arXiv subjects

Stefan J. Wagner

Publications and source records attributed to Stefan J. Wagner.

17 recordsLinked to original sources

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.

astro-ph.HE↗

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.

astro-ph.HE↗

arXiv2016

Attenuation of TeV $γ$-rays by the starlight photon field of the host galaxy

The absorption of TeV $γ$-ray photons produced in relativistic jets by surrounding soft photon fields is a long-standing problem of jet physics. In some cases the most likely emission site close to the central black hole is ruled out because of the high opacity caused by strong optical and infrared photon sources, such as the broad line region. Mostly neglected for jet modeling is the absorption of $γ$-rays in the starlight photon field of the host galaxy. Analyzing the absorption for arbitrary locations and observation angles of the $γ$-ray emission site within the host galaxy we find that the distance to the galaxy center, the observation angle, and the distribution of starlight in the galaxy are crucial for the amount of absorption. We derive the absorption value for a sample of $20$ TeV detected blazars with a redshift $z_r<0.2$. The absorption value of the $γ$-ray emission located in the galaxy center may be as high as $20\%$ with an average value of $6\%$. This is important in order to determine the intrinsic blazar parameters. We see no significant trends in our sample between the degree of absorption and host properties, such as starlight emissivity, galactic size, half-light radius, and redshift. While the uncertainty of the spectral properties of the extragalactic background light exceeds the effect of absorption by stellar light from the host galaxy in distant objects, the latter is a dominant effect in nearby sources. It may also be revealed in a differential comparison of sources with similar redshifts.

astro-ph.HE↗

arXiv2016

The Exceptional Flare of Mrk 501 in 2014: Combined Observations with H.E.S.S. and FACT

The BL Lac type object Mrk 501 was observed at very high energies (E > 100 GeV) in 2014 with the upgraded H.E.S.S. (High Energy Stereoscopic System) phase 2 array. The data collected with the central 28m telescope allow for a broader energy range extending to lower energies when compared to the one obtained with the four small telescopes alone. A strong flaring event with a flux level comparable to the 1997 historical maximum has been detected as a consequence of target of opportunity observations triggered by alerts from the FACT collaboration. The First G-APD Cherenkov Telescope (FACT) is continuously monitoring bright blazars at TeV energies providing important pre- and post-flare information. For the first time, the data and lightcurves from H.E.S.S. and FACT are compared. These contemporaneous observations allow for a better characterization of the source emission. In a multiwavelength context, more precise correlation studies between VHE and lower energies are possible thanks to the dense sampling of the FACT observations. The hard intrinsic spectrum detected by H.E.S.S. during the flare allows the derivation of strong constraints on the scale of Lorentz invariance violation via the non-detection of EBL opacity modifications and from time-of-flight studies.

astro-ph.HE↗

arXiv2016

The extended jet of AP Librae: Origin of the very high-energy gamma-ray emission?

The low-frequency peaked BL Lac object (LBL) AP Librae exhibits very-high-energy (VHE, $E>100$GeV) $γ$-ray emission and hosts an extended jet, which has been detected in radio and X-rays. The jet X-ray spectral index implies an inverse Compton origin. These observations are unusual for LBLs calling for a consistent explanation of this extraordinary source. The observationally constrained parameters necessary to describe the core emission within the standard one-zone model are unable to explain the broad-band spectrum, even if observationally unconstrained external photon fields are taken into account. We demonstrate that the addition of the extended jet emission successfully reproduces the total spectral energy distribution. In particular, the VHE radiation is produced in the $>100\,$kpc long extended jet via inverse Compton scattering of cosmic microwave background photons by highly relativistic electrons. We present several ways to test this theory. The extended jet is weakly magnetized ($B_0 = 2.5\,μ$G), while its minimum and maximum electron Lorentz factors are $γ_{min}=60$ and $γ_{max}=5\times 10^6$, respectively. The electron spectral index is $s=2.6$. These parameters are comparable to parameters of other blazars with extended X-ray jets dominated by inverse Compton scattering.

