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Kovalev Y.Y.,Max Planck Institute for Radio Astronomy | Kardashev N.S.,RAS Lebedev Physical Institute | Kellermann K.I.,U.S. National Radio Astronomy Observatory | Lobanov A.P.,Max Planck Institute for Radio Astronomy | And 20 more authors.
Astrophysical Journal Letters | Year: 2016

Inverse Compton cooling limits the brightness temperature of the radiating plasma to a maximum of 1011.5 K. Relativistic boosting can increase its observed value, but apparent brightness temperatures much in excess of 1013 K are inaccessible using ground-based very long baseline interferometry (VLBI) at any wavelength. We present observations of the quasar 3C 273, made with the space VLBI mission RadioAstron on baselines up to 171,000 km, which directly reveal the presence of angular structure as small as 26 μas (2.7 light months) and brightness temperature in excess of 1013 K. These measurements challenge our understanding of the non-thermal continuum emission in the vicinity of supermassive black holes and require a much higher Doppler factor than what is determined from jet apparent kinematics. © 2016. The American Astronomical Society. All rights reserved..

Petrov L.,Astrogeo Center | Mahony E.K.,Netherlands Institute for Radio Astronomy | Edwards P.G.,CSIRO | Sadler E.M.,University of Sydney | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

We report results of the first phase of observations with the Australia Telescope Compact Array (ATCA) at 5 and 9 GHz of the fields around 411 γ -ray sources with declinations < +10 ° detected by Fermi but marked as unassociated in the Fermi Large Area Telescope 2-Year Source Catalog (2FGL). We have detected 424 sources with flux densities in the range 2-6 Jy that lie within the 99 per cent localization uncertainty of 283 γ -ray sources. Of these, 146 objects were detected in both 5- and 9-GHz bands. We found 84 sources in our sample with a spectral index flatter than -0.5. The majority of detected sources are weaker than 100 mJy and for this reason were not found in previous surveys. Approximately one third of our sample, 128 objects, have a probability of being associated more than 10 times greater than the probability of being a background source found in the vicinity of a γ -ray object by chance.We present the catalogue of positions of these sources, estimates of their flux densities and spectral indices where available. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Strader J.,Michigan State University | Chomiuk L.,Michigan State University | Cheung C.C.,U.S. Navy | Sand D.J.,Texas Tech University | And 12 more authors.
Astrophysical Journal Letters | Year: 2015

We present multiwavelength observations of the persistent Fermi-Large Area Telescope unidentified γ-ray source 1FGL J1417.7-4407, showing it is likely to be associated with a newly discovered X-ray binary containing a massive neutron star (nearly ) and a giant secondary with a 5.4 day period. SOAR optical spectroscopy at a range of orbital phases reveals variable double-peaked Hα emission, consistent with the presence of an accretion disk. The lack of radio emission and evidence for a disk suggests the γ-ray emission is unlikely to originate in a pulsar magnetosphere, but could instead be associated with a pulsar wind, relativistic jet, or could be due to synchrotron self-Compton at the disk-magnetosphere boundary. Assuming a wind or jet, the high ratio of γ-ray to X-ray luminosity (∼20) suggests efficient production of γ-rays, perhaps due to the giant companion. The system appears to be a low-mass X-ray binary that has not yet completed the pulsar recycling process. This system is a good candidate to monitor for a future transition between accretion-powered and rotational-powered states, but in the context of a giant secondary. © 2015. The American Astronomical Society. All rights reserved.

Schinzel F.K.,University of New Mexico | Petrov L.,Astrogeo Center | Taylor G.B.,University of New Mexico | Taylor G.B.,U.S. National Radio Astronomy Observatory | And 3 more authors.
Astrophysical Journal, Supplement Series | Year: 2015

We present the results of an all-sky radio survey between 5 and 9 GHz of sky areas surrounding all unassociated γ-ray objects listed in the Fermi Large Area Telescope Second Source Catalog (2FGL). The goal of these observations is to find all new γ-ray active galactic nucleus (AGN) associations with radio sources >10 mJy at 8 GHz. We observed with the Very Large Array and the Australia Telescope Compact Array the areas around unassociated sources, providing localizations of weak radio point sources found in 2FGL fields at arcmin scales. Then we followed-up a subset of those with the Very Long Baseline and the Long Baseline Arrays to confirm detections of radio emission on parsec-scales. We quantified association probabilities based on known statistics of source counts and assuming a uniform distribution of background sources. In total we found 865 radio sources at arcsec scales as candidates for association and detected 95 of 170 selected for follow-up observations at milliarcsecond resolution. Based on this we obtained firm associations for 76 previously unknown γ-ray AGNs. Comparison of these new AGN associations with the predictions from using the Wide-field Infrared Survey Explorer color-color diagram shows that half of the associations are missed. We found that 129 out of 588 observed γ-ray sources at arcmin scales not a single radio continuum source was detected above our sensitivity limit within the γ-ray localization. These «empty» fields were found to be particularly concentrated at low Galactic latitudes. The nature of these Galactic γ-ray emitters is not yet determined. © 2015. The American Astronomical Society. All rights reserved.

Lee S.-S.,Korea Astronomy and Space Science Institute | Petrov L.,Astrogeo Center | Byun D.-Y.,Korea Astronomy and Space Science Institute | Kim J.,Korea Astronomy and Space Science Institute | And 22 more authors.
Astronomical Journal | Year: 2014

We report the very long baseline interferometry (VLBI) observing performance of the Korean VLBI Network (KVN). The KVN is the first millimeter-dedicated VLBI network in East Asia. The KVN consists of three 21 m radio telescopes with baseline lengths in a range of 305-476 km. The quasi-optical system equipped on the antennas allows simultaneous observations at 22, 43, 86, and 129 GHz. The first fringes of the KVN were obtained at 22 GHz on 2010 June 8. Test observations at 22 and 43 GHz on 2010 September 30 and 2011 April 4 confirmed that the full cycle of VLBI observations works according to specification: scheduling, antenna control system, data recording, correlation, post-correlation data processing, astrometry, geodesy, and imaging analysis. We found that decorrelation due to instability in the hardware at times up to 600 s is negligible. The atmosphere fluctuations at KVN baseline are partly coherent, which allows us to extend integration time under good winter weather conditions up to 600 s without significant loss of coherence. The post-fit residuals at KVN baselines do not exhibit systematic patterns, and the weighted rms of the residuals is 14.8 ps. The KVN is ready to image compact radio sources both in snapshot and full-track modes with residual noise in calibrated phases of less than 2 deg at 22 and 43 GHz and with dynamic ranges of 300 for snapshot mode and 1000 for full-track mode. With simultaneous multi-frequency observations, the KVN can be used to make parsec-scale spectral index maps of compact radio sources. © 2014. The American Astronomical Society. All rights reserved..

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