arXiv subjects

M. Tornikoski

Publications and source records attributed to M. Tornikoski.

At least 19 recordsLinked to original sources

arXiv2026

A Ring of Fire Orphan γ-Ray Flare in the Neutrino Candidate 3C 120

We present 43\,GHz VLBI observations of the radio galaxy 3C~120 during its brightest $γ$-ray outburst (March 2018), recently associated with the IceCube neutrino alert IC-180213A. Despite reaching $L_γ= 3.7 \times 10^{44}$\,erg\,s$^{-1}$, contemporaneous X-ray monitoring from INTEGRAL/ISGRI, MAXI/GSC, and \textit{Swift}/XRT revealed no variability across 0.3-200\,keV, nor in B, V, R, and I band optical observations or 37 \& 235\,GHz observations, establishing an orphan flare. High-cadence VLBI imaging identified a new jet disturbance (N) propagating at $β_{\rm app} = (2.8 \pm 1.3)$ through quasi-stationary features C1-C3. The $γ$-ray peak coincided spatially and temporally with N crossing C3 ($r \sim 0.38$\,mas), where we measured a factor-of-5 increase in fractional polarization ($m = 16\%$) and $Δχ\sim 24^\circ$ EVPA rotation, indicating localized magnetic field compression. The extreme Compton dominance ($L_γ/ L_{\rm syn,blob} \approx 160$) is naturally explained by the Ring of Fire scenario, in which N ($Γ_{\rm blob} = 6$, $B_{\rm blob} = 0.023$\,G) inverse-Compton scatters synchrotron photons from C3, reproducing the observed $γ$-ray luminosity for physically reasonable parameters. Unlike the 2014-2015 orphan flares attributed to rapid spine reorientation near the BLR, the 2018 event represents a distinct physical mechanism, a propagating disturbance interacting with stationary jet structure at $\sim10\times$ the BLR radius.This work provides the first direct observational link between VLBI-resolved jet dynamics and orphan $γ$-ray emission in a radio galaxy.

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arXiv2025

Insights into jet-NLR energetics in PMN J0948+0022

The analysis of the optical spectra of PMN J0948+0022 showed significant variations in the spectral lines that, when combined with the Fermi $γ$-ray light curve and radio observations reported by other authors, were interpreted as the result of interactions between the relativistic jet and the narrow-line region (NLR). In this work, we present order-of-magnitude calculations of the energetics associated with this proposed jet-NLR interaction. We demonstrate that the observed outflows are capable of absorbing a fraction of the jet energy and converting it into kinetic energy. This mechanism provides a natural explanation for the optical spectral variability recorded with the X-shooter and Multi-Unit Spectroscopic Explorer (MUSE) instruments. Our results support the scenario in which feedback from the relativistic jet can dynamically influence the circumnuclear gas, offering new insights into the coupling between jets and the NLR in $γ$-ray-emitting narrow-line Seyfert 1 galaxies.

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arXiv2025

IXPE Observations of the Blazar Mrk 501 in 2022: A Multiwavelength View

The blazar Markarian 501 (Mrk 501) was observed on three occasions over a 4-month period between 2022 March and 2022 July with the Imaging X-ray Polarimetry Explorer (IXPE). In this paper, we report for the first time on the third IXPE observation, performed between 2022 July 9 and 12, during which IXPE detected a linear polarization degree of $Π_X=6\pm2$ per cent at a polarization angle, measured east of north, of $Ψ_X=143^\circ\pm11^\circ$ within the 2-8 keV X-ray band. The X-ray polarization angle and degree during this observation are consistent with those obtained during the first two observations. The chromaticity of the polarization across radio, optical, and X-ray bands is likewise consistent with the result from the simultaneous campaigns during the first two observations. Furthermore, we present two types of models to explain the observed spectral energy distributions (SEDs) and energy-resolved polarization: a synchrotron self-Compton model with an anisotropic magnetic field probability distribution in the emitting volume, as well as an energy-stratified shock model. Our results support both the shock scenario as well as support that small levels of magnetic field anisotropy can explain the observed polarization.

