arXiv subjects

Andrea Gokus

Publications and source records attributed to Andrea Gokus.

17 recordsLinked to original sources

arXiv2026

Tracking down the broadband polarimetric properties of PG 1553+113

We report on a nine-month monitoring campaign of the blazar PG 1553+113, relying on three observations carried out in 2025 with the Imaging X-ray Polarimetry Explorer (IXPE) and supported by multi-wavelength facilities. The source displayed pronounced variability across the electromagnetic spectrum, with X-ray flux changes by up to a factor of $\sim5$ and complex evolution of the optical polarization properties, including one of the largest (exceeding $150^{\circ}$) and fastest rotations in the electric vector position angle (EVPA) ever recorded. This swing of the EVPA was also accompanied by a temporary drop of the optical polarization degree to nearly zero. Significant X-ray polarization was observed during the third IXPE pointing, with a polarization degree $\Pi_{\rm X}\,=(\,18.4\,\pm\,5.8)\%$ and $\Psi_{\rm X}\,=\,74^{\circ} \pm 9^{\circ}$ in the 2--8~keV band, while only upper limits were obtained in the first two epochs. The optical data show that the second IXPE observation occurred shortly after a dramatic optical polarization event characterized by a rapid EVPA swing and strong depolarization. Two possible scenarios may explain the broadband polarimetric behavior: (i) the superposition of two emitting regions with nearly orthogonal magnetic field configurations and variable relative contributions, and (ii) the interaction of a single emitting region with a shock that temporarily reorders the magnetic field. In both cases, the data support a picture in which the X-ray and optical emissions arise from closely related but not strictly co-spatial regions within a dynamically evolving, magnetically structured jet.

astro-ph.HE↗

arXiv2026

X-ray spectroscopy mass constraints on V1674 Her: the fastest nova does not have a near-Chandrasekhar white dwarf

V1674 Her (Nova Her 2021) is the fastest classical nova ever recorded, with an optical decline time of $t_2 \sim 1$ day, typically interpreted as evidence for a white dwarf mass close to the Chandrasekhar limit. We present a broadband X-ray study of V1674 Her combining contemporaneous XMM-Newton and NuSTAR observations in quiescence to directly constrain the white dwarf mass and magnetic field strength. The hard X-ray emission is modeled using a physically motivated post-shock accretion column model that accounts for the temperature gradient in the flow and reflection from the white dwarf surface. Under the assumption that the accretion disk is truncated at the co-rotation radius, we obtain a white dwarf mass of $M = 1.09^{+0.07}_{-0.06}\,M_\odot$. An independent constraint derived from timing analysis of the X-ray power spectrum yields a consistent value of $M = 1.12 \pm 0.06\,M_\odot$. These values are significantly lower than those inferred from empirical decline-time relations, suggesting that such relations may overestimate white dwarf masses in extreme fast novae. From the inferred accretion rate and magnetospheric radius, we estimate a surface magnetic field strength of $B = 21.3^{+6.6}_{-5.7}\,(\mathrm{stat})^{+12.9}_{-8.1}\,(\mathrm{sys})\,\mathrm{MG}$, placing V1674 Her at the high end of the magnetic field distribution for intermediate polars. Our results demonstrate that even the fastest novae do not necessarily host near-Chandrasekhar white dwarfs, highlighting the importance of direct X-ray constraints and suggesting that additional parameters beyond white dwarf mass play a key role in setting nova timescales.

astro-ph.HE↗

arXiv2026

The IXPE and multifrequency polarimetric view of the extreme blazars 1ES 1101-232 and RGB J0710+591

