scholarly journals A flat-spectrum flare in S4 0444+63 revealed by a new implementation of multiwavelength single-dish observations

2020 ◽  
Vol 492 (2) ◽  
pp. 2807-2817 ◽  
Author(s):  
M Giroletti ◽  
S Righini
Keyword(s):  
Very Long Baseline Interferometry ◽  
Gamma Ray ◽  
Peak Frequency ◽  
Efficient Tool ◽  
Analysis Pipeline ◽  
Long Baseline ◽  
Radio Wavelength ◽  
Doppler Factor ◽  
First Time ◽  
24 Ghz

ABSTRACT Relativistic amplification boosts the contribution of the jet base to the total emission in blazars, thus making single-dish observations useful and practical to characterize their physical state, particularly during episodes of enhanced multiwavelength activity. Following the detection of a new gamma-ray source by Fermi-LAT in 2017 July, we observed S4 0444+63 in order to secure its identification as a gamma-ray blazar. We conducted observations with the Medicina and Noto radio telescopes at 5, 8, and 24 GHz for a total of 12 epochs between 2017 August 1 and 2018 September 22. We carried out the observations with on-the-fly cross-scans and reduced the data with our newly developed Cross-scan Analysis Pipeline, which we present here in detail for the first time. We found the source to be in an elevated state of emission at radio wavelength, compared to historical values, which lasted for several months. The maximum luminosity was reached on 2018 May 16 at 24 GHz, with $L_{24}=(1.7\pm 0.3)\times 10^{27}\ \mathrm{W\, Hz}^{-1}$; the spectral index was found to evolve from slightly rising to slightly steep. Besides the new observations, which have proved to be an effective and efficient tool to secure the identification of the source, additional single dish and very long baseline interferometry data provide further insight on the physics of the source. We estimate a synchrotron peak frequency νpeak = 1012.97 Hz and a Doppler factor in excess of δ ∼ 5.0, with both quantities playing a role in the gamma-ray emission from the source.

scholarly journals Core-shifts and proper-motion constraints in the S5 polar cap sample at the 15 and 43 GHz bands

2018 ◽  
Vol 614 ◽  
pp. A74 ◽  
Author(s):  
F. J. Abellán ◽  
I. Martí-Vidal ◽  
J. M. Marcaide ◽  
J. C. Guirado
Keyword(s):  
Active Galactic Nuclei ◽  
Very Long Baseline Interferometry ◽  
Galactic Nuclei ◽  
Polar Cap ◽  
Motion Constraints ◽  
The Core ◽  
Long Baseline ◽  
Precision Phase ◽  
First Time ◽  
Core Shift

We have studied a complete radio sample of active galactic nuclei with the very-long-baseline-interferometry (VLBI) technique and for the first time successfully obtained high-precision phase-delay astrometry at Q band (43 GHz) from observations acquired in 2010. We have compared our astrometric results with those obtained with the same technique at U band (15 GHz) from data collected in 2000. The differences in source separations among all the source pairs observed in common at the two epochs are compatible at the 1σ level between U and Q bands. With the benefit of quasi-simultaneous U and Q band observations in 2010, we have studied chromatic effects (core-shift) at the radio source cores with three different methods. The magnitudes of the core-shifts are of the same order (about 0.1 mas) for all methods. However, some discrepancies arise in the orientation of the core-shifts determined through the different methods. In some cases these discrepancies are due to insufficient signal for the method used. In others, the discrepancies reflect assumptions of the methods and could be explained by curvatures in the jets and departures from conical jets.

scholarly journals Detection of a flaring blazar coincident with an IceCube high-energy neutrino

2019 ◽  
Vol 207 ◽  
pp. 02001
Author(s):  
Anna Franckowiak
Keyword(s):  
Gamma Ray ◽  
Spectral Energy Distribution ◽  
Neutrino Flux ◽  
High Energy ◽  
Spectral Energy ◽  
Large Area ◽  
High Energy Neutrino ◽  
Energy Neutrino ◽  
Radio Wavelength ◽  
First Time

