S. Kiehlmann's research while affiliated with University of Crete and other places
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Publications (141)
High-redshift ($z >3$) $\gamma$-ray blazars are rare, but they are crucial for our understanding of jet evolution, $\gamma$-ray production and propagation, and the growth of supermassive black holes in the early universe. A new analysis of Fermi-LAT data reveals a significant (5$\sigma$), spectrally soft ($\Gamma \simeq$ 3.0) $\gamma$-ray source in...
Aims. We present the first multiwavelength study of Mrk 501 that contains simultaneous very-high-energy (VHE) γ -ray observations and X-ray polarization measurements from the Imaging X-ray Polarimetry Explorer (IXPE).
Methods. We used radio-to-VHE data from a multiwavelength campaign carried out between March 1, 2022, and July 19, 2022 (MJD 59639 t...
Active galaxies, especially blazars, are among the most promising extragalactic candidates for high-energy neutrino sources. To date, ANTARES searches included these objects and used GeV–TeV γ -ray flux to select blazars. Here, a statistically complete blazar sample selected by their bright radio emission is used as the target for searches of origi...
PG 1553 + 113 is one of the few blazars with a convincing quasi-periodic emission in the gamma-ray band. The source is also a very high energy (VHE; >100 GeV) gamma-ray emitter. To better understand its properties and identify the underlying physical processes driving its variability, the MAGIC Collaboration initiated a multiyear, multiwavelength m...
We present the first degree-scale tomography map of the dusty magnetized interstellar medium (ISM) from stellar polarimetry and distance measurements. We used the RoboPol polarimeter at Skinakas Observatory to conduct a survey of the polarization of starlight in a region of the sky of about four square degrees. We propose a Bayesian method to decom...
Context . The calibration of optical polarimeters relies on the use of stars with negligible polarization (i.e., unpolarized standard stars) for determining the instrumental polarization zero point. For wide-field polarimeters, calibration is often done by imaging the same star over multiple positions in the field of view (FoV), which is a time-con...
Compact symmetric objects (CSOs) are jetted active galactic nuclei (AGN) with overall projected size <1 kpc. The classification was introduced to distinguish these objects from the majority of compact jetted AGN in centimeter-wavelength very long baseline interferometry observations, where the observed emission is relativistically boosted toward th...
Compact symmetric objects (CSOs) are compact (<1 kpc), jetted active galactic nuclei (AGN), whose jet axes are not aligned close to the line of sight, and whose observed emission is not predominantly relativistically boosted toward us. Two classes of CSOs have previously been identified: approximately one-fifth are edge dimmed and the rest are edge...
We use a sample of 54 compact symmetric objects (CSOs) to confirm that there are two unrelated CSO classes: an edge-dimmed, low-luminosity class (CSO 1), and an edge-brightened, high-luminosity class (CSO 2). Using blind tests, we show that CSO 2s consist of three subclasses: CSO 2.0, having prominent hot spots at the leading edges of narrow jets a...
Context. The high-mass X-ray binary LS I +61°303 is composed of a Be-type star and a compact object in an eccentric orbit. The emission from this source is variable and periodic across the electromagnetic spectrum, from radio to very high-energy γ rays. The orbital period has been determined as P 1 ≈ 26.5 d, and the source also features a super-orb...
Aims. We have performed the first broadband study of Mrk 421 from radio to TeV gamma rays with simultaneous measurements of the X-ray polarization from IXPE.
Methods. The data were collected as part of an extensive multiwavelength campaign carried out between May and June 2022 using MAGIC, Fermi -LAT, NuSTAR , XMM-Newton , Swift , and several optic...
Optical circular polarization observations can directly test the particle composition in black hole jets. We report the first observations of the BL Lac-type object S4 0954+65 in high linear polarized states. While no circular polarization was detected, we were able to place upper limits of $<0.5 $ at 99.7 confidence. Using a simple model and our n...
PKS 1830−211 is a γ-ray emitting, high-redshift (z =2.507 ± 0.002), lensed flat-spectrum radio quasar. During the period mid-February to mid-April 2019, this source underwent a series of strong γ-ray flares that were detected by both AGILE-GRID and Fermi-LAT, reaching a maximum γ-ray flux of FE > 100 MeV ≈ 2.3 × 10−5 photons cm−2 s−1. Here we repor...
