arXiv subjects

Alicja Wierzcholska

Publications and source records attributed to Alicja Wierzcholska.

At least 19 recordsLinked to original sources

arXiv2026

Comparative analysis of BL Lacertae in flaring and non-flaring states: timing and spectral studies

BL Lacertae is a blazar known for its high flux variability and occasional broadband flares of unknown origin. It was in an extended flaring state from July 2020 until the end of 2021, making it an ideal candidate to study spectral and temporal properties during different flux states. We analysed five XMM--Newton EPIC observations of BL Lacertae taken up to the end of 2021. Temporal properties were investigated using fractional variability, minimum variability timescale, and the discrete correlation function. Detailed spectral modeling was performed on the two most variable observations, including correlation analysis between the soft (0.3--2.0 keV) and hard (2.0--10.0 keV) bands. Two of five observations were found to be highly variable with $F_{\mathrm{var}} = 19.16 \pm 0.32$ and $6.27 \pm 0.43$. The 2021 observation corresponds to the highest flux state. The shortest variability timescale in the 0.3--10 keV band is 1.24 ks. Assuming synchrotron-dominated X-ray emission, this timescale constrains the emission region size. Under equipartition between the magnetic field and radiating particles, this implies $B \approx 0.4\,\mathrm{G}$. A softer-when-brighter spectral trend was found, as commonly seen in blazars. Spectra were modeled with single power-law, log-parabola, and broken power-law models; the broken power-law gave the best fit by Akaike Information Criterion in most cases, with a strong break energy--flux correlation. A thermal blackbody component showed a positive temperature--flux correlation in some observations. The spectral break, interpreted as the synchrotron cooling break, shifts to higher energies with increasing flux. The source consistently showed softer-when-brighter behavior. Only one observation showed significant soft--hard band correlation. The data suggest the synchrotron peak moves into or across the X-ray band as the source brightens.

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arXiv2026

Not so Swift: 20 years of multiwavelength observations of Mrk 421 and Mrk 501

Aims. The blazars Mrk 421 and Mrk 501 have shown multiwavelength variability on all observed timescales, and have been well studied at high energies on short timescales. We aim to characterise the long-term temporal behaviour of these blazars at synchrotron energies, namely optical, UV, and X-ray, in order to assess current models of these objects and their processes. Methods. Amongst the longest light curves ever studied for these sources, we investigated 20 years of data (2005-2025) from the Swift-UVOT and Swift-XRT telescopes. We examined spectral models, fractional variabilities, flux distributions, and X-ray photon index vs flux relations, as well as carrying out in-depth time series analysis using structure functions, Lomb-Scargle periodograms, and discrete correlation functions. Results. Mrk 421 and Mrk 501 both showed intriguing variability in all studied wavelengths; this variability has been found to be energy dependent, as has the trend of lognormality in flux distributions. X-ray photon indices fluctuated greatly throughout the entire period, showing an overall harder-when-brighter trend. Hints of a quasi-periodicity have been found in the X-ray of Mrk 501 (host frame time scale $\sim390$ days, >3$σ$) but not in the UV or X-ray of Mrk 421, or in the UV of Mrk 501. No correlation at any time lag was found between the optical/UV and X-ray bands in either source.

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arXiv2025

Disentangling two spectral components in the X-ray emission of the blazar 1ES 0229+200

X-ray observations are essential to achieve a deeper understanding of the broadband emission mechanism in blazars. Here, we present a long-term spectral and temporal analysis of X-ray and optical observations of 1E 0229+200 collected with the Neil Gehrels Swift Observatory from 2008 to 2024, complemented by hard X-ray observations from the Nuclear Spectroscopic Telescope Array NuSTAR. The blazar 1ES 0229+200 is a high-frequency, peaked BL Lac object, known for its exceptionally hard very high-energy (VHE) $γ$-ray spectrum extending up to 10 TeV. In August 2021, NuSTAR observed the source in a low X-ray state, revealing a concave spectral shape with a distinct upturn around 25 keV. This feature contrasts with previous observations performed with NuSTAR and Swift-BAT, which showed no such spectral upturn. Previous observations of 1ES 0229+200 and broadband SED (spectral energy distribution) modelling suggest that its X-ray emission extends beyond 100 keV without a significant cutoff. The newly detected spectral upturn may indicate a transition between the synchrotron and inverse Compton components or could be linked to photohadronic processes involving high-energy neutrinos. We discuss the implications of this finding in the context of blazar spectral energy distributions, particularly the potential existence of a third SED bump in the kiloelectronvolt to megaelectronvolt range. The observed spectral features support the hypothesis that 1ES 0229+200 could be a source of high-energy neutrino emission.