Multiwavelength polarimetry is a powerful tool to probe magnetic field and flow geometries in the relativistic jets of blazars. In this respect, particularly interesting are the sources whose synchrotron emission covers a broad range of frequencies, from radio to X-rays, such as the BL Lac objects of the HSP type. Previous measurements including radio, optical and X-ray data show a clear trend, with the degree of polarization increasing with frequency. Here we report radio, optical and X-ray observations ($Swift$, $Nustar$ and $IXPE$) of 1ES 1101-232 and RGB J0710+591, two blazars belonging to the puzzling subclass of extreme BL Lacs (EHBL). For 1ES 1101-232 we found a strong frequency-dependency of the degree of polarization (with a ratio $Π_X/Π_O\simeq 5.2$). For RGB J0710+591, IXPE derived a 1$σ$ upper limit $Π_X<11.6\%$, comparable with the measured optical degree of polarization (average $Π_O\sim 12\%$). We discuss the results in the framework of current interpretations and, in particular, we report an improved version of the stratified shock model that is able to reproduce the observed data of both sources.

astro-ph.HE↗

arXiv2025

Picture an Astronomer: Best Practices for Retaining Talent in Astrophysics

Women are consistently underrepresented in astrophysics yet are simultaneously subject to disproportionate attrition at every career stage. This disparity between demonstrated efficacy in job performance and ultimate career outcome was the primary motivation for the Picture an Astronomer series, which included both targeted public outreach to increase representation of women in astrophysics and high-level, solution-oriented discussions among professional astronomers. In March 2025, more than 200 astronomers came together in a hybrid-format symposium focused on the state of the field for female scientists, combining scientific exchange with discussions of policies and practices to strengthen retention of talent in the field. This white paper is the result of those discussions, offering a wide range of recommendations developed in the context of gendered attrition in astrophysics but which ultimately support a healthier climate for all scientists alike.

astro-ph.IM↗

arXiv2025

Polarization Dynamics of X-Ray Synchrotron Emission from a Multi-Zone Blazar Jet

The polarization of X-ray synchrotron emission in blazars directly probes the magnetic field geometry and particle acceleration processes in relativistic jets. We use particle-in-cell simulations of magnetic reconnection and magnetized turbulence, coupled to polarization-sensitive radiative transfer code, to interpret IXPE observations of Mrk 421 during a high flux state recorded in December of 2023. To evaluate the fitness of the two theoretical scenarios, we rely on a quantitative comparison of the statistical properties of simulated and observed X-ray flux and polarization light curves using five evaluation metrics, rather than attempting to fit individual data points. We propose a turbulence-driven multi-zone model where jet emission is represented as the sum of the radiative output of N independent cells, each described by a particle-in-cell simulation. Comparison of ensembles of simulated Stokes-parameter light curves with IXPE data shows that magnetic reconnection dominated models provide the best match to the observed X-ray flux and polarization dynamics. The optimal configuration corresponds to N = 15 emitting cells, which reproduces the observed amplitudes and timescales of the X-ray flux and polarization variations. Magnetized turbulence models underpredict both the flux and polarization variability. Our results indicate that a multi-zone, reconnection-powered emission scenario can describe the X-ray polarization behavior of Mrk 421 and establish a quantitative framework for testing theoretical models against IXPE observations of other high-synchrotron-peaked blazars.

astro-ph.HE↗

arXiv2025

Recurrence plot analysis of blazar gamma-ray light curves: Exploiting the time-domain capabilities of Fermi-LAT

Variability studies of jetted AGN, in particular blazars, have been used to gain a better understanding of the particle acceleration mechanisms in jets. However, statistical methods used for the characterization of variability often rely on stationary time series data, which is not fulfilled for most blazar light curves. We introduce the recurrence plot method for long-term $γ$-ray light curves sampled by Fermi-LAT and present our results for the BL Lac object Mkn 421 and the FSRQ PKS 1424-41. Using surrogates to determine the significance of our findings, we conclude that Mkn 421 exhibits more determinism than PKS 1424-41, and that both sources potentially show nonlinearity. However, the latter has to be tested against more advanced surrogates that are able to replicate the nonstationarity of the original light curves. In future work, we will extend our recurrence analysis to a sample of $\sim50$ $γ$-ray bright sources to probe the jet dynamics of different blazar classes.

