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Register a new userSurvey of nearby active galactic nuclei with the HAWC Observatory
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The High Altitude Water Cherenkov Observatory (HAWC) has a wide field-of-view (FOV, $sim$2sr) and a high duty cycle ($sim$95%), which make it a powerful survey and monitoring experiment for sources of TeV gamma rays. We present a systematic survey of gamma-ray sources based on the Fermi 3FHL catalog. Sources are restricted to HAWCs FOV (Declination 19$^circ$ $pm$ 40$^circ$) and to extragalactic sources with redshift: 0.001 $<$ z $<$ 0.3. Extragalactic gamma-ray sources are dominated by active galactic nuclei (AGN) and TeV gamma-ray sources are mostly BL Lac-type blazars. The study of AGNs through high energy gamma rays has opened a new window into the extreme processes of particle acceleration in the jets of these objects and provides a way to study the photon propagation and extra-galactic background light. We have improved the HAWC sensitivity at low energies (100 GeV to 1 TeV) based on the Crab pulsar, which is an excellent calibration source for TeV gamma rays. We will present the results of searching for and monitoring nearby AGNs with the improved analysis.
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The High Altitude Water Cherenkov Gamma-Ray Observatory (HAWC) continuously detects TeV photons and particles within its large field-of-view, accumulating every day a deeper exposure of two thirds of the sky. We analyzed 1523~days of HAWC live data acquired over four and a half years, in a follow-up analysis of {138} nearby ($z<0.3$) active galactic nuclei from the {em Fermi} 3FHL catalog culminating within $40^circ$ of the zenith at Sierra Negra, the HAWC site. This search for persistent TeV emission used a maximum-likelihood analysis assuming intrinsic power-law spectra attenuated by pair production of gamma-ray photons with the extragalactic background light. HAWC clearly detects persistent emission from Mkn~421 and Mkn~501, the two brightest blazars in the TeV sky, at 65$sigma$ and 17$sigma$ level, respectively. {Weaker evidence for long-term emission is found for three other known very-high energy emitters:} the radiogalaxy M87 and the BL Lac objects VER~J0521+211 and 1ES~1215+303, the later two at $zsim 0.1$. We find evidence for collective emission from the set of 30 previously reported very high-energy sources that excludes Mkn~421 and Mkn~501 with a random probability $sim 10^{-5}$. Upper limits are presented for the sample under the power-law assumption and in the predefined (0.5-2.0), (2.0-8.0) and (8.0-32.0) TeV energy intervals.
The Event Horizon Telescope (EHT), now with its first ever image of the photon ring around the supermassive black hole of M87, provides a unique opportunity to probe the physics of supermassive black holes through Very Long Baseline Interferometry (VLBI), such as the existence of the event horizon, the accretion processes as well as jet formation in Low Luminosity AGN (LLAGN). We build a theoretical model which includes an Advection Dominated Accretion Flow (ADAF) with emission from thermal and non-thermal electrons in the flow and a simple radio jet outflow. The predicted spectral energy distribution (SED) of this model is compared to sub-arcsec resolution observations to get the best estimates of the model parameters. The model-predicted radial emission profiles at different frequency bands are used to predict whether the inflow can be resolved by the EHT or with telescopes such as the Global 3-mm VLBI array (GMVA). In this work the model is initially tested with high resolution SED data of M87 and then applied to our sample of 5 galaxies (Cen A, M84, NGC 4594, NGC 3998 and NGC 4278). The model then allows us to predict if one can detect and resolve the inflow for any of these galaxies using the EHT or GMVA within an 8 hour integration time.
Galaxy mergers play a crucial role in the formation of massive galaxies and the buildup of their bulges. An important aspect of the merging process is the in-spiral of the supermassive black-holes (SMBHs) to the centre of the merger remnant and the eventual formation of a SMBH binary. If both the SMBHs are accreting they will form a dual or binary active galactic nucleus (DAGN). The final merger remnant is usually very bright and shows enhanced star formation. In this paper we summarize the current sample of DAGN from previous studies and describe methods that can be used to identify strong DAGN candidates from optical and spectroscopic surveys. These methods depend on the Doppler separation of the double peaked AGN emission lines, the nuclear velocity dispersion of the galaxies and their optical/UV colours. We describe two high resolution, radio observations of DAGN candidates that have been selected based on their double peaked optical emission lines (DPAGN). We also examine whether DAGN host galaxies have higher star formation rates (SFRs) compared to merging galaxies that do not appear to have DAGN. We find that the SFR is not higher for DAGN host galaxies. This suggests that the SFRs in DAGN host galaxies is due to the merging process itself and not related to the presence of two AGN in the system.
Based on RXTE scans and observations with the SWIFT/XRT telescope and INTEGRAL observatory, we report the identification of four X-ray sources discovered during the RXTE Slew Survey of the |b|>10deg sky with nearby (z ~ 0.017-0.098) luminous (log L_2-10keV ~ 42.7-44 erg/s) active galactic nuclei. Two of the objects exhibit heavily intrinsically absorbed X-ray spectra (NHL~10^23 cm^-2).
In this paper, we review the prospects for studies of active galactic nuclei (AGN) using the envisioned future Cherenkov Telescope Array (CTA). This review focuses on jetted AGN, which constitute the vast majority of AGN detected at gamma-ray energies. Future progress will be driven by the planned lower energy threshold for very high energy (VHE) gamma-ray detections to ~10 GeV and improved flux sensitivity compared to current-generation Cherenkov Telescope facilities. We argue that CTA will enable substantial progress on gamma-ray population studies by deepening existing surveys both through increased flux sensitivity and by improving the chances of detecting a larger number of low-frequency peaked blazars because of the lower energy threshold. More detailed studies of the VHE gamma-ray spectral shape and variability might furthermore yield insight into unsolved questions concerning jet formation and composition, the acceleration of particles within relativistic jets, and the microphysics of the radiation mechanisms leading to the observable high-energy emission. The broad energy range covered by CTA includes energies where gamma-rays are unaffected from absorption while propagating in the extragalactic background light (EBL), and extends to an energy regime where VHE spectra are strongly distorted. This will help to reduce systematic effects in the spectra from different instruments, leading to a more reliable EBL determination, and hence will make it possible to constrain blazar models up to the highest energies with less ambiguity.
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