We present the multiwavelength flaring activity of the blazar AO 0235+164 during its recent active period from 2013 to 2019. From a discrete correlation function (DCF) analysis, we find a significant ($>95~{{\%}}$) correlation between radio and γ-ray light curves with flares at longer wavelengths following flares at shorter wavelengths. We identify...
In 2022 the BL Lac object S4 0954+65 underwent a major variability phase, reaching its historical maximum brightness in the optical and γ-ray bands. We present optical photometric and polarimetric data acquired by the Whole Earth Blazar Telescope (WEBT) Collaboration from 2022 April 6 to July 6. Many episodes of unprecedented fast variability were...
We report on the low Galactic latitude (b = 4.○3) quasar 2005+403, the second active galactic nuclei, in which we detected a rare phenomenon of multiple imaging induced by refractive-dominated scattering. The manifestation of this propagation effect is revealed at different frequencies (≲ 8 GHz) and epochs of VLBA observations. The pattern formed b...
Context. Next-generation wide-field optical polarimeters such as Wide-Area Linear Optical Polarimeters (WALOPs) have a field of view (FoV) of tens of arcminutes. Wide-field polarimetric flat sources are essential to the efficient and accurate calibration of these instruments. However, no established wide-field polarimetric standard or flat sources...
Studying blazar radio variability on timescales ranging from months to years provides information on the sub-parsec-scale structures of the jets, and the physics of the central active galactic nuclei. In this study, we focus on the radio variability of 1158 blazars observed at 15 GHz through the Owens Valley Radio Observatory Blazar Monitoring Prog...
We report on the low Galactic latitude ($b=4.3^\circ$) quasar 2005$+$403, the second active galactic nuclei, in which we detected a rare phenomenon of multiple imaging induced by refractive-dominated scattering. The manifestation of this propagation effect is revealed at different frequencies ($\lesssim8$ GHz) and epochs of VLBA observations. The p...
Studying the long-term radio variability (timescales of months to years) of blazars enables us to gain a better understanding of the physical structure of these objects on sub-parsec scales, and the physics of super massive black holes. In this study, we focus on the radio variability of 1157 blazars observed at 15 GHz through the Owens Valley Radi...
Studying the long-term radio variability (timescales of months to years) of blazars enables us to gain a better understanding of the physical structure of these objects on sub-parsec scales, and the physics of super massive black holes. In this study, we focus on the radio variability of 1157 blazars observed at 15~GHz through the Owens Valley Radi...
Context. Optical polarimeters are typically calibrated using measurements of stars with known and stable polarization parameters. However, there is a lack of such stars available across the sky. Many of the currently available standards are not suitable for medium and large telescopes due to their high brightness. Moreover, as we find, some of the...
The magnetic-field conditions in astrophysical relativistic jets can be probed by multiwavelength polarimetry, which has been recently extended to X-rays. For example, one can track how the magnetic field changes in the flow of the radiating particles by observing rotations of the electric vector position angle Ψ. Here we report the discovery of a...
Optical polarimeters are typically calibrated using measurements of stars with known and stable polarization parameters. However, there is a lack of such stars available across the sky. Many of the currently available standards are not suitable for medium and large telescopes due to their high brightness. Moreover, as we find, some of the used pola...
We report the detection of very high energy gamma-ray emission from the blazar S3 1227+25 (VER J1230+253) with the Very Energetic Radiation Imaging Telescope Array System (VERITAS). VERITAS observations of the source were triggered by the detection of a hard-spectrum GeV flare on 2015 May 15 with the Fermi-Large Area Telescope (LAT). A combined 5 h...
The magnetic field conditions in astrophysical relativistic jets can be probed by multiwavelength polarimetry, which has been recently extended to X-rays. For example, one can track how the magnetic field changes in the flow of the radiating particles by observing rotations of the electric vector position angle $\Psi$. Here we report the discovery...
Next-generation wide-field optical polarimeters like the Wide-Area Linear Optical Polarimeters (WALOPs) have a field of view (FoV) of tens of arcminutes. For efficient and accurate calibration of these instruments, wide-field polarimetric flat sources will be essential. Currently, no established wide-field polarimetric standard or flat sources exis...
