arXiv subjects

Jeffrey A. Hodgson

Publications and source records attributed to Jeffrey A. Hodgson.

29 records · Page 2Linked to original sources

arXiv2018

Revealing the Nature of Blazar Radio Cores through Multi-Frequency Polarization Observations with the Korean VLBI Network

We study the linear polarization of the radio cores of eight blazars simultaneously at 22, 43, and 86 GHz with observations obtained by the Korean VLBI Network (KVN) in three epochs between late 2016 and early 2017 in the frame of the Plasma-physics of Active Galactic Nuclei (PAGaN) project. We investigate the Faraday rotation measure (RM) of the cores; the RM is expected to increase with observing frequency if core positions depend on frequency due to synchrotron self-absorption. We find a systematic increase of RMs at higher observing frequencies in our targets. The RM--$ν$ relations follow power-laws with indices distributed around 2, indicating conically expanding outflows serving as Faraday rotating media. Comparing our KVN data with contemporaneous optical polarization data from the Steward Observatory for a few sources, we find indication that the increase of RM with frequency saturates at frequencies of a few hundreds GHz. This suggests that blazar cores are physical structures rather than simple $τ=1$ surfaces. A single region, e.g. a recollimation shock, might dominate the jet emission downstream of the jet launching region. We detect a sign change in the observed RMs of CTA 102 on a time scale of $\approx$1 month, which might be related to new superluminal components emerging from its core undergoing acceleration/deceleration and/or bending. We see indication for quasars having higher core RMs than BL Lac objects, which could be due to denser inflows/outflows in quasars.

astro-ph.HE↗

arXiv2018

Gamma-rays in the radio galaxy 3C 84: A complex situation

3C 84 is a nearby Active Galactic Nucleus (AGN) that is unique in that is believed that we are observing near the true jet launching region - unlike blazars. The source is active in Gamma rays and has been detected with Fermi since its launch in 2008, including being detected at TeV energies with other instruments. Due to the relative proximity of the source (z=0.018), it provides a unique opportunity to pinpoint the location of the $γ$-ray emission by combining the Gamma ray data with very long baseline inteferometry (VLBI) data. A study using the Korean VLBI network (KVN) showed that the Gamma rays occur in both downstream jet emission and the region near where the jet is launched. Further analysis of the kinematics using Wavelet Image Segmentation and Evaluation (WISE) algorithm, which uses 2-dimensional cross-correlations to statistically derive the kinematics of high-resolution 7 mm VLBA data show that the Gamma ray emission is caused by a fast-travelling shock catching a slower moving shock and then interacting with the external medium, in behaviour reminiscent of a long duration gamma-ray burst (GRB). This could explain why such high energy flaring is seen in such low Doppler boosted sources. Finally, we show some early results from a study of the jet launching region using the Global mm-VLBI Array (GMVA). The nucleus appears to have a consistent double nuclear structure that is likely too broad to be the true jet base.

astro-ph.HE↗

arXiv2018

KVN observations suggest multiple $γ$-ray emission regions in 3C 84

3C 84 (NGC 1275) is a well-studied mis-aligned Active Galactic Nucleus (AGN), which has been active in Gamma rays since at least 2008. We have monitored the source at four wavelengths (14 mm, 7 mm, 3 mm and 2 mm) using the Korean VLBI network (KVN) since 2013 as part of the interferometric monitoring of $γ$-ray bright AGN (iMOGABA) program. 3C 84 exhibits bright radio emission both near the central supermassive black hole (SMBH) feature known as C1 and from a moving feature located to the south known as C3. Other facilities have also detected these short-term variations above a slowly rising trend at shorter wavelengths, such as in Gamma ray and 1 mm total intensity light-curves. We find that the variations in the $γ$ rays and 1 mm total intensity light-curves are correlated, with the $γ$ rays leading and lagging the radio emission. Analysis of the 2 mm KVN data shows that both the Gamma rays and 1 mm total intensity short-term variations are better correlated with the SMBH region than C3, likely placing the short-term variations in C1. We interpret the emission as being due to the random alignment of spatially separated emission regions. We place the slowly rising trend in C3, consistent with previous results. Additionally, we report that since mid-2015, a large mm-wave radio flare has been occurring in C3, with a large Gamma ray flare coincident with the onset of this flare at all radio wavelengths.

astro-ph.HE↗

arXiv2018

Long-term millimeter VLBI monitoring of M87 with KVN at milliarcsecond resolution: nuclear spectrum

