VLBA observations of radio faint Fermi-LAT sources above 10 GeV

Abstract

The first Fermi-LAT High-energy source catalog (1FHL), containing gamma-ray sources detected above 10 GeV, is an ideal sample to characterize the physical properties of the most extreme gamma-ray sources. We investigate the pc scale properties of a sub-sample of radio faint 1FHL sources with the aim to confirm the proposed blazar associations, by revealing a compact high brightness temperature radio core, and we propose new low-frequency counterparts for the unassociated gamma-ray sources (UGS). Moreover, we increase the number of 1FHL sources with high resolution observations to explore the possible connection between radio and gamma rays at E >10 GeV. We observed 84 1FHL sources, mostly blazars of High Synchrotron Peaked (HSP) type, in the northern sky with the Very Long Baseline Array (VLBA) at 5 GHz. These sources lack high resolution radio observations and have at least one NVSS counterpart within the 95% confidence radius. For those sources without a well identified radio counterpart we exploit the VLBA multiple phase-center correlation capability to discern among the possible low-frequency candidates. For about 93% of the sources of our sample we reveal a compact high brightness temperature radio core, supporting their proposed blazar association. The vast majority of the detected sources are radio weak, with a median VLBI flux density value of 16.3 mJy. For the detected sources we obtain an average brightness temperature of the order of $2\times 10^{10}\mathrm{K}$. We find a compact component for 16 UGS, for which we propose a new low-frequency association. We find brightness temperature values which do not require high Doppler factors, and are in agreement with the expected values for the equipartition of energy between particles and magnetic field. We find strong indications about the blazar nature of all of the detected UGS, for which we propose new low-frequency associations.

arXiv comment11 pages, 5 figures. accepted for publication in Astronomy & Astrophysics

References in corpus (28)

• Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Interpretation
• Fermi Large Area Telescope Second Source Catalog
• A Unifying View of the Spectral Energy Distributions of Blazars
• The Spectral Energy Distribution of Fermi bright blazars
• A theoretical unifying scheme for gamma-ray bright blazars
• DiFX2: A more flexible, efficient, robust and powerful software correlator
• The Third Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope
• The First Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope
• 2FHL: The Second Catalog of Hard Fermi-LAT Sources
• The Imprint of The Extragalactic Background Light in the Gamma-Ray Spectra of Blazars
• The First Fermi-LAT Catalog of Sources Above 10 GeV
• The radio/gamma-ray connection in Active Galactic Nuclei in the era of the Fermi Large Area Telescope
• The Jets of TeV Blazars at Higher Resolution: 43 GHz and Polarimetric VLBA Observations from 2005-2009
• Unveiling the nature of the unidentified gamma-ray sources I: a new method for the association of gamma-ray blazars
• Surveys with the Cherenkov Telescope Array
• A Sample of Low Redshift BL Lacs. I. The Radio Data
• Unveiling the nature of the unidentified gamma-ray sources II: radio, infrared and optical counterparts of the gamma-ray blazar candidates
• Unveiling the nature of the unidentified gamma-ray sources III: gamma-ray blazar-like counterparts at low radio frequencies
• Unveiling the nature of the unidentified gamma-ray sources IV: the $\text{Swift}$ catalog of potential X-ray counterparts
• Optical spectroscopic observations of gamma-ray blazar candidates II. The 2013 KPNO campaign in the Northern Hemisphere
• New Associations of Gamma-Ray Sources from the Fermi Second Source Catalog
• First-Epoch VLBA Imaging of Twenty New TeV Blazars
• VLBA monitoring of Mrk 421 at 15 GHz and 24 GHz during 2011
• VSOP and Ground-based VLBI Imaging of the TeV Blazar Markarian 421 at Multiple Epochs
• High-energy sources at low radio frequency: the Murchison Widefield Array view of Fermi blazars
• Unveiling the nature of the unidentified gamma-ray sources VI: gamma-ray blazar candidates in the WISH survey and their radio properties
• Very Long Baseline polarimetry and the gamma-ray connection in Markarian 421 during the broadband campaign in 2011
• Swift/XRT counterparts to unassociated Fermi high-energy LAT sources

Cited by in corpus (10)

• Multi-Epoch VLBA Imaging of Twenty New TeV Blazars: Apparent Jet Speeds
• A parsec-scale wobbling jet in the high-synchrotron peaked blazar PG 1553+113
• A persistent double nuclear structure in 3C 84
• Localizing the $\gamma$-ray emitting region in the blazar TXS 2013+370
• Exploring the connection between radio and GeV-TeV gamma-ray emission in the 1FHL and 2FHL AGN samples
• The $\gamma$-ray sky seen at X-ray energies II: the Swift hunt of Fermi BL Lac objects among unidentified gamma-ray sources
• Evaluating Optical Classification for {\em Fermi} Blazar Candidates with a Statistical method using Broadband Spectral Indices
• The loud and the quiet: searching for radio counterparts of two radio-weak BL Lac candidates with VLBI
• An XMM-Newton catalogue of BL Lacs
• Is 4C+29.48 a gamma-ray source?