We report the detection of very high energy gamma-ray emission from the blazar S3 1227+25 (VER J1230+253) with the Very Energetic Radiation Imaging Telescope Array System (VERITAS). VERITAS observations of the source were triggered by the detection of a hard-spectrum GeV flare on May 15, 2015 with the Fermi-Large Area Telescope (LAT). A combined fi...
Compact Symmetric Objects (CSOs) are a class of compact, jetted Active Galactic Nuclei (AGN) whose jet axes are not aligned close to the line of sight, and whose observed emission is not predominantly relativistically boosted towards us. Using complete samples of CSOs, we present three independent lines of evidence, based on their relative numbers,...
Compact Symmetric Objects (CSOs) form a distinct class of jetted active galactic nuclei (jetted-AGN). We examine a carefully selected sample of 54 CSOs, and confirm that there are two unrelated classes: an edge-dimmed, low-luminosity class (CSO 1), and an edge-brightened, high-luminosity class (CSO 2). Using statistically significant blind tests, w...
Compact Symmetric Objects (CSOs) are jetted Active Galactic Nuclei (AGN) with overall projected size < 1 kpc. The classification was introduced to distinguish these objects from the majority of known compact jetted-AGN, where the observed emission is relativistically boosted towards the observer. The original classification criteria for CSOs were:...
We present the first Bayesian method for tomographic decomposition of the plane-of-sky orientation of the magnetic field with the use of stellar polarimetry and distance. This standalone tomographic inversion method presents an important step forward in reconstructing the magnetized interstellar medium (ISM) in three dimensions within dusty regions...
Aims. We report on the follow-up of the extremely bright long gamma-ray burst (GRB) GRB 210619B with optical polarimetry.
Methods. We conducted optopolarimetric observations of the optical afterglow of GRB 210619B in the SDSS- r band in the time window ∼5967 − 8245 s after the burst, using the RoboPol instrument at the Skinakas observatory.
Results...
Blazars are a class of jet-dominated active galactic nuclei with a typical double-humped spectral energy distribution. It is of common consensus the Synchrotron emission to be responsible for the low frequency peak, while the origin of the high frequency hump is still debated. The analysis of X-rays and their polarization can provide a valuable too...
The BL Lac object 1ES 0647+250 is one of the few distant $\gamma$-ray emitting blazars detected at very high energies (VHE, $\gtrsim$100 GeV) during a non-flaring state. It was detected with the MAGIC telescopes during its low activity in the years 2009-2011, as well as during three flaring activities in the years 2014, 2019 and 2020, with the high...
Total intensity variability light curves offer a unique insight into the ongoing debate about the launching mechanism of jets. For this work, we utilised the availability of radio and γ -ray light curves over a few decades of the radio source 3C 84 ( NGC 1275 ). We calculated the multi-band time-lags between the flares identified in the light curve...
A black hole x-ray binary (XRB) system forms when gas is stripped from a normal star and accretes onto a black hole, which heats the gas sufficiently to emit x-rays. We report a polarimetric observation of the XRB Cygnus X-1 using the Imaging X-ray Polarimetry Explorer. The electric field position angle aligns with the outflowing jet, indicating th...
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 $\gamma$-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 curv...
We report on the follow-up of the extremely bright gamma-ray burst GRB 210619B with optical polarimetry. We conducted optopolarimetric observations of the optical afterglow of GRB 210619B in the SDSS-r band in the time window ~ 5967 - 8245 seconds after the burst, using the RoboPol instrument at the Skinakas observatory. We find signs of variabilit...
The Wide-Area Linear Optical Polarimeter (WALOP)-South instrument is an upcoming wide-field and high-accuracy optical polarimeter to be used as a survey instrument for carrying out the Polar-Areas Stellar Imaging in Polarization High Accuracy Experiment (PASIPHAE) program. Designed to operate as a one-shot four-channel and four-camera imaging polar...
The origin of Petaelectronvolt (PeV) astrophysical neutrinos is fundamental to our understanding of the high-energy Universe. Apart from the technical challenges of operating detectors deep below ice, oceans, and lakes, the phenomenological challenges are even greater than those of gravitational waves; the sources are unknown, hard to predict, and...