astro-ph.HE↗

arXiv2025

High-energy variability of the gravitationally lensed blazar PKS 1830-211

The production site and process responsible for the highly variable high-energy emission observed from blazar jets are still debated. Gravitational lenses can be used as microscopes to investigate the nature of such sources. We study the broad-band spectral properties and the high-energy variability of the gravitationally-lensed blazar PKS 1830-211, for which radio observations have revealed two images, to put constraints on the jet physics and the existence of a gravitationally-induced time delay and magnification ratio between the images. We utilize Swift/XRT, Nustar, and Fermi-LAT observations from 2016 and 2019 to compare periods of low activity and high activity in PKS 1830-211. Short-timescale variability is elucidated with an unbinned power spectrum analysis of time-tagged NuSTAR photon data. To study the gravitationally-induced time delay in the gamma-ray light curve observed with Fermi-LAT, we improve existing autocorrelation function based methods. Our modified auto-correlation method yields a delay of t_0=21.1 +/- 0.1 d and magnification factor a=0.13 +/- 0.01. These parameters remain time-invariant. In data from 2016 and 2019, the X-ray spectra remain remarkably stable, contrasting with extreme changes in gamma-rays. Both states can be fitted with a single component from Comptonisation of infrared emission from the dusty torus, with different gamma-ray states arising solely from a shift in the break of the electron energy distribution. The detection of a consistent lag throughout the whole light curve suggests that they originate from a persistent location in the jet.

astro-ph.HE↗

arXiv2025

The most distant $γ$-ray flare to date: a multiwavelength campaign on the $z = 4.715$ blazar GB6 B1428+4217

In November 2023, the Fermi Large Area Telescope detected a $γ$-ray flare from the high-redshift blazar GB6 B1428+4217 ($z=4.715$). We initiated a multi-wavelength follow-up campaign involving Swift, NuSTAR, the Sierra Nevada and Perkins Observatories, and the Effelsberg 100-m radio telescope. This source, also known as 5BZQ J1430+4204, has shown an anomalous soft X-ray spectrum in previous observations, including possible ionized absorption features or signatures of bulk Comptonization of thermal electrons, which are also detected during the flaring episode. Simultaneous optical data revealed a polarization fraction of ${\sim}8$\% in the R band, confirming that synchrotron emission dominated over thermal emission from the accretion disk. The hard X-ray flux was enhanced during the flare. Modeling of the broadband spectral energy distribution suggests that the high-energy component is dominated by Compton scattering by external seed photons from the accretion disk. The origin of the flare is consistent with the injection of a hard-spectrum electron population in the emission region. With a $γ$-ray luminosity among the top 5% of flaring events, GB6 B1428+4217 exemplifies a prototypical MeV blazar. Its Compton-dominated SED and extreme luminosity are in line with expectations from the blazar sequence. High-redshift flares like this are critical for understanding jet physics in the early Universe and may improve detection prospects with future missions such as COSI.

astro-ph.HE↗

arXiv2025

Unveiling blazar synchrotron emission: a multiwavelength polarimetric study of HSP and LSP populations

Polarimetric properties of blazars allow us to put constraints on the acceleration mechanisms that fuel their powerful jets. By studying the multiwavelength polarimetric behaviour of high synchrotron peaked (HSP) and low synchrotron peaked (LSP) blazars, we aim to explore differences in their emission mechanisms and magnetic field structure in the acceleration region. In this study, we take advantage of several X-ray polarisation observations of HSP by the IXPE, including four new observations of Mrk 501, and optical polarisation observations of LSP from RoboPol and many others. We find that the polarisation degree (PD) distribution of HSP in X-rays is systematically higher than in optical and mm-radio wavelengths, as reported in previous IXPE publications. The distribution of the X-ray electric vector position angles (PA) is centered around the jet axis with most of the observations consistent with zero difference within uncertainties. In fact, the distribution of the offset of the PA from the jet axis is consistent between the LSP and HSP populations (with PA measured in optical for the first, X-ray for the latter), suggesting a common magnetic field structure close to the acceleration region, in strong support of the emerging energy stratified picture of particle acceleration followed by energy loss in blazar jets.