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arXiv2024

Exceptional X-ray activity in BL Lacertae

BL Lacertae is a unique blazar for which the X-ray band can cover either the synchrotron or the inverse Compton, or both parts of the broadband spectral energy distribution. In the latter case, when the spectral upturn is located in the X-ray range, it allows contemporaneous study of the low- and high-energy ends of the electron distribution function. In this work, we study spectral and temporal variability using X-ray and optical observations of the blazar performed with the Neil Gehrels Swift Observatory from 2020 to 2023. The large set of observational data reveals intensive flaring activity, accompanied by spectral changes in both spectral branches. We conclude that the low-energy and high-energy ends of the particle distribution function are characterised by similar variability scales. Additionally, the hard X-ray observations of BL Lacertae performed with the Nuclear Spectroscopic Telescope Array (NuSTAR) confirm a concave spectral curvature for some epochs of the blazar activity and reveal that it can be shifted up to energies of as high as 8 keV. The time-resolved spectral analysis allows us to disentangle X-ray spectral variability features of the synchrotron from inverse Compton components. Despite significant variability of both spectral components, we find only small changes in the position of the spectral upturn. The different slopes and shapes of the X-ray spectrum of BL Lacertae demonstrate that the classification of this source is not constant, and BL Lacertae can exhibit features of either high-, intermediate-, or low-energy peaked blazar in different epochs of observation. This also indicates that the spectral upturn for this blazar can be located not only in the X-ray range of 0.3-10 keV, but also at lower or higher energies.

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arXiv2023

H.E.S.S. realtime follow-ups of IceCube high-energy neutrino alerts

The evidence for multi-messenger photon and neutrino emission from the blazar TXS 0506+056 has demonstrated the importance of realtime follow-up of neutrino events by various ground- and space-based facilities. The effort of H.E.S.S. and other experiments in coordinating observations to obtain quasi-simultaneous multiwavelength flux and spectrum measurements has been critical in measuring the chance coincidence with the high-energy neutrino event IC-170922A and constraining theoretical models. For about a decade, the H.E.S.S. transient program has included a search for gamma-ray emission associated with high-energy neutrino alerts, looking for gamma-ray activity from known sources and newly detected emitters consistent with the neutrino location. In this contribution, we present an overview of follow-up activities for realtime neutrino alerts with H.E.S.S. in 2021 and 2022. Our analysis includes both public IceCube neutrino alerts and alerts exchanged as part of a joint H.E.S.S.-IceCube program. We focus on interesting coincidences observed with gamma-ray sources, particularly highlighting the significant detection of PKS 0625-35, an AGN previously detected by H.E.S.S., and three IceCube neutrinos.

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arXiv2022

X-ray intraday variability of the TeV blazar Mrk 421 with {\it XMM-Newton}

Highly variable Markarian 421 is a bright high synchrotron energy peaked blazar showing wide featureless non-thermal spectrum making it a good candidate for our study of intraday flux and spectral variations over time. We analyse its X-ray observations of over 17 years taken with the EPIC-PN instrument to probe into the intraday variability properties. The photon energy band of 0.3 - 10.0 keV, and its sub-bands, soft 0.3-2.0 keV and hard 2.0-10.0 keV. To examine flux variability, fractional variability amplitude and the minimum variability timescale have been calculated. We also probed into the spectral variability by studying hardness ratio for each observation and the correlation between the two energy bands using discrete correlation function and inspecting the normalized light curves. The parameters obtained from these methods have been studied for any correlation or non-random trends. From this work, we speculate on the constraints for possible particle acceleration and emission processes in the jet, for better understanding of the processes involving a turbulent behaviour except of shocks. A positive discrete correlation function between the two sub-bands indicates the role of the same electron population in the emission of photons in the two bands. The correlation between the parameters of flux variability and parameters of spectral variation and lags in sub-energy bands provide the constraints to be considered for any modelling of emission processes.

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arXiv2021

Is PKS 0625-354 another variable TeV active galactic nucleus?