The origin of Petaelectronvolt (PeV) astrophysical neutrinos is fundamental to our understanding of the high-energy Universe. Apart from the technical challenges of operating detectors deep below ice, oceans, and lakes, the phenomenological challenges are even greater than those of gravitational waves; the sources are unknown, hard to predict, and...
We present the first Bayesian method for tomographic decomposition of the plane-of-sky orientation of the magnetic field with the use of stellar polarimetry and distance. This standalone tomographic inversion method presents an important step forward in reconstructing the magnetized interstellar medium (ISM) in 3D within dusty regions. We develop a...
![Fig. 3. [C II] emission line spectra of the four LOSs from the SOFIA...](publication/361793134/figure/fig2/AS:11431281095921898@1668052569373/C-II-emission-line-spectra-of-the-four-LOSs-from-the-SOFIA-FIFI-LS-spectrometer-The_Q320.jpg)
![Fig. 4. [C II] emission line spectra, I λ , versus λ, obtained with ISO...](publication/361793134/figure/fig3/AS:11431281095905989@1668052569407/C-II-emission-line-spectra-I-l-versus-l-obtained-with-ISO-Ingalls-et-al-2002_Q320.jpg)
![Fig. 6. [C II] integrated intensity versus H I column density. Colored...](publication/361793134/figure/fig4/AS:11431281095921899@1668052569494/C-II-integrated-intensity-versus-H-I-column-density-Colored-lines-show-the-expected-C_Q320.jpg)
Context. Atomic gas in the diffuse interstellar medium (ISM) is organized in filamentary structures. These structures usually host cold and dense molecular clumps. The Galactic magnetic field is considered to play an important role in the formation of these clumps.
Aims. Our goal is to explore the role of the magnetic field in the H I -H 2 transiti...
In a black hole X-ray binary (XRB) system, gas accreted from a normal star onto a black hole glows brightly in X-rays. We report on an observation of the XRB Cygnus X-1 (Cyg X-1) by the Imaging X-ray Polarimetry Explorer IXPE) yielding the first highly significant detection of X-ray polarization from an accreting black hole. The electric vector pol...
3C 84 ( NGC 1275 ) is one of the brightest radio sources in the millimetre radio bands, which led to a plethora of very-long-baseline interferometry (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 millim...
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 th...
Symmetric achromatic variability (SAV) is a rare form of radio variability in blazars that has been attributed to gravitational milli-lensing by a ∼10 ² –10 ⁵ M ⊙ 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 sam...
We present the results of variability power spectral density (PSD) analysis using multiwavelength radio to GeV γ -ray light curves covering timescales of decades/years to days/minutes for the blazars 3C 279 and PKS 1510−089. The PSDs are modeled as single power laws, and the best-fit spectral shape is derived using the “power spectral response” met...
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...
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 t...
Variability of a galaxy’s core radio source can be a significant consequence of AGN accretion. However, this variability has not been well studied, particularly at high radio frequencies. As such, we report on a campaign monitoring the high radio frequency variability of 20 nearby, cool-core brightest cluster galaxies. From our representative sampl...
We present the results of variability power spectral density (PSD) analysis using multiwavelength radio to GeV\,$\gamma$-ray light curves covering decades/years to days/minutes timescales for the blazars 3C\,279 and PKS\,1510$-$089. The PSDs are modeled as single power-laws, and the best-fit spectral shape is derived using the `power spectral respo...
We report multiwavelength observations of the gravitationally lensed blazar QSO B0218+357 in 2016-2020. Optical, X-ray and GeV flares were detected. The contemporaneous MAGIC observations do not show significant very-high-energy (VHE, ≳ 100 GeV) gamma-ray emission. The lack of enhancement in radio emission measured by OVRO indicates the multi-zone...
We report multiwavelength observations of the gravitationally lensed blazar QSO B0218+357 in 2016-2020. Optical, X-ray and GeV flares were detected. The contemporaneous MAGIC observations do not show significant very-high-energy (VHE, >= 100 GeV) gamma-ray emission. The lack of enhancement in radio emission measured by OVRO indicates the multi-zone...
Compact Symmetric Objects (CSOs), young jetted-AGN of overall projected size <1 kpc, are of great interest due to their youth and evolution. The classification was introduced to distinguish between ~95% of powerful compact extragalactic radio sources in flux density limited samples that are dominated by asymmetric emission due to relativistic beami...
