arXiv subjects

A. Gokus

Publications and source records attributed to A. Gokus.

15 recordsLinked to original sources

arXiv2026

Rapid jet ejection from PKS 0215+015 coincident with a high-energy neutrino event

Aims. We present a new neutrino-blazar multiwavelength flare coincidence observed in the blazar PKS 0215+015, which showed a strong multiwavelength outburst in coincidence with the IceCube neutrino track alert IC220225A, similar to the case of TXS 0506+056. We investigate the immediate response of the radio jet to the major flare. Methods. We performed target-of-opportunity observations with the Very Long Baseline Array (VLBA) at 15, 23, and 43 GHz in full polarization for six epochs with monthly cadence following the neutrino event. We combine the VLBA observations with monitoring data from the Effelsberg 100-m telescope, the Australia Telescope Compact Array, and Fermi/LAT. Results. Based on our VLBI kinematic analysis, we identified a new rapid jet component with an apparent speed of ~60-80c, which was ejected around the arrival of IC220225A. The fast component ejection is traced by a characteristic signature in polarization that suggests a shock-shock interaction with a quasi-stationary feature. By combining the VLBI results with radio variability data, we estimated a bulk Lorentz factor of $\Gamma = 105 \pm 56$ and a jet viewing angle of $\vartheta = (1.47 \pm 0.31)^\circ$. Conclusions. We note that the properties of the rapid component exceed previously reported maximum apparent jet speeds and Lorentz factors from continuous VLBI monitoring programs. This is likely only possible because we are observing an exceptional flaring event at high redshift (z=1.72) with higher observing cadence than in typical monitoring programs. We suggest that neutrino production in PKS 0215+015 can occur through p{\gamma}-interactions with protons possibly accelerated within the fast-moving feature. The target photon field could be external to the jet or explained by a multi-layered jet. The latter scenario is consistent with the presence of quasi-stationary features revealed in our analysis.

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arXiv2025

BlazEr1: The eROSITA Blazar Catalog. Blazars and Blazar Candidates in the First eROSITA Survey

Aims. eROSITA, on board the Spectrum Roentgen Gamma (SRG) spacecraft, performed its first X-ray all-sky survey (eRASS1) between December 2019 and June 2020. It detected about 930000 sources, providing us with an unprecedented opportunity for a detailed blazar census. We present the properties of blazars and blazar candidates in eRASS1 and the compilation of the eROSITA blazar catalog. Methods. We compiled a list of blazar and blazar candidates from the literature and matched it with the eRASS1 catalog, constructing the Blazars in eRASS1 (BlazEr1) catalog. For sources with more than 50 counts, we obtained their X-ray spectral properties. We compiled multiwavelength data from the radio to the gamma-ray regimes for all sources, including multiwavelength spectral indices and redshifts. The full catalog is available online. Results. We present the BlazEr1 catalog, containing 5865 sources, of which 2106 are associated with confirmed blazars. For 2966 sources, eROSITA provides the first X-ray data. The contamination from non-blazar sources of the entire sample is less than 11%. Most candidates exhibit properties typical for blazars. We present the properties of the entire X-ray detected blazar population, including the distributions of X-ray luminosities and photon indices, multiwavelength properties, and the blazar log N-log S distribution. Our catalog provides follow up targets, such as potential MeV and TeV blazars. Conclusions. The BlazEr1 catalog provides a compilation of X-ray detected blazars and blazar candidates. The catalog serves as a starting point for exploiting further eROSITA surveys using the same methodology, enabling us to study the X-ray variability and a large number of spectral energy distributions of blazars in the future.