The majority of the active galactic nuclei (AGN) detected at very-high-energies above 100 GeV belong to the class of blazars with a small angle between the jet-axis and the line-of-sight. Only about 10 percent of the gamma-ray AGN are objects with a larger viewing angle resulting in a smaller Doppler boosting of the emission. Originally, it was believed that gamma-ray emission can only be observed from blazars and those are variable in its brightness. Instead, the last years have shown that non-blazar active galaxies also show a fascinating variability behaviour which provide important new insights into the physical processes responsible for the gamma-ray production and especially for flaring events. Here, we report on the observation of gamma-ray variability of the active galaxy PKS 0625-354 detected with the H.E.S.S. telescopes in November 2018. The classification of PKS 0625-354 is a still matter of debate. The H.E.S.S. measurements were performed as part of a flux observing program and showed in the first night of the observation a detection of the object with >5sigma. A denser observation campaign followed for the next nine nights resulting in a decrease of the gamma-ray flux. Those observations were accompanied with Swift in the X-ray and UV/optical band allowing for the reconstruction of a multi-band broad-band spectral energy distribution. We will discuss the implications of the gamma-ray variability of the object.

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arXiv2021

The ablation of gas clouds by blazar jets and the long-lasting flare in CTA 102

Long-lasting, very bright multiwavelength flares of blazar jets are a curious phenomenon. The interaction of a large gas cloud with the jet of a blazar may serve as a reservoir of particles entrained by the jet. The size and density structure of the cloud then determine the duration and strength of the particle injection into the jet and the subsequent radiative outburst of the blazar. In this presentation, a comprehensive parameter study is provided showing the rich possibilities that this model offers. Additionally, we use this model to explain the 4-months long, symmetrical flare of the flat spectrum radio quasar CTA 102 in late 2016. During this flare, CTA 102 became one of the brightest blazars in the sky despite its large redshift of $z=1.032$.

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arXiv2020

Constraining X-ray emission in HBL blazars using multiwavelength observations

The X-ray spectrum of extreme HBL type blazars is located in the synchrotron branch of the broadband spectral energy distribution (SED), at energies below the peak. A joint fit of the extrapolated X-ray spectra together with a host galaxy template allows characterizing the synchrotron branch in the SED. The X-ray spectrum is usually characterized either with a pure or a curved power-law model. In the latter case, however, it is hard to distinguish an intrinsic curvature from excess absorption. In this paper, we focus on five well-observed blazars: 1ES 0229+200, PKS 0548-322, RX J1136+6737, 1ES 1741+196, 1ES 2344+514. We constrain the infrared-to-X-ray emission of these five blazars using a model that is characterized by the host galaxy, spectral curvature, absorption, and ultraviolet excess to separate these spectral features. In the case of four sources: 1ES 0229+200, PKS 0548-322, 1ES 1741+196, 1ES 2344+514 the spectral fit with the atomic neutral hydrogen from the Leiden Argentina Bonn Survey result in a significant UV excess present in the broadband spectral energy distribution. Such excess can be interpreted as an additional component, for example, a blue bump. However, in order to describe spectra of these blazars without such excess, additional absorption to the atomic neutral hydrogen from the Leiden Argentina Bonn Survey is needed.

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arXiv2019

Unraveling The Complex Nature Of The Very High-Energy $γ$-Ray Blazar PKS 2155-304

PKS 2155-304 is a blazar located in the Southern Hemisphere, monitored with the High Energy Stereoscopic System (H.E.S.S.) at very high energy (VHE, E>100 GeV) $γ$ rays every year since 2002. Thanks to the large data set collected in the VHE range and simultaneous coverage in optical, ultraviolet (UV), X-ray and high energy $γ$-ray ranges, this object is an excellent laboratory to study spectral and temporal variability in blazars. However, despite many years of dense monitoring, the nature of the variability observed in PKS 2155-304 remains puzzling. In this paper, we discuss the complex spectral and temporal variability observed in PKS 2155-304. The data discussed include VHE $γ$-ray data collected with H.E.S.S. between 2013 and 2016, complemented with multiwavelength (MWL) observations from Fermi-LAT, Swift-XRT, Swift-UVOT, SMARTS, and the ATOM telescope. During the period of monitoring, PKS 2155-304 was transitioning from its lower state to the flaring states, and exhibiting different flavors of outbursts. For the first time, orphan optical flare lasting a few months was observed. Correlation studies show an indication of correlation between the X-ray and VHE $γ$-ray fluxes. Interestingly, a comparison of optical and X-ray or VHE $γ$-ray fluxes does not show global correlation. However, two distinct tracks in the diagram were found, which correspond to the different flaring activity states of PKS 2155-304.