Compact symmetric objects (CSOs), young jetted-active galactic nuclei of overall projected size <1 kpc, are of great interest due to their youth and evolution. The classification was introduced to distinguish between ∼95% of powerful compact extragalactic radio sources in flux density limited samples that are dominated by asymmetric emission due to...
Most 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 flux density variations in the blazar PKS 2131-021, which strongly suggest a SMBHB with an orbital separation of $\sim 0.00...
![Fig. 3. [C ii] emission line spectra of the four LOSs from the SOFIA...](publication/355494521/figure/fig2/AS:1082675922251778@1635141463427/C-ii-emission-line-spectra-of-the-four-LOSs-from-the-SOFIA-FIFI-LS-spectrometer-The_Q320.jpg)
![Fig. 4. [C ii] emission line spectra, I λ vs λ, obtained with ISO...](publication/355494521/figure/fig3/AS:1082675922243584@1635141463445/C-ii-emission-line-spectra-I-l-vs-l-obtained-with-ISO-Ingalls-et-al-2002-The_Q320.jpg)
Atomic gas in the diffuse interstellar medium (ISM) is organized in filamentary structures. These structures usually host cold and dense molecular clumps. The Galactic magnetic field is considered to play an important role in the formation of these clumps. Our goal is to explore the role of the magnetic field in the HI - H$_{2}$ transition process....
Variability of a galaxy's core radio source can be a significant consequence of AGN accretion. However, this variability has not been well studied, particularly at high radio frequencies. As such, we report on a campaign monitoring the high radio frequency variability of 20 nearby, cool-core brightest cluster galaxies. Our highest cadence observati...
At optical wavelengths, blazar Electric Vector Position Angle (EVPA) rotations linked with gamma-ray activity have been the subject of intense interest and systematic investigation for over a decade. One difficulty in the interpretation of EVPA rotations is the inherent 180○ ambiguity in the measurements. It is therefore essential, when studying EV...
Context. Coherence in the characteristics of neighboring sources in the 2D and 3D space may suggest the existence of large-scale cosmic structures that are useful for cosmological studies. Numerous works have been conducted to detect such features in global scales as well as in confined areas of the sky. However, their results are often contradicto...
Context.Coherence in the characteristics of neighboring sources in 2D and 3D space may suggest the existence of large-scale cosmic structures, which are useful for cosmological studies. Numerous works have been conducted to detect such features in global scalesas well as in confined areas of the sky. However, results are often contradictory and the...
At optical wavelengths, blazar Electric Vector Position Angle (EVPA) rotations linked with gamma-ray activity have been the subject of intense interest and systematic investigation for over a decade. One difficulty in the interpretation of EVPA rotations is the inherent 180° ambiguity in the measurements. It is therefore essential, when studying EV...
At optical wavelengths, blazar Electric Vector Position Angle (EVPA) rotations linked with gamma-ray activity have been the subject of intense interest and systematic investigation for over a decade. One difficulty in the interpretation of EVPA rotations is the inherent 180{\deg} ambiguity in the measurements. It is therefore essential, when studyi...
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...
Context. QSO B1420+326 is a blazar classified as a Flat Spectrum Radio Quasar (FSRQ). In the beginning of 2020 it underwent an enhanced flux state. An extensive multiwavelength campaign allowed us to trace the evolution of the flare. Aims. We search for VHE gamma-ray emission from QSO B1420+326 during this flaring state. We aim to characterize and...
Context. QSO B1420+326 is a blazar classified as a Flat Spectrum Radio Quasar (FSRQ). In the beginning of 2020 it underwent an enhanced flux state. An extensive multiwavelength campaign allowed us to trace the evolution of the flare. Aims. We search for VHE gamma-ray emission from QSO B1420+326 during this flaring state. We aim to characterize and...
We present uniformly reprocessed and re-calibrated data from the RoboPol programme of optopolarimetric monitoring of active galactic nuclei (AGN), covering observations between 2013, when the instrument was commissioned, and 2017. In total, the dataset presented in this paper includes 5068 observations of 222 AGN with Dec > −25○. We describe the cu...
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...
