Low-frequency radio observations of Seyfert galaxies: A test of the unification scheme

Singh, Veeresh; Shastri, P.; Ishwara‐Chandra, C. H.; Athreya, R.

doi:10.1051/0004-6361/201221003

Cited by 12 publications

(13 citation statements)

References 90 publications

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“…The radio luminosities are k-corrected using a simple power law radio spectrum with the spectral index measured between 1.4 GHz and 150 MHz whenever available, otherwise an average spectral index of −0.7 is used. The average radio spectral index of −0.7 is generally found in Seyfert galaxies (Singh et al 2013). We note that the 1.4 GHz luminosity distribution of our NLS1s is primarily based on the FIRST measurements (i.e., 483/498 detections are from FIRST), and NVSS flux density is used only when FIRST detection is not available.…”

Section: Radio Luminositiesmentioning

confidence: 96%

Investigating kpc-scale radio emission properties of narrow-line Seyfert 1 galaxies

Singh

Chand

2018

Monthly Notices of the Royal Astronomical Society

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In recent years, several Radio-Loud Narrow-Line Seyfert 1 galaxies (RL-NLS1) possessing relativistic jets have come into attention with their detections in Very Large Baseline Array (VLBA) and in γ-ray observations. In this paper we attempt to understand the nature of radio-jets in NLS1s by examining the kpc-scale radio properties of, hitherto, the largest sample of 11101 optically-selected NLS1s. Using 1.4 GHz FIRST, 1.4 GHz NVSS, 327 MHz WENNS, and 150 MHz TGSS catalogues we find the radiodetection of merely ∼ 4.5 per cent (498/11101) NLS1s, with majority (407/498 ∼ 81.7 per cent) of them being RL-NLS1s. Our study yields the highest number of RL-NLS1s and it can only be a lower limit. We find that the most of our radio-detected NLS1s are compact (< 30 kpc), exhibit both flat as well as steep radio spectra, and are distributed across a wide range of 1.4 GHz radio luminosities (10 22 − 10 27 W Hz −1 ). At the high end of radio luminosity our NLS1s often tend to show blazar-like properties such as compact radio-size, flat/inverted radio spectrum, radio variability and polarization. The diagnostic plots based on the mid-IR colours suggest that the radio emission in NLS1s is mostly powered by AGN, while nuclear star-formation may have a significant contribution in NLS1s of low radio luminosities. The radio luminosity versus radio-size plot infers that the radio-jets in NLS1s are either in the early evolutionary phase or possibly remain confined within the nuclear region due to low-power or intermittent AGN activity.

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Section: Radio Luminositiesmentioning

confidence: 96%

Investigating kpc-scale radio emission properties of narrow-line Seyfert 1 galaxies

Singh

Chand

2018

Monthly Notices of the Royal Astronomical Society

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“…Other attempts have been made in the construction of radio SED for radio AGN, but these have been limited to small samples and/or single spectral index studies (e.g. Laing & Peacock 1980;Mauch et al 2013;Singh et al 2013;Hardcastle et al 2016;Kharb et al 2016). Understanding the physical processes that drive integrated radio spectra of AGNs is not an easy task due to the multiple physical conditions contributing to its total emission, as multicomponent structures and orientation effects, and hence studies have focused on spectral index maps (α maps) of spatially resolved sources (Harwood et al 2015;Vardoulaki et al 2015).…”

Section: Agnmentioning

confidence: 99%

“…Multifrequency spectral studies of the integrated radio SED of AGNs are not widespread in the literature (though there are a few exceptions, Laing & Peacock 1980;Ker et al 2012;Singh et al 2013;Kharb et al 2016;Mahony et al 2016). Recent literature has focused on morphological studies through spectral index maps (e.g.…”

Section: Introductionmentioning

confidence: 99%

The LOFAR window on star-forming galaxies and AGNs – curved radio SEDs and IR–radio correlation at 0<z<2.5