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arXiv2019

Observations of blazar PKS 2023-07 in flaring state with HESS and Fermi-LAT in 2016-2017 and constraints on an intrinsic cut-off

PKS 2023-07 is a flat spectrum radio quasar located at a redshift $z=1.388$, farther than any source currently detected at very high energies ($E>100$ GeV). At such energies, absorption by the extragalactic background light (EBL) renders the detection of distant sources particularly challenging. The High Energy Stereoscopic System (H.E.S.S.) observed the source following reports from AGILE (April 2016) and Fermi-LAT (April 2016, September and October 2017) on high-flux states in gamma rays. During each of the three flaring periods, near-simultaneous observations were obtained with H.E.S.S., Fermi-LAT and multiple telescopes at other wavelengths. Though the source was not significantly detected by H.E.S.S., upper limits were derived for each observation period. Through constraints given by Fermi-LAT in the MeV--GeV domain and differential upper limits by H.E.S.S., we searched for an intrinsic cutoff in the EBL-corrected gamma ray spectrum of PKS 2023-07.

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arXiv2019

H.E.S.S. Monitoring of PKS 2155-304 in 2015 and 2016

PKS 2155-304 is one of the brightest blazar located in Southern Hemisphere, monitored with H.E.S.S. since the first light of the experiment. Here we report multiwavelength monitoring observations collected during the period of 2015-2016 with H.E.S.S.,Fermi-LAT, Swift-XRT, Swift-UVOT, and ATOM. Two years of multiwavelength data with very good temporal coverage allowed to characterize broadband emission observed from the region of PKS 2155-304 and study potential multifrequency correlations. During the period of monitoring, PKS 2155-304 revealed complex multiwavelength variability with two outbursts characterized by completely different multiband properties. The 2015 activity of the blazar is characterized by a flare observed at all wavelengths studied. The broadband emission observed during the outburst is well correlated without any time lags. Contrary to 2015, in 2016, only orphan outburst in the optical and ultraviolet wavelengths was observed. Such an orphan activity is reported for the first time for the blazar PKS 2155-304.

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arXiv2019

The VHE Gamma-Ray View of the FSRQ PKS~1510-089

The flat spectrum radio quasar PKS 1510-089 is a monitored target in many wavelength bands due to its high variability. It was detected as a very-high-energy (VHE) $γ$-ray emitter with H.E.S.S. in 2009, and has since been a regular target of VHE observations by the imaging Cherenkov observatories H.E.S.S. and MAGIC. In this paper, we summarize the current state of results focusing on the monitoring effort with H.E.S.S. and the discovery of a particularly strong VHE flare in 2016 with H.E.S.S. and MAGIC. While the source has now been established as a weak, but regular emitter at VHE, no correlation with other energy bands has been established. This is underlined by the 2016 VHE flare, where the detected optical and high-energy $γ$-ray counterparts evolve differently than the VHE flux.

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arXiv2019

The Long-Lasting Activity in the Flat Spectrum Radio Quasar (FSRQ) CTA~102

The flat spectrum radio quasar CTA 102 ($z = 1.032$) went through a tremendous phase of variability. Since early 2016 the gamma-ray flux level has been significantly higher than in previous years. It was topped by a four month long giant outburst, where peak fluxes were more than 100 times higher than the quiescence level. Similar trends are observable in optical and X-ray energies. We have explained the giant outburst as the ablation of a gas cloud by the relativistic jet that injects additional matter into the jet and can self-consistently explain the long-term light curve. Here, we argue that the cloud responsible for the giant outburst is part of a larger system that collides with the jet and is responsible for the years-long activity in CTA 102.

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arXiv2018

The extended flare in CTA 102 in 2016 and 2017 within a hadronic model through cloud ablation by the relativistic jet

The flat spectrum radio quasar CTA 102 (redshift 1.037) exhibited a tremendously bright 4-months long outburst from late 2016 to early 2017. In a previous paper, we interpreted the event as the ablation of a gas cloud by the relativistic jet. The multiwavelength data have been reproduced very well within this model using a leptonic emission scenario. Here we expand that work by using a hadronic scenario, which gives us greater freedom with respect to the location of the emission region within the jet. This is important, since the inferred gas cloud parameters depend on the distance from the black hole. While the hadronic model faces the problem of invoking super-Eddington jet luminosities, it reproduces well the long-term trend and also days-long subflares. While the latter result in inferred cloud parameters that match those expected for clouds of the broad-line region, the long-term trend is not compatible with such an interpretation. We explore the possibilities that the cloud is from the atmosphere of a red giant star or comes from a star-forming region that passes through the jet. The latter could also explain the much longer-lasting activity phase of CTA 102 from late 2015 till early 2018.

