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Register a new userRadio Imaging of the Very-High-Energy Gamma-Ray Emission Region in the Central Engine of a Radio Galaxy
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The accretion of matter onto a massive black hole is believed to feed the relativistic plasma jets found in many active galactic nuclei (AGN). Although some AGN accelerate particles to energies exceeding 10^12 electron Volts (eV) and are bright sources of very-high-energy (VHE) gamma-ray emission, it is not yet known where the VHE emission originates. Here we report on radio and VHE observations of the radio galaxy M87, revealing a period of extremely strong VHE gamma-ray flares accompanied by a strong increase of the radio flux from its nucleus. These results imply that charged particles are accelerated to very high energies in the immediate vicinity of the black hole.
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The radio source 1146+596 is hosted by an elliptical/S0 galaxy NGC,3894, with a low-luminosity active nucleus. The radio structure is compact, suggesting a very young age of the jets in the system. Recently, the source has been confirmed as a high-energy (HE, $>0.1$,GeV) $gamma$-ray emitter, in the most recent accumulation of the {it Fermi} Large Area Telescope (LAT) data. Here we report on the analysis of the archival {it Chandra} X-ray Observatory data for the central part of the galaxy, consisting of a single 40,ksec-long exposure. We have found that the core spectrum is best fitted by a combination of an ionized thermal plasma with the temperature of $simeq 0.8$,keV, and a moderately absorbed power-law component (photon index $Gamma = 1.4pm 0.4$, hydrogen column density $N_{rm H}/10^{22}$,cm$^{-2}$,$= 2.4pm 0.7$). We have also detected the iron K$alpha$ line at $6.5pm 0.1$,keV, with a large equivalent width of EW,$= 1.0_{-0.5}^{+0.9}$,keV. Based on the simulations of the {it Chandra}s Point Spread Function (PSF), we have concluded that, while the soft thermal component is extended on the scale of the galaxy host, the hard X-ray emission within the narrow photon energy range 6.0--7.0,keV originates within the unresolved core (effectively the central kpc radius). The line is therefore indicative of the X-ray reflection from a cold neutral gas in the central regions of NGC,3894. We discuss the implications of our findings in the context of the X-ray Baldwin effect. NGC,3894 is the first young radio galaxy detected in HE $gamma$-rays with the iron K$alpha$ line.
We present results from deep observations towards the Cygnus region using 300 hours of very-high-energy (VHE) $gamma$-ray data taken with the VERITAS Cherenkov telescope array and over seven years of high-energy $gamma$-ray data taken with the Fermi satellite at an energy above 1 GeV. As the brightest region of diffuse $gamma$-ray emission in the northern sky, the Cygnus region provides a promising area to probe the origins of cosmic rays. We report the identification of a potential Fermi-LAT counterpart to VER J2031+415 (TeV J2032+4130), and resolve the extended VHE source VER J2019+368 into two source candidates (VER J2018+367* and VER J2020+368*) and characterize their energy spectra. The Fermi-LAT morphology of 3FGL 2021.0+4031e (the Gamma-Cygni supernova remnant) was examined and a region of enhanced emission coincident with VER J2019+407 was identified and jointly fit with the VERITAS data. By modeling 3FGL J2015.6+3709 as two sources, one located at the location of the pulsar wind nebula CTB 87 and one at the quasar QSO J2015+371, a continuous spectrum from 1 GeV to 10 TeV was extracted for VER J2016+371 (CTB 87). An additional 71 locations coincident with Fermi-LAT sources and other potential objects of interest were tested for VHE $gamma$-ray emission, with no emission detected and upper limits on the differential flux placed at an average of 2.3% of the Crab Nebula ux. We interpret these observations in a multiwavelength context and present the most detailed $gamma$-ray view of the region to date.
