arXiv subjects

E. Järvelä

Publications and source records attributed to E. Järvelä.

At least 19 recordsLinked to original sources

arXiv2026

EMU detection of mysterious multiple component radio-continuum emission from nearby spiral Seyfert I type galaxy 'Sand-timer' (NGC 7213)

We present an Australian Square Kilometre Array Pathfinder (ASKAP) Evolutionary Map of the Universe (EMU) radio-continuum detection of the nearby spiral Seyfert I type galaxy NGC 7213 which we nickname `Sand-timer' due to its morphological appearance. We find intriguing, peculiar emission extending from the central supermassive black hole (SMBH), which we refer to as galaxy hyperbubbles and/or active galactic nucleus (AGN) jets. Finally, we find two previously undetected kiloparsec-scale features in the polarised intensity image, which appear most likely also related to the central SMBH, as restarted bipolar radio jets, though somewhat obscured in the radio-continuum total intensity image. They resemble the bow shocks often seen in Fanaroff-Riley Class II (FR-II) radio galaxies, and so we propose they most likely represent an early episode of the `Sand-timer's' AGN jet activity. An alternative scenario is that the core of NGC 7213 may be a supermassive binary black hole (SMBBH) undergoing a tidal disruption event (TDE), potentially causing the peculiar radio-continuum emission.

astro-ph.GA↗

arXiv2026

Polarization Angle Swings in Blazars Detected in the Millimeter-wave with the South Pole Telescope

We present the first systematic search for electric vector position angle (EVPA) swings in the millimeter-wave (mm-wave) emission of blazars, using five years of observations from the South Pole Telescope SPT-3G camera at 95 and 150 GHz, and investigate their connection to gamma-ray flares. Of the 168 bright sources in the ~1500 square degrees SPT-3G Main Field, eight have sufficient polarization signal-to-noise for reliable EVPA measurement, four of which have continuous Fermi LAT gamma-ray detections. We detect EVPA swings in all four gamma-ray-active blazars and in none of the remaining four, consistent with the established connection between EVPA swings and high-energy emission seen at optical wavelengths. The observed swing amplitudes and rotation rates are smaller than those found in optical studies, consistent with mm-wave EVPA variability being slower than at shorter wavelengths. Random walk simulations of the polarization angle using a multi-cell model fail to reproduce both the number, amplitude, and duration of the observed swings, suggesting that this mechanism is insufficient to explain the observed swing population. Analysis of EVPA variability on one-week timescales is consistent with the anti-correlation between polarization degree and EVPA rotation rate previously observed at optical wavelengths. Of the 14 detected swings, eight are within 30 days of a gamma-ray flare. While many individual swing-flare associations are found to have a very low probability of happening by chance, the full ensembles of 95 and 150 GHz swing time lags with respect to gamma-ray flares are found to be consistent with random coincidence.

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arXiv2026

An X-ray and optical spectral study of the changing-look narrow-line Seyfert 1 2MASX J0413-0050

Active galactic nuclei (AGN) showing dramatic spectral and flux variations, either due to changes in the accretion rate (changing-state, CS-AGN) of the supermassive black hole or in the line-of-sight column density (changing-obscuration, CO-AGN), have been classified as changing-look (CL) AGN. Here we present a peculiar source, 2MASX J0413-0050, first identified as a narrow-line Seyfert 1 (NLS1s) galaxy in 2004. When re-observed twice in 2021, it showed a transition in the spectral type (towards a Seyfert 1.9) and the complete and mysterious disappearance of the Hbeta line while source was in a high accretion state. In the meantime, the X-ray flux decreased between observations taken in 2020 and 2022, and again in the most recent spectrum of 2023. Shortly after this, another optical spectrum revealed the re-emergence of both the narrow and broad Hbeta components (Seyfert 1.8). Despite the fact that it was not possible to retrieve the line-of-sight column density from the X-ray spectra, which would have helped in assessing whether this event could be attributed to a CO AGN scenario, the observational evidence does not necessarily support such an interpretation. J0413-0050 may have undergone several switch-on switch-off phases over the past 20 years, on an unknown timescale, which could have affected the accretion power and, consequently, the optical continuum and so the emission lines coming from the broad-line region (BLR). For these reasons, it is reasonable to classify this source as a CS-AGN. The case of J0413-0050 supports the hypothesis that NLS1s can indeed experience CL phenomena.