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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 $\Pi_X=6\pm2$ per cent at a polarization angle, measured east of north, of $\Psi_X=143^\circ\pm11^\circ$ 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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arXiv2024

Very-long-baseline interferometry study of the flaring blazar TXS 1508+572 in the early Universe

High-redshift blazars provide valuable input to studies of the evolution of active galactic nuclei (AGN) jets and provide constraints on cosmological models. Detections at high energies ($0.1<\mathrm{E}<100$ GeV) of these distant sources are rare, but when they exhibit bright gamma-ray flares, we are able to study them. However, contemporaneous multi-wavelength observations of high-redshift objects ($z>4$) during their different periods of activity have not been carried out so far. An excellent opportunity for such a study arose when the blazar TXS 1508+572 ($z=4.31$) exhibited a $\gamma$-ray flare in 2022 February in the $0.1-300$ GeV range with a flux 25 times brighter than the one reported in the in the fourth catalog of the \textit{Fermi} Large Area Telescope. Our goal is to monitor the morphological changes, spectral index and opacity variations that could be associated with the preceding $\gamma$-ray flare in TXS 1508+572 to find the origin of the high-energy emission in this source. We also plan to compare the source characteristics in the radio band to the blazars in the local Universe ($z<0.1$). In addition, we aim to collect quasi-simultaneous data to our multi-wavelength observations of the object, making TXS 1508+572 the first blazar in the early Universe ($z>4$) with contemporaneous multi-frequency data available in its high state. In order to study the parsec-scale structure of the source, we performed three epochs of very-long-baseline interferometry (VLBI) follow-up observations with the Very Long Baseline Array (VLBA) supplemented with the Effelsberg 100-m Telescope at 15, 22, and 43 GHz, which corresponds to 80, 117, and 228 GHz in the rest frame of TXS 1508+572. In addition, one 86 GHz (456 GHz) measurement was performed by the VLBA and the Green Bank Telescope during the first epoch.

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arXiv2024

Rapid Variability of Mrk 421 During Extreme Flaring as Seen Through the Eyes of XMM-Newton

By studying the variability of blazars across the electromagnetic spectrum, it is possible to resolve the underlying processes responsible for rapid flux increases, so-called flares. We report on an extremely bright X-ray flare in the high-peaked BL Lacertae object Mrk 421 that occurred simultaneously with enhanced $\gamma$-ray activity detected at very high energies (VHE) by FACT on 2019 June 9. We triggered an observation with XMM-Newton, which observed the source quasi-continuously for 25 hours. We find that the source was in the brightest state ever observed using XMM-Newton, reaching a flux of $2.8\times10^{-9}$ erg cm$^{-2}$ s$^{-1}$ over an energy range of 0.3 - 10 keV. We perform a spectral and timing analysis to reveal the mechanisms of particle acceleration and to search for the shortest source-intrinsic time scales. Mrk 421 exhibits the typical harder-when-brighter behaviour throughout the observation and shows a clock-wise hysteresis pattern, which indicates that the cooling dominates over the acceleration process. While the X-ray emission in different sub-bands is highly correlated, we can exclude large time lags as the computed zDCFs are consistent with a zero lag. We find rapid variability on time scales of 1 ks for the 0.3 - 10 keV band and down to 300s in the hard X-ray band (4 - 10 keV). Taking these time scales into account, we discuss different models to explain the observed X-ray flare, and find that a plasmoid-dominated magnetic reconnection process is able to describe our observation best.

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arXiv2024

The SRG/eROSITA all-sky survey: First X-ray catalogues and data release of the western Galactic hemisphere

The eROSITA telescope array aboard the Spektrum Roentgen Gamma (SRG) satellite began surveying the sky in December 2019, with the aim of producing all-sky X-ray source lists and sky maps of an unprecedented depth. Here we present catalogues of both point-like and extended sources using the data acquired in the first six months of survey operations (eRASS1; completed June 2020) over the half sky whose proprietary data rights lie with the German eROSITA Consortium. We describe the observation process, the data analysis pipelines, and the characteristics of the X-ray sources. With nearly 930000 entries detected in the most sensitive 0.2-2.3 keV energy range, the eRASS1 main catalogue presented here increases the number of known X-ray sources in the published literature by more than 60%, and provides a comprehensive inventory of all classes of X-ray celestial objects, covering a wide range of physical processes. A smaller catalogue of 5466 sources detected in the less sensitive but harder 2.3-5 keV band is the result of the first true imaging survey of the entire sky above 2 keV. We show that the number counts of X-ray sources in eRASS1 are consistent with those derived over narrower fields by past X-ray surveys of a similar depth, and we explore the number counts variation as a function of the location in the sky. Adopting a uniform all-sky flux limit (at 50% completeness) of F_{0.5-2 keV} > 5 \times 10^{-14}$ erg\,s$^{-1}$\,cm$^{-2}$, we estimate that the eROSITA all-sky survey resolves into individual sources about 20% of the cosmic X-ray background in the 1-2 keV range. The catalogues presented here form part of the first data release (DR1) of the SRG/eROSITA all-sky survey. Beyond the X-ray catalogues, DR1 contains all detected and calibrated event files, source products (light curves and spectra), and all-sky maps. Illustrative examples of these are provided.