FR0s are compact radio sources that represent the bulk of the Radio-Loud (RL) AGN population, but they are still poorly understood. Pilot studies on these sources have been already performed at radio and optical wavelengths: here we present the first X-ray study of a sample of 19 FR0 radio galaxies selected from the SDSS/NVSS/FIRST sample of Best & Heckman (2012), with redshift $leq$ 0.15, radio size $leq$ 10 kpc and optically classified as low-excitation galaxies (LEG). The X-ray spectra are modeled with a power-law component absorbed by Galactic column density with, in some cases, a contribution from thermal extended gas. The X-ray photons are likely produced by the jet as attested by the observed correlation between X-ray (2-10 keV) and radio (5 GHz) luminosities, similar to FRIs. The estimated Eddington-scaled luminosities indicate a low accretion rate. Overall, we find that the X-ray properties of FR0s are indistinguishable from those of FRIs, thus adding another similarity between AGN associated with compact and extended radio sources. A comparison between FR0s and low luminosity BL Lacs, rules out important beaming effects in the X-ray emission of the compact radio galaxies. FR0s have different X-ray properties with respect to young radio sources (e.g. GPS/CSS sources), generally characterized by higher X-ray luminosities and more complex spectra. In conclusion, the paucity of extended radio emission in FR0s is probably related to the intrinsic properties of their jets that prevent the formation of extended structures, and/or to intermittent activity of their engines.
The radio galaxy NGC 1275, recently identified as a very high energy (VHE, >100 GeV) gamma-ray emitter by MAGIC, is one of the few non-blazar AGN detected in the VHE regime. In order to better understand the origin of the gamma-ray emission and locate it within the galaxy, we studied contemporaneous multi-frequency observations of NGC 1275 and modeled the overall spectral energy distribution (SED). We analyzed unpublished MAGIC observations carried out between Oct. 2009 and Feb. 2010, and the previously published ones taken between Aug. 2010 and Feb. 2011. We studied the multi-band variability and correlations by analyzing data of Fermi-LAT (0.1-100 GeV), as well as Chandra (X-ray), KVA (optical) and MOJAVE (radio) data taken during the same period. Using customized Monte Carlo simulations corresponding to early MAGIC stereo data, we detect NGC 1275 also in the earlier campaign. The flux level and energy spectra are similar to the results of the second campaign. The monthly light curve >100 GeV shows a hint of variability at the 3.6 sigma level. In the Fermi-LAT band, both flux and spectral shape variabilities are reported. The optical light curve is variable and shows a clear correlation with the gamma-ray flux >100 MeV. In radio, 3 compact components are resolved in the innermost part of the jet. One of them shows a similar trend as the LAT and KVA light curves. The 0.1-650 GeV spectra measured simultaneously with MAGIC and Fermi-LAT can be well fitted either by a log-parabola or by a power-law with a sub-exponential cutoff for both campaigns. A single-zone synchrotron-self-Compton model, with an electron spectrum following a power-law with an exponential cutoff, can explain the broadband SED and the multi-band behavior of the source. However, this model suggests an untypical low bulk-Lorentz factor or a velocity alignment closer to the line of sight than the pc-scale radio jet.
We present high cadence multi-frequency radio observations of the long Gamma-Ray Burst (GRB) 190829A, which was detected at photon energies above 100 GeV by the High Energy Stereoscopic System (H.E.S.S.). Observations with the Meer Karoo Array Telescope (MeerKAT, 1.3 GHz), and Arcminute Microkelvin Imager - Large Array (AMI-LA, 15.5 GHz) began one day post-burst and lasted nearly 200 days. We used complementary data from Swift X-Ray Telescope (XRT), which ran to 100 days post-burst. We detected a likely forward shock component with both MeerKAT and XRT up to over 100 days post-burst. Conversely, the AMI-LA light curve appears to be dominated by reverse shock emission until around 70 days post-burst when the afterglow flux drops below the level of the host galaxy. We also present previously unpublished observations of the other H.E.S.S.-detected GRB, GRB 180720B from AMI-LA, which shows likely forward shock emission that fades in less than 10 days. We present a comparison between the radio emission from the three GRBs with detected very high energy (VHE) gamma-ray emission and a sensitivity-limited radio afterglow sample. GRB 190829A has the lowest isotropic radio luminosity of any GRB in our sample, but the distribution of luminosities is otherwise consistent, as expected, with the VHE GRBs being drawn from the same parent distribution as the other radio-detected long GRBs.
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