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arXiv2026

NOCTURNE. I. The radio spectrum of narrow-line Seyfert 1 galaxies

The origin of the radio emission in active galactic nuclei (AGN) is still debated. Multiple physical mechanisms can contribute to the spectrum at these frequencies, including relativistic jets, the jet base, outflows, star formation, and synchrotron emission from the hot corona. Recently, new extreme radio variability has been observed in the class of low-mass/high-Eddington AGN known as narrow-line Seyfert 1 (NLS1) galaxies, suggesting that another, more exotic mechanism may also play a role, especially at frequencies above 10 GHz. To investigate this relatively unexplored area of the radio spectrum, we observed a sample of 50 NLS1s with the Karl G. Jansky Very Large Array (JVLA), and 20 of them were observed twice. In this sample, 24 sources were not detected, while the others are typically characterized by a steep spectrum that can be modeled with a power law. We also identified two new candidate jetted NLS1s, including a high-frequency peaker, which is an extremely young relativistic jet. We found no significant variability in the sources observed twice. We conclude that the radio spectrum of NLS1s is typically dominated by optically thin emission, likely from low-power outflows, or by circumnuclear star formation, with a limited contribution from relativistic jets. Further studies at different spatial scales and at other wavelengths are necessary to fully constrain the origin of the radio emission in this class of active galaxies.

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arXiv2025

Revised optical classification of a sample of gamma-ray emitting AGN with GTC and VLT

We report the results of the optical follow-up of a sample of $γ$-ray-emitting active galactic nuclei (AGN). New high-quality optical spectra were obtained using Gran Telescopio Canarias with Optical System for Imaging and low- Intermediate-Resolution Integrated Spectroscopy and the European Southern Observatory Very Large Telescope Unit Telescope 1 with the FOcal Reducer and low dispersion Spectrograph 2. From the analysis of these spectra, we confirmed the previous classification as narrow-line Seyfert 1 (NLS1) for four objects and discovered two new NLS1s, bringing the total number of optically confirmed $γ$-NLS1s to 26. We also identified two ambiguous cases between NLS1 and intermediate Seyfert (IS), three IS, one broad-line Seyfert 1, one Seyfert 2. Based on the new spectra, we calculated black hole masses ranging from $10^{6.25}$ to $10^{9.32}$ $M_\odot$, and Eddington ratios spanning 0.05 to 2.07. This reclassification reinforces the scenario in which AGN with relatively small black hole masses are capable of launching powerful relativistic jets, contributing to our broader understanding of $γ$-ray-emitting AGN.

astro-ph.GA↗

arXiv2025

A comprehensive radio study of narrow-line Seyfert 1 galaxies

Narrow-line Seyfert 1 (NLS1) galaxies are a type of active galactic nuclei (AGN) that were originally classified as sources with little to no radio emission. Although the class is rather unified from an optical perspective, their radio characteristics are diverse. One of the biggest curiosities found in these sources is their ability to form and maintain powerful relativistic jets. We studied the radio properties of a sample of 3998 NLS1 galaxies which is the largest clean sample available, thus allowing us to study the population-wide characteristics. We used both historical and ongoing surveys: the LOw-Frequency ARray (LOFAR) Two-metre Sky Survey (LoTSS; 144~MHz), the Faint Images of the Radio Sky at Twenty-centimeters (FIRST; 1.4~GHz), the National Radio Astronomy Observatory (NRAO) Very Large Array (VLA) Sky Survey (NVSS; 1.4~GHz), and the VLA Sky Survey (VLASS; 3~GHz). We were able to obtain a radio detection for $\sim40\%$ of our sources, with the most detections by LoTSS. The majority of the detected NLS1 galaxies are faint ($\sim1-2$ mJy) and non-variable, suggesting considerable contributions from star formation activities, especially at 144~MHz. However, we identified samples of extreme sources, for example, in fractional variability and radio luminosity, indicating significant AGN activity. Our results highlight the heterogeneity of the NLS1 galaxy population in radio and lays the foundation for targeted future studies.