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arXiv2024

TELAMON: Effelsberg monitoring of AGN jets with very-high-energy astroparticle emission -- I. Program description and sample characterization

Aims. We introduce the TELAMON program which is using the Effelsberg 100-m telescope to monitor the radio spectra of active galactic nuclei (AGN) under scrutiny in astroparticle physics, specifically TeV blazars and candidate neutrino-associated AGN. Here, we present and characterize our main sample of TeV-detected blazars. Methods. We analyze the data sample from the first ~2.5 years of observations between August 2020 and February 2023 in the range from 14 GHz to 45 GHz. During this pilot phase, we have observed all 59 TeV-detected blazars in the Northern Hemisphere (i.e., Dec. >0{\deg}) known at the time of observation. We discuss the basic data reduction and calibration procedures used for all TELAMON data and introduce a sub-band averaging method used to calculate average light curves for the sources in our sample. Results. The TeV-selected sources in our sample exhibit a median flux density of 0.12 Jy at 20 mm, 0.20 Jy at 14 mm and 0.60 Jy at 7 mm. The spectrum for most of the sources is consistent with a flat radio spectrum and we find a median spectral index ($S(\nu)\propto\nu^\alpha$) of $\alpha=-0.11$. Our results on flux density and spectral index are consistent with previous studies of TeV-selected blazars. Compared to the GeV-selected F-GAMMA sample, TELAMON sources are significantly fainter in the radio band. This is consistent with the double-humped spectrum of blazars being shifted towards higher frequencies for TeV-emitters (in particular for high-synchrotron peaked BL Lac type objects), which results in a lower radio flux density. The spectral index distribution of our TeV-selected blazar sample is not significantly different from the GeV-selected F-GAMMA sample. Moreover, we present a strategy to track the light curve evolution of sources in our sample for future variability and correlation analysis.

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arXiv2023

TELAMON: Effelsberg Monitoring of AGN Jets with Very-High-Energy Astroparticle Emissions -- Polarization properties

We present recent results of the TELAMON program, which is using the Effelsberg 100-m telescope to monitor the radio spectra of active galactic nuclei (AGN) under scrutiny in astroparticle physics, namely TeV blazars and neutrino-associated AGN. Our sample includes all known Northern TeV-emitting blazars as well as blazars positionally coincident with IceCube neutrino alerts. Polarization can give additional insight into the source properties, as the polarized emission is often found to vary on different timescales and amplitudes than the total intensity emission. Here, we present an overview of the polarization properties of the TeV-emitting TELAMON sources at four frequencies in the 20 mm and 7 mm bands. While at 7 mm roughly $82\,\%$ of all observed sources are found to be significantly polarized, for 20 mm the percentage is $\sim58\,\%$. We find that most of the sources exhibit mean fractional polarizations of $<5\%$, matching the expectations of rather low polarization levels in these sources from previous studies at lower radio frequencies. Nevertheless, we demonstrate examples of how the polarized emission can provide additional information over the total intensity.

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arXiv2023

Multiwavelength study of extreme variability in LEDA 1154204: A changing-look event in a type 1.9 Seyfert