In September 22, 2017, IceCube released a public alert announcing the detection of a 290 TeV neutrino track event with an angular uncertainty of one square degree (90% containment). A multi-messenger follow-up campaign was initiated resulting in the detection of a GeV gamma-ray flare by the Fermi Large Area Telescope positionally consistent with the location of the known Bl Lac object, TXS 0506+056 , located only 0.1 degrees from the best-fit neutrino position. The probability of finding a GeV gamma-ray flare in coincidence with a high-energy neutrino event assuming a correlation of the neutrino flux with the gamma-ray energy flux in the energy band between 1 and 100 GeV was calculated to be 3σ (after trials correction). Following the detection of the flaring blazar the imaging air Cherenkov telescope MAGIC detected the source for the first time in the > 100 GeV gamma-ray band. The activity of the source was confirmed in X-ray, optical and radio wavelength. Several groups have developed lepto-hadronic models which succeed to explain the multi-messenger spectral energy distribution.

scholarly journals The third realization of the International Celestial Reference Frame by very long baseline interferometry

2020 ◽  
Vol 644 ◽  
pp. A159 ◽  
Author(s):  
P. Charlot ◽  
C. S. Jacobs ◽  
D. Gordon ◽  
S. Lambert ◽  
A. de Witt ◽  
...  
Keyword(s):  
Reference Frame ◽  
Very Long Baseline Interferometry ◽  
Galactic Center ◽  
Motion Field ◽  
Dual Frequency ◽  
Extended Source ◽  
Long Baseline ◽  
Radio Frequencies ◽  
Celestial Reference Frame ◽  
24 Ghz

A new realization of the International Celestial Reference Frame (ICRF) is presented based on the work achieved by a working group of the International Astronomical Union (IAU) mandated for this purpose. This new realization follows the initial realization of the ICRF completed in 1997 and its successor, ICRF2, adopted as a replacement in 2009. The new frame, referred to as ICRF3, is based on nearly 40 years of data acquired by very long baseline interferometry at the standard geodetic and astrometric radio frequencies (8.4 and 2.3 GHz), supplemented with data collected at higher radio frequencies (24 GHz and dual-frequency 32 and 8.4 GHz) over the past 15 years. State-of-the-art astronomical and geophysical modeling has been used to analyze these data and derive source positions. The modeling integrates, for the first time, the effect of the galactocentric acceleration of the solar system (directly estimated from the data) which, if not considered, induces significant deformation of the frame due to the data span. The new frame includes positions at 8.4 GHz for 4536 extragalactic sources. Of these, 303 sources, uniformly distributed on the sky, are identified as “defining sources” and as such serve to define the axes of the frame. Positions at 8.4 GHz are supplemented with positions at 24 GHz for 824 sources and at 32 GHz for 678 sources. In all, ICRF3 comprises 4588 sources, with three-frequency positions available for 600 of these. Source positions have been determined independently at each of the frequencies in order to preserve the underlying astrophysical content behind such positions. They are reported for epoch 2015.0 and must be propagated for observations at other epochs for the most accurate needs, accounting for the acceleration toward the Galactic center, which results in a dipolar proper motion field of amplitude 0.0058 milliarcsecond yr−1 (mas yr−1). The frame is aligned onto the International Celestial Reference System to within the accuracy of ICRF2 and shows a median positional uncertainty of about 0.1 mas in right ascension and 0.2 mas in declination, with a noise floor of 0.03 mas in the individual source coordinates. A subset of 500 sources is found to have extremely accurate positions, in the range of 0.03–0.06 mas, at the traditional 8.4 GHz frequency. Comparing ICRF3 with the recently released Gaia Celestial Reference Frame 2 in the optical domain, there is no evidence for deformations larger than 0.03 mas between the two frames, in agreement with the ICRF3 noise level. Significant positional offsets between the three ICRF3 frequencies are detected for about 5% of the sources. Moreover, a notable fraction (22%) of the sources shows optical and radio positions that are significantly offset. There are indications that these positional offsets may be the manifestation of extended source structures. This third realization of the ICRF was adopted by the IAU at its 30th General Assembly in August 2018 and replaced the previous realization, ICRF2, on January 1, 2019.

