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Register a new userRadio and gamma-ray loud narrow-line Seyfert 1 galaxies in the spotlight
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Added by
Vassilis Karamanavis
Publication date
2017
fields
Physics
and research's language is
English
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Narrow-line Seyfert 1 (NLS1) galaxies provide us with unique insights into the drivers of AGN activity under extreme conditions. Given their low black hole (BH) masses and near-Eddington accretion rates, they represent a class of galaxies with rapidly growing supermassive BHs in the local universe. Here, we present the results from our multi-frequency radio monitoring of a sample of {gamma}-ray loud NLS1 galaxies ({gamma}NLS1s), including systems discovered only recently, and featuring both the nearest and the most distant {gamma}NLS1s known to date. We also present high-resolution radio imaging of 1H 0323+342, which is remarkable for its spiral or ring-like host. Finally, we present new radio data of the candidate {gamma}-emitting NLS1 galaxy RX J2314.9+2243, characterized by a very steep radio spectrum, unlike other {gamma}NLS1s.
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The recent detection of gamma-ray emission from four radio-loud narrow-line Seyfert 1 galaxies suggests that the engine driving the AGN activity of these objects share some similarities with that of blazars, namely the presence of a gamma-ray emitting, variable, jet of plasma closely aligned to the line of sight. In this work we analyze the gamma-ray light curves of the four radio-loud narrow-line Seyfert 1 galaxies for which high-energy gamma-ray emission has been discovered by Fermi/LAT, in order to study their variability. We find significant flux variability in all the sources. This allows us to exclude a starburst origin of the gamma-ray photons and confirms the presence of a relativistic jet. Furthermore we estimate the minimum e-folding variability timescale (3 - 30 days) and infer an upper limit for the size of the emitting region (0.2 - 2 pc, assuming a relativistic Doppler factor delta=10 and a jet aperture of theta=0.1 rad).
Gamma-ray detected radio-loud narrow-line Seyfert 1 (g-NLS1) galaxies constitute a small but interesting sample of the gamma-ray loud AGN. The radio-loudest g-NLS1 known, PKS 2004-447, is located in the southern hemisphere and is monitored in the radio regime by the multiwavelength monitoring program TANAMI. We aim for the first detailed study of the radio morphology and long-term radio spectral evolution of PKS 2004-447, which are essential to understand the diversity of the radio properties of g-NLS1s. The TANAMI VLBI monitoring program uses the Australian Long Baseline Array (LBA) and telescopes in Antarctica, Chile, New Zealand, and South Africa to monitor the jets of radio-loud active galaxies in the southern hemisphere. Lower resolution radio flux density measurements at multiple radio frequencies over four years of observations were obtained with the Australia Telescope Compact Array (ATCA). The TANAMI VLBI image at 8.4 GHz shows an extended one-sided jet with a dominant compact VLBI core. Its brightness temperature is consistent with equipartition, but it is an order of magnitude below other g-NLS1s with the sample value varying over two orders of magnitude. We find a compact morphology with a projected large-scale size <11 kpc and a persistent steep radio spectrum with moderate flux-density variability. PKS 2004-447 appears to be a unique member of the g-NLS1 sample. It exhibits blazar-like features, such as a flat featureless X-ray spectrum and a core dominated, one-sided parsec-scale jet with indications for relativistic beaming. However, the data also reveal properties atypical for blazars, such as a radio spectrum and large-scale size consistent with Compact-Steep-Spectrum (CSS) objects, which are usually associated with young radio sources. These characteristics are unique among all g-NLS1s and extremely rare among gamma-ray loud AGN.
Most of the radio-loud narrow-line Seyfert 1 (NLS1) galaxies resemble compact steep-spectrum sources. However, the extremely radio-loud ones show blazar-like characteristics, like flat radio spectra, compact radio cores, substantial variability and high brightness temperatures. These objects are thought to be similar to blazars as they possess relativistic jets seen at small angle to the line of sight. This claim has been further supported by the Fermi satellite discovery of gamma-ray emission from a handful of these sources. Using the Wide-Field Infrared Survey Explorer (WISE) data, we analyzed the mid-infrared variability characteristics of $42$ radio-loud NLS1 at $3.4$ and $4.6,mu$m. We found that $27$ out of the studied $42$ sources showed variability in at least one of the two infrared bands. In some cases, significant changes in the infrared colors can alter the location of the source in the WISE color-color diagram which might lead to different classification. More than $60$% of the variable sources also showed variability within a $1-1.5$ day interval. Such short time scales argue for a compact emission region like those associated with the jets. This connection is further strengthened by the fact that the brightest $gamma$-ray emitters of the sample ($6$ sources), all showed short time scale infrared variability.
Before the launch of the Fermi Gamma-ray Space Telescope satellite only two classes of active galactic nuclei (AGN) were known to generate relativistic jets and thus to emit up to the $gamma$-ray energy range: blazars and radio galaxies, both hosted in giant elliptical galaxies. The discovery by the Large Area Telescope (LAT) on-board the Fermi satellite of variable $gamma$-ray emission from a few radio-loud narrow-line Seyfert 1 galaxies (NLSy1) revealed the presence of an emerging third class of AGN with powerful relativistic jets. Considering that NLSy1 are usually hosted in late-type galaxies with relatively small black hole masses, this finding opened new challenging questions about the nature of these objects, the disc/jet connection, the emission mechanisms at high energies, and the formation of relativistic jets. In this review, I will discuss the broad-band properties of the $gamma$-ray-emitting NLSy1 included in the Fourth Fermi LAT source catalog, highlighting major findings and open questions regarding jet physics, black hole mass estimation, host galaxy and accretion process of these sources in the Fermi era.
We report the analysis of all Swift observations available up to 2019 April of $gamma$-ray-emitting narrow-line Seyfert 1 galaxies (NLSy1). The distribution of X-ray luminosities (and fluxes) indicates that the jet radiation significantly contributes to their X-ray emission, with Doppler boosting making values higher than other radio-loud NLSy1. The 0.3-10 keV photon indices are on average harder with respect to radio-quiet and radio-loud NLSy1, confirming a dominant jet contribution in X-rays. However, the lower variability amplitude with respect to blazars and the softening of the spectrum in some periods suggests that also the corona radiation contributes to the X-ray emission. In optical and ultraviolet (UV) significant flux changes have been observed on daily, weekly, and monthly time-scale, providing a clear indication of the significant contribution of the jet radiation in this part of spectrum. A strong correlation between X-ray, UV, and optical emission and simultaneous flux variations have been observed in 1H 0323+342, SBS 0846+513, PMN J0948+0022 as expected in case the jet radiation is the dominant mechanism. Correlated multiband variability favours the jet-dominated scenario also in FBQS J1644+2619 and PKS 2004-447. The summed X-ray Telescope spectra of 1H 0323+342, SBS 0846+513, PMN J0948+0022, and FBQS J1644+2619 are well fitted by a broken power law with a break around 2 keV. The spectrum above 2 keV is dominated by the non-thermal emission from a beamed relativistic jet, as suggested by the hard photon index. A Seyfert-like feature like the soft X-ray excess has been observed below 2 keV, making these $gamma$-ray-emitting NLSy1 different from typical blazars.
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