Context. Multiwavelength studies of transients in actively accreting supermassive black holes have revealed that large-amplitude variability is frequently linked to significant changes in the optical spectra -- a phenomenon known as changing-look AGN (CLAGN).} Aims. In 2020, the Zwicky Transient Facility detected a transient flaring event in the type 1.9 AGN LEDA 1154204, wherein brightness sharply increased by 0.55 mag in one month, then began to decay. Spectrum Roentgen Gamma (SRG)/eROSITA also observed the object as part of its all-sky X-ray surveys, after the flare had started decaying. Methods. We performed a three-year, multiwavelength follow-up campaign to track the source's spectral and temporal characteristics, during the post-flare fading. This campaign included optical spectroscopy, X-ray spectroscopy and photometry, and UV, optical, and IR continuum photometry. Results. Optical spectra taken near the flare peak revealed a broad double-peaked H$\beta$ emission and a blue continuum, both undetected in a 2005 archival spectrum; broad H$\beta$ had increased by a factor $>$5--6. Then, from late 2020 through 2023, broad Balmer line flux faded as the continuum faded, with Balmer decrement increasing by $\sim$2.2, consistent with the expected ionization response. The X-ray spectrum exhibits no significant spectral variability despite dramatic flux variation -- a factor of 17. There is no evidence of a soft X-ray excess, indicating an energetically unimportant warm corona. Conclusions. The transient event was likely triggered by a disk instability in a pre-existing AGN-like accretion flow, culminating in the observed multiwavelength variability -- X-rays via thermal Comptonization, BLR illumination, and IR dust echo -- and CLAGN event.

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arXiv2023

VLBI Scrutiny of a New Neutrino-Blazar Multiwavelength-Flare Coincidence

In the past years, evidence has started piling up that some high-energy cosmic neutrinos can be associated with blazars in flaring states. On February 26, 2022, a new blazar-neutrino coincidence has been reported: the track-like neutrino event IC220225A detected by IceCube is spatially coincident with the flat-spectrum radio quasar PKS 0215+015. Like previous associations, this source was found to be in a high optical and $γ$-ray state. Moreover, the source showed a bright radio outburst, which substantially increases the probability of a true physical association. We have performed six observations with the VLBA shortly after the neutrino event with a monthly cadence and are monitoring the source with the Effelsberg 100m-Telescope, and with the Australia Compact Telescope Array. Here, we present first results on the contemporary parsec-scale jet structure of PKS 0215+015 in total intensity and polarization to constrain possible physical processes leading to neutrino emission in blazars.

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arXiv2023

Discovery of the luminous X-ray ignition eRASSt J234402.9$-$352640; I. Tidal disruption event or a rapid increase in accretion in an active galactic nucleus?

In November 2020, a new, bright object, eRASSt J234402.9$-$352640, was discovered in the second all-sky survey of SRG/eROSITA. The object brightened by a factor of at least 150 in 0.2--2.0 keV flux compared to an upper limit found six months previous, reaching an observed peak of $1.76_{-0.24}^{+0.03} \times 10^{-11}$ erg cm$^{-2}$ s$^{-1}$. The X-ray ignition is associated with a galaxy at $z=0.10$, making the peak luminosity log$_{10}(L_{\rm 0.2-2keV}/[\textrm{erg s}^{-1}])$=$44.7\pm0.1$. Around the time of the rise in X-ray flux, the nucleus of the galaxy brightened by approximately 3 mag. in optical photometry, after correcting for the host. We present data from Swift, XMM-Newton, and NICER, which reveal a very soft spectrum as well as strong 0.2--2.0 keV flux variability on multiple timescales. Optical spectra taken in the weeks after the ignition event show a blue continuum with broad, asymmetric Balmer emission lines, and high-ionisation ([OIII]$λλ$4959,5007) and low-ionisation ([NII]$λ$6585, [SII]$λλ$6716,6731) narrow emission lines. Following the peak in the optical light curve, the X-ray, UV, and optical photometry all show a rapid decline. The X-ray light curve shows a decrease in luminosity of $\sim$0.45 over 33 days and the UV shows a drop of $\sim$0.35. eRASSt J234402.9$-$352640 also shows a brightening in the mid-infrared, likely powered by a dust echo of the luminous ignition. We find no evidence in Fermi-LAT $γ$-ray data for jet-like emission. The event displays characteristics of a tidal disruption event (TDE) as well as of an active galactic nucleus (AGN), complicating its classification. Based on the softness of the X-ray spectrum, the presence of high-ionisation optical emission lines, and the likely infrared echo, we find that a TDE within a turned-off AGN best matches our observations.

