Gamma-Ray and VLBI Radio Emission from Gamma-ray-loud Blazars

Based on the γ-ray data of 51 γ-ray-loud blazars in the third EGRET catalog and VLBI radio data at 2.29 GHz in high state. we studied the correlations between the VLBI and γ-rayluminosities and the fluxes using the partial correlation analysis method. The results show that there is a strong correlation between γ-ray and VLBI radio fluxes and a significant correlation between γ-ray and VLBI radio luminosities. These results are likely to provide a possible support for self-Compton model of the γ-ray emission in blazars. It also shows that the γ-ray radiation may come from the region within the order of 1 pc which is consistent with the results given by luminosity variation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Characterization of the atmosphere above a site for millimeter wave astronomy

The Sardinia Radio Telescope (SRT) is a challeging scientific project managed by the National Institute for Astrophysics (INAF), it is being developed at 30 km North of the city of Cagliari, Italy. The goal of the SRT project is to build a general purpose, fully steerable, 64 m diameter radio telescope, capable of operating with high efficiency in the centimeter and millimeter frequency range (0.3–100 GHz). In portions of this frequency range, especially towards the high end, astronomical observations can be heavily deteriorated by non-optimal atmospheric conditions, especially by water vapor content. The water molecule permanent electric dipole in fact, leads to pressure broadened rotational transitions around the 22.23 GHz spectral line. Furthermore, water vapor’s continuum absorption and emission may influence higher frequency observations too. To a lower degree, cloud liquid black body radiation can also affect centimeter and millimeter observations. In addition to this, inhomogeneities in water vapor distributions can cause signal phase errors which introduce a great amount of uncertainty to VLBI mode observations. The Astronomical Observatory of Cagliari (OA-CA) has obtained historical timeseries of radiosonde profiles conducted at the airport of Cagliari. Through the radiosonde measurements and an appropriate radiative transfer model, we have performed a statistical analysis of the SRT site’s atmosphere which accounts for atmospheric opacity at different frequencies, integrated water vapor (IWV), integrated liquid water (ILW) and cloud cover distributions during the year. This will help to investigate in which period of the year astronomical observations at different frequencies should be performed preferably. The results show that, at the SRT site, K-band astronomical observations are possible all year round, the median opacity at 22.23 GHz is 0.10 Np in the winter (Dec-Jan-Feb) and 0.16 Np in the summer (Jun-Jul-Aug). Integrated water vapor during winter months ranges, on average, between 7 and 15 mm. Cloud cover is usually not present for more than 36% of the time during the year. The atmospheric opacity study indicates that observations at higher frequencies (50–100 GHz) may be performed usefully: the median opacity at 100 GHz is usually below or equal to 0.2 Np in the period that ranges from January to April.     

RATAN-600 7.6-cm deep sky strip surveys at the declination of the SS433 source during the 1980–1999 period. Data reduction and the catalog of radio sources in the right-ascension interval 7h ≤ R.A. < 17h

We use two independent methods to reduce the data of the surveys made with RATAN-600 radio telescope at 7.6 cm in 1988–1999 at the declination of the SS433 source. We also reprocess the data of the “Cold” survey (1980–1981). The resulting RCR (RATAN COLD REFINED) catalog contains the right ascensions and fluxes of objects identified with those of the NVSS catalog in the right-ascension interval 7 ^h ≤ R.A. < 17 ^h . We obtain the spectra of the radio sources and determine their spectral indices at 3.94 and 0.5 GHz. The spectra are based on the data from all known catalogs available from the CATS, Vizier, and NED databases, and the flux estimates inferred from the maps of the VLSS and GB6 surveys. For 245 of the 550 objects of the RCR catalog the fluxes are known at two frequencies only: 3.94 GHz (RCR) and 1.4 GHz (NVSS). These are mostly sources with fluxes smaller than 30mJy. About 65% of these sources have flat or inverse spectra (α > −0.5). We analyze the reliability of the results obtained for the entire list of objects and construct the histograms of the spectral indices and fluxes of the sources. Our main conclusion is that all 10–15 mJy objects found in the considered right-ascension interval were already included in the decimeter-wave catalogs.     

