iMOGABA (being updated)
A KVN Key Science Program with the title of Origins of Gamma-ray flares in AGNs (PI: Sang-Sung Lee, firstname.lastname@example.org) is a three-year project consisting of VLBI monitoring observations (or iMOGABA) and single dish (SD) rapid response observations (RRO, or MOGABA). The VLBI monitoring observations are comprised of ten 24-hr observations per year (every month) of about 30 gamma-ray brigt active galactic nuclei (AGNs)(see http://radio.kasi.re.kr/sslee) with Korea VLBI Network (KVN) at 22, 43, 86, and 129 GHz. The SD RROs may consist of twelve 7-hr observations per source (every week for 3 months after triggering) of gamma-ray flaring sources with two KVN SD telescopes at 22, 43, and 86 GHz (and/or 129 GHz)in dual polarization. We expect one or two sources per year for the SD RROs. Gamma-ray flares of AGNs are known to be occured in innermost regions of relativistic jets which radiate in whole ranges of electromagnetic spectra due to synchrotron radiation, syschrotron self absorption, inverse-compton scttering, doppler boosting etc. Here we may eraise two questions on the natures of the gamma-ray flares of AGN such as: a) What is the basic casue of the gamma-ray flares from AGNs? b) What is the physical process of the causes? For the first question, there are several suggestions like 1) a relativistic jet of high energy plasma (Marscher et al. 2008), 2) Doppler boosting of synchrotron radiation of the jet (Dermer 1995), 3) inverse Compton scattering by relativistic electrons, etc. For the second question, we may find some candidates and detail mechanism for the gamma-ray flares such as 1) compression and heating of the plasma in the relativistic jets, 2) generation of the relativistic particles, 3) rapid variability in flux and magnetic field. In order to answer to the questions, we may conduct either 1) studies of large samples of flaring AGNs for investigating statistics and correlation of observed properties (Lister et al. 2011), 2) multi-wavelength observations of individual objects for testing time profiles of flares (Jorstad et al. 2010), for studying physical properties of emission features (jet knots) (Agudo et al. 2011), and studying evolution of SEDs (Wehrle et al. 2013), or 3) polarization observations for looking at magnetic field environments (Jorstad et al. 2013). Possible explanations of the gamma-ray flares in AGNs are a) shocks-in-jets propagating within jet flow and b) bending of the whole jets. For both cases, we should expect changes in polarization, luminosity, particle distribution, and structures of jets at mas-scale. The multifrequency simultaneous VLBI/SD observations with KVN are the best tool for detecting such changes correlated with gamma-ray flares. This KSP aims to answer the fundamental questions about the basic nature of the flares of AGN.
The observations consist of ten 24-hr VLBI observations per year from 2015 January to 2018 January of about 30 gamma-ray brigt active galactic nuclei (AGNs) with Korea VLBI Network (KVN) at 22, 43, 86, and 129 GHz. Ten observations will be conducted every month. Our first priority is to measure changes of jet structures at mas-scale of about 30 γ-ray bright AGNs when they are flaring in the gamma-ray. Our targets are chosen to have flares in gamma-ray detected with Fermi-LAT gamma-ray space telescope (see http://radio.kasi.re.kr/sslee). Since most of them are very bright at KVN’s operating frequencies, they are able to be detected at KVN baselines within coherence times (20-100 sec) at correponding frequencies. However, we have confirmed that it was possible to improve fringe detection sensitivity with longer integration time than canonical coherence times. Therefore, we observe each source with several 5-min-long scans distributed over its LST time range in order to have uniform uv-coverages. With this snap-shot mode observations, we are able to detect sizes and flux densities of compact jets. However, some target sources with complicated jet structures at KVN’s resolution should be carefully images. This issue may be resolved with full-track observations for each sources. Our second priority is to measure flux density changes of AGN jets at mas-scale. Since atmospheric fluctuation are very high at millimeter wavelengths, careful amplitude calibration should be conducted for individual observations. For usual VLBI imaging observations, one can selfc-calibrate the amplitude with closure-amplitudes. However, unfortunately, with only KVN three stations, the amplitude self-calibration is not possible. Therefore, we conduct very frequent measurements for atmospheric opacity (every hour) and antenna system noise temperature (every scan). More importantly, constant monitor and adjustment for antenna pointing (and/or focus) are performed.
Please see http://radio.kasi.re.kr/sslee for the list of target sources in addition to calibrators in this project.
The KVN KSP is a joint project based on the contributions from the following members:
Practial information of the KSP observations starting in 2014 Aug is uploaded and summarized in this wiki page for the members of the KSP. You may find the public web page for the KSP at http://radio.kasi.re.kr/sslee. Please choose one of the following for updating, checking, etc the status of the KSP observations.