We present uniformly reprocessed and re-calibrated data from the RoboPol programme of optopolarimetric monitoring of active galactic nuclei (AGN), covering observations between 2013, when the instrument was commissioned, and 2017. In total, the dataset presented in this paper includes 5068 observations of 222 AGN with Dec > -25 deg. We describe the...
The stellar binary system LS I +61303, composed of a compact object in an eccentric orbit around a B0 Ve star, emits from radio up to gamma-ray energies. The orbital modulation of radio spectral index, X-ray, and GeV gamma-ray data suggests the presence of two peaks. This two-peaked profile is in line with the accretion theory predicting two accret...
The quasar 3C 454.3 is a blazar known for its rapid and violent outbursts seen across the electromagnetic spectrum. Using γ -ray, X-ray, multiband optical, and very-long-baseline interferometric data we investigate the nature of two such events that occurred in 2013 and 2014 accompanied by strong variations in optical polarization, including a ∼230...
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, blazars, have been considered to be one of the main candidates due to their ab...
The stellar binary system LS I +61°303, composed of a compact object in an eccentric orbit around a B0 Ve star, emits from radio up to γ-ray energies. The orbital modulation of radio spectral index, X-ray, and GeV γ-ray data suggests the presence of two peaks. This two-peaked profile is in line with the accretion theory predicting two accretion–eje...
Context. 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 can...
The quasar 3C 454.3 is a blazar known for its rapid and violent outbursts seen across the electromagnetic spectrum. Using gamma-ray, X-ray, multi-band optical, and very long baseline interferometric data we investigate the nature of two such events that occurred in 2013 and 2014 accompanied by strong variations in optical polarization, including a...
Extreme high-frequency BL Lacs (EHBL) feature their synchrotron peak of the broad-band spectral energy distribution (SED) at ν_s ≥ 10¹⁷ Hz. The BL Lac object 1ES 2344+514 was included in the EHBL family because of its impressive shift of the synchrotron peak in 1996. During the following years, the source appeared to be in a low state without showi...
Context. It has become evident that one-zone synchrotron self-Compton models are not always adequate for very high-energy (VHE) gamma-ray-emitting blazars. While two-component models perform better, they are difficult to constrain due to the large number of free parameters. Aims. In this work, we make a first attempt at taking into account the obse...
Context. It has become evident that one-zone synchrotron self-Compton models are not always adequate for very-high-energy (VHE) gamma-ray emitting blazars. While two-component models are performing better, they are difficult to constrain due to the large number of free parameters. Aims. In this work, we make a first attempt to take into account the...
Motivated by the identification of the blazar TXS 0506+056 as the first promising high-energy neutrino counterpart candidate, we search for additional neutrino blazar candidates among the Fermi–Large Area Telescope detected blazars. We investigate the multiwavelength behavior from radio to GeV gamma-rays of blazars found to be in spatial coincidenc...
Blazars are known for their variability on a wide range of timescales at all wavelengths. Most studies of TeV gamma-ray blazars focus on short timescales, especially during flares. With a decade of observations from the Fermi-LAT and VERITAS, we present an extensive study of the long-term multiwavelength radio-to-gamma-ray flux-density variability,...
We report the results of decade-long (2008-2018) γ-ray to 1 GHz radio monitoring of the blazar 3C 279, including GASP/WEBT, Fermi and Swift data, as well as polarimetric and spectroscopic data. The X-ray and γ-ray light curves correlate well, with no delay 3 hours, implying general co-spatiality of the emission regions. The γ-ray-optical flux-flux...
Blazars are known for their variability on a wide range of timescales at all wavelengths. Most studies of TeV gamma-ray blazars focus on short timescales, especially during flares. With a decade of observations from the Fermi-LAT and VERITAS, we present an extensive study of the long-term multi-wavelength radio-to-gamma-ray flux-density variability...
Aims. The γ-ray production mechanism and its localization in blazars are still a matter of debate. The main goal of this paper is to constrain the location of the high-energy emission in the blazar TXS 2013+370 and to study the physical and geometrical properties of the inner jet region on sub-pc scales. Methods. TXS 2013+370 was monitored during 2...
Motivated by the identification of the blazar TXS 0506+056 as the first promising high-energy neutrino counterpart candidate, we search for additional neutrino blazars candidates among the Fermi-LAT detected blazars. We investigate the multi-wavelength behavior from radio to GeV gamma rays of blazars found to be in spatial coincidence with single h...
