

et al. 2004

A&A

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Abstract.We have carried out a multifrequency analysis of the radio variability of blazars, exploiting the data obtained during the extensive monitoring programs carried out at the University of Michigan Radio Astronomy Observatory (UMRAO, at 4.8, 8, and 14.5 GHz) and at the Metsähovi Radio Observatory (22 and 37 GHz). Two different techniques detect, in the Metsähovi light curves, evidence of periodicity at both frequencies for 5 sources (0224 + 671, 0945 + 408, 1226 + 023, 2200 + 420, and 2251 + 158). For the last three sources, consistent periods are found also at the three UMRAO frequencies and the Scargle (1982) method yields an extremely low false-alarm probability. On the other hand, the 22 and 37 GHz periodicities of 0224+671 and 0945 + 408 (which were less extensively monitored at Metsähovi and for which we get a significant false-alarm probability) are not confirmed by the UMRAO database, where some indications of ill-defined periods of about a factor of two longer are retrieved. We have also investigated the variability index, the structure function, and the distribution of intensity variations of the most extensively monitored sources. We find a statistically significant difference in the distribution of the variability index for BL Lac objects compared to flat-spectrum radio quasars (FSRQs), in the sense that the former objects are more variable. For both populations the variability index steadily increases with increasing frequency. The distribution of intensity variations also broadens with increasing frequency, and approaches a log-normal shape at the highest frequencies. We find that variability enhances by 20-30% the high frequency counts of extragalactic radio-sources at bright flux densities, such as those of the WMAP and P surveys. In all objects with detected periodicity we find evidence for the existence of impulsive signals superimposed on the periodic component.

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A 15 years monitoring program at 408 MHz

Cited by 36 publications

References 4 publications

Radio-optical scrutiny of compact AGN: correlations between properties of pc-scale jets and optical nuclear emission

Radio-optical scrutiny of compact AGN: correlations between properties of pc-scale jets and optical nuclear emission

Implications of 21-cm observations for damped Lyalpha systems

A multifrequency analysis of radio variability of blazars

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