ADS list of publications
ADS list of refereed publications


Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A:
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Refereed Publications

(First/corresponding author papers marked in blue)
35. Gamma-ray emission in radio galaxies under the VLBI scope — II. The relationship between gamma-ray emission and parsec-scale jets in radio galaxies
Angioni R., Ros E., Kadler M., …, Krauß F. et al. 2020, A&A, accepted for publication.
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Following our study of the radio and high-energy properties of γ-ray-emitting radio galaxies, here we investigate the kinematic and spectral properties of the parsec-scale jets of radio galaxies that have not yet been detected by Fermi-LAT. We take advantage of the regular VLBI observations provided by the TANAMI monitoring program, and explore the kinematic properties of six γ-ray-faint radio galaxies. We include publicly available VLBI kinematics of γ-ray-quiet radio galaxies monitored by the MOJAVE program and perform a Fermi-LAT analysis, deriving upper limits. We combine these results with those from our previous paper to construct the largest sample of radio galaxies with combined VLBI and γ-ray measurements to date. We find superluminal motion up to βapp=3.6 in the jet of PKS 2153-69. We find a clear trend of higher apparent speed as a function of distance from the jet core on scales of ∼105Rs, corresponding to the end of the collimation and acceleration zone in nearby radio galaxies. We find evidence of subluminal apparent motion in the jets of PKS 1258−321 and IC 4296, and no measurable motion for PKS 1549−79, PKS 1733−565 and PKS 2027−308. We compare the VLBI properties of γ-ray-detected and undetected radio galaxies, and find significantly different distributions of median core flux density, and, possibly, of median core brightness temperature. We find a significant correlation between median core flux density and γ-ray flux, but no correlation with typical Doppler boosting indicators such as median core brightness temperature and core dominance. Our study suggests that high-energy emission from radio galaxies is related to parsec-scale radio emission from the inner jet, but is not driven by Doppler boosting effects, in contrast to the situation in their blazar counterparts.
34. Multimessenger observations of counterparts to IceCube-190331A
Krauß F., Calamari E., Azadeh K. et al. 2020, MNRAS, accepted for publication.
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High-energy neutrinos are a promising tool for identifying astrophysical sources of high and ultra-high energy cosmic rays (UHECR). Prospects of detecting neutrinos at high energies (≳TeV) from blazars have been boosted after the recent association of IceCube-170922A and TXS 0506+056. We investigate the high-energy neutrino, IceCube-190331A, a high-energy starting event (HESE) with a high likelihood of being astrophysical in origin. We initiated a Swift/XRT and UVOT tiling mosaic of the neutrino localisation, and followed up with ATCA radio observations, compiling a multiwavelength SED for the most likely source of origin. NuSTAR observations of the neutrino location and a nearby X-ray source were also performed. We find two promising counterpart in the 90% confidence localisation region and identify the brightest as the most likely counterpart. However, no Fermi/LAT γ-ray source and no prompt Swift/BAT source is consistent with the neutrino event. At this point it is unclear whether any of the counterparts produced IceCube-190331A. We note that the Helix Nebula is also consistent with the position of the neutrino event, and we calculate that associated particle acceleration processes cannot produce the required energies to generate a high-energy HESE neutrino.
33. The Fourth Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope
Ajello M., Angioni R., Axelsson M., …, Krauß F., et al. 2020, ApJ 892, 105.
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The fourth catalog of active galactic nuclei (AGNs) detected by the Fermi Gamma-ray Space Telescope Large Area Telescope (4LAC) between 2008 August 4 and 2016 August 2 contains 2863 objects located at high Galactic latitudes (| b|>r; 10°). It includes 85% more sources than the previous 3LAC catalog based on 4 yr of data. AGNs represent at least 79% of the high-latitude sources in the fourth Fermi-Large Area Telescope Source Catalog (4FGL), which covers the energy range from 50 MeV to 1 TeV. In addition, 344 gamma-ray AGNs are found at low Galactic latitudes. Most of the 4LAC AGNs are blazars (98%), while the remainder are other types of AGNs. The blazar population consists of 24% Flat Spectrum Radio Quasars (FSRQs), 38% BL Lac-type objects, and 38% blazar candidates of unknown types (BCUs). On average, FSRQs display softer spectra and stronger variability in the gamma-ray band than BL Lacs do, confirming previous findings. All AGNs detected by ground-based atmospheric Cerenkov telescopes are also found in the 4LAC.
32. Apparent superluminal core expansion and limb brightening in the candidate neutrino blazar TXS 0506+056
Ros E., Kadler M., Perucho M., …, Krauß F. et al. 2020, A&A 633, L1.
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Context. IceCube has reported a very-high-energy neutrino (IceCube-170922A) in a region containing the blazar TXS 0506+056. Correlated gamma-ray activity has led to the first high-probability association of a high-energy neutrino with an extragalactic source. This blazar has been found to be in a radio outburst during the neutrino event.
Aims: Our goal is to probe the sub-milliarcsecond properties of the radio jet right after the neutrino detection and during the further evolution of the radio outburst.
Methods: We performed target of opportunity observations at 43 GHz frequency using very long baseline interferometry imaging, corresponding to 7 mm in wavelength, with the Very Long Baseline Array two and eight months after the neutrino event.
Results: We produced two images of the radio jet of TXS 0506+056 at 43 GHz with angular resolutions of (0.2 × 1.1) mas and (0.2 × 0.5) mas, respectively. The source shows a compact, high brightness temperature core, albeit not approaching the equipartition limit and a bright and originally very collimated inner jet. Beyond approximately 0.5 mas from the millimeter-VLBI core, the jet loses this tight collimation and expands rapidly. During the months after the neutrino event associated with this source, the overall flux density is rising. This flux density increase happens solely within the core. Notably, the core expands in size with apparent superluminal velocity during these six months so that the brightness temperature drops by a factor of three despite the strong flux density increase.
Conclusions: The radio jet of TXS 0506+056 shows strong signs of deceleration and/or a spine-sheath structure within the inner 1 mas, corresponding to about 70-140 pc in deprojected distance, from the millimeter-VLBI core. This structure is consistent with theoretical models that attribute the neutrino and gamma-ray production in TXS 0506+056 to interactions of electrons and protons in the highly relativistic jet spine with external photons originating from a slower moving jet region. Proton loading due to jet-star interactions in the inner host galaxy is suggested as the possible cause of deceleration.
31. X-ray spectral and flux variability of the microquasar GRS 1758-258 on timescales from weeks to years
Hirsch M., Pottschmidt K., Smith D. M., …, Krauß F. et al. 2020, A&A, accepted for publication.
