A Strong Radio Brightening At The Jet Base Of M 87 During The Elevated Very High Energy Gamma-Ray State In 2012
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- Istituto Nazionale di Astrofisica (INAF), Bologna (Italy); National Astronomical Observatory of Japan, Tokyo (Japan)
- Istituto Nazionale di Astrofisica (INAF), Bologna (Italy)
- Japan Aerospace Exploration Agency, Tsukuba (Japan); Korea Astronomy and Space Science Inst. (KASI), Daejeon (Korea)
- Istituto Nazionale di Astrofisica (INAF), Bologna (Italy); Univ. of Bologna (Italy)
- Naval Research Lab. (NRL), Washington, DC (United States)
- Washington Univ., St. Louis, MO (United States)
- National Astronomical Observatory of Japan, Tokyo (Japan)
- Japan Aerospace Exploration Agency, Tsukuba (Japan)
- National Astronomical Observatory of Japan, Tokyo (Japan); Univ. of Tokyo (Japan)
- National Astronomical Observatory of Japan, Tokyo (Japan); The Graduate Univ. for Advanced Sciences, Tokyo (Japan)
- The Graduate Univ. for Advanced Sciences, Tokyo (Japan)
- Spanish National Research Council (CSIC), Granada (Spain)
- National Astronomical Observatory of Japan, Tokyo (Japan); Japan Aerospace Exploration Agency, Tsukuba (Japan); Univ. of Tokyo (Japan)
- Univ. of Trento (Italy)
- Kagoshima Univ. (Japan)
- Harvard-Smithsonian Center for Astrophysics, Cambridge, MA (United States)
We report our intensive, high-angular-resolution radio monitoring observations of the jet in M 87 with the VLBI Exploration of Radio Astrometry (VERA) and the European VLBI Network (EVN) from February 2011 to October 2012, together with contemporaneous high-energy (HE; 100 MeV< E <100 GeV) -ray light curves obtained by the Fermi Large Area Telescope (LAT). During this period (specifically from February 2012 to March 2012), an elevated level of the M 87 flux is reported at very-high-energy (VHE; E > 100 GeV) -rays by VERITAS. We detected a remarkable (up to ~70%) increase of the radio flux density from the unresolved jet base (radio core) with VERA at 22 and 43 GHz coincident with the VHE activity. Meanwhile, we confirmed with EVN at 5 GHz that the peculiar knot HST-1, which is an alternative favored -ray production site located at &120 pc from the nucleus, remained quiescent in terms of its flux density and structure. These results in the radio bands strongly suggest that the VHE -ray activity in 2012 originates in the jet base within 0.03 pc or 56 Schwarzschild radii (the VERA spatial resolution of 0.4 mas at 43 GHz) from the central supermassive black hole. We further conducted VERA astrometry for the M 87 core at six epochs during the flaring period, and detected core shifts between 22 and 43 GHz, a mean value of which is similar to that measured in the previous astrometric measurements. We also discovered a clear frequency-dependent evolution of the radio core flare at 43, 22 and 5 GHz; the radio flux density increased more rapidly at higher frequencies with a larger amplitude, and the light curves clearly showed a time-lag between the peaks at 22 and 43 GHz, the value of which is constrained to be within ~ 35 - 124 days. This indicates that a new radio-emitting component was created near the black hole in the period of the VHE event, and then propagated outward with progressively decreasing synchrotron opacity. By combining the obtained core shift and time-lag, we estimated an apparent speed of the newborn component propagating through the opaque region between the cores at 22 and 43 GHz. We derived a sub-luminal speed (less than ~0.2c) for this component. This value is significantly slower than the super-luminal (~1.1c) features that appeared from the core during the prominent VHE flaring event in 2008, suggesting that the stronger VHE activity can be associated with the production of the higher Lorentz factor jet in M 87.
- Research Organization:
- SLAC National Accelerator Lab., Menlo Park, CA (United States)
- Sponsoring Organization:
- USDOE
- Grant/Contract Number:
- AC02-76SF00515
- OSTI ID:
- 1356468
- Journal Information:
- The Astrophysical Journal, Vol. 788, Issue 2; ISSN 0004-637X
- Publisher:
- Institute of Physics (IOP)Copyright Statement
- Country of Publication:
- United States
- Language:
- English
Web of Science
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