astro-ph.HE↗

arXiv2025

New TeV-emitting BL Lac candidates from the eROSITA X-ray survey

TeV-emitting BL Lac type blazars represent the extreme end of the blazar population. They are characterized by relatively weak jets and radiatively inefficient accretion disks. Particles accelerated in these jets experience fewer radiative losses, allowing them to reach energies beyond the TeV scale and produce TeV gamma-ray emission. The study of TeV blazars is constrained by the limited number of known sources in this category. Currently, only 56 high synchrotron-peaked BL Lacs have been detected at energies above 0.1 TeV. Searches for TeV emission from BL Lacs typically target sources with bright X-ray emission and a synchrotron peak at or above 1 keV. The recently released eRASS catalog by the eROSITA collaboration, which covers half of the sky, represents the deepest X-ray survey in the soft X-ray band to date. Utilizing the eROSITA survey, combined with infrared data from WISE and archival radio observations, we have identified 121 TeV-emitting blazar candidates. Our search introduces selection criteria based on the radio to infrared that remove quasar-like objects that have similar infrared spectra and X-ray fluxes as TeV-emitting BL Lacs. In our search, we find 23 objects that had not been detected in the ROSAT X-ray survey and 11 that have not been previously associated with blazars. The candidates resulting from our search are suitable for follow-up observations with currently operating imaging atmospheric Cherenkov telescopes, as well as future facilities like the CTAO Observatory.

astro-ph.HE↗

arXiv2024

A gamma-ray flare from TXS 1508+572: characterizing the jet of a $z=4.31$ blazar in the early Universe

Blazars can be detected from very large distances due to their high luminosity. However, the detection of $γ$-ray emission of blazars beyond $z=3$ has only been confirmed for a small number of sources. Such observations probe the growth of supermassive black holes close to the peak of star formation in the history of galaxy evolution. As a result from a continuous monitoring of a sample of 80 $z>3$ blazars with Fermi-LAT, we present the first detection of a $γ$-ray flare from the $z=4.31$ blazar TXS 1508+572. This source showed high $γ$-ray activity from February to August 2022, reaching a peak luminosity comparable to the most luminous flares ever detected with Fermi -LAT. We conducted a multiwavelength observing campaign involving XMM-Newton, Swift, the Effelsberg 100-m radio telescope and the Very Long Baseline Array. In addition, we make use of the monitoring programs by the Zwicky Transient Facility and NEOWISE at optical and infrared wavelengths, respectively. We find that the source is particularly variable in the infrared band on daily time scales. The spectral energy distribution collected during our campaign is well described by a one-zone leptonic model, with the $γ$-ray flare originating from an increase of external Compton emission as a result of a fresh injection of accelerated electrons.

astro-ph.HE↗

arXiv2024

Astronomy's climate emissions: Global travel to scientific meetings in 2019

Travel to academic conferences -- where international flights are the norm -- is responsible for a sizeable fraction of the greenhouse gas (GHG) emissions associated with academic work. In order to provide a benchmark for comparison with other fields, as well as for future reduction strategies and assessments, we estimate the CO2-equivalent emissions for conference travel in the field of astronomy for the prepandemic year 2019. The GHG emission of the international astronomical community's 362 conferences and schools in 2019 amounted to 42,500 tCO2e, assuming a radiative-forcing index factor of 1.95 for air travel. This equates to an average of 1.0 $\pm$ 0.6 tCO2e per participant per meeting. The total travel distance adds up to roughly 1.5 Astronomical Units, that is, 1.5 times the distance between the Earth and the Sun. We present scenarios for the reduction of this value, for instance with virtual conferencing or hub models, while still prioritizing the benefits conferences bring to the scientific community.

