842 research outputs found
An Improved Measurement of the Hubble Constant from the Sunyaev-Zeldovich Effect
We present a determination of the Hubble constant from measurements of the
Sunyaev-Zeldovich Effect (SZE) in an orientation-unbiased sample of 7 z < 0.1
galaxy clusters. With improved X-ray models and a more accurate 32-GHz
calibration, we obtain H_O = 64+14-11 +/- 14_sys km/s/Mpc. for a standard CDM
cosmology, or 66+14-11 +/- 15_sys km/s/Mpc for a flat LambdaCDM cosmology. In
combination with X-ray cluster measurements and the BBN value for Omega_B, we
find Omega_M = 0.32 +/- 0.05.Comment: 5 pp., Accepted for publication in ApJ
A New Component in the Radio Continua of PNe
A byproduct of experiments designed to map the CMB is the recent detection of a new component of foreground galactic emission. The anomalous foreground at 10–30 GHz, unexplained by traditional emission mechanisms, correlates with 100 mum dust emission, and is thus presumably due to dust.Is the anomalous foreground ubiquitous in the Galaxy? I will present evidence obtained with the CBI and SIMBA+SEST supporting the existence of the new component in the ISM at large, and in specific objects, in the form of a 31 GHz excess over free-free emission in PNe
MAGIC detection of short-term variability of the high-peaked BL Lac object lES 0806+524
The high-frequency-peaked BL Lac (HBL) 1ES 0806+524 (z = 0.138) was discovered in very high energy (VHE) γ-rays in 2008. Until now, the broad-band spectrum of 1ES 0806+524 has been only poorly characterized, in particular at high energies. We analysed multiwavelength observations from γ-rays to radio performed from 2011 January to March, which were triggered by the high activity detected at optical frequencies. These observations constitute the most precise determination of the broad-band emission of 1ES 0806+524 to date. The stereoscopic Major Atmospheric Gamma-Ray Imaging Cherenkov (MAGIC) observations yielded a γ-ray signal above 250 GeV of (3.7 ± 0.7) per cent of the Crab Nebula flux with a statistical significance of 9.9σ. The multiwavelength observations showed significant variability in essentially all energy bands, including a VHE γ-ray flare that lasted less than one night, which provided unprecedented evidence for short-term variability in 1ES 0806+524. The spectrum of this flare is well described by a power law with a photon index of 2.97 ± 0.29 between ∼150 GeV and 1 TeV and an integral flux of (9.3 ± 1.9) per cent of the Crab nebula flux above 250 GeV. The spectrum during the non-flaring VHE activity is compatible with the only available VHE observation performed in 2008 with VERITAS when the source was in a low optical state. The broad-band spectral energy distribution can be described with a one-zone synchrotron self-Compton model with parameters typical for HBLs, indicating that 1ES 0806+524 is not substantially different from the HBLs previously detected
Simultaneous Radio to (Sub-) mm-Monitoring of Variability and Spectral Shape Evolution of potential GLAST Blazars
The Large Area Telescope (LAT) instrument onboard GLAST offers a tremendous
opportunity for future blazar studies. In order to fully benefit from its
capabilities and to maximize the scientific return from the LAT, it is of great
importance to conduct dedicated multi-frequency monitoring campaigns that will
result comprehensive observations. Consequently, we initiated an effort to
conduct a GLAST-dedicated, quasi-simultaneous, broad-band flux-density (and
polarization) monitoring of potential GLAST blazars with the Effelsberg and
OVRO radio telescopes (11cm to 7mm wavelength). Here, we present a short
overview of these activities which will complement the multi-wavelengths
activities of the GLAST/LAT collaboration towards the 'low-energy' radio bands.
Further we will give a brief outlook including the extension of this
coordinated campaign towards higher frequencies and future scientific aims.Comment: 3 pages, to appear in the Proceedings of the First GLAST Symposium,
Stanford University, February 200
VLBI Survey of a Complete Sample of Active Nuclei and Quasars
We have conducted a VLBI survey of a complete, flux-density limited sample of 65 extragalactic radio sources, selected at 5 GHz. We have made hybrid maps at 5 GHz of all of the sources accessible to the Mark-II system. The sources can be divided provisionally into a number of classes with different properties: central components of extended double sources, steep-spectrum compact sources, very compact (almost unresolved) sources, asymmetric sources (sometimes called “core-jet” sources), and “compact double” sources. It is not yet clear whether any of these classes is physically distinct from the others, or whether there is a continuous range of properties
Rapid TeV Gamma-Ray Flaring of BL Lacertae
We report on the detection of a very rapid TeV gamma-ray flare from BL Lacertae on 2011 June 28 with the Very Energetic Radiation Imaging Telescope Array System (VERITAS). The flaring activity was observed during a 34.6 minute exposure, when the integral flux above 200 GeV reached (3.4 ± 0.6) × 10^(–6) photons m^(–2) s^(–1), roughly 125% of the Crab Nebula flux measured by VERITAS. The light curve indicates that the observations missed the rising phase of the flare but covered a significant portion of the decaying phase. The exponential decay time was determined to be 13 ± 4 minutes, making it one of the most rapid gamma-ray flares seen from a TeV blazar. The gamma-ray spectrum of BL Lacertae during the flare was soft, with a photon index of 3.6 ± 0.4, which is in agreement with the measurement made previously by MAGIC in a lower flaring state. Contemporaneous radio observations of the source with the Very Long Baseline Array revealed the emergence of a new, superluminal component from the core around the time of the TeV gamma-ray flare, accompanied by changes in the optical polarization angle. Changes in flux also appear to have occurred at optical, UV, and GeV gamma-ray wavelengths at the time of the flare, although they are difficult to quantify precisely due to sparse coverage. A strong flare was seen at radio wavelengths roughly four months later, which might be related to the gamma-ray flaring activities. We discuss the implications of these multiwavelength results
Constraining the limiting brightness temperature and Doppler factors for the largest sample of radio bright blazars
Relativistic effects dominate the emission of blazar jets complicating our
understanding of their intrinsic properties. Although many methods have been
proposed to account for them, the variability Doppler factor method has been
shown to describe the blazar populations best. We use a Bayesian hierarchical
code called {\it Magnetron} to model the light curves of 1029 sources observed
by the Owens Valley Radio Observatory's 40-m telescope as a series of flares
with an exponential rise and decay, and estimate their variability brightness
temperature. Our analysis allows us to place the most stringent constraints on
the equipartition brightness temperature i.e., the maximum achieved intrinsic
brightness temperature in beamed sources which we found to be . Using our findings we estimated the
variability Doppler factor for the largest sample of blazars increasing the
number of available estimates in the literature by almost an order of
magnitude. Our results clearly show that -ray loud sources have faster
and higher amplitude flares than -ray quiet sources. As a consequence
they show higher variability brightness temperatures and thus are more
relativistically beamed, with all of the above suggesting a strong connection
between the radio flaring properties of the jet and -ray emission.Comment: 14 pages, 8 figures, accepted for publication in AP
- …