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We present the spectral and timing evolution of the persistent black hole X-ray binary GRS 1758-258 based on almost 12 years of observations using the Rossi X-ray Timing Explorer Proportional Counter Array. While the source was predominantly found in the hard state during this time, it entered the thermally dominated soft state seven times. In the soft state GRS 1758-258 shows a strong decline in flux above 3 keV rather than the pivoting flux around 10 keV more commonly shown by black hole transients. In its 3-20 keV hardness intensity diagram, GRS 1758-258 shows a hysteresis of hard and soft state fluxes typical for transient sources in outburst. The RXTE-PCA and RXTE-ASM long-term light curves do not show any orbital modulations in the range of 2 to 30 d. However, in the dynamic power spectra significant peaks drift between 18.47d and 18.04d for the PCA data, while less significant signatures between 19d and 20d are seen for the ASM data as well as for the Swift/BAT data. We discuss different models for the hysteresis behavior during state transitions as well as possibilities for the origin of the long term variation in the context of a warped accretion disk.
30. The unique case of the active galactic nucleus core of M87: a misaligned low-power blazar?
Lucchini M., Krauß F., Markoff S. 2019, MNRAS 489, p.1633-1643.
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M87 hosts one of the closest jetted active galactic nuclei (AGN) to Earth. Thanks to its vicinity and to the large mass of its central black hole, M87 is the only source in which the jet can be directly imaged down to near-event horizon scales with radio very large baseline interferometry. This property makes M87 a unique source to isolate and study jet launching, acceleration, and collimation. In this paper, we employ a multizone model designed as a parametrization of general relativistic magnetohydrodynamics (GRMHD); for the first time, we reproduce the jet’s observed shape and multiwavelength spectral energy distribution simultaneously. We find strong constraints on key physical parameters of the jet, such as the location of particle acceleration and the kinetic power. However, we underpredict the (unresolved) gamma-ray flux of the source, implying that the high-energy emission does not originate in the magnetically dominated inner jet regions. Our results have important implications both for comparisons of GRMHD simulations with observations and for unified models of AGN classes.
29. Gamma-ray emission in radio galaxies under the VLBI scope. I. Parsec-scale jet kinematics and high-energy properties of γ-ray-detected TANAMI radio galaxies
Angioni R., Ros E., Kadler M., …, Krauß F. et al. 2019, A&A 627, A148.
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Aims: In the framework of the multi-wavelength and very long baseline interferometry (VLBI) monitoring program TANAMI (Tracking Active Nuclei with Austral Milliarcsecond Interferometry), we study the evolution of the parsec-scale radio emission in radio galaxies in the southern hemisphere and their relationship to the γ-ray properties of the sources. Our study investigates systematically, for the first time, the relationship between the two energy regimes in radio galaxies. In this first paper, we focus on Fermi-LAT-detected sources.
Methods: The TANAMI program monitors a large sample of radio-loud AGN at 8.4 GHz and 22.3 GHz with the Australian long baseline array (LBA) and associated telescopes in Antarctica, Chile, New Zealand and South Africa. We performed a kinematic analysis for five γ-ray detected radio galaxies using multi-epoch 8.4 GHz VLBI images, deriving limits on intrinsic jet parameters such as speed and viewing angle. We analyzed 103 months of Fermi-LAT data in order to study possible connections between the γ-ray properties and the pc-scale jets of Fermi-LAT-detected radio galaxies, both in terms of variability and average properties. We discuss the individual source results and draw preliminary conclusions on sample properties including published VLBI results from the MOJAVE (Monitoring Of Jets in Active galactic nuclei with VLBA Experiments) survey, with a total of fifteen sources.
Results: We find that the first γ-ray detection of Pictor A might be associated with the passage of a new VLBI component through the radio core, which appears to be a defining feature of high-energy emitting Fanaroff-Riley type II radio galaxies. We detect subluminal parsec-scale jet motions in the peculiar AGN PKS 0521-36, and we confirm the presence of fast γ-ray variability in the source down to timescales of six hours, which is not accompanied by variations in the VLBI jet. We robustly confirm the presence of significant superluminal motion, up to βapp∼3, in the jet of the TeV radio galaxy PKS 0625-35. Our VLBI results constrain the jet viewing angle to be Θ < 53°, allowing for the possibility of a closely aligned jet. Finally, by analyzing the first pc-scale multi-epoch images of the prototypical compact symmetric object (CSO) PKS 1718-649, we place an upper limit on the separation speed between the two mini-lobes. This in turn allows us to derive a lower limit on the age of the source.
Conclusions: We can draw some preliminary conclusions on the relationship between pc-scale jets and γ-ray emission in radio galaxies, based on Fermi-LAT-detected sources with available multi-epoch VLBI measurements. We find that the VLBI core flux density correlates with the γ-ray flux, as seen in blazars. On the other hand, the γ-ray luminosity does not show any dependence on the core brightness temperature and core dominance, which are two common indicators of jet Doppler boosting. This seems to indicate that γ-ray emission in radio galaxies is not driven by orientation-dependent effects, as in blazars, in accordance with the unified model of jetted AGN.
28. Breaking degeneracy in jet dynamics: multi-epoch joint modelling of the BL Lac PKS 2155-304
Lucchini M., Markoff S., Crumley P., Krauß F. et al. 2019,
MNRAS 482, 4798-4812.
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Supermassive black holes can launch powerful jets which can be some of the most luminous multiwavelength sources; decades after their discovery their physics and energetics are still poorly understood. The past decade has seen a dramatic improvement in the quality of available data, but despite this improvement the semi-analytical modelling of jets has advanced slowly: simple one-zone models are still the most commonly employed method of interpreting data, in particular for active galactic nucleus (AGN) jets. These models can roughly constrain the properties of jets but they cannot unambiguously couple their emission to the launching regions and internal dynamics, which can be probed with simulations. However, simulations are not easily comparable to observations because they cannot yet self-consistently predict spectra. We present an advanced semi-analytical model which accounts for the dynamics of the whole jet, starting from a simplified parametrization of relativistic magnetohydrodynamics in which the magnetic flux is converted into bulk kinetic energy. To benchmark the model, we fit six quasi-simultaneous, multiwavelength spectral energy distributions of the BL Lac PKS 2155–304 obtained by the TANAMI (Tracking Active Galactic Nuclei with Austral Milliarc-second Interferometry) program, and we address the degeneracies inherent to such a complex model by employing a state-of-the-art exploration of parameter space, which so far has been mostly neglected in the study of AGN jets. We find that this new approach is much more effective than a single-epoch fit in providing meaningful constraints on model parameters.
27. Fermi-LAT counterparts of IceCube neutrinos above 100 TeV
Krauß F., Deoskar K. et al. 2018,
A&A 620, A174.
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The IceCube Collaboration has published four years of data and the observed neutrino flux is significantly in excess of the expected atmospheric background. Due to the steeply falling atmospheric background spectrum, events at the highest energies are most likely extraterrestrial. In our previous approach we have studied blazars as the possible origin of the High-Energy Starting Events (HESE) neutrino events at PeV energies. In this work we extend our study to include all HESE neutrinos (which does not include IC 170922A) at or above a reconstructed energy of 100 TeV, but below 1 PeV. We study the X-ray and γ-ray data of all (∼200) 3LAC blazars that are positionally consistent with the neutrino events above 100 TeV to determine the maximum neutrino flux from these sources. This larger sample allows us to better constrain the scaling factor between the observed and maximum number of neutrino events. We find that when we consider a realistic neutrino spectrum and other factors, the number of neutrinos is in good agreement with the detected number of IceCube HESE events. We also show that there is no direct correlation between Fermi-LAT γ-ray flux and the IceCube neutrino flux and that the expected number of neutrinos is consistent with the non-detection of individual bright blazars.
26. Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A
The IceCube Collaboration, the Fermi/LAT collaboration, et al. 2018,
Science 361, 6398.
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Previous detections of individual astrophysical sources of neutrinos are limited to the Sun and the supernova 1987A, whereas the origins of the diffuse flux of high-energy cosmic neutrinos remain unidentified. On 22 September 2017, we detected a high-energy neutrino, IceCube-170922A, with an energy of ~290 tera–electron volts. Its arrival direction was consistent with the location of a known γ-ray blazar, TXS 0506+056, observed to be in a flaring state. An extensive multiwavelength campaign followed, ranging from radio frequencies to γ-rays. These observations characterize the variability and energetics of the blazar and include the detection of TXS 0506+056 in very-high-energy γ-rays. This observation of a neutrino in spatial coincidence with a γ-ray–emitting blazar during an active phase suggests that blazars may be a source of high-energy neutrinos.
[ADS] [ArXiV] [Journal]
25. Extended X-ray emission in PKS 1718-649
Beuchert T., Rodríguez-Ardila, A, Moss V. A., …, Krauß F. et al. 2018,
A&A 612, L4.
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PKS 1718-649 is one of the closest and most comprehensively studied candidates of a young active galactic nucleus (AGN) that is still embedded in its optical host galaxy. The compact radio structure, with a maximal extent of a few parsecs, makes it a member of the group of compact symmetric objects (CSO). Its environment imposes a turnover of the radio synchrotron spectrum towards lower frequencies, also classifying PKS 1718-649 as gigahertz-peaked radio spectrum (GPS) source. Its close proximity has allowed the first detection of extended X-ray emission in a GPS/CSO source with Chandra that is for the most part unrelated to nuclear feedback. However, not much is known about the nature of this emission. By co-adding all archival Chandra data and complementing these datasets with the large effective area of XMM-Newton, we are able to study the detailed physics of the environment of PKS 1718$-$649. Not only can we confirm that the bulk of the ≲kiloparsec-scale environment emits in the soft X-rays, but we also identify the emitting gas to form a hot, collisionally ionized medium. While the feedback of the central AGN still seems to be constrained to the inner few parsecs, we argue that supernovae are capable of producing the observed large-scale X-ray emission at a rate inferred from its estimated star formation rate.
[ADS] [ArXiV][Journal]
24. Investigating source confusion in PMN J1603-4904
Krauß F., Kreter M., Müller C. et al. 2018,
A&A 610, L8.
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PMN J1603-4904 is a likely member of the rare class of γ-ray emitting young radio galaxies. Only one other source, PKS 1718-649, has been confirmed so far. These objects, which may transition into larger radio galaxies, are a stepping stone to understanding AGN evolution. It is not completely clear how these young galaxies, seen edge-on, can produce high-energy γ rays. PMN J1603-4904 has been detected by TANAMI Very Long Baseline Interferometry (VLBI) observations and has been followed-up with multiwavelength observations. A Fermi Gamma-ray Space Telescope Large Area Telescope (Fermi-LAT) γ-ray source has been associated with this young galaxy in the LAT catalogs. We have obtained Chandra observations of the source to consider the possibility of source confusion due to the relatively large positional uncertainty of Fermi-LAT. The goal was to investigate the possibility of other X-ray bright sources in the vicinity of PMN J1603-4904 that could be counterparts to the γ-ray emission. With Chandra/ACIS, we find no other sources in the uncertainty ellipse of Fermi-LAT data, which includes an improved localization analysis of eight years of data. We further study the X-ray fluxes and spectra. We conclude that PMN J1603-4904 is indeed the second confirmed γ-ray bright young radio galaxy.
[ADS] [ArXiV][Journal]
23. TANAMI: Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry – II. Additional Sources
Müller C., Kadler M., Ojha R., …, Krauß F., et al. 2018,
A&A 610, A1.
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Context. TANAMI is a multiwavelength program monitoring active galactic nuclei (AGN) south of −30° declination including high resolution Very Long Baseline Interferometry (VLBI) imaging, radio, optical/UV, X-ray and γ-ray studies. We have previously published
first-epoch 8.4 GHz VLBI images of the parsec-scale structure of the initial sample. In this paper, we present images of 39 additional sources. The full sample comprises most of the radio- and γ-ray brightest AGN in the southern quarter of the sky, overlapping with the region from which high-energy (> 100 TeV) neutrino events have been found.
Aims. We characterize the parsec-scale radio properties of the jets and compare with the quasi-simultaneous Fermi/LAT γ-ray data. Furthermore, we study the jet properties of sources which are in positional coincidence with high-energy neutrino events as compared
to the full sample. We test the positional agreement of high-energy neutrino events with various AGN samples.
Methods. TANAMI VLBI observations at 8.4 GHz are made with Southern-Hemisphere radio telescopes located in Australia, Antarctica, Chile, New Zealand, and South Africa.
Results. Our observations yield the first images of many jets below −30◦ declination at milliarcsecond resolution. We find that γ-ray loud TANAMI sources tend to be more compact on parsec-scales and have higher core brightness temperatures than γ-ray faint jets, indicating higher Doppler factors. No significant structural difference is found between sources in positional coincidence with high-energy neutrino events and other TANAMI jets. The 22 γ-ray brightest AGN in the TANAMI sky show only a weak positional agreement with high-energy neutrinos demonstrating that the > 100 TeV IceCube signal is not simply dominated by a small number of the γ-ray brightest blazars. Instead, a larger number of sources have to contribute to the signal with each individual source having only a small Poisson probability for producing an event in multi-year integrations of current neutrino detectors.
[ADS] [ArXiV][Journal]
22. First multi-wavelength campaign on the gamma-ray-loud active galaxy IC 310
Ahnen M. L., Ansoldi S., .. (MAGIC Coll.), Krauß F., et al. 2017,
A&A 603, A25.
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Context. The extragalactic very-high-energy gamma-ray sky is rich in blazars. These are jetted active galactic nuclei that are viewed at a small angle to the line-of-sight. Only a handful of objects viewed at a larger angle are so far known to emit above 100 GeV. Multi-wavelength studies of such objects up to the highest energies provide new insights into the particle and radiation processes of active galactic nuclei.
Aims. We aim to report the results from the first multi-wavelength campaign observing the TeV detected nucleus of the active galaxy IC 310, whose jet is observed at a moderate viewing angle of 10°−20°.
Methods. The multi-instrument campaign was conducted between 2012 November and 2013 January, and involved observations with MAGIC, Fermi, INTEGRAL, Swift, OVRO, MOJAVE and EVN. These observations were complemented with archival data from the AllWISE and 2MASS catalogs. A one-zone synchrotron self-Compton model was applied to describe the broadband spectral energy distribution.