scholarly journals Recent developments in supernova research with VLBI

2013 ◽  
Vol 9 (S296) ◽  
pp. 53-57 ◽  
Author(s):  
Norbert Bartel ◽  
Michael F. Bietenholz
Keyword(s):  
Very Long Baseline Interferometry ◽  
Gamma Ray ◽  
Gamma Ray Bursts ◽  
Expansion Velocity ◽  
Radial Density ◽  
Density Profiles ◽  
Long Baseline ◽  
Vlbi Observations ◽  
Recent Developments ◽  
Circumstellar Medium

AbstractVery long baseline interferometry (VLBI) observations during the last 30 years have resolved many supernovae and provided detailed measurements of the expansion velocity and deceleration. Such measurements are useful for estimating the radial density profiles of both the ejecta and the circumstellar medium left over from the progenitor. VLBI measurements are also the most direct way of confirming the relativistic expansion velocities thought to occur in supernovae associated with gamma-ray bursts. Well-resolved images of a few supernovae have been obtained, and the interaction of the ejecta as it expands into the circumstellar medium could be monitored in detail. We discuss recent results, for SN 1979C, SN 1986J, and SN 1993J, and note that updated movies of the latter two of the supernovae from soon after the explosion to the present are available from the first author's personal website.

scholarly journals Global alignments of parsec-scale AGN radio jets and their polarization planes

2020 ◽  
Vol 635 ◽  
pp. A102 ◽  
Author(s):  
D. Blinov ◽  
C. Casadio ◽  
N. Mandarakas ◽  
E. Angelakis
Keyword(s):  
Active Galactic Nuclei ◽  
Large Scale ◽  
Very Long Baseline Interferometry ◽  
Parallel Transport ◽  
Polarization Plane ◽  
Galactic Nuclei ◽  
Radio Jets ◽  
Long Baseline ◽  
Flux Density Distribution ◽  
First Time

Context. A number of works have reported that the polarization plane of extragalactic sources as well as the structural axes of radio sources are aligned on the large scale. However, both the claims and their interpretation remain controversial. Aims. For the first time, we explore the alignment of parsec-scale jets. Additionally, we use archival polarimetric data at different wavelengths in order to compare the relative orientations of the jets and the polarization planes of their emission. Methods. Using the flux density distribution in very long baseline interferometry radio maps from the Astrogeo database, we determine the parsec-scale jet orientation for the largest sample of active galactic nuclei to date. Employing the method of parallel transport and a sample statistics characterizing the jet orientation dispersion among neighbors, we test whether the identified jets are significantly aligned. Results. We show that the parsec-scale jets in our sample do not demonstrate any significant global alignments. Moreover, the jet direction is found to be weakly correlated with the direction of the polarization plane at different frequencies.

scholarly journals Realization of a multifrequency celestial reference frame through a combination of normal equation systems

2019 ◽  
Vol 630 ◽  
pp. A101 ◽  
Author(s):  
M. Karbon ◽  
A. Nothnagel
Keyword(s):  
Radio Source ◽  
Reference Frame ◽  
Reference Frames ◽  
Very Long Baseline Interferometry ◽  
Normal Equation ◽  
Long Baseline ◽  
Combination Approach ◽  
Celestial Reference Frame ◽  
Right Ascension ◽  
24 Ghz

