Identification of the Early Fermi LAT Gamma-Ray Bright Objects with Extragalactic VLBI sources
Y. Y. Kovalev
备注：
5 pages, 4 figures, 1 table, accepted by the Astrophysical Journal Letters; minor corrections to the text are made, one panel is added to Figure 1; full table 1 in electronic form can be extracted from the preprint source
A list of 205 gamma-ray strong objects was reported recently as a result of a 3-month integration with the Large Area Telescope on board the Fermi Gamma-Ray Space Telescope. We attempted identification of these objects, cross-correlating the gamma-ray positions with VLBI positions of a large all-sky sample of extragalactic radio sources selected on the basis of their parsec-scale flux density. The original associations reported by the Fermi team are confirmed and six new identifications are suggested. A Monte-Carlo analysis shows that the fraction of chance associations in our analysis is less than 5 per cent, and confirms that the vast majority of gamma-ray bright extragalactic sources are radio loud blazars with strong parsec-scale jets. A correlation between the parsec-scale radio and gamma-ray flux is supported by our analysis of a complete VLBI flux-density-limited sample of extragalactic jets. The effectiveness of using a VLBI catalog to find associations between gamma-ray detections and compact extragalactic radio sources, especially near the Galactic plane, is demonstrated. It is suggested that VLBI catalogs should be used for future identification of Fermi LAT objects.
If we assume that nuclei of extragalactic radio sources contain binary black hole systems, the two black holes can eject VLBI components in which case two families of different VLBI trajectories will be observed. Another important consequence of a binary black hole system is that the VLBI core is associated with one black hole, and if a VLBI component is ejected by the second black hole, one expects to be able to detect the offset of the origin of the VLBI component ejected by the black hole that is not associated with the VLBI core. The ejection of VLBI components is perturbed by the precessi…
The point source list of the Wilkinson Microwave Anisotropy Probe (WMAP) is a uniform, all-sky catalogue of bright sources with flux density measurements at high (up to 94 GHz) radio frequencies. We investigated the five-year WMAP list to compile a new catalogue of bright and compact extragalactic radio sources to be potentially studied with Very Long Baseline Interferometry at millimeter wavelengths (mm-VLBI) and Space VLBI (SVLBI). After comparing the WMAP data with the existing mm-VLBI catalogues, we sorted out the yet unexplored sources. Using the 41, 61 and 94 GHz WMAP flux densities, we …
We present very long baseline interferometry (VLBI) observations of the milliarcsecond-scale radio structure of the supernova remnant SNR 4449$-$1 in the galaxy NGC 4449. This young and superluminous remnant was observed at 1.6 GHz ($λ= 18$\,cm) with the European VLBI Network. The observations confirm earlier identifications of this object with a supernova remnant (SNR) while revealing a somewhat different morphology compared with the structure reported by Bietenholz et al. from VLBI observations at 1.4 GHz. This difference is discussed here in the context of structural sensitivity of both obs…
Very Long Baseline Interferometry is capable of measuring the gravitational delay caused by the Sun and planet gravitational fields. The post-Newtonian parameter $γ$ is now estimated with accuracy of $σ_γ=2\cdot 10^{-4}$ using a global set of VLBI data from 1979 to present (Lambert, Gontier, 2009), and $σ_γ=2\cdot10^{-5}$ by the Cassini spacecraft (Bertotti et. al, 2003). Unfortunately, VLBI observations in S- and X-bands very close to the Solar limb (less than 2-3 degrees) are not possible due to the strong turbulence in the Solar corona. Instead, the close approach of big planets to the line…
The TANAMI program has been monitoring the parsec-scale radio jets of southern gamma-ray bright AGN with VLBI techniques simultaneously with Fermi/LAT monitoring of their gamma-ray emission. Here we present the gamma-ray properties of the TANAMI sources based on an analysis of the preliminary 1-year LAT source list. We present upper limits on the gamma-ray flux for TANAMI sources not detected by LAT.