We report the results of decade-long (2008-2018) γ-ray to 1 GHz radio monitoring of the blazar 3C 279, including GASP/WEBT, Fermi and Swift data, as well as polarimetric and spectroscopic data. The X-ray and γ-ray light curves correlate well, with no delay 3 hours, implying general co-spatiality of the emission regions. The γ-ray-optical flux-flux...
We present a multiwavelength study of the flat-spectrum radio quasar CTA 102 during 2013–2017. We use radio-to-optical data obtained by the Whole Earth Blazar Telescope, 15 GHz data from the Owens Valley Radio Observatory, 91 and 103 GHz data from the Atacama Large Millimeter Array, near-infrared data from the Rapid Eye Monitor telescope, as well a...
Aims. The γ -ray production mechanism and its localization in blazars are still a matter of debate. The main goal of this paper is to constrain the location of the high-energy emission in the blazar TXS 2013+370 and to study the physical and geometrical properties of the inner jet region on sub-pc scales.
Methods. TXS 2013+370 was monitored during...
The $\gamma$-ray production mechanism and its localization in blazars are still a matter of debate. The main goal of this paper is to constrain the location of the high-energy emission in the blazar TXS 2013+370 and to study the physical and geometrical properties of the inner jet region on sub-pc scales. VLBI observations at 86 GHz and space-VLBI...
We have conducted the first systematic search for interday variability in a large sample of extragalactic radio sources at 15 GHz. From the sample of 1158 radio-selected blazars monitored over an ∼10 yr span by the Owens Valley Radio Observatory 40-m telescope, we identified 20 sources exhibiting significant flux density variations on 4-d time-scal...
We present a multiwavelength study of the flat-spectrum radio quasar CTA 102 during 2013-2017. We use radio-to-optical data obtained by the Whole Earth Blazar Telescope, 15 GHz data from the Owens Valley Radio Observatory, 91 and 103 GHz data from the Atacama Large Millimeter Array, near-infrared data from the Rapid Eye Monitor telescope, as well a...
We have conducted the first systematic search for interday variability in a large sample of extragalactic radio sources at 15 GHz. From the sample of 1158 radio-selected blazars monitored over a $\sim$10 year span by the Owens Valley Radio Observatory 40-m telescope, we identified 20 sources exhibiting significant flux density variations on 4-day t...
We investigate the relationship between the 5 GHz interstellar scintillation (ISS) and the 15 GHz intrinsic variability of the compact, radio-selected active galactic nuclei (AGN) common to the Microarcsecond Scintillation-Induced Variability (MASIV) Survey and the Owens Valley Radio Observatory blazar flux density monitoring program. As part of th...
We present the design and performance of RoboPol, a four-channel optical polarimeter operating at the Skinakas Observatory in Crete, Greece. RoboPol is capable of measuring both relative linear Stokes parameters q and u (and the total intensity I) in one sky exposure. Though primarily used to measure the polarization of point sources in the R band,...
Citations
... The IceCube Neutrino Observatory, for instance, has utilized a cubic kilometer of Antarctic ice since 2011 to observe neutrinos in the TeV-PeV range [25]. Following the discovery of HE neutrinos in 2013 [26,27], further observations by IceCube [28][29][30][31][32][33][34], ANTARES [35], and Baikal-GVD [36] initiated an ongoing phase of characterizing the diffuse neutrino flux and searching for candidate neutrino sources [37][38][39][40][41]. However, despite the recent identification of the first neutrino sources in the northern hemisphere, the origins of most of the diffuse flux are still unknown [42,43], and the absence of strong anisotropies in the flux indicates an extragalactic origin, with a minor galactic contribution at the level of 10% [44][45][46]. ...
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... In contrast, current optical polarization catalogs include measurements for about 10,000 stars. 2 The primary scientific goal of the PASIPHAE survey is to combine this highly accurate stellar polarimetric data with the GAIA survey's stellar distance measurements to create a 3D tomography map of the dust and magnetic field in the Milky Way Galaxy's polar regions. 3,4 For a detailed discussion of the scientific motivations and objectives, refer to the PASIPHAE program's white paper. 1 The unique scientific objectives of the PASIPHAE survey drive the technical design goals for the WALOP instruments, applicable to both WALOP-North and WALOP-South, as detailed in Table 1. The rationale and justification for the target values of each design parameter are explained in the optical design paper of the WALOP-South instrument by Maharana et al. 2021, 5 hereafter referred to as Paper I. ...