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arXiv2024

The host galaxy of low-luminosity compact sources

The term 'active galactic nuclei' (AGN) subtends a huge variety of objects, classified on their properties at different wavelengths. Peaked sources (PS) represent a class of AGN at the first stage of evolution, characterised by a peaked radio spectrum. Among these radio sources, low-luminosity compact (LLC) sources can be identified as PS accreting with a high Eddington rate, harbouring low-power jets and hosting low-mass black holes. These properties are also shared by narrow-line Seyfert 1 galaxies (NLS1s). In 2016, LLCs were hypothesised to be the parent population of NLS1s with a flat radio spectrum (F-NLS1s), suggesting the former to be the same objects as the latter, seen at higher inclination. Based on radio luminosity functions and optical spectra analysis, 10 LLCs were identified as valid candidates for F-NLS1s. To account for the missing puzzle piece, verifying if these LLCs could be hosted in late-type galaxies as NLS1s, we performed the photometric decomposition of their Pan-STARRS1 images in all five filters. We used the 2D fitting algorithm GALFIT for the single-band analysis, and its extension GALFITM for the multi-band analysis. Considering that the morphological type and the structural parameters of the host can be dependent on the wavelength, we found six out of ten LLCs hosted in late-type galaxies, probably with pseudo-bulges, three point-like sources and one object of uncertain classification. Although this study is based on a small sample, it represents the first morphological analysis of LLC host galaxies. The results confirm the trend observed in NLS1s, indicating late-type/disc-like host galaxies for LLCs and supporting the validity of the parent population scenario.

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arXiv2024

A Hot Mess: The Rich and Complex Soft Emitting Regions Surrounding the Reflection Dominated Flaring Central Engine of Mrk 1239

Previous X-ray works on Mrk 1239 have revealed a complex Narrow Line Seyfert 1 (NLS1) that exhibits substantial absorption and strong emission from both collisional (CIE) and photoionized (PIE) plasmas. Here, we report on deep-pointed observations with $XMM{\rm -}Newton$ and $NuSTAR$, along with $Swift$ monitoring, to understand the $0.3-30$ keV continuum emission and the central engine geometry. A strong X-ray flare, where the AGN brightens by a factor of five in $\sim30$ ks, is captured between $4-30$ keV and can be attributed to a brightening of the primary continuum. However, the lack of any variability below $\sim3$ keV on long- or short-time scales requires complete absorption of the AGN continuum with a neutral medium of column density $\sim 10^{23.5}{\rm cm}^{-2}$. The timing and spectral properties are consistent with a blurred reflection interpretation for the primary emission. The variability and presence of a Compton hump disfavours ionized partial covering. The neutral absorber, if outflowing, could be crashing into the surrounding medium and ISM to produce the low-energy continuum and CIE. Scattered emission off the inner torus could produce the PIE. The intricate scenario is demanded by the data and highlights the complexity of the environment that is normally invisible when overwhelmed by the AGN continuum. Objects like Mrk 1239 serve as important sources for unveiling the interface between the AGN and host galaxy environments.

astro-ph.HE↗

arXiv2023

Unprecedented extreme high-frequency radio variability in early-stage active galactic nuclei

We report on the discovery of one of the most extreme cases of high-frequency radio variability ever measured in active galactic nuclei (AGN), observed on timescales of days and exhibiting variability amplitudes of three to four orders of magnitude. These sources, all radio-weak narrow-line Seyfert 1 (NLS1) galaxies, were discovered some years ago at Aalto University Metsähovi Radio Observatory (MRO) based on recurring flaring at 37 GHz, strongly indicating the presence of relativistic jets. In subsequent observations with the Karl G. Jansky Very Large Array (JVLA) at 1.6, 5.2, and 9.0~GHz no signs of jets were seen. To determine the cause of their extraordinary behaviour, we observed them with the JVLA at 10, 15, 22, 33, and 45 GHz, and with the Very Long Baseline Array (VLBA) at 15 GHz. These observations were complemented with single-dish monitoring at 37 GHz at MRO, and at 15 GHz at Owens Valley Radio Observatory (OVRO). Intriguingly, all but one source either have a steep radio spectrum up to 45 GHz, or were not detected at all. Based on the 37 GHz data the timescales of the radio flares are a few days, and the derived variability brightness temperatures and variability Doppler factors comparable to those seen in blazars. We discuss alternative explanations for their extreme behaviour, but so far no definite conclusions can be made. These sources exhibit radio variability at a level rarely, if ever, seen in AGN. They might represent a new type of jetted AGN, or a new variability phenomenon, and thus deserve our continued attention.