Results. IC 310 showed an extraordinary TeV flare at the beginning of the campaign, followed by a low, but still detectable TeV flux. Compared to previous measurements in this energy range, the spectral shape was found to be steeper during the low emission state. Simultaneous observations in the soft X-ray band showed an enhanced energy flux state and a harder-when-brighter spectral shape behavior. No strong correlated flux variability was found in other frequency regimes. The broadband spectral energy distribution obtained from these observations supports the hypothesis of a double-hump structure.
Conclusions. The harder-when-brighter trend in the X-ray and VHE emission, observed for the first time during this campaign, is consistent with the behavior expected from a synchrotron self-Compton scenario. The contemporaneous broadband spectral energy distribution is well described with a one-zone synchrotron self-Compton model using parameters that are comparable to those found for other gamma-ray-emitting misaligned blazars.
[ADS] [ArXiV] [Journal]
21. MAGIC detection of very high energy gamma-ray emission from the low-luminosity blazar 1ES 1741+196
Ahnen M. L., Ansoldi S., .. (MAGIC Coll.), …, Krauß F., et al. 2017,
MNRAS 468, 1534-1541.
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We present the first detection of the nearby (z=0.084) low-luminosity BL Lac object 1ES 1741+196 in the very high energy (VHE: E>100 GeV) band. This object lies in a triplet of interacting galaxies. Early predictions had suggested 1ES 1741+196 to be, along with several other high-frequency BL Lac sources, within the reach of MAGIC detectability. Its detection by MAGIC, later confirmed by VERITAS, helps to expand the small population of known TeV BL Lacs. The source was observed with the MAGIC telescopes between 2010 April and 2011 May, collecting 46 h of good quality data. These observations led to the detection of the source at 6.0 σ confidence level, with a steady flux F(>100GeV)=(6.4±1.7stat±2.6syst)⋅10−12 ph cm−2 s−1 and a differential spectral photon index Γ=2.4±0.2stat±0.2syst in the range of ∼80 GeV – 3 TeV. To study the broad-band spectral energy distribution (SED) simultaneous with MAGIC observations, we use KVA, Swift/UVOT and XRT, and Fermi/LAT data. One-zone synchrotron-self-Compton (SSC) modeling of the SED of 1ES 1741+196 suggests values for the SSC parameters that are quite common among known TeV BL Lacs except for a relatively low Doppler factor and slope of electron energy distribution. A thermal feature seen in the SED is well matched by a giant elliptical’s template. This appears to be the signature of thermal emission from the host galaxy, which is clearly resolved in optical observations.
[ADS] [ArXiV][Journal]
20. Multiband Observations of the Quasar PKS 2326–502 during Active and Quiescent Gamma-Ray States in 2010-2012
Dutka M. S., Carpenter B. D., Ojha R., …, Krauß F., et al. 2017,
ApJ 853, 182.
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Quasi-simultaneous observations of the Flat Spectrum Radio Quasar PKS 2326–502 were carried out in the gamma-ray, X-ray, UV, optical, near-infrared, and radio bands. Thanks to these observations we are able to characterize the spectral energy distribution of the source during two flaring and one quiescent gamma-ray states. These data were used to constrain one-zone leptonic models of the spectral energy distributions of each flare and investigate the physical conditions giving rise to them. While modeling one flare only required changes to the electron spectrum, the other flare needed changes in both the electron spectrum and the size of the emitting region with respect to the quiescent state. These results are consistent with an emerging pattern of two broad classes of flaring states seen in blazars.
[ADS] [Journal] [ArXiV]
19. The MHz-peaked radio spectrum of the unusual gamma-ray source PMN J1603–4904
Müller C., Burd P. R., Schulz R., …, Krauß F., et al. 2016,
A&A 593, L19.
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Context. The majority of bright extragalactic gamma-ray sources are blazars. Only a few radio galaxies have been detected by Fermi/LAT. Recently, the GHz-peaked spectrum source PKS 1718–649 was confirmed to be gamma-ray bright, providing further evidence for the existence of a population of gamma-ray loud, compact radio galaxies. A spectral turnover in the radio spectrum in the MHz to GHz range is a characteristic feature of these objects, which are thought to be young due to their small linear sizes. The multiwavelength properties of the gamma-ray source PMN J1603–4904 suggest that it is a member of this source class.
Aims. The known radio spectrum of PMN J1603–4904 can be described by a power law above 1 GHz. Using observations from the Giant Metrewave Radio Telescope (GMRT) at 150, 325, and 610 MHz, we investigate the behaviour of the spectrum at lower frequencies to search for a low-frequency turnover.
Methods. Data from the TIFR GMRT Sky Survey (TGSS ADR) catalogue and archival GMRT observations were used to construct the first MHz to GHz spectrum of PMN J1603–4904. Results. We detect a low-frequency turnover of the spectrum and measure the peak position at about 490 MHz (rest-frame), which, using the known relation of peak frequency and linear size, translates into a maximum linear source size of ~1.4 kpc.
Conclusions. The detection of the MHz peak indicates that PMN J1603–4904 is part of this population of radio galaxies with turnover frequencies in the MHz to GHz regime. Therefore it can be considered the second, confirmed object of this kind detected in gamma-rays. Establishing this gamma-ray source class will help to investigate the gamma-ray production sites and to test broadband emission models.
[ADS] [Journal] [ArXiV]
18. The TANAMI Multiwavelength Program: Dynamic SEDs of Southern Blazars
Krauß F., Wilms J., Kadler M. et al. 2016,
A&A 591, A130.
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Simultaneous broadband spectral and temporal studies of blazars are an important tool for investigating active galactic nuclei (AGN) jet physics. We study the spectral evolution between quiescent and flaring periods of 22 radio-loud AGN through multi-epoch, quasi-simultaneous broadband spectra. For many of these sources these are the first broadband studies. We use a Bayesian block analysis of Fermi/LAT light curves in order to determine time ranges of constant flux for constructing quasi-simultaneous SEDs. The shapes of the resulting 81 SEDs are described by two logarithmic parabolas and a blackbody spectrum where needed. For low states the peak frequencies and luminosities agree well with the blazar sequence, higher luminosity implying lower peak frequencies. This is not true for sources in a high state. The γ-ray photon index in Fermi/LAT correlates with the synchrotron peak frequency in low and intermediate states. No correlation is present in high states. The black hole mass cannot be determined from the SEDs. Surprisingly, the thermal excess often found in FSRQs at optical/UV wavelengths can be described by blackbody emission and not an accretion disk spectrum. The “harder-when-brighter” trend, typically seen in X-ray spectra of flaring blazars, is visible in the blazar sequence. Our results for low and intermediate states, as well as the Compton dominance, are in agreement with previous results. Black hole mass estimates using the parameters from Bonchi (2013) are in agreement with some of the more direct measurements. For two sources, estimates disagree by more than four orders of magnitude, possibly due to boosting effects. The shapes of the thermal excess seen predominantly in flat spectrum radio quasars are inconsistent with a direct accretion disk origin.
[ADS] [Journal] [ArXiV]
17. A significant hardening and rising shape detected in the MeV/GeV nuFnu spectrum from the recently-discovered very-high-energy blazar S4 0954+65 during the bright optical flare in 2015 February
Tanaka Y. T., Becerra Gonzalez J., Itoh R., …, Krauß F. et al. 2016,
PASJ 68, 51.
