arXiv subjects

S. Kiehlmann

Publications and source records attributed to S. Kiehlmann.

At least 37 records · Page 2Linked to original sources

arXiv2023

Bright-Moon Sky as a Wide-Field Linear Polarimetric Flat Source for Calibration

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 exist. This paper tests the feasibility of using the polarized sky patches of the size of around ten-by-ten arcminutes, at a distance of up to 20 degrees from the Moon, on bright-Moon nights as a wide-field linear polarimetric flat source. We observed 19 patches of the sky adjacent to the bright-Moon with the RoboPol instrument in the SDSS-r broadband filter. These were observed on five nights within two days of the full-Moon across two RoboPol observing seasons. We find that for 18 of the 19 patches, the uniformity in the measured normalized Stokes parameters $q$ and $u$ is within 0.2 %, with 12 patches exhibiting uniformity within 0.07 % or better for both $q$ and $u$ simultaneously, making them reliable and stable wide-field linear polarization flats. We demonstrate that the sky on bright-Moon nights is an excellent wide-field linear polarization flat source. Various combinations of the normalized Stokes parameters $q$ and $u$ can be obtained by choosing suitable locations of the sky patch with respect to the Moon

astro-ph.IM↗

arXiv2023

Starlight-polarization-based tomography of the magnetized interstellar medium: PASIPHAE's line-of-sight inversion method

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 model in which the polarization signal from the magnetized and dusty ISM is described by thin layers at various distances. Our modeling makes it possible to infer the mean polarization (amplitude and orientation) induced by individual dusty clouds and to account for the turbulence-induced scatter in a generic way. We present a likelihood function that explicitly accounts for uncertainties in polarization and parallax. We develop a framework for reconstructing the magnetized ISM through the maximization of the log-likelihood using a nested sampling method. We test our Bayesian inversion method on mock data taking into account realistic uncertainties from Gaia and as expected for the optical polarization survey PASIPHAE according to the currently planned observing strategy. We demonstrate that our method is effective at recovering the cloud properties as soon as the polarization induced by a cloud to its background stars is higher than $\sim 0.1\%$ for the adopted survey exposure time and level of systematic uncertainty. Our method makes it possible to recover not only the mean polarization properties but also to characterize the intrinsic scatter, thus creating new ways to characterize ISM turbulence and the magnetic field strength. Finally, we apply our method to an existing data set of starlight polarization with known line-of-sight decomposition, demonstrating agreement with previous results and an improved quantification of uncertainties in cloud properties.

astro-ph.GA↗

arXiv2022

GRB 210619B optical afterglow polarization

We report on the follow-up of the extremely bright long 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 report a $5\,σ$ detection of polarization $P=1.5\pm0.3$ at polarization angle $EVPA=8\pm6^\circ$. We find that during our observations the polarization is likely constant. These values are corrected for polarization induced by the interstellar medium of the Milky Way and host-induced polarization is likely negligible. Thus the polarization we quote is intrinsic to the GRB afterglow.

astro-ph.HE↗

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.

astro-ph.HE↗

arXiv2022

The hunt for extraterrestrial high-energy neutrino counterparts

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 lack clear signatures. Neutrino astronomy therefore represents the greatest challenge faced by the astronomy and physics communities thus far. The possible neutrino sources range from accretion disks and tidal disruption events, to relativistic jets and galaxy clusters with blazar TXS~0506+056 the most compelling association thus far. Since that association, immense effort has been put into proving or disproving that jets are indeed neutrino emitters, but to no avail. By generating simulated neutrino counterpart samples, we explore the potential of detecting a significant correlation of neutrinos with jets from active galactic nuclei. We find that, given the existing challenges, even our best experiments could not have produced a $>3σ$ result. Larger programs over the next few years will be able to detect a significant correlation only if the brightest radio sources, rather than all jetted active galactic nuclei, are neutrino emitters. We discuss the necessary strategies required to steer future efforts into successful experiments.

astro-ph.HE↗

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.

astro-ph.HE↗

arXiv2022

HI-H$_2$ transition: exploring the role of the magnetic field

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. We targeted a filamentary cloud where gas transitions from atomic to molecular. This cloud is located at the edges of an expanding structure, known as the North Celestial Pole Loop (NCPL). We probed the magnetic field properties of the cloud with optical polarization observations. We performed multi-wavelength spectroscopic observations of different species in order to probe the gas phase properties of the cloud. We identified two distinct sub-regions within the cloud. One of the regions hosts purely atomic gas, while the other is dominated by molecular gas although most of it is CO-dark. The estimated plane-of-the-sky magnetic field strength between the two regions remains constant within uncertainties and lies in the range 20 ~ 30$~μ$G. The total magnetic field strength does not scale with density which implies that gas is compressed along the field lines. We also found that turbulence is sub-Alfvénic. The HI velocity gradients are in general perpendicular to the mean magnetic field orientation, except for the region close to the CO clump where they tend to become parallel. The latter is likely related to gas undergoing gravitational infall. The magnetic field morphology of the target cloud is parallel to the HI column density structure of the cloud in the atomic region, while it tends to become perpendicular to the HI structure in the molecular region. If this is verified in more cases it has important consequences for the ISM magnetic field modeling with HI data.