astro-ph.HE↗

arXiv2015

Rapid variability at very high energies in Mrk 501

A major flaring state of the BL Lac object Mrk 501 was observed by the High Energy Stereoscopic System (H.E.S.S.) in June, 2014. Flux levels higher than one Crab unit were recorded and rapid variability at very high energies ($\sim$2-20 TeV) was revealed. The high statistics afforded by the flares allowed us to probe the presence of minutes timescale variability and study its statistical characteristics exclusively at TeV energies owing to the high energy threshold of approximately 2 TeV. Doubling times of a few minutes are estimated for fluxes greater than 2 TeV. Statistical tests on the light curves show interesting temporal structure in the variations including deviations from a normal flux distribution similar to those found in the PKS 2155-304 flare of July 2006, at nearly an order of magnitude higher threshold energy. Rapid variations at such high energies put strong constraints on the physical mechanisms in the blazar jet.

astro-ph.HE↗

arXiv2015

Spectral characteristics of Mrk 501 during the 2012 and 2014 flaring states

Observations at Very High Energies (VHE, E > 100 GeV) of the BL Lac object Mrk 501 taken with the High Energy Stereoscopic System (H.E.S.S.) in four distinct periods between 2004 and 2014 are presented, with focus on the 2012 and 2014 flaring states. The source is detected with high significance above $\sim$ 2 TeV in $\sim$ 13.1 h livetime. The observations comprise low flux states and strong flaring events, which in 2014 show a flux level comparable to the 1997 historical maximum. Such high flux states enable spectral variability and flux variability studies down to a timescale of four minutes in the 2-20 TeV energy range. During the 2014 flare, the source is clearly detected in each of these bins. The intrinsic spectrum is well described by a power law of index $Γ=2.15\pm0.06$ and does not show curvature in this energy range. Flux dependent spectral analyses show a clear harder-when-brighter behaviour. The high flux levels and the high sensitivity of H.E.S.S. allow studies in the unprecedented combination of short timescales and an energy coverage that extends significantly above 20 TeV. The high energies allow us to probe the effect of EBL absorption at low redshifts, jet physics and LIV. The multiwavelength context of these VHE observations is presented as well.

astro-ph.HE↗

arXiv2015

Long term lightcurve of the BL Lac object 1ES 0229+200 at TeV energies

The high-frequency peaked BL Lac object 1ES 0229+200 (z = 0.14) was first detected in very high energy (VHE, E > 100 GeV) $γ$-rays by the H.E.S.S. (High Energy Stereoscopic System) collaboration in 2006. No flux variability was reported in the initial study and its spectral characteristics have been used to derive constraints on the extragalactic background light (EBL) and on the intergalactic magnetic field (IGMF). 1ES 0229+200 has been observed with H.E.S.S. for $\sim$130 hours from 2004 to 2013: the full dataset analysed with a more sensitive method will be presented here. The results indicate that the source is not constant and displays flux variability on yearly and monthly timescales. The existence of flux variability affects the derivation of the constraints on the IGMF. The H.E.S.S. observations cover several simultaneous multi-frequency campaigns and the VHE variations are compared with those reported in different bands.

astro-ph.HE↗

arXiv2009

A new strategy to select candidate blazars for VHE observation

A new strategy to select candidate blazars for VHE observations, that gives an inherently complete sample from an all-sky compilation (excluding a narrow region around the galactic plane) of GeV-bright blazars is presented. The strategy is based on selecting candidate VHE blazars from a GeV-detected blazar sample, namely the LAT-bright-AGN-sample (LBAS) of the Fermi Gamma-ray Space Telescope (FGST). The intrinsic spectra in the VHE regime is estimated based on the extrapolation of the GeV spectra measured with FGST. This is corrected for the unavoidable extinction due to the Extragalactic Background Light (EBL) in the UV to mid-IR. The resulting observable spectra ranked by the integrated flux is used to extract candidates for observations in the VHE regime with Imaging Atmospheric Cherenkov Telescopes (IACTs). The predictions are cross-checked using the sample of known VHE blazars.

astro-ph.HE↗

arXiv2009

Correlation between the synchrotron and the inverse-Compton components, in the longterm light curve of GeV blazars

To explore the correlation in the GeV band (i.e. the inverse-Compton component) with the optical band (i.e. the synchrotron component) of blazars that are bright in high energies, the light curves of a number of blazars monitored with the Fermi Gamma-ray Space Telescope (FGST), were compared to the corresponding light curves recorded with the Automatic Telescope for Optical Monitoring (ATOM). The results are presented.