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We report on Fermi Large Area Telescope (LAT) and multi-wavelength results on the recently-discovered very-high-energy (VHE, E > 100 GeV) blazar S4 0954+65 (z=0.368) during an exceptionally bright optical flare in 2015 February. During the time period (2015 February, 13/14, or MJD 57067) when the MAGIC telescope detected VHE γ-ray emission from the source, the Fermi-LAT data indicated a significant spectral hardening at GeV energies, with a power-law photon index of 1.8±0.1-compared with the 3FGL value (averaged over four years of observation) of 2.34±0.04. In contrast, Swift/XRT data showed a softening of the X-ray spectrum, with a photon index of 1.72±0.08 (compared with 1.38 ± 0.03 averaged during the flare from MJD 57066 to 57077), possibly indicating a modest contribution of synchrotron photons by the highest-energy electrons superposed on the inverse Compton component. Fitting of the quasi-simultaneous (<1 day) broadband spectrum with a one-zone synchrotron plus inverse- Compton model revealed that GeV/TeV emission could be produced by inverse-Compton scattering of external photons from the dust torus. We emphasize that a flaring blazar showing high flux of ≥ 1.0 × 10-6 photons cm-2 s-1 (E > 100 MeV) and a hard spectral index of ΓGeV ≤ 2.0 detected by Fermi-LAT on daily time scales is a promising target for TeV follow-up by ground-based Cherenkov telescopes to discover high-redshift blazars, investigate their temporal variability and spectral features in the VHE band, and also constrain the intensity of the extragalactic background light.
[ADS] [Journal] [ADS] [ArXiV]
16. NuSTAR and XMM-Newton Observations of the Hard X-Ray Spectrum of Centaurus A
Fürst F., Müller C., Madsen K., …, Krauß F. et al. 2016,
ApJ 819, 150
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We present simultaneous XMM-Newton and Nuclear Spectroscopic Telescope Array (NuSTAR) observations spanning 3-78 keV of the nearest radio galaxy, Centaurus A (Cen A). The accretion geometry around the central engine in Cen A is still debated, and we investigate possible configurations using detailed X-ray spectral modeling. NuSTAR imaged the central region of Cen A with subarcminute resolution at X-ray energies above 10 keV for the first time, but found no evidence for an extended source or other off-nuclear point sources. The XMM-Newton and NuSTAR spectra agree well and can be described with an absorbed power law with a photon index Γ=1.815±0.005 and a fluorescent Fe Kα line in good agreement with literature values. The spectrum does not require a high-energy exponential rollover, with a constraint of Efold > 1 MeV. A thermal Comptonization continuum describes the data well, with parameters that agree with values measured by INTEGRAL, in particular an electron temperature kte between ≈100-300 keV and seed photon input temperatures between 5 and 50 eV. We do not find evidence for reflection or a broad iron line and put stringent upper limits of R ≤ 0.01 on the reflection fraction and accretion disk illumination. We use archival Chandra data to estimate the contribution from diffuse emission, extra-nuclear point sources, and the outer X-ray jet to the observed NuSTAR and XMM-Newton X-ray spectra and find the contribution to be negligible. We discuss different scenarios for the physical origin of the observed hard X-ray spectrum and conclude that the inner disk is replaced by an advection-dominated accretion flow or that the X-rays are dominated by synchroton self-Compton emission from the inner regions of the radio jet or a combination thereof.
[ADS] [Journal] [ArXiV]
15. Coincidence of a high-fluence blazar outburst with a PeV-energy neutrino event
Kadler M., Krauß F., Mannheim K. et al. 2016,
Nature Physics 12, 807.
 Toggle abstract
Context. The Fermi/LAT instrument has detected about two thousand extragalactic high energy (E ≥ 100 MeV) γ-ray sources. One of the brightest is 3FGL J1603.9-4903; it is associated to the radio source PMN J1603-4904. Its nature is not yet clear, it could be either a very peculiar BL Lac or a compact symmetric object radio source which are considered as the early stage of a radio galaxy. The latter, if confirmed, would be the first detection in γ-rays for this class of objects. A redshift z = 0.18 ± 0.01 has recently been claimed on the basis of the detection of a single X-ray line at 5.44 ± 0.05 keV which has been interpreted as a 6.4 keV (rest frame) fluorescent line. Aims. We aim to investigate the nature of 3FGL J1603.9-4903/PMN J1603-4904 using optical-to near-IR (NIR) spectroscopy. Methods. We observed PMN J1603-4904 with the UV-NIR VLT/X-Shooter spectrograph for two hours. We extracted spectra in the visible and NIR range that we calibrated in flux and corrected for telluric absorption. We systematically searched for absorption and emission features. Results. The source was detected starting from ~6300 Å down to 24 000 Å with an intensity similar to that of its 2MASS counterpart and a mostly featureless spectrum. The continuum lacks absorption features and thus is non-stellar in origin and most likely non-thermal. In addition to this spectrum, we detected three emission lines that we interpret as the Hα-[NII] complex, the [SII]λ,λ6716, 6731 doublet and the [SIII]λ 9530 line; we obtain a redshift estimate of z = 0.2321 ± 0.0004. The line ratios suggest that a LINER/Seyfert nucleus powers the emission. This new redshift measurement implies that the X-ray line previously detected should be interpreted as a 6.7 keV line which is very peculiar.
[ADS] [Journal] [ArXiV]
14. Optical-NIR spectroscopy of the puzzling γ-ray source 3FGL 1603.9-4903/PMN J1603-4904 with X-Shooter
Goldoni P., Pita S., Boisson C., …, Krauß F., et al. 2016,
A&A 586, L2
 Toggle abstract

Context. The Fermi/LAT instrument has detected about two thousand extragalactic high energy (E ≥ 100 MeV) γ-ray sources. One of the brightest is 3FGL J1603.9-4903; it is associated to the radio source PMN J1603-4904. Its nature is not yet clear, it could be either a very peculiar BL Lac or a compact symmetric object radio source which are considered as the early stage of a radio galaxy. The latter, if confirmed, would be the first detection in γ-rays for this class of objects. A redshift z = 0.18 ± 0.01 has recently been claimed on the basis of the detection of a single X-ray line at 5.44 ± 0.05 keV which has been interpreted as a 6.4 keV (rest frame) fluorescent line.
Aims. We aim to investigate the nature of 3FGL J1603.9-4903/PMN J1603-4904 using optical-to near-IR (NIR) spectroscopy.
Methods. We observed PMN J1603-4904 with the UV-NIR VLT/X-Shooter spectrograph for two hours. We extracted spectra in the visible and NIR range that we calibrated in flux and corrected for telluric absorption. We systematically searched for absorption and emission features.
Results. The source was detected starting from ∼6300 Å down to 24 000 Å with an intensity similar to that of its 2MASS counterpart and a mostly featureless spectrum. The continuum lacks absorption features and thus is non-stellar in origin and most likely non-thermal. In addition to this spectrum, we detected three emission lines that we interpret as the Hα-[NII] complex, the [SII]λ,λ6716, 6731 doublet and the [SIII]λ 9530 line; we obtain a redshift estimate of z = 0.2321 ± 0.0004. The line ratios suggest that a LINER/Seyfert nucleus powers the emission. This new redshift measurement implies that the X-ray line previously detected should be interpreted as a 6.7 keV line which is very peculiar.