A Broadband Solar Radio Spectrometer and Some New Observational Results

A broadband solar radio spectrometer with a bandwidth of about 7 GHz has been developed in China for solar maximum 23. This work is a cooperative project of Beijing Astronomical Observatory (BAO), Purple Mountain Observatory (PMO), Yunnan Observatory (YNO), and Nanjing University. The spectrometer of PMO worked in the waveband of 4.5–7.5 GHz, that of BAO in 1–2 GHz, 2.6–3.8 GHz, and 5.2–7.6 GHz, and that of YNO in 0.7–1.5 GHz. The spectrometer of PMO is a multichannel and frequency-agile one with a time resolution of 1–5 ms and a frequency resolution of 10 MHz. It started to operate in August 1999 and since then more than 300 spectral events have been observed, and some type III or type III-like structures have also been found. In this paper, some selected typical events, for example, the events on 25 August 1999 and 27 October 1999, are presented, and some new observed features are also described and discussed.     

Broadband Radio Bursts and Fine Structures during the Great Solar Event on 14 July 2000

25 MHz–7.6 GHz global and detailed (fine structure – FS) radio spectra are presented, which were observed in the NOAA 9077 active region for the Bastille Day (14 July 2000) flare at 10:10–11:00 UT. Besides broadband radio bursts, high-resolution dynamic spectra reveal metric type II burst, decimetric type IV burst and various decimetric and microwave FSs, such as type III bursts, type U bursts, reverse-slope (RS)-drifting burst, fiber bursts, patch and drifting pulsation structure (DPS). The peak-flux-density spectrum of the radio bursts over the range 1.0–7.6 GHz globally appears as a U-shaped signature. Analyzing the features of backbone and herringbones of the type II burst, the speeds of shock and relevant energetic electron beams were estimated to be 1100 km s⁻¹ and 58 500 km s⁻¹, respectively. Also the time sequence of the radio emission is analyzed by comparing with the hard X-rays (HXRs) and the soft X-rays (SXRs) in this flare. After the maxima of the X-rays, the radio emission in the range 1.0–7.6 GHz reached maxima first at the higher frequency, then drifted to the lower frequency. This comparison suggested that the flare included three successive processes: firstly the X-rays rose and reached maxima at 10:10–10:23 UT, accompanied by fine structures only in the range 2.6–7.6 GHz; secondly the microwave radio emission reached maxima accompanied by many fine structures over the range 1.0–7.6 GHz at 10:23–10:34 UT; then a decimetric type IV burst and its associated FSs (fibers) in the range 1.0–2.0 GHz appeared after 10:40 UT.     

Origin of the Submillimeter Radio Emission During the Time-Extended Phase of a Solar Flare

Solar flares observed in the 200 – 400 GHz radio domain may exhibit a slowly varying and time-extended component which follows a short (few minutes) impulsive phase and can last for a few tens of minutes to more than one hour. The few examples discussed in the literature indicate that such long-lasting submillimeter emission is most likely thermal bremsstrahlung. We present a detailed analysis of the time-extended phase of the 27 October 2003 (M6.7) flare, combining 1 – 345 GHz total-flux radio measurements with X-ray, EUV, and Hα observations. We find that the time-extended radio emission is, as expected, radiated by thermal bremsstrahlung. Up to 230 GHz, it is entirely produced in the corona by hot and cool materials at 7 – 16 MK and 1 – 3 MK, respectively. At 345 GHz, there is an additional contribution from chromospheric material at a few 10⁴ K. These results, which may also apply to other millimeter–submillimeter radio events, are not consistent with the expectations from standard semiempirical models of the chromosphere and transition region during flares, which predict observable radio emission from the chromosphere at all frequencies where the corona is transparent.     

Positively Drifting Structures During the 18 March 2003 Solar Flare

We analyze the high-frequency drift radio structures observed by the spectrometer at Purple Mountain Observatory (PMO) over the frequency range of 4.5 – 7.5 GHz during the 18 March 2003 solar flare. The drifting structures take place before the soft X-ray maximum, almost at the maximum of hard X-ray flux at 25 – 50 keV. For the first time, the positive drift in this kind of radio structures is detected in such a high frequency range. Their global drifting rate is roughly estimated as 3.6 GHz s⁻¹. They appear in four groups, lasting in total for less than 6 s, and have a broad bandwidth of more than 2 GHz but a smaller ratio of the bandwidth of the drifting structures to mean frequency than that of the lower frequency range. The lifetime of each individual burst in this event can be derived by using the high temporal resolution of the spectrometer at PMO and has an average value of 36.3 ms. Since the negative drifting structures observed in the 0.6 – 4.5 GHz frequency range were interpreted to be a radio signature of a plasmoid ejected upward (moving out of the Sun), the present observation may imply that it is possible for a plasmoid to move downward during a solar flare. However, for a confirmation of this suggestion direct radio imaging observation would be needed.     