We study the centimeter- to millimeter-wavelength synchrotron spectrum of the core of the radio galaxy M87 at $\lesssim0.8\,{\rm mas}~\sim110R_{s}$ spatial scales using four years of fully simultaneous, multi-frequency VLBI data obtained by the Korean VLBI Network (KVN). We find a core spectral index $α$ of $\gtrsim-0.37$ ($S\propto ν^{+α}$) between 22GHz and 129GHz. By combining resolution-matched flux measurements from the Very Long Baseline Array (VLBA) at 15GHz and taking the Event Horizon Telescope (EHT) 230GHz core flux measurements in epochs 2009 and 2012 as lower limits, we find evidence of a nearly flat core spectrum across 15GHz and 129GHz, which could naturally connect the 230GHz VLBI core flux. The extremely flat spectrum is a strong indication that the jet base does not consist of a simple homogeneous plasma, but of inhomogeneous multi-energy components, with at least one component with the turn-over frequency $\gtrsim100$GHz. The spectral shape can be qualitatively explained if both the strongly (compact, optically thick at $>$100GHz) and the relatively weakly magnetized (more extended, optically thin at $<$100GHz) plasma components are colocated in the footprint of the relativistic jet.

astro-ph.GA↗

arXiv2017

3 mm GMVA Observations of Total and Polarized Emission from Blazar and Radio Galaxy Core Regions

We present total and linearly polarized 3 mm Global mm-VLBI Array images of a sample of blazars and radio galaxies from the VLBA-BU-BLAZAR 7 mm monitoring program designed to probe the innermost regions of active galactic nuclei (AGN) jets and locate the sites of gamma-ray emission observed by the Fermi-LAT. The lower opacity at 3 mm and improved angular resolution, on the order of 50 microarcseconds, allow us to distinguish features in the jet not visible in the 7 mm VLBA data. We also compare two different methods used for the calibration of instrumental polarisation and we analyze the resulting images for some of the sources in the sample.

astro-ph.GA↗

arXiv2017

The Millimeter-Radio Emission of BL Lacertae During Two gamma-ray Outbursts

We present a study of the inexplicit connection between radio jet activity and gamma-ray emission of BL Lacertae (BL Lac; 2200+420). We analyze the long-term millimeter activity of BL Lac via interferometric observations with the Korean VLBI Network (KVN) obtained at 22, 43, 86, and 129 GHz simultaneously over three years (from January 2013 to March 2016); during this time, two gamma-ray outbursts (in November 2013 and March 2015) can be seen in gamma-ray light curves obtained from Fermi observations. The KVN radio core is optically thick at least up to 86 GHz; there is indication that it might be optically thin at higher frequencies. To first order, the radio light curves decay exponentially over the time span covered by our observations, with decay timescales of 411+/-85 days, 352+/-79 days, 310+/-57 days, and 283+/-55 days at 22, 43, 86, and 129 GHz, respectively. Assuming synchrotron cooling, a cooling time of around one year is consistent with magnetic field strengths B~2microT and electron Lorentz factors gamma~10,000. Taking into account that our formal measurement errors include intrinsic variability and thus over-estimate the statistical uncertainties, we find that the decay timescale tau scales with frequency nu like tau~nu^{-0.2}. This relation is much shallower than the one expected from opacity effects (core shift), but in agreement with the (sub-)mm radio core being a standing recollimation shock. We do not find convincing radio flux counterparts to the gamma$ray outbursts. The spectral evolution is consistent with the `generalized shock model' of Valtaoja et al. (1992). A temporary increase in the core opacity and the emergence of a knot around the time of the second gamma-ray event indicate that this gamma-ray outburst might be an `orphan' flare powered by the `ring of fire' mechanism.

astro-ph.GA↗

arXiv2017

Interferometric Monitoring of Gamma-ray Bright AGNs: S5 0716+714

We present the results of very long baseline interferometry (VLBI) observations of gamma-ray bright blazar S5 0716+714 using the Korean VLBI Network (KVN) at the 22, 43, 86, and 129 GHz bands, as part of the Interferometric Monitoring of Gamma-ray Bright AGNs (iMOGABA) KVN key science program. Observations were conducted in 29 sessions from January 16, 2013 to March 1, 2016, with the source being detected and imaged at all available frequencies. In all epochs, the source was compact on the milliarcsecond (mas) scale, yielding a compact VLBI core dominating the synchrotron emission on these scales. Based on the multi-wavelength data between 15 GHz (Owens Valley Radio Observatory) and 230 GHz (Submillimeter Array), we found that the source shows multiple prominent enhancements of the flux density at the centimeter (cm) and millimeter (mm) wavelengths, with mm enhancements leading cm enhancements by -16$\pm$8 days. The turnover frequency was found to vary between 21 to 69GHz during our observations. By assuming a synchrotron self-absorption model for the relativistic jet emission in S5 0716+714, we found the magnetic field strength in the mas emission region to be $\le$5 mG during the observing period, yielding a weighted mean of 1.0$\pm$0.6 mG for higher turnover frequencies (e.g., >45 GHz).