astro-ph.HE↗

arXiv2009

Constraints on cosmological parameters from the EBL attenuation of VHE spectra of extragalactic sources

The Extragalactic Background Light (EBL) causes attenuation of the VHE spectra of extragalactic sources, via pair-production. The amount of attenuation depends on the distance of the source, i.e. on the integration over the line of sight path taken by the VHE photons. The line of sight path calculation, depends on the values of the cosmological parameters and the evolution of these parameters. With the recent indications of a lower EBL level from observations and the greater sensitivity of the proposed future Cherenkov experiments it will be possible to detect higher redshift sources. We study whether the EBL attenuation of extragalactic sources is sensitive enough to put constrains on the cosmological parameters.

astro-ph.CO↗

arXiv2009

The Nature of a Cosmic-Ray Accelerator, CTB37 B, Observed with Suzaku and Chandra

We report on Suzaku and Chandra observations of the young supernova remnant CTB37B, from which TeV gamma-rays were detected by the H.E.S.S. Cherenkov telescope. The 80 ks Suzaku observation provided us with a clear image of diffuse emission and high-quality spectra. The spectra revealed that the diffuse emission is comprised of thermal and non-thermal components. The thermal component can be represented by an NEI model with a temperature, a pre-shock electron density and an age of 0.9(0.7-1.1) keV, 0.4(0.3-0.5) cm^{-3} and 650(350-3150) yr, respectively. This suggests that the explosion of CTB37B occurred in a low-density space. A non-thermal power-law component was found from the southern region of CTB37B. Its photon index of ~1.5 and a high roll-off energy (>15 keV) indicate efficient cosmic-ray acceleration. A comparison of this X-ray spectrum with the TeV gamma-ray spectrum leads us to conclude that the TeV gamma-ray emission seems to be powered by either multi-zone Inverse Compton scattering or the decay of neutral pions. The point source resolved by Chandra near the shell is probably associated with CTB37B, because of the common hydrogen column density with the diffuse thermal emission. Spectral and temporal characteristics suggest that this source is a new anomalous X-ray pulsar.

astro-ph↗

arXiv2002

Complex shock structure in the western hot-spot of Pictor A

We have carried out simulations of supersonic light jets in order to model the features observed in optical and radio images of the western hot-spot in the radio galaxy Pictor A. We have considered jets with density ratios eta=1e-2 - 1e-4, and Mach numbers ranging between 5 and 50. From each simulation, we have generated ray-traced maps of radio surface brightness at a variety of jet inclinations, in order to study the appearance of time-dependent luminous structures in the vicinity of the western hot-spot. We compare these rendered images with observed features of Pictor A. A remarkable feature of Pictor A observations is a bar-shaped "filament" inclined almost at right angles to the inferred jet direction and extending 24" (10.8 /h kpc) along its longest axis. The constraints of reproducing the appearance of this structure in simulations indicate that the jet of Pictor A lies nearly in the plane of the sky. The results of the simulation are also consistent with other features found in the radio image of Pictor A. This filament arises from the surging behaviour of the jet near the hot-spot; the surging is provoked by alternate compression and decompression of the jet by the turbulent backflow in the cocoon. We also examine the arguments for the jet in Pictor A being at a more acute angle to the line of sight and find that our preferred orientation is just consistent with the limits on the brightness ratio of the X-ray jet and counter-jet. We determine from our simulations, the structure function of hot-spot brightness and also the cumulative distribution of the ratio of intrinsic hot-spot brightnesses. The latter may be used to quantify the use of hot-spot ratios for the estimation of relativistic effects.

astro-ph↗

arXiv2001

Variability Time Scales of TeV Blazars Observed in the ASCA Continuous Long-Look X-ray Monitoring