astro-ph.GA↗

arXiv2023

A host galaxy study of southern narrow-line Seyfert 1 galaxies

We studied seven nearby narrow-line Seyfert 1 (NLS1) galaxies in $J$- and $Ks$-bands with redshifts varying from 0.019 to 0.092. This is the first multi-source study targeting the hosts of southern NLS1 galaxies. Our data was obtained with the FourStar instrument of the 6.5~m Magellan Baade telescope at the Las Campanas Observatory (Chile). The aim of our study is to determine the host galaxy morphologies of these sources by using GALFIT. We were able to model six out of the seven sources reliably. Our conclusion is that all of the reliably modelled sources are disk-like galaxies, either spirals or lenticulars. None of these sources present elliptical morphology. Our findings are in agreement with the hypothesis that disk-like galaxies are the main host of jetted NLS1 galaxies. Taking advantage of observations in two bands, we also produced a $J - Ks$ colour map of each source. Five of the six colour maps show significant dust extinction near the core of the galaxy -- a feature often seen in gamma-ray-detected jetted NLS1 galaxies, and interpreted to be a consequence of a past minor merger.

astro-ph.GA↗

arXiv2023

Long-term Swift and Metsähovi monitoring of SDSS J164100.10+345452.7 reveals multi-wavelength correlated variability

We report on the first multi-wavelength Swift monitoring campaign performed on SDSS J164100.10+345452.7, a nearby narrow-line Seyfert 1 galaxy formerly known as radio quiet which was recently detected both in the radio (at 37 GHz) and in the $γ$-rays, which hints at the presence of a relativistic jet. During our 20-month Swift campaign, while pursuing the primary goal of assessing the baseline optical/UV and X-ray properties of J1641, we caught two radio flaring episodes, one each year. Our strictly simultaneous multi-wavelength data closely match the radio flare and allow us to unambiguously link the jetted radio emission of J1641. Indeed, for the X-ray spectra preceding and following the radio flare a simple absorbed power-law model is not an adequate description, and an extra absorption component is required. The average spectrum of J1641 can be best described by an absorbed power law model with a photon index $Γ=1.93\pm0.12$, modified by a partially covering neutral absorber with a covering fraction $f=0.91_{-0.03}^{+0.02}$. On the contrary, the X-ray spectrum closest to the radio flare does not require such extra absorber and is much harder ($Γ_{\rm flare} \sim 0.7\pm0.4$), thus implying the emergence of a further, harder spectral component. We interpret this as the jet emission emerging from a gap in the absorber. The fractional variability we derive in the optical/UV and X-ray bands are found to be lower than the typical values reported in the literature, since our observations of J1641 are dominated by the source being in a low state. Under the assumption that the origin of the 37 GHz radio flare is the emergence of a jet from an obscuring screen also observed in the X-rays, the derived total jet power is $P^{\rm tot}_{\rm jet}=3.5\times10^{42}$ erg s$^{-1}$, comparable to the lowest measured in the literature. [Abridged]

astro-ph.HE↗

arXiv2022

Spectacular 240 kpc double-sided relativistic jets in a spiral-hosted narrow-line Seyfert 1 galaxy