Rivera

Williams

Hardcastle

et al. 2017

Monthly Notices of the Royal Astronomical Society

134

123

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We present a study of the low-frequency radio properties of star-forming (SF) galaxies and active galactic nuclei (AGNs) up to redshift z = 2.5. The new spectral window probed by the Low Frequency Array (LOFAR) allows us to reconstruct the radio continuum emission from 150 MHz to 1.4 GHz to an unprecedented depth for a radio-selected sample of 1542 galaxies in ∼7 deg 2 of the LOFAR Boötes field. Using the extensive multiwavelength data set available in Boötes and detailed modelling of the far-infrared to ultraviolet spectral energy distribution (SED), we are able to separate the star formation (N = 758) and the AGN (N = 784) dominated populations. We study the shape of the radio SEDs and their evolution across cosmic time and find significant differences in the spectral curvature between the SF galaxy and AGN populations. While the radio spectra of SF galaxies exhibit a weak but statistically significant flattening, AGN SEDs show a clear trend to become steeper towards lower frequencies. No evolution of the spectral curvature as a function of redshift is found for SF galaxies or AGNs. We investigate the redshift evolution of the infrared-radio correlation for SF galaxies and find that the ratio of total infrared to 1.4-GHz radio luminosities decreases with increasing redshift: q 1.4 GHz = (2.45 ± 0.04) (1 + z) −0.15 ± 0.03. Similarly, q 150 MHz shows a redshift evolution following q 150 GHz = (1.72 ± 0.04) (1 + z) −0.22 ± 0.05. Calibration of the 150 MHz radio luminosity as a star formation rate tracer suggests that a single power-law extrapolation from q 1.4 GHz is not an accurate approximation at all redshifts.

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“…Radio emission from Radio Quiet (RQ) AGN (as defined by Kellermann et al 1989) is lower by several orders of magnitude than that of RL AGN. RQ AGN show, on average, smaller radio structures some of which are often unresolved down to parsec scales (Blundell et al 1996;Blundell & Beasley 1998;Lal et al 2004;Nagar et al 1999Anderson et al 2004;Singh et al 2013), and only sub-relativistic velocities (Middelberg et al 2004;. The physical origin of the radio emission in RQ AGN, whether a down-⋆ E-mail: behar@physics.technion.ac.il scaled version of the RL, collimated jets (Barvainis et al 1996;Gallimore et al 2006), or coronal emission from magnetic activity above the accretion disk (Field & Rogers 1993), remains to be resolved.…”

Section: Introductionmentioning

confidence: 99%

Discovery of millimetre-wave excess emission in radio-quiet active galactic nuclei

Behar

Baldi

Laor

et al. 2015

Monthly Notices of the Royal Astronomical Society

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The physical origin of radio emission in Radio Quiet Active Galactic Nuclei (RQ AGN) remains unclear, whether it is a downscaled version of the relativistic jets typical of Radio Loud (RL) AGN, or whether it originates from the accretion disk. The correlation between 5 GHz and X-ray luminosities of RQ AGN, which follows L R = 10 −5 L X observed also in stellar coronae, suggests an association of both X-ray and radio sources with the accretion disk corona. Observing RQ AGN at higher (mm-wave) frequencies, where synchrotron self absorption is diminished, and smaller regions can be probed, is key to exploring this association. Eight RQ AGN, selected based on their high X-ray brightness and variability, were observed at 95 GHz with the CARMA and ATCA telescopes. All targets were detected at the 1 − 10 mJy level. Emission excess at 95 GHz of up to ×7 is found with respect to archival low-frequency steep spectra, suggesting a compact, optically-thick core superimposed on the more extended structures that dominate at low frequencies. Though unresolved, the 95 GHz fluxes imply optically thick source sizes of 10 −4 − 10 −3 pc, or ∼ 10 − 1000 gravitational radii. The present sources lie tightly along an L R (95 GHz) = 10 −4 L X (2-10 keV) correlation, analogous to that of stellar coronae and RQ AGN at 5 GHz, while RL AGN are shown to have higher L R /L X ratios. The present observations argue that simultaneous mm-wave and X-ray monitoring of RQ AGN features a promising method for understanding accretion disk coronal emission.

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Low-frequency radio observations of Seyfert galaxies: A test of the unification scheme

Cited by 12 publications

References 90 publications

Investigating kpc-scale radio emission properties of narrow-line Seyfert 1 galaxies

Investigating kpc-scale radio emission properties of narrow-line Seyfert 1 galaxies

The LOFAR window on star-forming galaxies and AGNs – curved radio SEDs and IR–radio correlation at 0<z<2.5

Discovery of millimetre-wave excess emission in radio-quiet active galactic nuclei

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