physics.soc-ph↗

arXiv2024

eROSITA Detection of a Cloud Obscuration Event in the Seyfert AGN EC 04570-5206

Recent years have seen broad observational support for the presence of a clumpy component within the circumnuclear gas around SMBHs. In the X-ray band, individual clouds can manifest themselves when they transit the line of sight to the X-ray corona, temporarily obscuring the X-ray continuum and thereby indicating the characteristics and location of these clouds. X-ray flux monitoring with SRG/eROSITA has revealed that in the Seyfert 1 AGN EC 04570-5206, the soft X-ray flux dipped abruptly for about 10-18 months over 2020-2021, only to recover and then drop a second time by early 2022. Here, we investigate whether these flux dips and recoveries could be associated with cloud occultation events. We complemented the eROSITA scans with multiwavelength follow-up observations, including X-ray/UV observations with Swift, XMM-Newton, and NICER, along with ground-based optical photometric and spectroscopic observations to investigate the spectral and flux variability. XMM-Newton spectra confirm that the soft X-ray flux dips were caused by partial-covering obscuration by two separate clouds. The 2020-2021 event was caused by a cloud with column density near 1e22 /cm2 and a covering fraction near 0.6. The cloud in the 2022 event had a column density near 3e23 /cm2 and a covering fraction near 0.8. The optical/UV continuum flux varied minimally and the optical emission line spectra showed no variability in Balmer profiles or intensity. The transiting gas clouds are neutral or lowly-ionized, while the lower limits on their radial distances are commensurate with the dust sublimation zone (cloud 1) or the optical broad line region (cloud 2). One possible explanation is a dust-free, outflowing wind with embedded X-ray clumps. These events are the first cloud obscuration events detected in a Seyfert galaxy using eROSITA's X-ray monitoring capabilities.

astro-ph.HE↗

arXiv2023

The High Energy X-ray Probe (HEX-P): the most powerful jets through the lens of a superb X-ray eye

A fraction of the active supermassive black holes at the centers of galaxies in our Universe are capable of launching extreme kiloparsec-long relativistic jets. These jets are known multiband (radio to $γ$-ray) and multimessenger (neutrino) emitters, and some of them have been monitored over several decades at all accessible wavelengths. However, many open questions remain unanswered about the processes powering these highly energetic phenomena. These jets intrinsically produce soft-to-hard X-ray emission that extends from $E\sim0.1\,\rm keV$ up to $E>100\,\rm keV$. Simultaneous broadband X-ray coverage, combined with excellent timing and imaging capabilities, is required to uncover the physics of jets. Indeed, truly simultaneous soft-to-hard X-ray coverage, in synergy with current and upcoming high-energy facilities (such as IXPE, COSI, CTAO, etc.) and neutrino detectors (e.g., IceCube), would enable us to disentangle the particle population responsible for the high-energy radiation from these jets. A sensitive hard X-ray survey ($F_{8-24\,\rm keV}<10^{-15}\,\rm erg~cm^{-2}~s^{-1}$) could unveil the bulk of their population in the early Universe. Acceleration and radiative processes responsible for the majority of their X-ray emission would be pinned down by microsecond timing capabilities at both soft and hard X-rays. Furthermore, imaging jet structures for the first time in the hard X-ray regime could unravel the origin of their high-energy emission. The proposed Probe-class mission concept High Energy X-ray Probe (HEX-P) combines all these required capabilities, making it the crucial next-generation X-ray telescope in the multi-messenger, time-domain era. HEX-P will be the ideal mission to unravel the science behind the most powerful accelerators in the universe.