[ADS] [Journal] [ArXiV]
13. Radio and Gamma-ray Properties of Extragalactic Jets from the TANAMI Sample
Böck M., Kadler M., Müller C., …, Krauß F., et al. 2016,
A&A 590, A40
 Toggle abstract
Using high-resolution radio imaging with VLBI techniques, the TANAMI program has been observing the parsec-scale radio jets of southern (declination south of −30° ) γ-ray bright AGN simultaneously with Fermi /LAT monitoring of their γ-ray emission. We present the radio and γ-ray properties of the TANAMI sources based on one year of contemporaneous TANAMI and Fermi /LAT data. A large fraction (72%) of the TANAMI sample can be associated with bright γ-ray sources for this time range. Association rates differ for different optical classes with all BL Lacs, 76% of quasars and just 17% of galaxies detected by the LAT. Upper limits were established on the γ-ray flux from TANAMI sources not detected by LAT. This analysis led to the identification of three new Fermi sources whose detection was later confirmed. The γ-ray and radio luminosities are related by Lγ ∝ Lr 0.89±0.04 . The brightness temperatures of the radio cores increase with the average γ-ray luminosity, and the presence of brightness temperatures above the inverse Compton limit implies strong Doppler boosting in those sources. The undetected sources have lower γ/radio luminosity ratios and lower contemporaneous brightness temperatures. Unless the Fermi /LAT-undetected blazars are strongly γ-ray-fainter than the Fermi /LAT-detected ones, their γ-ray luminosity should not be significantly lower than the upper limits calculated here.
[ADS] [Journal] [ArXiV]
12. The Third Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope
Ackermann M., Ajello M., Atwood W., …, Krauß F., et al.
2015, ApJ 810, 14A
 Toggle abstract
The third catalog of active galactic nuclei (AGNs) detected by the Fermi-LAT (3LAC) is presented. It is based on the third Fermi-LAT catalog (3FGL) of sources detected between 100 MeV and 300 GeV with a Test Statistic greater than 25, between 2008 August 4 and 2012 July 31. The 3LAC includes 1591 AGNs located at high Galactic latitudes (| b| > 10° ), a 71% increase over the second catalog based on 2 years of data. There are 28 duplicate associations, thus 1563 of the 2192 high-latitude gamma-ray sources of the 3FGL catalog are AGNs. Most of them (98%) are blazars. About half of the newly detected blazars are of unknown type, i.e., they lack spectroscopic information of sufficient quality to determine the strength of their emission lines. Based on their gamma-ray spectral properties, these sources are evenly split between flat-spectrum radio quasars (FSRQs) and BL Lacs. The most abundant detected BL Lacs are of the high-synchrotron-peaked (HSP) type. About 50% of the BL Lacs have no measured redshifts. A few new rare outliers (HSP-FSRQs and high-luminosity HSP BL Lacs) are reported. The general properties of the 3LAC sample confirm previous findings from earlier catalogs. The fraction of 3LAC blazars in the total population of blazars listed in BZCAT remains non-negligible even at the faint ends of the BZCAT-blazar radio, optical, and X-ray flux distributions, which hints that even the faintest known blazars could eventually shine in gamma-rays at LAT-detection levels. The energy-flux distributions of the different blazar populations are in good agreement with extrapolation from earlier catalogs.
[ADS] [Journal] [ArXiV]
11. A Variable-Density Absorption Event in NGC 3227 mapped with Suzaku and Swift
Beuchert T., Markowitz A., Krauß F., et al. 2015,
A&A 584, A82.
 Toggle abstract
The morphology of the circumnuclear gas accreting onto supermassive black holes in Seyfert galaxies remains a topic of much debate. As the innermost regions of Active Galactic Nuclei (AGN) are spatially unresolved, X-ray spectroscopy, and in particular line-of-sight absorption variability, is a key diagnostic to map out the distribution of gas. Observations of variable X-ray absorption in multiple Seyferts and over a wide range of timescales indicate the presence of clumps/clouds of gas within the circumnuclear material. Eclipse events by clumps transiting the line of sight allow us to explore the properties of the clumps over a wide range of radial distances from the optical/UV Broad Line Region (BLR) to beyond the dust sublimation radius. Time-resolved absorption events have been extremely rare so far, but suggest a range of density profiles across Seyferts. We resolve a weeks-long absorption event in the Seyfert NGC 3227. We examine six Suzaku and twelve Swift observations from a 2008 campaign spanning 5 weeks. We use a model accounting for the complex spectral interplay of three differently-ionized absorbers. We perform time-resolved spectroscopy to discern the absorption variability behavior. We also examine the IR-to-X-ray spectral energy distribution (SED) to test for reddening by dust. The 2008 absorption event is due to moderately-ionized (log ξ∼1.2−1.4) gas covering 90% of the line of sight. We resolve the density profile to be highly irregular, in contrast to a previous symmetric and centrally-peaked event mapped with RXTE in the same object. The UV data do not show significant reddening, suggesting that the cloud is dust-free. The 2008 campaign has revealed a transit by a filamentary, moderately-ionized cloud of variable density that is likely located in the BLR, and possibly part of a disk wind.
[ADS] [Journal] [ArXiV]
10. PSR J1906+0722: An elusive gamma-ray pulsar
Clark C.J., Pletsch H.J., …, Krauß F., et al. 2015,
ApJ 809, L2
 Toggle abstract
We report the discovery of PSR J1906+0722, a gamma-ray pulsar detected as part of a blind survey of unidentified Fermi Large Area Telescope (LAT) sources being carried out on the volunteer distributed computing system, Einstein@Home. This newly discovered pulsar previously appeared as the most significant remaining unidentified gamma-ray source without a known association in the second Fermi-LAT source catalog (2FGL) and was among the top ten most significant unassociated sources in the recent third catalog (3FGL). PSR J1906+0722 is a young, energetic, isolated pulsar, with a spin frequency of 8.9 Hz, a characteristic age of 49 kyr, and spin-down power 1.0×1036 erg s-1. In 2009 August it suffered one of the largest glitches detected from a gamma-ray pulsar (Δf/f≈4.5×10−6). Remaining undetected in dedicated radio follow-up observations, the pulsar is likely radio-quiet. An off-pulse analysis of the gamma-ray flux from the location of PSR J1906+0722 revealed the presence of an additional nearby source, which may be emission from the interaction between a neighboring supernova remnant and a molecular cloud. We discuss possible effects which may have hindered the detection of PSR J1906+0722 in previous searches and describe the methods by which these effects were mitigated in this survey. We also demonstrate the use of advanced timing methods for estimating the positional, spin and glitch parameters of difficult-to-time pulsars such as this.