Context. We present a celestial reference frame (CRF) based on the combination of independent, multifrequency radio source position catalogs using nearly 40 years of very long baseline interferometry observations at the standard geodetic frequencies at SX band and about 15 years of observations at higher frequencies (K and XKa). The final catalog contains 4617 sources. Aims. We produce a multifrequency catalog of radio source positions with full variance–covariance information across all radio source positions of all input catalogs. Methods. We combined three catalogs, one observed at 8 GHz (X band), one at 24 GHz (K band) and one at 32 GHz (Ka band). Rather than only using the radio source positions, we developed a new, rigorous combination approach by carrying over the full covariance information through the process of adding normal equation systems. Special validation routines were used to characterize the random and systematic errors between the input reference frames and the combined catalog. Results. The resulting CRF contains precise positions of 4617 compact radio astronomical objects, 4536 measured at 8 GHz, 824 sources also observed at 24 GHz, and 674 at 32 GHz. The frame is aligned with ICRF3 within ±3 μas and shows an average positional uncertainty of 0.1 mas in right ascension and declination. No significant deformations can be identified. Comparisons with Gaia-CRF remain inconclusive, nonetheless significant differences between all frames can be attested.

scholarly journals VLBI Constraints on Type I b/c Supernovae

2014 ◽  
Vol 31 ◽  
Author(s):  
Michael F. Bietenholz
Keyword(s):  
Very Long Baseline Interferometry ◽  
Gamma Ray ◽  
Expansion Velocity ◽  
Type I ◽  
Density Profiles ◽  
Circumstellar Material ◽  
Spatially Resolved ◽  
Long Baseline ◽  
Forward Shock

AbstractVery long baseline interferometry observations of supernovae and gamma-ray bursts provide almost the only way of obtaining spatially resolved information about the sources. In particular, a determination of the expansion velocity of the forward shock, as well as the geometry of the fireball and its evolution with time are possible for relatively nearby events, provided they are radio bright. Monitoring the expansion of the shock front can provide information on the density profiles of both the circumstellar material and on the ejecta. Very long baseline interferometry observations can also potentially resolve gamma-ray burst jets which are not directed along the line of sight, providing crucial confirmation of relativistic expansion in such objects. This review gives an overview of recent results from supernovae, including the Type I b/c SNe 2011dh, 2009bb, and 2007gr, and discusses the prospects for future observations.

scholarly journals Very Long Baseline Interferometry Observations of the Proposed Radio Counterpart of an EGRET Source

Symmetry ◽  
2020 ◽  
Vol 12 (9) ◽  
pp. 1516
Author(s):  
Patrik Milán Veres ◽  
Krisztina Éva Gabányi ◽  
Sándor Frey
Keyword(s):  
Very Long Baseline Interferometry ◽  
Gamma Ray ◽  
Interferometric Imaging ◽  
High Brightness ◽  
The Core ◽  
Long Baseline ◽  
Superluminal Motion ◽  
Γ Ray ◽  
Previous Claim ◽  
Jet Structure

We present high-resolution radio interferometric imaging observations of the radio source NVSS J182659+343113 (hereafter J1826+3431), the proposed radio counterpart of the γ-ray source, 3EG J1824+3441 detected by the Energetic Gamma Ray Experiment Telescope (EGRET) on board the Compton Gamma Ray Observatory satellite. We analyzed eight epochs of archival multi-frequency very long baseline interferometry data. We imaged the asymmetric core–jet structure of the source, and detected apparent superluminal motion in the jet. At the highest observing frequency, 15.3 GHz, the core shows high brightness temperature indicating Doppler boosting. Additionally, the radio features undergo substantial flux density variability. These findings strengthen the previous claim of the association of the blazar J1826+3431 with the possible γ-ray source, 3EG J1824+3441.

scholarly journals Very long baseline interferometry imaging of the advancing ejecta in the first gamma-ray nova V407 Cygni

2020 ◽  
Vol 638 ◽  
pp. A130 ◽  
Author(s):  
M. Giroletti ◽  
U. Munari ◽  
E. Körding ◽  
A. Mioduszewski ◽  
J. Sokoloski ◽  
...  
Keyword(s):  
Equatorial Plane ◽  
Very Long Baseline Interferometry ◽  
Gamma Ray ◽  
Orbital Plane ◽  
Polar Axis ◽  
Expansion Velocity ◽  
Large Area ◽  
North West ◽  
Long Baseline ◽  
5 Ghz