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arXiv2021

Multi-wavelength study of Mrk 421 during a TeV flare

The blazar Mrk 421 shows frequent, short flares in the TeV energy regime. Due to the fast nature of such episodes, we often fail to obtain sufficient simultaneous information about flux variations in several energy bands. To overcome this lack of multi-wavelength (MWL) coverage, especially for the pre- and post-flare periods, we have set up a monitoring program with the FACT telescope (TeV energies) and the Neil Gehrels Swift Observatory (X-rays). On 2019 June 9, Mrk 421 showed a TeV outburst reaching a flux level of more than two times the flux of the Crab Nebula at TeV energies. We acquired simultaneous data in the X-rays with additional observations by XMM-Newton and INTEGRAL. For the first time, we can study a TeV blazar in outburst taking advantage of highly sensitive X-ray data from XMM-Newton and INTEGRAL combined. Our dataset is complemented by pointed radio observations by Effelsberg at GHz frequencies. We present our first results, including the γ-ray and X-ray light curves, a timing analysis of the X-ray data obtained with XMM-Newton , as well as the radio spectra before, during and after the flare.

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arXiv2021

TELAMON: Effelsberg Monitoring of AGN Jets with Very-High-Energy Astroparticle Emissions

We introduce the TELAMON program, which is using the Effelsberg 100-m telescope to monitor the radio spectra of active galactic nuclei (AGN) under scrutiny in astroparticle physics, namely TeV blazars and candidate neutrino-associated AGN. Thanks to its large dish aperture and sensitive instrumentation, the Effelsberg telescope can yield radio data superior over other programs in the low flux-density regime down to several 10 mJy. This is a particular strength in the case of TeV-emitting blazars, which are often comparatively faint radio sources of the high-synchrotron peaked type. We perform high-cadence high-frequency observations every 2-4 weeks at multiple frequencies up to 44 GHz. This setup is well suited to trace dynamical processes in the compact parsec-scale jets of blazars related to high-energy flares or neutrino detections. Our sample currently covers about 40 sources and puts its focus on AGN with very-high-energy astroparticle emission, i.e., TeV blazars and neutrino-associated AGN. Here, we introduce the TELAMON program characteristics and present first results obtained since fall 2020.

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arXiv2020

Search for high-redshift blazars with Fermi/LAT

High-$z$ blazars (z $\geq 2.5$) are the most powerful class of persistent $γ$-ray sources in the Universe. These objects possess the highest jet powers and luminosities and have black hole masses often in excess of $10^9$ solar masses. In addition, high-$z$ blazars are important cosmological probes and serve as test objects for blazar evolution models. Due to their large distance, their high-energy emission typically peaks below the GeV range, which makes them difficult to study with Fermi/LAT. Therefore, only the very brightest objects are detectable and, to date, only a small number of high-z blazars have been detected with Fermi/LAT. In this work, we studied the monthly binned long-term $γ$-ray emission of a sample of 176 radio and optically detected blazars that have not been reported as known $γ$-ray sources in the 3FGL catalog. In order to account for false-positive detections, we calculated monthly Fermi/LAT light curves for a large sample of blank sky positions and derived the number of random fluctuations that we expect at various test statistic (TS) levels. For a given blazar, a detection of TS > 9 in at least one month is expected $\sim 15\%$ of the time. Although this rate is too high to secure detection of an individual source, half of our sample shows such single-month $γ$-ray activity, indicating a population of high-energy blazars at distances of up to z=5.2. Multiple TS > 9 monthly detections are unlikely to happen by chance, and we have detected several individual new sources in this way, including the most distant $γ$-ray blazar, BZQ J1430+4204 (z = 4.72). Finally, two new $γ$-ray blazars at redshifts of z = 3.63 and z = 3.11 are unambiguously detected via very significant (TS > 25) flares in individual monthly time bins.

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arXiv2018

Decomposing blazar spectra into lepto-hadronic emission components

Recently reported coincidences between high-energy neutrino events and major blazar outbursts reinforce the relevance of lepto-hadronic emission models for blazars. We study the influence of physical parameters on the neutrino output modeling blazar spectral energy distributions self-consistently assuming a relativistically propagating acceleration zone surrounded by a larger cooling zone. We find that the gross features of the spectral energy distribution can readily be explained with the model. A rigorous test requires time-resolved measurements of blazar spectral energy distributions during an outburst and high-statistics neutrino measurements to discriminate the leptonic and hadronic emission components.

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