Extragalactic jets are a common feature of radio-loud active galaxies. The nature of the observed jets in relation to the bulk flow is still unclear. In particular it is not clear whether the observations of parsec-scale jets using the very long baseline interferometric technique (VLBI) reveal wave-like structures that develop and propagate along the jet, or trace the jet flow itself. In this contribution I review the evidence collected during the last years showing that the ridge-lines of helical radio-jets do not correspond to observational artifacts. This conclusion was reached by studying …
It is expected that the European Space Agency mission Gaia will make possible to determine coordinates in the optical domain of more than 500000 quasars. In 2006, a radio astrometry project was launched with the overall goal to make comparison of coordinate systems derived from future space-born astrometry instruments with the coordinate system constructed from analysis of the global very long baseline interferometry (VLBI) more robust. Investigation of their rotation, zonal errors, and the non-alignment of the radio and optical positions caused by both radio and optical structures are importa…
It is anticipated that future space-born missions, such as Gaia, will be able to determine in optical domain positions of more than 100,000 bright quasars with sub-mas accuracies that are comparable to very long baseline interferometry (VLBI) accuracies. Comparisons of coordinate systems from space-born missions and from VLBI will be very important, first for investigation of possible systematic errors, second for investigation of possible shift between centroids of radio and optical emissions in active galaxy nuclea. In order to make such a comparison more robust, a program of densification o…
Most extragalactic jets in radio-loud Active Galactic Nuclei are bright and variable gamma-ray sources, which are continuously monitored with Fermi/LAT. We present the gamma-ray properties of the MOJAVE and TANAMI AGN samples of radio-loud AGN. Both programs provide properties of the parsec-scale radio jets using Very Long Baseline Interferometry (VLBI) techniques. This information is important to understand the broad-band emission mechanism of these sources. In this work we compare the radio and gamma -ray properties of the two samples and present upper limits on the gamma-ray flux of the rad…
Modeling VLBI ejections of nuclei of extragalactic radio sources, indicates that their nuclei contain a binary black hole system. One can derive the distance and the positions of the two black holes in the plane of the sky. We can also use the RMS of the time series of the ICRF2 survey to obtain an estimate of the structure and the size of the nuclei. We will discuss the possible problems to link VLBI observations and GAIA optical observations of radio quasars if they contain a binary black hole system.
We present a catalogue of positions and correlated flux densities of 410 flat-spectrum, compact extragalactic radio sources, previously detected in the AT20G survey. The catalogue spans the declination range -90deg, -40deg and was constructed from four 24 hour VLBI observing sessions with the Australian Long Baseline Array made at 8.3 GHz. The detection rate in these experiments is 97%. The median uncertainty of source positions is 2.6 mas, the median correlated flux density at baseline projections lengths longer than 1000 km is 0.14 Jy. The goal of this work is 1) to provide a pool of sources…
The Southern Hemisphere VLBI monitoring program TANAMI provides dual-frequency (8 GHz and 22 GHz), milliarcsecond monitoring of extragalactic jets south of -30 degrees declination. The TANAMI sample consists of a combined radio and gamma-ray selected subsample of currently 80 AGN jets, with new gamma-ray bright sources being added upon detections by Fermi/LAT. Supporting programs provide simultaneous multiwavelength coverage of all sources, in order to construct broadband spectral energy distributions (SEDs) of flaring and quiescence source states, as well as a rapid follow-up of high-energy f…
The TANAMI (Tracking AGN with Austral Milliarcsecond Interferometry) and associated programs provide comprehensive radio monitoring of extragalactic gamma-ray sources south of declination -30 degrees. Joint quasi-simultaneous observations between the Fermi Gamma-ray Space Telescope and ground based observatories allow us to discriminate between competing theoretical blazar emission models. High resolution VLBI observations are the only way to spatially resolve the sub-parsec level emission regions where the high-energy radiation originates. The gap from radio to gamma-ray energies is spanned w…
One of the open questions in extragalactic jet Astrophysics is related to the nature of the observed radio jet, namely whether it traces a pattern or the flow structure itself. In this paper I summarize the evidence collected for the presence of waves in extragalactic jets. The evidence points towards the peak of emission in helical jets corresponding to pressure-maxima of a wave that is generated within the core region and propagates downstream. Making use of a number of very long baseline interferometry (VLBI) observations of the radio jet in the quasar S5~0836+710 at different frequencies a…
