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Harris D.E.,University of Sao Paulo | Massaro F.,University of Sao Paulo | Massaro F.,Kavli Institute for Particle Astrophysics and Cosmology | Cheung C.C.,National Academy of science | And 13 more authors.
Astrophysical Journal | Year: 2011

We describe a Chandra X-ray target-of-opportunity project designed to isolate the site of TeV flaring in the radio galaxy M87. To date, we have triggered the Chandra observations only once (2010 April) and by the time of the first of our nine observations, the TeV flare had ended. However, we found that the X-ray intensity of the unresolved nucleus was at an elevated level for our first observation. Of the more than 60 Chandra observations we have made of the M87 jet covering nine years, the nucleus was measured at a comparably high level only three times. Two of these occasions can be associated with TeV flaring, and at the time of the third event, there were no TeV monitoring activities. From the rapidity of the intensity drop of the nucleus, we infer that the size of the emitting region is of order a few light days × the unknown beaming factor; comparable to the same sort of estimate for the TeV emitting region. We also find evidence of spectral evolution in the X-ray band which seems consistent with radiative losses affecting the non-thermal population of the emitting electrons within the unresolved nucleus. © 2011 The American Astronomical Society. All rights reserved.

Corbel S.,University Paris Diderot | Corbel S.,Institut Universitaire de France | Dubus G.,CNRS Grenoble Institute for Particle Astrophysics and Cosmology Laboratory | Tomsick J.A.,University of California at Berkeley | And 19 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

With frequent flaring activity of its relativistic jets, Cygnus X-3 (Cyg X-3) is one of the most active microquasars and is the only Galactic black hole candidate with confirmed high-energy γ-ray emission, thanks to detections by Fermi Large Area Telescope (Fermi/LAT) and AGILE. In 2011, Cyg X-3 was observed to transit to a soft X-ray state, which is known to be associated with high-energy γ-ray emission. We present the results of a multiwavelength campaign covering a quenched state, when radio emission from Cyg X-3 is at its weakest and the X-ray spectrum is very soft. A giant (∼20Jy) optically thin radio flare marks the end of the quenched state, accompanied by rising non-thermal hard X-rays. Fermi/LAT observations (E≥ 100 MeV) reveal renewed γ-ray activity associated with this giant radio flare, suggesting a common origin for all non-thermal components. In addition, current observations unambiguously show that the γ-ray emission is not exclusively related to the rare giant radio flares. A three-week period of γ-ray emission is also detected when Cyg X-3 was weakly flaring in radio, right before transition to the radio quenched state. No γ-rays are observed during the ∼1-month long quenched state, when the radio flux is weakest. Our results suggest transitions into and out of the ultrasoft X-ray (radio-quenched) state trigger γ-ray emission, implying a connection to the accretion process, and also that the γ-ray activity is related to the level of radio flux (and possibly shock formation), strengthening the connection to the relativistic jets. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Weisskopf M.C.,NASA | Tennant A.F.,NASA | Arons J.,University of California at Berkeley | Blandford R.,Kavli Institute for Particle Astrophysics and Cosmology | And 25 more authors.
Astrophysical Journal | Year: 2013

We present results from our analysis of Chandra X-Ray Observatory, W. M. Keck Observatory, and Karl G. Jansky Very Large Array (VLA) images of the Crab Nebula that were contemporaneous with the γ-ray flare of 2011 April. Despite hints in the X-ray data, we find no evidence for statistically significant variations that pinpoint the specific location of the flares within the Nebula. The Keck observations extend this conclusion to the "inner knot," i.e., the feature within an arcsecond of the pulsar. The VLA observations support this conclusion. We also discuss theoretical implications of the γ-ray flares and suggest that the most dramatic γ-ray flares are due to radiation-reaction-limited synchrotron emission associated with sudden, dissipative changes in the current system sustained by the central pulsar. © 2013. The American Astronomical Society. All rights reserved.

Giroletti M.,National institute for astrophysics | Hada K.,Graduate University for Advanced Studies | Hada K.,Japan National Astronomical Observatory | Giovannini G.,National institute for astrophysics | And 10 more authors.
Astronomy and Astrophysics | Year: 2012

Aims. We aim to constrain the structural variations within the HST-1 region downstream of the radio jet of M 87, in general as well as in connection to the episodes of activity at very high energy (VHE). Methods. We analyzed and compared 26 VLBI observations of the M 87 jet, obtained between 2006 and 2011 with the Very Long Baseline Array (VLBA) at 1.7 GHz and the European VLBI Network (EVN) at 5 GHz. Results. HST-1 is detected at all epochs; we model-fitted its complex structure with two or more components, the two outermost of which display a significant proper motion with a superluminal velocity around ∼4 c. The motion of a third feature that is detected upstream is more difficult to characterize. The overall position angle of HST-1 has changed during the time of our observations from-65° to-90°, while the structure has moved by over 80 mas downstream. Our results on the component evolution suggest that structural changes at the upstream edge of HST-1 can be related to the VHE events. © 2012 ESO.

McConville W.,NASA | McConville W.,University of Maryland University College | Ostorero L.,University of Turin | Ostorero L.,National Institute of Nuclear Physics, Italy | And 22 more authors.
Astrophysical Journal | Year: 2011

We report Fermi Large Area Telescope (LAT) observations and broadband spectral modeling of the radio-loud active galaxy 4C+55.17 (z = 0.896), formally classified as a flat-spectrum radio quasar. Using 19 months of all-sky survey Fermi-LAT data, we detect a γ-ray continuum extending up to an observed energy of 145GeV, and furthermore we find no evidence of γ-ray variability in the source over its observed history. We illustrate the implications of these results in two different domains. First, we investigate the origin of the steady γ-ray emission, where we re-examine the common classification of 4C+55.17 as a quasar-hosted blazar and consider instead its possible nature as a young radio source. We analyze and compare constraints on the source physical parameters in both blazar and young radio source scenarios by means of a detailed multiwavelength analysis and theoretical modeling of its broadband spectrum. Second, we show that the γ-ray spectrum may be formally extrapolated into the very high energy (VHE, ≥100GeV) range at a flux level detectable by the current generation of ground-based Cherenkov telescopes. This enables us to place constraints on models of extragalactic background light within LAT energies and features the source as a promising candidate for VHE studies of the universe at an unprecedented redshift of z = 0.896. © 2011. The American Astronomical Society. All rights reserved.

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