Aims. We study the multi-band variability and correlations of the TeV blazar Mrk 421 on year timescales, which can bring additional insight on the processes responsible for its broadband emission. ...Methods. We observed Mrk 421 in the very high energy (VHE) γ-ray range with the Cherenkov telescope MAGIC-I from March 2007 to June 2009 for a total of 96 h of effective time after quality cuts. The VHE flux variability is quantified using several methods, including the Bayesian Block algorithm, which is applied to data from Cherenkov telescopes here for the first time. The 2.3 yr long MAGIC light curve is complemented with data from the Swift/BAT and RXTE/ASM satellites and the KVA, GASP-WEBT, OVRO, and Metsähovi telescopes from February 2007 to July 2009, allowing for an excellent characterisation of the multi-band variability and correlations over year timescales. Results. Mrk 421 was found in different γ-ray emission states during the 2.3 yr long observation period: The flux above 400 GeV spans from the minimum nightly value of (1.3 ± 0.4)×10-11 cm-2 s-1 to the maximum flux, that is about 24 times higher, at (3.1 ± 0.1)×10-10 cm-2 s-1. Flares and different levels of variability in the γ-ray light curve could be identified with the Bayesian Block algorithm. The same behaviour of a quiet and active emission was found in the X-ray light curves measured by Swift/BAT and the RXTE/ASM, with a direct correlation in time. The behaviour of the optical light curve of GASP-WEBT and the radio light curves by OVRO and Metsähovi are different as they show no coincident features with the higher energetic light curves and a less variable emission. Overall, the fractional variability increases with energy. The comparable variability in the X-ray and VHE bands and their direct correlation during both high- and low-activity periods spanning many months show that the electron populations radiating the X-ray and γ-ray photons are either the same, as expected in the synchrotron-self-Compton mechanism, or at least strongly correlated, as expected in electromagnetic cascades.
ABSTRACT We report on simultaneous broadband observations of the TeV-emitting blazar Markarian 501 between 2013 April 1 and August 10, including the first detailed characterization of the synchrotron ...peak with Swift and NuSTAR. During the campaign, the nearby BL Lac object was observed in both a quiescent and an elevated state. The broadband campaign includes observations with NuSTAR, MAGIC, VERITAS, the Fermi Large Area Telescope, Swift X-ray Telescope and UV Optical Telescope, various ground-based optical instruments, including the GASP-WEBT program, as well as radio observations by OVRO, Metsähovi, and the F-Gamma consortium. Some of the MAGIC observations were affected by a sand layer from the Saharan desert, and had to be corrected using event-by-event corrections derived with a Light Detection and Ranging (LIDAR) facility. This is the first time that LIDAR information is used to produce a physics result with Cherenkov Telescope data taken during adverse atmospheric conditions, and hence sets a precedent for the current and future ground-based gamma-ray instruments. The NuSTAR instrument provides unprecedented sensitivity in hard X-rays, showing the source to display a spectral energy distribution (SED) between 3 and 79 keV consistent with a log-parabolic spectrum and hard X-ray variability on hour timescales. None (of the four extended NuSTAR observations) show evidence of the onset of inverse-Compton emission at hard X-ray energies. We apply a single-zone equilibrium synchrotron self-Compton (SSC) model to five simultaneous broadband SEDs. We find that the SSC model can reproduce the observed broadband states through a decrease in the magnetic field strength coinciding with an increase in the luminosity and hardness of the relativistic leptons responsible for the high-energy emission.
A 2.5 m long liquid hydrogen target for COMPASS Bielert, E; Bremer, J; Doshita, N ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
05/2014, Letnik:
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Journal Article
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A 2.5 m long liquid hydrogen target has been developed for the COMPASS experiment at CERN to investigate the nucleon spin structure via the Deeply Virtual Compton Scattering (DVCS) process. To ...recognize exclusive DVCS events, produced photons and slow protons need to be detected. In order to do so, the material budget around the target has to be minimal. A 0.125 mm thick Kapton super( registered )Kapton registered target cell and a 1 mm thick carbon fiber vacuum chamber with a Mylar super( registered )Mylar registered window have been constructed and tested. Finally, the target system was successfully employed during the DVCS pilot run in COMPASS at the end of 2012. The objective of this paper is to give a detailed description of this newly developed liquid hydrogen target apparatus.
The presence of a population of point sources in a data set modifies the underlying neutrino-count statistics from the Poisson distribution. This deviation can be exactly quantified using the ...non-Poissonian template fitting technique, and in this work we present the first application of this approach to the IceCube high-energy neutrino data set. Using this method, we search in 7 yr of IceCube data for point-source populations correlated with the disk of the Milky Way, the Fermi bubbles, the Schlegel, Finkbeiner, and Davis dust map, or with the isotropic extragalactic sky. No evidence for such a population is found in the data using this technique, and in the absence of a signal, we establish constraints on population models with source-count distribution functions that can be described by a power law with a single break. The derived limits can be interpreted in the context of many possible source classes. In order to enhance the flexibility of the results, we publish the full posterior from our analysis, which can be used to establish limits on specific population models that would contribute to the observed IceCube neutrino flux.