Many of the compact extragalactic radio sources that are used as fiducial points to define the celestial reference frame are known to have proper motions detectable with long-term geodetic/astrometric Very Long Baseline Interferometry (VLBI) measurements. These changes can be as high as several hundred micro-arcseconds per year for certain objects. When imaged with VLBI at milli-arcsecond (mas) angular resolution, these sources (radio-loud active galactic nuclei) typically show structures dominated by a compact, often unresolved "core" and a one-sided "jet". The positional instability of compa…
The Compact Symmetric Objects (CSOs) are small (<1 kiloparsec) and powerful extragalactic radio sources showing emission on both sides of an active galactic nucleus and no signs of strong relativistic beaming. They may be young radio sources, progenitors of large FRII radio galaxies. We aim to study the statistical properties of CSOs by constructing and investigating a new large sample of CSO candidates on the basis of dual-frequency, parsec-scale morphology. For the candidate selection we utilized VLBI data for 4170 extragalactic objects obtained simultaneously at 2.3 and 8.6 GHz (S and X ban…
Aims. We have analysed VLBA 92cm archive data of 44 extragalactic sources in order to identify early targets and potential calibrator sources for the LOFAR radio telescope and the RadioAstron space VLBI mission. Some of these sources will also be suitable as in-beam calibrators, permitting deep, wide-field studies of other faint sources in the same field of view. Methods. All publicly available VLBA 92cm data observed between 1 January 2003 to December 31 2006 have been analysed via an automatic pipeline, implemented within AIPS. The vast majority of the data are unpublished. Results. The samp…
Radio galaxies are relatively faint at $γ$-ray energies, where they make up only 1-2% of all AGN detected by Fermi-LAT. However, they offer a unique perspective to study the intrinsic properties of AGN jets. For this reason, the combination of $γ$-ray and multi-wavelength data with high-resolution VLBI monitoring is a powerful tool to tackle the basic unanswered questions about AGN jets. Here we present preliminary results from a sample study of radio galaxies in the Southern hemisphere observed by the TANAMI VLBI monitoring program. We obtain high-resolution maps at 8.4 and 22.3 GHz, and stud…
This paper presents the catalog of correlated flux densities in three ranges of baseline projection lengths of 637 sources from a 43 GHz (Q-band) survey observed with the Korean VLBI Network. Of them, 623 sources have not been observed before at Q-band with VLBI. The goal of this work in the early science phase of the new VLBI array is twofold: to evaluate the performance of the new instrument that operates in a frequency range of 22-129 GHz and to build a list of objects that can be used as targets and as calibrators. We have observed the list of 799 target sources with declinations down to -…
Observations of ten Compact Symmetric Objects ({\rm CSO}) candidates have been made with the Very Long Baseline Array at 8.4 GHz in 2005 and with a combined Chinese and European VLBI array at 8.4 GHz in 2009. The 2009 observations incorporate for the first time the two new Chinese telescopes at Miyun and Kunming for international astrophysical observations. The observational data, in combination with archival VLBA data from previous epochs, have been used to derive the proper motions of the VLBI components. Because of the long time baseline of $\sim$16 years of the VLBI data sets, the expansio…
The TANAMI (Tracking AGN with Austral Milliarcsecond Interferometry) program provides comprehensive VLBI monitoring of extragalactic gamma-ray sources south of declination -30 degrees. Operating at two radio frequencies (8 and 22 GHz), this program is a critical component of the joint quasi-simultaneous observations with the Fermi Gamma-ray Space Telescope and ground based observatories to discriminate between competing theoretical blazar emission models. We describe the TANAMI program and present early results on the 75 sources currently being monitored.
Context. Blazars are among the most energetic objects in the Universe. In 2008 August, Fermi/LAT detected the blazar PKS 1502+106 showing a rapid and strong gamma-ray outburst followed by high and variable flux over the next months. This activity at high energies triggered an intensive multi-wavelength campaign covering also the radio, optical, UV, and X-ray bands indicating that the flare was accompanied by a simultaneous outburst at optical/UV/X-rays and a delayed outburst at radio bands. Aims: In the current work we explore the phenomenology and physical conditions within the ultra-relativi…
Using high-resolution radio imaging with VLBI techniques, the TANAMI program has been observing the parsec-scale radio jets of southern (declination south of -30°) gamma-ray bright AGN simultaneously with Fermi/LAT monitoring of their gamma-ray emission. We present the radio and gamma-ray properties of the TANAMI sources based on one year of contemporaneous TANAMI and Fermi/LAT data. A large fraction (72%) of the TANAMI sample can be associated with bright gamma-ray sources for this time range. Association rates differ for different optical classes with all BL Lacs, 76% of quasars and just 17%…
We report the observation in the GeV band of the blazar 1ES 0229+200, which over recent years has become one the primary sources used to put constraints on the Extragalactic Background Light (EBL) and Extragalactic Magnetic Field (EGMF). We derive constraints on both the EBL and EGMF from the combined Fermi-HESS data set taking into account the direct and cascade components of the source spectrum. We show that the limit on the EBL depends on the EGMF strength and vice versa. In particular, an EBL density twice as high as that derived by Franceschini et al. (2008) is allowed if the EGMF is stro…