[ADS] [Journal] [ArXiV]
9. ANTARES constrains a blazar origin of two IceCube PeV neutrino events
Adrián-Martínez S. Albert A., …, Krauß F. et al. 2015,
A&A 576, L8
 Toggle abstract
Context. The source(s) of the neutrino excess reported by the IceCube Collaboration is unknown. The TANAMI Collaboration recently reported on the multiwavelength emission of six bright, variable blazars which are positionally coincident with two of the most energetic IceCube events. Objects like these are prime candidates to be the source of the highest-energy cosmic rays, and thus of associated neutrino emission.
Aims. We present an analysis of neutrino emission from the six blazars using observations with the ANTARES neutrino telescope.
Methods. The standard methods of the ANTARES candidate list search are applied to six years of data to search for an excess of muons – and hence their neutrino progenitors – from the directions of the six blazars described by the TANAMI Collaboration, and which are possibly associated with two IceCube events. Monte Carlo simulations of the detector response to both signal and background particle fluxes are used to estimate the sensitivity of this analysis for different possible source neutrino spectra. A maximum-likelihood approach, using the reconstructed energies and arrival directions of through-going muons, is used to identify events with properties consistent with a blazar origin.
Results. Both blazars predicted to be the most neutrino-bright in the TANAMI sample (1653−329 and 1714−336) have a signal flux fitted by the likelihood analysis corresponding to approximately one event. This observation is consistent with the blazar-origin hypothesis of the IceCube event IC 14 for a broad range of blazar spectra, although an atmospheric origin cannot be excluded. No ANTARES events are observed from any of the other four blazars, including the three associated with IceCube event IC20. This excludes at a 90% confidence level the possibility that this event was produced by these blazars unless the neutrino spectrum is flatter than −2.4.
[ADS] [Journal] [ArXiV]
8. 5.9 keV Mn K-shell X-ray luminosity from the decay of 55Fe in Type Ia supernova models
Seitenzahl I.R., Summa A., Krauß F. et al. 2015,
MNRAS 447, 1484
 Toggle abstract
We show that the X-ray line flux of the Mn Kα line at 5.9 keV from the decay of 55Fe is a promising diagnostic to distinguish between Type Ia supernova (SN Ia) explosion models. Using radiation transport calculations, we compute the line flux for two three-dimensional explosion models: a near-Chandrasekhar mass delayed detonation and a violent merger of two (1.1 and 0.9 M) white dwarfs. Both models are based on solar metallicity zero-age main-sequence progenitors. Due to explosive nuclear burning at higher density, the delayed-detonation model synthesizes ∼3.5 times more radioactive 55Fe than the merger model. As a result, we find that the peak Mn Kα line flux of the delayed-detonation model exceeds that of the merger model by a factor of ∼4.5. Since in both models the 5.9-keV X-ray flux peaks five to six years after the explosion, a single measurement of the X-ray line emission at this time can place a constraint on the explosion physics that is complementary to those derived from earlier phase optical spectra or light curves. We perform detector simulations of current and future X-ray telescopes to investigate the possibilities of detecting the X-ray line at 5.9 keV. Of the currently existing telescopes, XMM–Newton/pn is the best instrument for close (≲1–2 Mpc), non-background limited SNe Ia because of its large effective area. Due to its low instrumental background, Chandra/ACIS is currently the best choice for SNe Ia at distances above ∼2 Mpc. For the delayed-detonation scenario, a line detection is feasible with Chandra up to ∼3 Mpc for an exposure time of 106 s. We find that it should be possible with currently existing X-ray instruments (with exposure times ≲5 × 105 s) to detect both of our models at sufficiently high S/N to distinguish between them for hypothetical events within the Local Group. The prospects for detection will be better with future missions. For example, the proposed Athena/X-IFU instrument could detect our delayed-detonation model out to a distance of ∼5 Mpc. This would make it possible to study future events occurring during its operational life at distances comparable to those of the recent supernovae SN 2011fe (∼6.4 Mpc) and SN 2014J (∼3.5 Mpc).
[ADS] [Journal] [arXiV]
7. Redshifted Fe Kα line from the unusual γ-ray source PMN J1603–4904
Müller C., Krauß F. et al. 2015,
A&A 574, A117
 Toggle abstract
Multiwavelength observations have revealed the highly unusual properties of the gamma-ray source PMN J1603-4904, which are difficult to reconcile with any other well established gamma-ray source class. The object is either a very atypical blazar or compact jet source seen at a larger angle to the line of sight. In order to determine the physical origin of the high-energy emission processes in PMN J1603-4904, we study the X-ray spectrum in detail. We performed quasi-simultaneous X-ray observations with XMM-Newton and Suzaku in 2013 September, resulting in the first high signal-to-noise X-ray spectrum of this source. The 2-10 keV X-ray spectrum can be well described by an absorbed power law with an emission line at 5.44±0.05 keV (observed frame). Interpreting this feature as a Kα line from neutral iron, we determine the redshift of PMN J1603-4904 to be z=0.18±0.01, corresponding to a luminosity distance of 872±54 Mpc. The detection of a redshifted X-ray emission line further challenges the original BL Lac classification of PMN J1603-4904. This result suggests that the source is observed at a larger angle to the line of sight than expected for blazars, and thus the source would add to the elusive class of gamma-ray loud misaligned-jet objects, possibly a γ-ray bright young radio galaxy.
[ADS] [Journal] [arXiV]
6. Black hole lightning due to particle acceleration at subhorizon scales
Aleksić J., …, Krauß F. et al. 2014,
Science 346, 1080
 Toggle abstract
Supermassive black holes with masses of millions to billions of solar masses are commonly found in the centers of galaxies. Astronomers seek to image jet formation using radio interferometry but still suffer from insufficient angular resolution. An alternative method to resolve small structures is to measure the time variability of their emission. Here we report on gamma-ray observations of the radio galaxy IC 310 obtained with the MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) telescopes, revealing variability with doubling time scales faster than 4.8 min. Causality constrains the size of the emission region to be smaller than 20% of the gravitational radius of its central black hole. We suggest that the emission is associated with pulsar-like particle acceleration by the electric field across a magnetospheric gap at the base of the radio jet.
[ADS] [Journal] [arXiV]
5. TANAMI blazars in the IceCube PeV-neutrino fields
Krauß F. et al. 2014,
A&A 566, L7
 Toggle abstract
The IceCube Collaboration has announced the discovery of a neutrino flux in excess of the atmospheric background. Owing to the steeply falling atmospheric background spectrum, events at PeV energies most likely have an extraterrestrial origin. We present the multiwavelength properties of the six radio-brightest blazars that are positionally coincident with these events using contemporaneous data of the TANAMI blazar sample, including high-resolution images and spectral energy distributions. Assuming the X-ray to γ-ray emission originates in the photoproduction of pions by accelerated protons, the integrated predicted neutrino luminosity of these sources is high enough to explain the two detected PeV events.
[ADS] [Journal] [arXiV]
4. Rapid and multiband variability of the TeV bright active nucleus of the galaxy IC 310
Aleksić J., …, Krauss F. et al. 2014,
A&A 563, A91
 Toggle abstract
Context. The radio galaxy IC 310 has recently been identified as a gamma-ray emitter based on observations at GeV energies with Fermi-LAT and at very high energies (VHE, E > 100 GeV) with the MAGIC telescopes. Originally classified as a head-tail radio galaxy, the nature of this object is subject of controversy since its nucleus shows blazar-like behavior.