Context. In 2010 March, the Large Area Telescope on board Fermi revealed a transient gamma-ray source that is positionally coincident with the optical nova in the symbiotic binary, V407 Cyg. This event marked the first discovery of gamma-ray emission from a nova. Aims. We aim to obtain resolved radio imaging of the material involved in the nova event, to determine the ejecta geometry and advance velocity directly in the image plane, and to constrain the physical conditions of the system. Methods. We observed the source with the European VLBI (Very Long Baseline Interferometry) Network in real time mode, at 1.6 and 5 GHz, and the Very Long Baseline Array at 1.6, 5, and 8.4 GHz. In total, we observed the source over 16 epochs, starting 20 days after the optical discovery and continuing for over six months. Results. Milliarcsecond-scale radio emission is detected in 10/16 epochs of observations. The source is initially very dim but it later shows a substantial increase in brightness and a resolved shell-like structure 40–90 days after the optical event. The shell has a projected elliptical shape and is asymmetric in brightness and spectral index, being brighter and characterised by a rising spectrum at the south-eastern edge. We determine a projected expansion velocity of ∼3500 km s−1 in the initial phase (for an adopted 2.7 kpc distance), and ∼2100 km s−1 between day 20 and 91. We also found an emitting feature about 350 mas (940 AU) to the north-west, advancing at a projected velocity of ∼700 km s−1 along the polar axis of the binary. The total flux density in the VLBI images is significantly lower than that previously reported at similar epochs and over much wider angular scales with the VLA. Conclusions. Optical spectra convincingly demonstrated that in 2010 we were viewing V407 Cyg along the equatorial plane and from behind the Mira. Our radio observations image the bipolar flow of the ejecta perpendicular to the orbital plane, where deceleration is much lower than through the equatorial plane probed by the truncated profile of optical emission lines. The separated polar knot at 350 mas and the bipolar flow strictly resemble a similar arrangement seen in Hen 2-104, another symbiotic Mira seen equator-on that went through a large outburst ∼5700 yrs ago. The observed ∼700 km s−1 expansion constrains the launch date of the polar knot around 2004, during the accretion-fed active phase preceding the 2010 nova outburst.

scholarly journals Very Long Baseline Interferometry observations: the closest look at the cores of AGN

2013 ◽  
Vol 9 (S304) ◽  
pp. 71-77
Author(s):  
Marcello Giroletti ◽  
Rocco Lico ◽  
Kazuhiro Hada ◽  
Gabriele Giovannini
Keyword(s):  
Very Long Baseline Interferometry ◽  
High Energy ◽  
Physical Parameters ◽  
Long Baseline ◽  
Superluminal Motion ◽  
Multi Wavelength ◽  
Very High Energy ◽  
Doppler Factor ◽  
Jet Structure ◽  
Very High

AbstractVery Long Baseline Interferometry (VLBI) provides the highest angular resolution achievable in astronomy, reaching sub-milliarcsecond scales. For radio loud AGNs, this offers the unique opportunity to directly image and monitor the fine details of the jet structure, approaching the event horizon in the nearest and most supermassive black holes, like in M87. After a quick review of various VLBI facilities, we present results from recent VLBI monitoring projects on two remarkable radio and high energy sources: the radio galaxy M87 and the BL Lac object Mrk421. For the latter, we present a detailed analysis of the jet structure in total intensity and polarization through the whole 2011, during which a multi-wavelength campaign took place. We reveal flux density variability but no significant changes in the jet structure. In M87, we detect and follow the evolution of the core and of the jet feature HST-1; we reveal superluminal motion of components within HST-1, with a possible connection between ejection of new such components and the occurrence of very high energy flares. Estimates on the physical parameters for both sources are given (Doppler factor, viewing angle, magnetic field, etc.).

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