astro-ph.GA↗

arXiv2016

Multiple simultaneous Gamma-ray emission sites in 3C 84

The mis-aligned active galactic nuclei (AGN), 3C\,84, has been observed approximately monthly at four frequencies simultaneously (22\,GHz -- 129\,GHz) on the Korean VLBI Network (KVN) since 2013 as part of the interferometric monitoring of $γ$-ray bright AGN (iMOGABA) program. 3C\,84 is known to have to sites of bright radio emission, with the first thought to be near the central super-massive black hole and the second in a slowly moving emission feature, several milliarcseconds south of the core. Analysis of this data suggests that the highly variable Gamma-ray emission originates near the SMBH and the slowly rising $γ$-ray emission originates in the slow moving feature.

astro-ph.HE↗

arXiv2016

Interferometric Monitoring of Gamma-ray Bright AGNs I: Results of Single-epoch Multifrequency Observations

We present results of single-epoch very long baseline interferometry (VLBI) observations of gamma-ray bright active galactic nuclei (AGNs) using the Korean VLBI Network (KVN) at 22, 43, 86, and 129~GHz bands, which are part of a KVN key science program, Interferometric Monitoring of Gamma-ray Bright AGNs (iMOGABA). We selected a total of 34 radio-loud AGNs of which 30 sources are gamma-ray bright AGNs with flux densities of $>6\times10^{-10}$~ph~cm$^{-2}$~s$^{-1}$. Single-epoch multi-frequency VLBI observations of the target sources were conducted during a 24-hr session on 2013 November 19 and 20. All observed sources were detected and imaged at all frequency bands with or without a frequency phase transfer technique which enabled the imaging of 12 faint sources at 129~GHz, except for one source. Many of the target sources are resolved on milliarcsecond scales, yielding a core-jet structure with the VLBI core dominating the synchrotron emission on the milliarcsecond scale. CLEAN flux densities of the target sources are 0.43-28~Jy, 0.32-21~Jy, 0.18-11~Jy, and 0.35-8.0~Jy in the 22, 43, 86, and 129~GHz bands, respectively.

astro-ph.GA↗

arXiv2016

The automatic calibration of Korean VLBI Network data

The calibration of Very Long Baseline Interferometry (VLBI) data has long been a time consuming process. The Korean VLBI Network (KVN) is a simple array consisting of three identical antennas. Because four frequencies are observed simultaneously, phase solutions can be transferred from lower frequencies to higher frequencies in order to improve phase coherence and hence sensitivity at higher frequencies. Due to the homogeneous nature of the array, the KVN is also well suited for automatic calibration. In this paper we describe the automatic calibration of single-polarisation KVN data using the KVN Pipeline and comparing the results against VLBI data that has been manually reduced. We find that the pipelined data using phase transfer produces better results than a manually reduced dataset not using the phase transfer. Additionally we compared the pipeline results with a manually reduced phase-transferred dataset and found the results to be identical.

astro-ph.IM↗

arXiv2014

The latest results from the Global mm-VLBI Array

The Global mm-VLBI Array (GMVA) is the highest angular resolution imaging interferometer currently available as a common user facility. It is capable of angular resolutions on the order of 40 microarcseconds. Currently 14 stations in the United States and Europe participate in global 3 mm VLBI observations. The GMVA is used for continuum and spectroscopic imaging, probing the central regions of active galaxies and the origin of jets as these regions are typically not observable at longer wavelengths due to synchrotron self-absorption. In early 2012, fringes were detected to the three stations of the Korean VLBI Network (KVN), opening the possibility of extending the baseline coverage of the VLBI array to the East. In these proceedings, we will present recent images from a monitoring program of gamma-ray blazars using the GMVA, including the sources 3C454.3 and 0235+164, and an update of its current status and abilities.

astro-ph.HE↗