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arXiv2025

Multi-epoch leptohadronic modeling of neutrino source candidate blazar PKS 0735+178

The origin of the astrophysical neutrino flux discovered by IceCube remains largely unknown. Several individual neutrino source candidates were observed. Among them is the gamma-ray flaring blazar TXS 0506+056. A similar coincidence of a high-energy neutrino and a gamma-ray flare was found in blazar PKS 0735+178. By modeling the spectral energy distributions of PKS 0735+178, we expect to investigate the physical conditions for neutrino production during different stages of the source activity. We analyze the multi-wavelength data during the selected periods of time. Using numerical simulations of radiation processes in the source, we study the parameter space of one-zone leptonic and leptohadronic models and find the best-fit solutions that explain the observed photon fluxes. We show the impact of model parameter degeneracy on the prediction of the neutrino spectra. We show that the available mutli-wavelength data are not sufficient to predict the neutrino spectrum unambiguously. Still, under the condition of maximal neutrino flux, we propose a scenario in which 0.2 neutrino events are produced during the 50 days flare.

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arXiv2025

The polarisation behaviour of OJ 287 viewed through radio, millimetre and optical observations between 2015 and 2017

OJ 287 is a bright blazar with century-long observations, and one of the strongest candidates to host a supermassive black hole binary. Its polarisation behaviour between 2015 and 2017 (MJD 57300-58000) contains several interesting events that we re-contextualise in this study. We collected optical photometric and polarimetric data from several telescopes and obtained high-cadence light curves from this period. In the radio band, we collected mm-wavelength polarisation data from the AMAPOLA program. We combined these with existing multifrequency polarimetric radio results and the results of very-long-baseline-interferometry imaging with the Global mm-VLBI Array at 86 GHz. In December 2015, an optical flare was seen according to the general relativistic binary black hole model. We suggest that the overall activity near the accretion disk and the jet base during this time may be connected to the onset of a new moving component K seen in the jet in March 2017. With the additional optical data, we find a fast polarisation angle rotation of 210 degrees coinciding with the December 2015 flare, hinting at a possible link between these events. Based on the 86-GHz images, we calculated a new speed of 0.12 mas/yr for K, which places it inside the core at the time of the 2015 flare. This speed also supports the scenario where the passage of K through the quasi-stationary feature S1 could have been the trigger for the very-high-energy gamma-ray flare of OJ 287 seen in February 2017. With the mm-polarisation data, we established that these bands follow the cm-band data but show a difference during the time of K passing through S1. This indicates that the mm-bands trace the substructures of the jet still unresolved in the cm-bands.

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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.

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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]

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arXiv2022

A multiband study and exploration of the radio wave - $γ$-ray connection in 3C 84

Total intensity variability light curves offer a unique insight into the ongoing debate about the launching mechanism of jets. For this work, we utilise the availability of radio and $γ$-ray light curves over a few decades of the radio source 3C 84 (NGC 1275). We calculate the multiband time lags between the flares identified in the light curves via discrete cross-correlation and Gaussian process regression. We find that the jet particle and magnetic field energy densities are in equipartition ($k_\textrm{r} = 1.08\pm0.18$). The jet apex is located $z_\textrm{91.5 GHz}= 22 - 645$ $R_\textrm{s}$ ($2 - 20 \times 10^{-3}$ pc) upstream of the 3 mm radio core; at that position, the magnetic field amplitude is $B_\textrm{core}^\textrm{91.5 GHz}= 3 - 10$ G. Our results are in good agreement with earlier studies, which utilised very-long-baseline interferometry. Furthermore, we investigate the temporal relation between the ejection of radio and $γ$-ray flares. Our results are in favour of the $γ$-ray emission being associated with the radio emission. We are able to tentatively connect the ejection of features identified at 43 and 86 GHz to prominent $γ$-ray flares. Finally, we compute the multiplicity parameter $λ$ and the Michel magnetisation $σ_\textrm{M}$ and find that they are consistent with a jet launched by the Blandford & Znajek 1977 mechanism.