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... With this picture in mind, we model the broadband spectral energy distributions (SEDs) of three objects: J1407+2827 (also known as OQ+208 or Mrk 668), J1511+0518, and J2022 +6137 (2021+614), to quantitatively explore the validity of the above expectation. These objects were selected from a sample of compact symmetric objects (CSOs; a subset of AGNs known for their compact, young radio structures; see, e.g., O'Dea 1998;O'Dea & Saikia 2021;Kiehlmann et al. 2024aKiehlmann et al. , 2024bReadhead et al. 2024) considered in Sobolewska et al. 2019a based on their distinctive X-ray spectral features derived based on extensive observations by Chandra, XMM-Newton, and NuSTAR (see Sobolewska et al. 2019b, 2023, hereafter S19 andS23 respectively). Key features are a high intrinsic X-ray absorption (equivalent hydrogen column densities, N H > 10 23 cm −2 ), as well as a narrow fluorescent Fe Kα line and the broad Compton reflection component indicating a reflection from a toroidal obscurer. ...
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... Very recent work on the kinematics of a carefully selected sample of mostly CSO-2 suggests that they should be considered a distinct class of 'short-lived' jets rather than simply young versions of more classical powerful radio-loud AGN (Kiehlmann et al. 2023). This fits well with the idea that CSO-type jet activity is powered by a "single fueling event", e.g. the capture of a single star by the central black hole (Readhead et al. 1994;An & Baan 2012;Kiehlmann et al. 2024). ...
... Recent work finds that stars embedded in AGN accretion disks should rapidly accrete to masses of O(10 2 )M ⊙ and that they may not collapse while they remain embedded in the disk (e.g., Cantiello et al. 2021;Jermyn et al. 2022). A range of rich dynamical encounters between these stars and other objects, including intermediate mass black holes (Leigh et al. 2018;McKernan et al. 2020), could potentially fill the AGN loss cone and generate high mass TDEs which might account for some turn-on jetted AGN (Readhead et al. 2024). ...
... PKS 1830−211 is a γ-ray-emitting, high-redshift (z = 2.507 ± 0.002), lensed flatspectrum radio quasar (see [95]). Recently, Ref. [96] investigated the γ-ray flare detected by AGILE/GRID and Fermi/LAT of PKS 1830−211. Figure 10 shows the multiwavelength light curves from radio (5 GHz) to γ-ray (E > 100 MeV). ...
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... In addition, unlike the high energy emission, which varies rapidly, the radio emission is believed to emanate from an extended region, and it varyies over months to years (e.g., Ciaramella et al. 2004;Richards et al. 2014;Fuhrmann et al. 2016). Nevertheless, as seen in PKS 0201+113 and a few other blazars, quasi-simultaneous radio and γ-ray flares (Max-Moerbeck et al. 2014; Karamanavis et al. 2016;Liao et al. 2016;Wang et al. 2023;Cheong et al. 2024) suggest a possible close connection between the radio and the γ-ray emission. Several other high-redshift objects are noteworthy. ...
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... In addition to intrinsic characteristics, polarization properties can be modeled using geometrical frameworks such as twisted or helical jet models (see e.g. Raiteri et al. 2023, and references therein). The effects and strength of the accretion disc component should also be considered, as FSRQs generally exhibit more powerful accretion discs than BLLs. ...
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... One of the simple explanations is that the youngest quasars are more compact, and, as a consequence, the variability time scale is shorter. This can be confirmed by the recently reported relationship between variability timescales and core sizes in [126]. On the other hand, the most bright sources can be detected at such high redshifts, and these bright sources may be more variable on average. ...
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... In the K band, Wilking et al. (1980) found p(K ) = 0.0133 ± 0.0005, and Bailey & Hough (1982) found p(K ) = 0.0113 ± 0.0003. Blinov et al. (2023) report substantial variations of U/I and Q/I over 2400 days of observations. ...
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