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arXiv2018

Characterizing long-term optical, ultraviolet and X-ray variability in different activity states of OJ 287

We have studied long-term optical, ultraviolet (UV) and X-ray observations of OJ 287 collected with the UVOT and XRT instruments mounted on board the Swift satellite to quantify spectral and temporal variability patterns observed during different activity states. We characterized the flux variations using the data collected during almost 11 yr of the monitoring of the blazar. Significant variability of the blazar has been detected both in the flux and spectral index from the optical to X-ray regimes. We noted that the variability patterns observed in the optical range are more pronounced than the ones in the X-ray band. There is no clear relation between the optical/UV and X-ray emission, neither during the quiescence state nor during outbursts. The most significant flares in the optical/UV regime were detected in 2015 December-2016 January. The shortest variability time-scale is one day and it is limited by the observation pointing. A low activity state of OJ 287 was observed at the end of 2014, while the beginning of 2015 revealed a flat X-ray spectrum, which has been observed for the first time. On one hand, this can be a spectral upturn where the synchrotron and inverse Compton components meet, but on the other hand, it can be generated by an additional emission component. The spectral studies have not revealed any bluer-when-brighter or redder-when-brighter chromatism in the colour-magnitude diagram for OJ 287 in any state of the source's activity. A harder-when-brighter behaviour was noticed for OJ 287 only in the case of the X-ray observations.

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arXiv2017

Signature of Inverse Compton emission from blazars

Blazars are classified into high, intermediate and low energy peaked sources based on the location of their synchrotron peak. This lies in infra-red/optical to ultra-violet bands for low and intermediate peaked blazars. The transition from synchrotron to inverse Compton emission falls in the X-ray bands for such sources. We present the spectral and timing analysis of 14 low and intermediate energy peaked blazars ob- served with XMMNewton spanning 31 epochs. Parametric fits to X-ray spectra helps constrain the possible location of transition from the high energy end of the syn- chrotron to the low energy end of the inverse Compton emission. In seven sources in our sample, we infer such a transition and constrain the break energy in the range 0.6 10 keV. The Lomb-Scargle periodogram is used to estimate the power spectral density (PSD) shape. It is well described by a power law in a majority of light curves, the index being flatter compared to general expectation from AGN, ranging here between 0.01 and 1.12, possibly due to short observation durations resulting in an absence of long term trends. A toy model involving synchrotron self-Compton (SSC) and exter- nal Compton (EC; disk, broad line region, torus) mechanisms are used to estimate magnetic field strength 6 0.03 - 0.88 G in sources displaying the energy break and infer a prominent EC contribution. The timescale for variability being shorter than synchrotron cooling implies steeper PSD slopes which are inferred in these sources.

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arXiv2017

Monitoring of the FSRQ PKS 1510-089 with H.E.S.S

The flat spectrum radio quasar (FSRQ) PKS 1510-089 (z=0.361) is known for its complex multiwavelength behavior. It has been monitored regularly at very high energy (VHE, $E>100\,$GeV) gamma-rays with H.E.S.S. since its discovery in 2009 in order to study the unknown behavior of FSRQs in quiescence at VHE, as well as the flux evolution around flaring events. Given the expected strong cooling of electrons and the absorption of VHE emission within the broad-line region, a detection of PKS 1510-089 at VHE in a quiescent state would be an important result, implying an acceleration and emission region on scales beyond the broad-line region. The H.E.S.S. monitoring has been intensified since 2015 and is complemented by monitoring at high energy ($E>100\,$MeV) gamma-rays with Fermi, at X-rays with Swift-XRT, and at optical frequencies with ATOM. The dense lightcurves allow for the first time detailed comparison studies between these energy bands. The source has been active in several frequency bands for a large fraction of the observation time frames. Yet, we do not find obvious correlations between the VHE and the other bands over the observed time frame indicating a non-trivial interplay of the acceleration, cooling and radiative processes. It also implies a rich variety in flaring behavior, which makes this source difficult to interpret within a unique theoretical framework.

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