We estimate the contribution to the extragalactic gamma-ray background (EGRB) from both intrinsic and cascade emissions produced by blazars using a simple semi- analysis method for two models of the blazar gamma-ray luminosity function (GLF). For the cascade emission, we consider two possible contributions: one is due to that the flux of the cascade emission is lower than the Fermi LAT sensitivity (case I), which is independent on the extragalactic magnetic field (EGMF), another is due to the fact that the flux of the cascade emission is larger than the Fermi LAT sensitivity but the emission a…
The Very Long Baseline Interferometry (VLBI) monitoring program TANAMI provides bi-monthly, dualfrequency (8GHz and 22GHz) observations of extragalactic jets with milliarcsecond resolution south of -30 deg declination using the Australian Long Baseline Array (LBA) and additional radio telescopes in Antarctica, Chile, New Zealand and South Africa. Supporting programs provide multiwavelength coverage of the Fermi/LAT sources of the TANAMI sample, in order to construct simultaneous broadband spectral energy distributions (SEDs), as well as rapid follow-ups of high energy flares. The main purpose …
The radio galaxy 3C 84 is a representative of gamma-ray-bright misaligned active galactic nuclei (AGNs) and one of the best laboratories to study the radio properties of the sub-pc jet in connection with the gamma-ray emission. In order to identify possible radio counterparts of the gamma-ray emissions in 3C 84, we study the change in structure within the central 1 pc and the light curve of sub-pc-size components C1, C2, and C3. We search for any correlation between changes in the radio components and the gamma-ray flares by making use of VLBI and single dish data. Throughout the radio monitor…
We report multi-epoch Very Long Baseline Interferometry (VLBI) study of the innermost jet of 3C 84. We carried out 14-epoch VLBI observations during 2006-2009 with the Japanese VLBI Network (JVN) and the VLBI Exploration of Radio Astrometry (VERA), immediately following the radio outburst that began in 2005. Comparison between VLBI lightcurve and single-dish lightcurve indicates that the outburst was associated with the central ~1 pc core. We found that this outburst accompanied the emergence of a new component, and the projected speed of this new component was 0:23c from 2007/297 (2007 Octobe…
The detection of gamma-ray emission by Fermi-LAT from the radio loud Narrow Line Seyfert 1 PMN J0948+0022 (Abdo et al. 2009, ApJ 699, 976) triggered a multi-wavelength campaign between March and July 2009. Given its high compactness (Doi et al. 2006, PASJ 58, 829), inverted spectrum, and 0deg declination, the source was an ideal target to observe at 22 GHz with a Global VLBI array extending from Europe to East Asia and Australia. In order to deliver prompt results to be analysed in combination with the other instruments participating in the campaign, the observations were carried out with real…
High-redshift radio-loud quasars are used to, among other things, test the predictions of cosmological models, set constraints on black hole growth in the early universe and understand galaxy evolution. Prior to this paper, 20 extragalactic radio sources at redshifts above 4.5 have been imaged with very long baseline interferometry (VLBI). Here we report on observations of an additional ten z>4.5 sources at 1.7 and 5 GHz with the European VLBI Network (EVN), thereby increasing the number of imaged sources by 50%. Combining our newly observed sources with those from the literature, we create a …
The origin of the high-energy emission of blazars is still a matter of debate. To investigate the emission mechanism of extragalactic outflows and to pin down the location of the emission, we have constructed a broadband spectral energy distribution (SED) database covering from the radio to the gamma-ray band for the complete MOJAVE sample, which consists of 135 relativistically beamed AGN with well-studied parsec-scale jets. Typically, the broadband SEDs of blazars shows a double-humped profile. It is believed that the lower-energy hump is due to synchrotron emission from the radio jet, and t…
GHz-peaked spectrum (GPS) radio sources are thought to be young objects which later evolve into FR-I and FR-II radio galaxies. We have used the Australia Telescope 20GHz (AT20G) survey catalogue to select a uniform sample of GPS sources with spectral peaks above 5GHz, which should represent the youngest members of this class. In this paper, we present e-VLBI observations of ten such objects which are associated with nearby (z<0.15) galaxies and so represent a new population of local, low--power GPS sources. Our e-VLBI observations were carried out at 4.8GHz with the Australia Telescope Long Ba…
We investigate statistical and individual astrophysical properties of active galactic nuclei (AGNs), such as parsec-scale flux density, core dominance, angular and linear sizes, maximum observed brightness temperatures of VLBI core components, spectral index distributions for core and jet components, and evolution of brightness temperature along the jets. Furthermore, we statistically compare core flux densities and brightness temperature as well as jet spectral indices of gamma-ray bright and weak sources. We used 19 very long baseline interferometry (VLBI) observing sessions carried out simu…