astro-ph.GA↗

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.

astro-ph.HE↗

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.

astro-ph.GA↗

arXiv2021

The Time-Dependent Distribution of Optical Polarization Angle Changes in Blazars

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 EVPA rotations, to ensure that the typical time-interval between successive observations -- i.e. the cadence -- is short enough to ensure that the correct modulo 180° value is selected. This optimal cadence depends on the maximum intrinsic EVPA rotation speed in blazars, which is currently not known. In this paper we address the following questions for the RoboPol sample: What range of rotation speeds for rotations greater than 90° can we expect? What observation cadence is required to detect such rotations? Have rapid rotations been missed in EVPA rotation studies thus far? What fraction of data is affected by the ambiguity? And how likely are detected rotations affected by the ambiguity? We answer these questions with three seasons of optical polarimetric observations of a statistical sample of blazars sampled weekly with the RoboPol instrument and an additional season with daily observations. We model the distribution of EVPA changes on time scales from 1-30 days and estimate the fraction of changes exceeding 90°. We show that at least daily observations are necessary to measure >96% of optical EVPA variability in the RoboPol sample of blazars correctly and that intra-day observations are needed to measure the fastest rotations that have been seen thus far.

astro-ph.HE↗

arXiv2021

Local alignments of parsec-scale AGN radiojets

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 their interpretation remains controversial. Aims.We investigate the potential alignment of parsec-scale radio jets in localized regions of the coordinates-redshift space. Methods.We use data from the Astrogeo VLBI FITS image database to deduce jet directions of radio sources. We perform the search for statistical alignments between nearby sources and explore the impact of instrumental biases. Results.We unveil four regions for which the alignment between jet directions deviates from randomness at a significance level of more than 5 sigma and is unlikely due to instrumental systematics. Intriguingly, their locations coincide with other known large-scale cosmic structures and/or regions of alignments. Conclusions.If the alignments found are the result of physical processes, the discovered regions may designate some of the largest structures known to date.

astro-ph.CO↗

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.

astro-ph.HE↗

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.

astro-ph.GA↗

arXiv2020

RoboPol: AGN polarimetric monitoring data

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 current version of the RoboPol pipeline that was used to process and calibrate the entire dataset, and we make the data publicly available for use by the astronomical community. Average quantities summarising optopolarimetric behaviour (average degree of polarization, polarization variability index) are also provided for each source we have observed and for the time interval we have followed it.

astro-ph.HE↗

arXiv2020

Evidence for periodic accretion-ejection in LSI+61303

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 accretion-ejection events for LS I +61303 along the 26.5 d orbit. However, the existing multiwavelength data are not simultaneous. In this paper, we report the results of a campaign covering radio, X-ray, and gamma-ray observations of the system along one single orbit. Our results confirm the two predicted events along the orbit and in addition show that the positions of radio and gamma-ray peaks are coincident with X-ray dips as expected for radio and gamma-ray emitting ejections depleting the X-ray emitting accretion flow. We discuss future observing strategies for a systematic study of the accretion-ejection physical processes in LS I +61303.

astro-ph.HE↗

arXiv2020

Two Flares with One Shock: the Interesting Case of 3C 454.3

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 230 degree electric vector position angle (EVPA) rotation. Our results suggest that a single disturbance was responsible for both flaring events. We interpret the disturbance as a shock propagating down the jet. Under this interpretation the 2013-flare originated most likely due to changes in the viewing angle caused by perhaps a bent or helical trajectory of the shock upstream of the radio core. The 2014-flare and optical polarization behavior are the result of the shock exiting the 43 GHz radio core, suggesting that shock crossings are one of the possible mechanisms for EVPA rotations.

astro-ph.HE↗

arXiv2020

Patterns in the multi-wavelength behavior of candidate neutrino blazars

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 high-energy neutrinos and lower-energy neutrino flare candidates. In addition, we compare the average gamma-ray emission of the potential neutrino-emitting sources to the entire sample of gamma-ray blazars. We find that neutrino-emitting blazar candidates are statistically compatible with both hypothesis of a linear correlation and of no correlation between neutrino and gamma-ray energy flux.

astro-ph.HE↗

arXiv2020

A decade of multi-wavelength observations of the TeV blazar 1ES 1215+303: Extreme shift of the synchrotron peak frequency and long-term optical-gamma-ray flux increase

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, with the addition of a couple of short-time radio-structure and optical polarization observations of the blazar 1ES 1215+303 (z=0.130), with a focus on its gamma-ray emission from 100 MeV to 30 TeV. Multiple strong GeV gamma-ray flares, a long-term increase in the gamma-ray and optical flux baseline and a linear correlation between these two bands are observed over the ten-year period. Typical HBL behaviors are identified in the radio morphology and broadband spectrum of the source. Three stationary features in the innermost jet are resolved by VLBA at 43.1, 22.2, and 15.3 GHz. We employ a two-component synchrotron self-Compton model to describe different flux states of the source, including the epoch during which an extreme shift in energy of the synchrotron peak frequency from infrared to soft X-rays is observed.

astro-ph.HE↗