Narrow-line Seyfert 1 (NLS1) galaxies are a peculiar subclass of active galactic nuclei (AGN). They have demonstrated that the presence of relativistic jets in an AGN is not strictly related to its radio-loudness, the black hole mass, or the host galaxy type. Here we present a remarkable example of a radio-quiet NLS1, 6dFGS gJ035432.8-134008 (J0354-1340). In our Karl G. Jansky Very Large Array observations at 5.5 GHz the source shows a bright core with a flat spectral index, and extended emission corresponding to very elongated jets. These are the largest double-sided radio jets found to date in an NLS1, with a deprojected linear size of almost 250 kpc. We also analysed near-infrared and optical images obtained by the Magellan Baade and the European Southern Observatory New Technology Telescope. By means of photometric decomposition and colour maps, we determined that J0354-1340 is hosted by a spiral/disk galaxy. Fully evolved relativistic jets have traditionally been associated with high-mass elliptical galaxies hosting the most massive black holes, but our results confirm that also less massive black holes in spiral galaxies can launch and sustain powerful jets, implying that the launching of the jets is governed by factors other than previously believed.

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arXiv2021

A New Sample of Gamma-Ray Emitting Jetted Active Galactic Nuclei -- Preliminary Results

We are compiling a new list of gamma-ray jetted active galactic nuclei (AGN), starting from the fourth catalog of point sources of the Fermi Large Area Telescope (LAT). Our aim is to prepare a list of jetted AGN with known redshifts and classifications to be used to calibrate jet power. We searched in the available literature for all the published optical spectra and multiwavelength studies useful to characterize the sources. We found new, missed, or even forgotten information leading to a substantial change in the redshift values and classification of many sources. We present here the preliminary results of this analysis and some statistics based on the gamma-ray sources with right ascension within the interval $0^{\rm h}-12^{\rm h}$ (J2000). Although flat-spectrum radio quasars and BL Lac objects are still the dominant populations, there is a significant increase in the number of other objects, such as misaligned AGN, narrow-line Seyfert 1 galaxies, and Seyfert galaxies. We also introduced two new classes of objects: changing-look AGN and ambiguous sources. About one third of the sources remain unclassified.

astro-ph.HE↗

arXiv2021

Hunting for the nature of the enigmatic narrow-line Seyfert 1 galaxy PKS 2004-447

Narrow-line Seyfert 1 (NLS1) galaxies are a class of active galactic nuclei (AGN) that, in some cases, can harbor powerful relativistic jets. One of them, PKS 2004-447, shows gamma-ray emission, and underwent its first recorded multifrequency flare in 2019. However, past studies revealed that in radio this source can be classified as a compact steep-spectrum source (CSS), suggesting that, unlike other gamma-ray sources, the relativistic jets of PKS 2004-447 have a large inclination with respect to the line of sight. We present here a set of spectroscopic observations of this object, aimed at carefully measuring its black hole mass and Eddington ratio, determining the properties of its emission lines, and characterizing its long term variability. We find that the black hole mass is $(1.5\pm0.2)\times10^7$ M$_\odot$, and the Eddington ratio is 0.08. Both values are within the typical range of NLS1s. The spectra also suggest that the 2019 flare was caused mainly by the relativistic jet, while the accretion disk played a minor role during the event. In conclusion, we confirm that PKS 2004-447 is one of the rare examples of gamma-ray emitting CSS/NLS1s hybrid, and that these two classes of objects are likely connected in the framework of AGN evolution.

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arXiv2020

Investigating the mini and giant radio flare episodes of Cygnus X-3

The microquasar Cygnus X-3 underwent a giant radio flare in April 2017, reaching a maximum flux of $\sim 16.5$ Jy at 8.5 GHz. We present results from a long monitoring campaign carried out with Medicina at 8.5, 18.6 and 24.1 GHz, in parallel to the Metsähovi radio telescope at 37 GHz, from 4 to 11 April 2017. We observe a spectral steepening from $α= 0.2$ to 0.5 (with $S_ν \propto ν^{-α}$) within 6 h around the epoch of the peak maximum of the flare, and rapid changes in the spectral slope in the following days during brief enhanced emission episodes while the general trend of the radio flux density indicated the decay of the giant flare. We further study the radio orbital modulation of Cyg X-3 emission associated with the 2017 giant flare and with six mini-flares observed in 1983, 1985, 1994, 1995, 2002 and 2016. The enhanced emission episodes observed during the decline of the giant flare at 8.5 GHz coincide with the orbital phase $ϕ\sim 0.5$ (orbital inferior conjunction). On the other hand the light curves of the mini-flares observed at $15-22$ GHz peak at $ϕ\sim 0$, except for the 2016 light curve which is shifted of 0.5 w.r.t. the other ones. We attribute the apparent phase shift to the variable location of the emitting region along the bent jet. This might be explained by the different accretion states of the flaring episodes (the 2016 mini-flare occurred in the hypersoft X-ray state).