Three uninterrupted, long (lasting respectively 7, 10, and 10 days) ASCA observations of the well-studied TeV-bright blazars Mrk 421, Mrk 501 and PKS 2155-304 all show continuous strong X-ray flaring. Despite the relatively faint intensity states in 2 of the 3 sources, there was no identifiable quiescent period in any of the observations. Structure function analysis shows that all blazars have a characteristic time scale of ~ a day, comparable to the recurrence time and to the time scale of the stronger flares. On the other hand, examination of these flares in more detail reveals that each of the strong flares is not a smooth increase and decrease, but exhibits substructures of shorter flares having time scales of ~10 ks. We verify via simulations that in order to explain the observed structure function, these shorter flares ("shots") are unlikely to be fully random, but in some way are correlated with each other. The energy dependent cross-correlation analysis shows that inter-band lags are not universal in TeV blazars. This is important since in the past, only positive detections of lags were reported. In this work, we determine that the sign of a lag may differ from flare to flare; significant lags of both signs were detected from several flares, while no significant lag was detected from others. However, we also argue that the nature of the underlying component can affect these values. The facts that all flares are nearly symmetric and that fast variability shorter than the characteristic time scale is strongly suppressed, support the scenario where the light crossing time dominates the variability time scales of the day-scale flares.

astro-ph↗

arXiv2001

Characteristic X-ray Variability of TeV Blazars: Probing the Link between the Jet and the Central Engine

We have studied the rapid X-ray variability of three extragalactic TeV gamma-ray sources; Mrk 421, Mrk 501 and PKS 2155-304. Analyzing the X-ray light curves obtained from ASCA and/or RXTE observations between 1993 and 1998, we have investigated the variability in the time domain from 10^3 to 10^8 sec. For all three sources, both the power spectrum density (PSD) and the structure function (SF) show a roll-over with a time-scale of the order of 1 day or longer, which may be interpreted as the typical time-scale of successive flare events. Although the exact shape of turn-over is not well constrained and the low-frequency (long time-scale) behavior is still unclear, the high-frequency (short time-scale) behavior is clearly resolved. We found that, on time-scales shorter than 1 day, there is only small power in the variability, as indicated by a steep power spectrum density of f^{-2~-3}. This is very different from other types of mass-accreting black-hole systems for which the short time-scale variability is well characterized by a fractal, flickering-noise PSD (f^{-1~-2}). The steep PSD index and the characteristic time-scale of flares imply that the X-ray emitting site in the jet is of limited spatial extent; D > 10^{17} cm distant from the base of the jet, which corresponds to > 100 Schwarzschild radii for 10^{7-10} M_solar black-hole systems.

astro-ph↗

arXiv1997

Fast Variations of Gamma-Ray Emission in Blazars

The largest group of sources identified by EGRET are Blazars. This sub-class of AGN is well known to vary in flux in all energy bands on time-scales ranging from a few minutes (in the optical and X-ray bands) up to decades (radio and optical regimes). In addition to variations of the gamma-ray flux between different viewing periods, the brightest of these sources showed a few remarkable gamma-ray flares on time-scales of about one day, confirming the extension of the ``Intraday-Variability (IDV)'' phenomenon into the GeV range. We present first results of a systematic approach to study fast variability with EGRET data. This statistical approach confirms the existence of IDV even during epochs when no strong flares are detected. This provides additional constraints on the site of the gamma-ray emission and allows cross-correlation analyses with light curves obtained at other frequencies even during periods of low flux. We also find that some stronger sources have fluxes systematically above threshold even during quiescent states. Despite the low count rates this allows explicit comparisons of flare amplitudes with other energy bands.

astro-ph↗

arXiv1995

Statistics of optical intraday variability in a complete sample of radio-selected BL Lac objects

We present a study of the intraday variability behaviour of a complete sample of radio-selected BL Lac objects taken from the 1 Jy catalogue. In 28 out of 34 BL Lac objects (82\%) we detected intraday variability. 7 objects were observed during several campaigns. None of them changed its variability behaviour. The duty cycle in radio-loud BL Lac objects is very high, i.e. at least 0.8. The typical peak-to-peak amplitudes of the variability were 30\%. By means of structure function and autocorrelation analyses we investigated the typical time-scales of the variability and determined the activity of the BL Lac objects. In 21 of the variable BL Lac objects we were able to measure a typical time-scale, which lies in the range between 0.5 and 3 days. In general, the results can be explained by the standard model, where shocks are propagating down a relativistic jet. However, in the two most densely sampled observations we found variability characteristics, which cannot easily be explained by the standard model alone, demonstrating that at least in these objects alternative models should be taken into account. Nevertheless, all models, which are able to explain variability on time-scales of days must be able to consider the high duty cycle of these BL Lac objects.

astro-ph↗