35th International Cosmic Ray Conference, ICRC 2017

PKS 1510-089 (z=0.361) is one of only a handful of flat spectrum radio quasars that have been detected at very high energy (VHE, E > 100GeV) gamma rays. It is a very active source across the entire electromagnetic spectrum. VHE observations in May 2016 with H.E.S.S. and MAGIC revealed an exceptionally strong flare, which lasted for less than two nights, and exhibited a peak flux of about 0.8 times the flux of the Crab Nebula above 200GeV. The flare provides the first evidence of intranight variability at VHE in this source. While optical observations with ATOM reveal a counterpart at optical frequencies, Fermi-LAT observations reveal only low flux variability at high energy (HE, E > 100 MeV) gamma rays. Interestingly, the HE spectral index significantly hardens during the peak of the VHE flare, indicating a strong shift of the peak frequency of the high energy component. Given the expected strong absorption due to the broad-line region, the VHE emission region cannot be located deep within that region.</p

Broadband Multi-wavelength Properties of M87 during the 2017 Event Horizon Telescope Campaign

Abstract: In 2017, the Event Horizon Telescope (EHT) Collaboration succeeded in capturing the first direct image of the center of the M87 galaxy. The asymmetric ring morphology and size are consistent with theoretical expectations for a weakly accreting supermassive black hole of mass ∼6.5 × 109 M ⊙. The EHTC also partnered with several international facilities in space and on the ground, to arrange an extensive, quasi-simultaneous multi-wavelength campaign. This Letter presents the results and analysis of this campaign, as well as the multi-wavelength data as a legacy data repository. We captured M87 in a historically low state, and the core flux dominates over HST-1 at high energies, making it possible to combine core flux constraints with the more spatially precise very long baseline interferometry data. We present the most complete simultaneous multi-wavelength spectrum of the active nucleus to date, and discuss the complexity and caveats of combining data from different spatial scales into one broadband spectrum. We apply two heuristic, isotropic leptonic single-zone models to provide insight into the basic source properties, but conclude that a structured jet is necessary to explain M87’s spectrum. We can exclude that the simultaneous γ-ray emission is produced via inverse Compton emission in the same region producing the EHT mm-band emission, and further conclude that the γ-rays can only be produced in the inner jets (inward of HST-1) if there are strongly particle-dominated regions. Direct synchrotron emission from accelerated protons and secondaries cannot yet be excluded

Weather and atmosphere observation with the ATOM all-sky camera

The Automatic Telescope for Optical Monitoring (ATOM) for H.E.S.S. is an 75 cm optical telescope which operates fully automated. As there is no observer present during observation, an auxiliary all-sky camera serves as weather monitoring system. This device takes an all-sky image of the whole sky every three minutes. The gathered data then undergoes live-analysis by performing astrometric comparison with a theoretical night sky model, interpreting the absence of stars as cloud coverage. The sky monitor also serves as tool for a meteorological analysis of the observation site of the the upcoming Cherenkov Telescope Array. This overview covers design and benefits of the all-sky camera and additionally gives an introduction into current efforts to integrate the device into the atmosphere analysis programme of H.E.S.S

Unraveling The Complex Nature Of The Very High-Energy γ\gamma-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 gamma-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 2155-30

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

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

International audienceThe 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

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

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

International audienceLong-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 $z=1.032$