astro-ph.HE↗

arXiv2023

The peculiar variable X-ray spectrum of the active galactic nucleus PKS 2005-489

PKS 2005-489 is a well-known, bright southern BL Lac object that has been detected up to TeV energies. In a low-flux state it exhibits the expected multiwavelength double-peaked spectrum in the radio -- $γ$-ray band. The high-flux state shows extreme flux variations in the X-ray band with a hardening as well as a peculiar curved feature in the spectrum. Thus far, PKS 2005-489 is the only source to exhibit such a feature. To study the X-ray variability further, we obtained the first hard X-ray spectrum of the source with NuSTAR. We compare quasi-simultaneous radio, optical, UV, soft and hard X-ray, and $γ$-ray data of PKS 2005-489 to archival data in order to study its broadband behavior. We find a very consistent quiet state in the SED, with little variation in spectral shape or flux between the 2012 and 2020 data. A possible explanation for the peculiar X-ray spectrum in the flaring state is an additional component in the jet, possibly accelerated via magnetic reconnection, that is not co-spatial to the low-flux state emission region.

astro-ph.HE↗

arXiv2021

Statistical properties of flux variations in blazar light curves at GeV and TeV energies

Despite numerous detections of individual flares, the physical origin of the rapid variability observed from blazars remains uncertain. Using Bayesian blocks and the Eisenstein-Hut HOP algorithm, we characterize flux variations of high significance in the $γ$-ray light curves of two samples of blazars. Daily binned long-term light curves of TeV-bright blazars observed with the First G-APD Cherenkov Telescope (FACT) are compared to those of GeV-bright blazars observed with the Large Area Telescope on board the $Fermi$ Gamma-ray Space Telescope ($Fermi$-LAT). We find no evidence for systematic asymmetry of the flux variations based on the derived rise and decay time scales. Additionally, we show that the daily-binned blazar light curves can be described by an exponential stochastic Ornstein-Uhlenbeck (OU) process with parameters depending on energy. Our analysis suggests that the flux variability in both samples is a superposition of faster fluctuations. This is, for instance, challenging to explain by shock-acceleration but expected for magnetic reconnection.

astro-ph.HE↗

arXiv2021

The first GeV flare of the radio-loud narrow-line Seyfert 1 galaxy PKS 2004-447

On 2019 October 25, the Fermi-Large Area Telescope observed the first gamma-ray flare from the radio-loud narrow-line Seyfert 1 (NLSy 1) galaxy PKS 2004$-$447 ($z=0.24$). We report on follow-up observations in the radio, optical-UV, and X-ray bands that were performed by ATCA, the Neil Gehrels Swift observatory, XMM-Newton, and NuSTAR, respectively, and our multi-wavelength analysis. We study the variability across all energy bands and additionally produce $γ$-ray light curves with different time binnings to study the variability on short timescales during the flare. We examine the X-ray spectrum from 0.5$-$50 keV by describing the spectral shape with an absorbed power law. We analyse multi-wavelength datasets before, during, and after the flare and compare these with a low activity state of the source by modelling the respective SEDs with a one-zone synchrotron inverse Compton radiative model. Finally, we compare our results to gamma-ray flares previously observed from other $γ$-loud NLSy 1 galaxies. At gamma-ray energies (0.1$-$300 GeV) the flare reached a total maximum flux of $(2.7\pm0.6)\times10^{-6}$~ph~cm$^{-2}$~s$^{-1}$ in 3-hour binning. With a photon index of $Γ_{0.1-300\mathrm{GeV}}=2.42\pm0.09$ during the flare, this corresponds to an isotropic gamma-ray luminosity of $(2.9\pm0.8)\times10^{47}\,\mathrm{erg}\,\mathrm{s}^{-1}$. The $γ$-ray, X-ray, and optical-UV light curves covering the end of September to the middle of November show significant variability, and we find indications for flux-doubling times of $\sim 2.2$~hours at $γ$-ray energies. During the flare, the SED exhibits large Compton dominance. While the increase in the optical-UV range can be explained by enhanced synchrotron emission, the elevated $γ$-ray flux can be accounted for by an increase in the bulk Lorentz factor of the jet, similarly observed for flaring gamma-ray blazars.

astro-ph.HE↗