Aims: To understand the nature of IC 310 and the origin of the VHE emission, we studied the spectral and flux variability of IC 310 from the X-ray band to the VHE γ-ray regime.
Methods: The light curve of IC 310 above 300 GeV has been measured with the MAGIC telescopes from 2009 October to 2010 February. Contemporaneous Fermi-LAT data (2008-2011) in the 10-500 GeV energy range were also analyzed. In the X-ray regime, archival observations from 2003 to 2007 with XMM-Newton, Chandra, and Swift-XRT in the 0.5-10 keV band were studied.
Results: The VHE light curve reveals several high-amplitude and short-duration flares. Day-to-day flux variability is clearly present (>5σ). The photon index between 120 GeV and 8 TeV remains at the value Γ ~ 2.0 during both low and high flux states. The VHE spectral shape does not show significant variability, whereas the flux at 1 TeV changes by a factor of ~7. Fermi-LAT detected only eight gamma-ray events in the energy range 10 GeV-500 GeV in three years of observation. The measured photon index of Γ = 1.3 ± 0.5 in the Fermi-LAT range is very hard. The X-ray measurements show strong variability in both flux and photon index. The latter varied from 1.76 ± 0.07 to 2.55 ± 0.07.
Conclusions: The rapid variability measured in γ-rays and X-rays confirms the blazar-like behavior of IC 310. The multi-TeV γ-ray emission seems to originate from scales of less than 80 Schwarzschild radii (for a black hole mass of 2 × 108 M) within the compact core of its FR I radio jet with orientation angle 10°-38°. The spectral energy distribution resembles that of an extreme blazar, albeit the luminosity is more than two orders of magnitude lower.
[ADS] [Journal] [arXiV]
3. The unusual multiwavelength properties of the gamma-ray source PMN J1603-4904
Müller C., …, Krauss F. et al. 2014,
A&A 562, A4
 Toggle abstract
Context. We investigate the nature and classification of PMN J1603−4904, a bright radio source close to the Galactic plane, which is associated with one of the brightest hard-spectrum γ-ray sources detected by Fermi/LAT. It has previously been classified as a low-peaked BL Lac object based on its broadband emission and the absence of optical emission lines. Optical measurements, however, suffer strongly from extinction and the absence of pronounced short-time γ-ray variability over years of monitoring is unusual for a blazar.
Aims. In this paper, we are combining new and archival multiwavelength data of PMN J1603−4904 in order to reconsider the classification and nature of this unusual γ-ray source.
Methods. For the first time, we study the radio morphology of PMN J1603−4904 at 8.4 GHz and 22.3 GHz, and its spectral properties on milliarcsecond scales, based on VLBI observations from the TANAMI program. We combine the resulting images with multiwavelength data in the radio, IR, optical/UV, X-ray, and γ-ray regimes.
Results. PMN J1603−4904 shows a symmetric brightness distribution at 8.4 GHz on milliarcsecond scales, with the brightest, and most compact component in the center of the emission region. The morphology is reminiscent of a compact symmetric object (CSO). Such objects, thought to be young radio galaxies, have been predicted to produce γ-ray emission but have not been detected as a class by the Fermi γ-ray telescope so far. Sparse (u,v)-coverage at 22.3 GHz prevents an unambiguous modeling of the source morphology at this higher frequency. Moreover, infrared measurements reveal an excess in the spectral energy distribution (SED), which can be modeled with a blackbody with a temperature of about 1600 K, and which is usually not present in blazar SEDs.
Conclusions. The TANAMI VLBI data and the shape of the broadband SED challenge the current blazar classification of one of the brightest γ-ray sources in the sky. PMN J1603−4904 seems to be either a highly peculiar BL Lac object or a misaligned jet source. In the latter case, the intriguing VLBI structure opens room for a possible classification of PMN J1603−4904 as a γ-ray bright CSO.
[ADS] [Journal] [arXiV]
2. Multi-wavelength Observations of PKS 2142-75 during Active and Quiescent Gamma-Ray States
Dutka M., …, Krauss F. et al. 2013,
ApJ 779, 174
 Toggle abstract
PKS 2142-75 (a.k.a. 2FGL J2147.4-7534) is a flat-spectrum radio quasar that was observed quasi-simultaneously by a suite of instruments across the electromagnetic spectrum during two flaring states in 2010 April and 2011 August as well as a quiescent state from 2011 December through 2012 January. The results of these campaigns and model spectral energy distributions (SEDs) from the active and quiescent states are presented. The SED model parameters of PKS 2142-75 indicate that the two flares of the source are created by unique physical conditions. SED studies of flat-spectrum radio quasars are beginning to indicate that there might be two types of flares, those that can be described purely by changes in the electron distribution and those that require changes in other parameters, such as the magnetic field strength or the size of the emitting region.
[ADS] [Journal] [arXiV]
1. The simultaneous low state spectral energy distribution of 1ES 2344+514 from radio to very high energies
Aleksić J., …, Krauss F. et al. 2013,
A&A 556, 67
 Toggle abstract
Context. BL Lacertae objects are variable at all energy bands on time scales down to minutes. To construct and interpret their spectral energy distribution (SED), simultaneous broad-band observations are mandatory. Up to now, the number of objects studied during such campaigns is very limited and biased towards high flux states.
Aims. We present the results of a dedicated multi-wavelength study of the high-frequency peaked BL Lacertae (HBL) object and known TeV emitter 1ES 2344+514 by means of a pre-organised campaign.
Methods. The observations were conducted during simultaneous visibility windows of MAGIC and AGILE in late 2008. The measurements were complemented by Metsähovi, RATAN-600, KVA+Tuorla, Swift and VLBA pointings. Additional coverage was provided by the ongoing long-term F-GAMMA and MOJAVE programs, the OVRO 40-m and CrAO telescopes as well as the Fermi satellite. The obtained SEDs are modelled using a one-zone as well as a self-consistent two-zone synchrotron self-Compton model.
Results. 1ES 2344+514 was found at very low flux states in both X-rays and very high energy gamma rays. Variability was detected in the low frequency radio and X-ray bands only, where for the latter a small flare was observed. The X-ray flare was possibly caused by shock acceleration characterised by similar cooling and acceleration time scales. MOJAVE VLBA monitoring reveals a static jet whose components are stable over time scales of eleven years, contrary to previous findings. There appears to be no significant correlation between the 15 GHz and R-band monitoring light curves. The observations presented here constitute the first multi-wavelength campaign on 1ES 2344+514 from radio to VHE energies and one of the few simultaneous SEDs during low activity states. The quasi-simultaneous Fermi-LAT data poses some challenges for SED modelling, but in general the SEDs are described well by both applied models. The resulting parameters are typical for TeV emitting HBLs. Consequently it remains unclear whether a so-called quiescent state was found in this campaign.
[ADS] [Journal] [arXiV]

Updated: July 2020