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arXiv2022

Jet kinematics in the transversely stratified jet of 3C 84 A two-decade overview

3C84 (NGC1275) is one of the brightest radio sources in the mm radio-bands, which led to a plethora of VLBI observations at numerous frequencies over the years. They reveal a two-sided jet structure, with an expanding but not well-collimated parsec-scale jet, pointing southward. High resolution mm-VLBI observations allow the study and imaging of the jet base on the sub-parsec scale. This could facilitate the investigation of the nature of the jet origin, also in view of the previously detected two-railed jet structure and east-west oriented core region seen with RadioAstron at 22 GHz. We produce VLBI images of this core and inner jet region, observed during the past twenty years at 15, 43, and 86 GHz. We determine the kinematics of the inner jet and ejected features at 43 and 86 GHz and compare their ejection times with radio and $γ$-ray variability. For the moving jet features, we find an average velocity of $β^\textrm{avg}_\textrm{app} = 0.055-0.22$c ($μ^\textrm{avg} = 0.04-0.18\,$mas/yr). From the time-averaged VLBI images at the three frequencies, we measure the transverse jet width along the bulk flow. On the $\leq 1.5$ parsec scale, we find a clear trend of the jet width being frequency dependent, with the jet being narrower at higher frequencies. This stratification is discussed in the context of a spine-sheath scenario, and is compared to other possible interpretations. From quasi-simultaneous observations at 43 and 86\,GHz we obtain spectral index maps, revealing a time-variable orientation of the spectral index gradient, due to structural variability of the inner jet.

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arXiv2022

The Unanticipated Phenomenology of the Blazar PKS~2131$-$021: A Unique Super-Massive Black Hole Binary Candidate

Most large galaxies host supermassive black holes in their nuclei and are subject to mergers, which can produce a supermassive black hole binary (SMBHB), and hence periodic signatures due to orbital motion. We report unique periodic radio flux density variations in the blazar PKS~2131$-$021, which strongly suggest an SMBHB with an orbital separation of $\sim 0.001-0.01$ pc. Our 45.1-year radio light curve shows two epochs of strong sinusoidal variation with the same period and phase to within $<2\%$ and $\sim 10\%$, respectively, straddling a 20-year period when this variation was absent. Our simulated light curves accurately reproduce the ``red noise'' of this object, and Lomb-Scargle, weighted wavelet Z-transform, and least-squares sine wave analyses demonstrate conclusively, at the $4.6σ$ significance level, that the periodicity in this object is not due to random fluctuations in flux density. The observed period translates to $2.082\pm 0.003$ years in the rest frame at the $z=1.285$ redshift of PKS~2131$-$021. The periodic variation in PKS~2131$-$021 is remarkably sinusoidal. We present a model in which orbital motion, combined with the strong Doppler boosting of the approaching relativistic jet, produces a sine-wave modulation in the flux density which easily fits the observations. Given the rapidly-developing field of gravitational wave experiments with pulsar timing arrays, closer counterparts to PKS~2131$-$021 and searches using the techniques we have developed are strongly motivated. These results constitute a compelling demonstration that the phenomenology, not the theory, must provide the lead in this field.

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arXiv2022

New Tests of Millilensing in the Blazar PKS 1413+135

Symmetric Achromatic Variability (SAV) is a rare form of radio variability in blazars that has been attributed to gravitational millilensing by a ~$10^2 - 10^5$ $M_\odot$ mass condensate. Four SAVs have been identified between 1980 and 2020 in the long-term radio monitoring data of the blazar PKS 1413+135. We show that all four can be fitted with the same, unchanging, gravitational lens model. If SAV is due to gravitational millilensing, PKS 1413+135 provides a unique system for studying active galactic nuclei with unprecedented microarcsecond resolution, as well as for studying the nature of the millilens itself. We discuss two possible candidates for the putative millilens: a giant molecular cloud hosted in the intervening edge-on spiral galaxy, and an undetected dwarf galaxy with a massive black hole. We find a significant dependence of SAV crossing time on frequency, which could indicate a fast shock moving in a slower underlying flow. We also find tentative evidence for a 989-day periodicity in the SAVs, which, if real, makes possible the prediction of future SAVs: the next three windows for possible SAVs begin in August 2022, May 2025, and February 2028.

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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.