The measurement of diffuse PeV gamma-ray emission from the Galactic plane would provide information about the energy spectrum and propagation of Galactic cosmic rays, and the detection of a pointlike ...source of PeV gamma-rays would be strong evidence for a Galactic source capable of accelerating cosmic rays up to at least a few PeV. This paper presents several unbinned maximum-likelihood searches for PeV gamma-rays in the Southern Hemisphere using 5 yr of data from the IceTop air shower surface detector and the in-ice array of the IceCube Observatory. The combination of both detectors takes advantage of the low muon content and deep shower maximum of gamma-ray air showers and provides excellent sensitivity to gamma-rays between ∼0.6 and 100 PeV. Our measurements of pointlike and diffuse Galactic emission of PeV gamma-rays are consistent with the background, so we constrain the angle-integrated diffuse gamma-ray flux from the Galactic plane at 2 PeV to 2.61 × 10−19 cm−2 s−1 TeV−1 at 90% confidence, assuming an E−3 spectrum, and we estimate 90% upper limits on pointlike emission at 2 PeV between 10−21 and 10−20 cm−2 s−1 TeV−1 for an E−2 spectrum, depending on decl. Furthermore, we exclude unbroken power-law emission up to 2 PeV for several TeV gamma-ray sources observed by the High Energy Spectroscopic System and calculate upper limits on the energy cutoffs of these sources at 90% confidence. We also find no PeV gamma-rays correlated with neutrinos from IceCube's high-energy starting event sample. These are currently the strongest constraints on PeV gamma-ray emission.
In order to identify the sources of the observed diffuse high-energy neutrino flux, it is crucial to discover their electromagnetic counterparts. To increase the sensitivity of detecting counterparts ...of transient or variable sources by telescopes with a limited field of view, IceCube began releasing alerts for single high-energy (Eν > 60 TeV) neutrino detections with sky localisation regions of order 1° radius in 2016. We used Pan-STARRS1 to follow-up five of these alerts during 2016–2017 to search for any optical transients that may be related to the neutrinos. Typically 10–20 faint (miP1 ≲ 22.5 mag) extragalactic transients are found within the Pan-STARRS1 footprints and are generally consistent with being unrelated field supernovae (SNe) and AGN. We looked for unusual properties of the detected transients, such as temporal coincidence of explosion epoch with the IceCube timestamp, or other peculiar light curve and physical properties. We found only one transient that had properties worthy of a specific follow-up. In the Pan-STARRS1 imaging for IceCube-160427A (probability to be of astrophysical origin of ∼50%), we found a SN PS16cgx, located at 10.0′ from the nominal IceCube direction. Spectroscopic observations of PS16cgx showed that it was an H-poor SN at redshift z = 0.2895 ± 0.0001. The spectra and light curve resemble some high-energy Type Ic SNe, raising the possibility of a jet driven SN with an explosion epoch temporally coincident with the neutrino detection. However, distinguishing Type Ia and Type Ic SNe at this redshift is notoriously difficult. Based on all available data we conclude that the transient is more likely to be a Type Ia with relatively weak Si II absorption and a fairly normal rest-frame r-band light curve. If, as predicted, there is no high-energy neutrino emission from Type Ia SNe, then PS16cgx must be a random coincidence, and unrelated to the IceCube-160427A. We find no other plausible optical transient for any of the five IceCube events observed down to a 5σ limiting magnitude of miP1 ≈ 22 mag, between 1 day and 25 days after detection.
Context. Blazars are variable sources on various timescales over a broad energy range spanning from radio to very high energy (>100 GeV, hereafter VHE). Mrk 501 is one of the brightest blazars at TeV ...energies and has been extensively studied since its first VHE detection in 1996. However, most of the γ-ray studies performed on Mrk 501 during the past years relate to flaring activity, when the source detection and characterization with the available γ-ray instrumentation was easier toperform. Aims. Our goal is to characterize the source γ-ray emission in detail, together with the radio-to-X-ray emission, during the non-flaring (low) activity, which is less often studied than the occasional flaring (high) activity. Methods. We organized a multiwavelength (MW) campaign on Mrk 501 between March and May 2008. This multi-instrument effort included the most sensitive VHE γ-ray instruments in the northern hemisphere, namely the imaging atmospheric Cherenkov telescopes MAGIC and VERITAS, as well as Swift, RXTE, the F-GAMMA, GASP-WEBT, and other collaborations and instruments. This provided extensive energy and temporal coverage of Mrk 501 throughout the entire campaign. Results. Mrk 501 was found to be in a low state of activity during the campaign, with a VHE flux in the range of 10%–20% of the Crab nebula flux. Nevertheless, significant flux variations were detected with various instruments, with a trend of increasing variability with energy and a tentative correlation between the X-ray and VHE fluxes. The broadband spectral energy distribution during the two different emission states of the campaign can be adequately described within the homogeneous one-zone synchrotron self-Compton model, with the (slightly) higher state described by an increase in the electron number density. Conclusions. The one-zone SSC model can adequately describe the broadband spectral energy distribution of the source during the two months covered by the MW campaign. This agrees with previous studies of the broadband emission of this source during flaring and non-flaring states. We report for the first time a tentative X-ray-to-VHE correlation during such a low VHE activity. Although marginally significant, this positive correlation between X-ray and VHE, which has been reported many times during flaring activity, suggests that the mechanisms that dominate the X-ray/VHE emission during non-flaring-activity are not substantially different from those that are responsible for the emission during flaring activity.
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