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arXiv2020

The Radio Structure of the Narrow-Line Seyfert 1 Mrk 783 with VLBA and e-MERLIN

In this paper, we present the analysis of new radio and optical observations of the narrow-line Seyfert 1 galaxy Mrk 783. 1.6 GHz observations performed with the e-MERLIN interferometer confirm the presence of the diffuse emission previously observed. The Very Long Baseline Array (VLBA) also detects the nuclear source both at 1.6 GHz (L-Band) and 5 GHz (C-band). While the L-band image shows only an unresolved core, the C-band image shows the presence of a partially resolved structure, at a position angle of 60°. The brightness temperature of the emission in both bands ($>10^6$ K) suggests that it is a pc-scale jet produced by the AGN. The relatively steep VLBA spectral index ($α_{VLBA} = 0.63\pm0.03$) is consistent with the presence of optically thin emission on milliarcsecond scales. Finally, we investigated two possible scenarios that can result in the misalignment between the kpc and pc-scale radio structure detected in the galaxy. We also analysed the optical morphology of the galaxy, which suggests that Mrk 783 underwent a merging in relatively recent times.

astro-ph.GA↗

arXiv2020

Absorbed relativistic jets in radio-quiet narrow-line Seyfert 1 galaxies

Narrow-line Seyfert 1 (NLS1) galaxies are peculiar active galactic nuclei (AGN). Most of them do not show strong radio emission, but recently seven radio-quiet (or -silent) NLS1s have been detected flaring multiple times at 37 GHz by the Metsähovi Radio Telescope, indicating the presence of relativistic jets in these peculiar sources. We observed them with the Karl G. Jansky Very Large Array (JVLA) in A configuration at 1.6, 5.2, and 9.0 GHz. Our results show that these sources are either extremely faint or not detected in the JVLA bands. At those frequencies, the radio emission from their relativistic jet must be absorbed, either via synchrotron self-absorption as it occurs in gigahertz-peaked sources or, more likely, via free-free absorption by a screen of ionized gas associated with starburst activity or shocks. Our findings cast new shadows on the radio-loudness criterion, which seems to be more and more frequently a misleading parameter. New high-frequency and high-resolution radio observations are essential to test our hypotheses.

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arXiv2019

Properties of flat-spectrum radio-loud Narrow-Line Seyfert 1 Galaxies

We have conducted a multiwavelength survey of 42 radio loud narrow-1ine Seyfert 1 galaxies (RLNLS1s), selected by searching among all the known sources of this type and omitting those with steep radio spectra. We analyse data from radio frequencies to X-rays, and supplement these with information available from online catalogs and the literature in order to cover the full electromagnetic spectrum. This is the largest known multiwavelength survey for this type of source. We detected 90% of the sources in X-rays and found 17% at gamma rays. Extreme variability at high energies was also found, down to timescales as short as hours. In some sources, dramatic spectral and flux changes suggest interplay between a relativistic jet and the accretion disk. The estimated masses of the central black holes are in the range $\sim 10^{6-8}M_{\odot}$, smaller than those of blazars, while the accretion luminosities span a range from $\sim 0.01$ to $\sim 0.49$ times the Eddington limit, similar to those of quasars. The distribution of the calculated jet power spans a range from $\sim 10^{42.6}$ to $\sim 10^{45.6}$ erg s$^{-1}$, generally lower than quasars and BL Lac objects, but partially overlapping with the latter. Once normalised by the mass of the central black holes, the jet power of the three types of active galactic nuclei are consistent with each other, indicating the scalability of the jet. Despite the observational differences, the central engine of RLNLS1s is apparently quite similar to that of blazars. The historical difficulties in finding radio-loud narrow-line Seyfert 1 galaxies might be due to their low power and to intermittent jet activity.

astro-ph.HE↗