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arXiv2021

Identifying changing jets through their radio variability

Supermassive black holes launch highly relativistic jets with velocities reaching Lorentz factors as high as $Γ>50$. How the jets accelerate to such high velocities and where along the jet do they reach terminal velocity are open questions that are tightly linked to their structure, launching and dissipation mechanisms. Changes in the beaming factor along the jets could potentially reveal jet acceleration, deceleration, or bending. We aim to (1) quantify the relativistic effects in multiple radio frequencies and (2) study possible jet velocity--viewing angle variations at parsec scales. We used the state-of-the-art code Magnetron to model light curves from the University of Michigan Radio Observatory and the Metsähovi Radio Observatory's monitoring programs in five frequencies covering about 25 years of observations in the 4.8-37~GHz range for 61 sources. We supplement our data set with high-frequency radio observations in the 100-340~GHz range from ALMA, CARMA, and SMA. For each frequency we estimate the Doppler factor which we use to quantify possible changes in the relativistic effects along the jets. The majority of our sources do not show any statistically significant difference in their Doppler factor across frequencies. This is consistent with constant velocity in a conical jet, as expected at parsec scales. However, our analysis reveals 17 sources where relativistic beaming changes as a function of frequency. In the majority of cases the Doppler factor increases towards lower frequencies. Only 1253-053 shows the opposite behavior. By exploring their jet properties we find that the jet of 0420-014 is likely bent across the 4.8-340~GHz range. For 0212+735 the jet is likely parabolic, and still accelerating in the 4.8-37~GHz range. We discuss possible interpretations for the trends found in the remaining sources.

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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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arXiv2021

Association of IceCube neutrinos with radio sources observed at Owens Valley and Metsähovi Radio Observatories

Identifying the most likely sources for high-energy neutrino emission has been one of the main topics in high-energy astrophysics ever since the first observation of high-energy neutrinos by the IceCube Neutrino Observatory. Active galactic nuclei with relativistic jets, also known as blazars, have been considered to be one of the main candidates because of their ability to accelerate particles to high energies. We study the connection between radio emission and IceCube neutrino events using data from the Owens Valley Radio Observatory (OVRO) and Metsähovi Radio Observatory blazar monitoring programs. We identify sources in our radio monitoring sample that are positionally consistent with IceCube high-energy neutrino events. We estimate their mean flux density and variability amplitudes around the neutrino arrival time, and compare these with values from random samples to establish the significance of our results. We find radio source associations within our samples with 15 high-energy neutrino events detected by IceCube. Nearly half of the associated sources are not detected in the $γ$-ray energies, but their radio variability properties and Doppler boosting factors are similar to the $γ$-ray detected objects in our sample, meaning that they could still be potential neutrino emitters. We find that the number of strongly flaring objects in our statistically complete OVRO samples is unlikely to be a random coincidence (at $2σ$ level). Based on our results, we conclude that although it is clear that not all neutrino events are associated with strong radio flaring blazars, observations of large-amplitude radio flares in a blazar at the same time as a neutrino event are unlikely to be a random coincidence.

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arXiv2020

The Relativistic Jet Orientation and Host Galaxy of the Peculiar Blazar PKS 1413+135

PKS 1413+135 is one of the most peculiar blazars known. Its strange properties led to the hypothesis almost four decades ago that it is gravitationally lensed by a mass concentration associated with an intervening galaxy. It exhibits symmetric achromatic variability, a rare form of variability that has been attributed to gravitational milli-lensing. It has been classified as a BL Lac object, and is one of the rare objects in this class with a visible counterjet. BL Lac objects have jet axes aligned close to the line of sight. It has also been classified as a compact symmetric object, which have jet axes not aligned close to the line of sight. Intensive efforts to understand this blazar have hitherto failed to resolve even the questions of the orientation of the relativistic jet, and the host galaxy. Answering these two questions is important as they challenge our understanding of jets in active galactic nuclei and the classification schemes we use to describe them. We show that the jet axis is aligned close to the line of sight and PKS 1413+135 is almost certainly not located in the apparent host galaxy, but is a background object in the redshift range $0.247 < z < 0.5$. The intervening spiral galaxy at $z = 0.247$ provides a natural host for the putative lens responsible for symmetric achromatic variability and is shown to be a Seyfert 2 galaxy. We also show that, as for the radio emission, a "multizone" model is needed to account for the high-energy emission.

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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

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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