Relationship between infrared and radio emission of spiral galaxies

The relationship between the IR luminosity of spiral galaxies in the 40-120 mm range and their radio luminosity at 0.408,1.49,2.38,5.0, and 10.7 GHz is investigated. At all frequencies there is a close correlation between the radio luminosity and the infrared luminosity of spiral galaxies. The slopes of the relationships between radio and IR luminosities are less than unity at all frequencies, but they differ little from it within the error limits. A close correlation also exists between the fluxes of radio and IR emission. The correlation coefficients are high for objects of all morphological subtypes, while the relationships between radio and IR luminosities are nearly linear. They are also nearly linear for spiral galaxies of different classes in the Byurakan classification. It is also shown that the correlation between radio and IR luminosities is weakened for high IR luminosities. Translated from Astrofizika, Vol. 43, No. 1, pp. 33-44, January–March, 2000.     

Radio, Hard X-Ray, EUV and Optical Study of September 9, 2002 Solar Flare

The multi-wavelength analysis is performed on a flare on September 9, 2002 with data of Owens Valley Solar Arrays (OVSA), Big Bear Solar Observatory (BBSO), Ramaty High Energy Solar Spectroscopic Imager (RHESSI), and Extreme UV Imager Telescope (EIT), and The Michelson Doppler Imager (MDI) on board of the Solar and Heliospheric Observatory (SOHO). The radio sources at 4.8 and 6.2 GHz located in the intersection of two flaring loops at 195 of SOHO/EIT respectively with two dipole magnetic fields of SOHO/MDI, in which one EIT loop was coincident with an X-ray loop of RHESSI at 12–25 keV, and two H_αbright kernels a1 and a2 of BBSO, respectively at the two footpoints of this loop; the second EIT loop connected another two H_αkernels b1 and b2 and radio sources at 7.8 and 8.2 GHz of OVSA. The maximum phase of microwave bursts was evidently later than that of hard X-ray bursts and H_αkernels a1 and a2, but consistent with that of H_αkernels b1 and b2. Moreover, the flare may be triggered by the interaction of the two flaring loops, which is suggested by the cross-correlation of radio, optical, and X-ray light curves of a common quasi-periodic oscillation in the rising phase, as well as two peaks at about 7 and 9 GHz of the microwave spectra at the peak times of the oscillation, while the bi-directional time delays at two reversal frequencies respectively at 7.8 and 9.4 GHz (similar to the peak frequencies of the microwave spectra) may indicate two reconnection sites at different coronal levels. The microwave and hard X-ray footpoint sources located in different EUV and optical loops may be explained by different magnetic field strength and the pitch angle distribution of nonthermal electrons in these two loops.     

A statistical study of Galactic SNRs using the PMN survey

The Parkes–MIT–NRAO (PMN) radio survey has been used to generate a quasi all-sky study of Galactic Supernova Remnants (SNRs) at a common frequency of 4.85 GHz (λ=6 cm). We present flux densities estimated for the sample of 110 Southern Galactic SNRs (up to δ=−65°) observed with the Parkes 64-m radio telescope and an additional sample of 54 from the Northern PMN (up to δ=+64°) survey undertaken with the Green Bank 43-m (20 SNRs) and 91-m (34 SNRs) radio telescopes. Out of this total sample of 164 selected SNRs (representing 71% of the currently 231 known SNRs in the Green catalogue) we consider 138 to provide reliable estimates of flux density and surface brightness distribution. This sub-sample represents those SNRs which fall within carefully chosen selection criteria which minimises the effects of the known problems in establishing reliable fluxes from the PMN survey data. Our selection criteria are based on a judicious restriction of source angular size and telescope beam together with careful evaluation of fluxes on a case by case basis. Direct comparison of our new fluxes with independent literature values gives excellent overall agreement. This gives confidence in the newly derived PMN fluxes when the selection criteria are respected. We find a sharp drop off in the flux densities for Galactic SNRs beyond 4 Jy and then a fairly flat distribution from 5 to 9 Jy, a slight decline and a further flat distribution from 9 to 20 Jy though the numbers of SNR in each Jy bin are low. We also re-visit the contentious Σ–D (radio surface brightness–SNRs diameter) relation to determine a new power law index for a sub-sample of shell type SNRs which yields β=−2.2±0.6. This new evaluation of the Σ–D relation, applied to the restricted sample, provides new distance estimates and their Galactic scale height distribution. We find a peak in the SNR distribution between 7–11 kpc with most restricted to ±100 pc Galactic scale height.     

High Energetic Electrons Accelerated by a Coronal Shock Wave

Combined SOHO (Solar and Helisopheric Observatory) and ground based radio observations show evidently signatures of electrons accelerated by a shock wave during the event on July 9, 1996. A solar type II radio burst has been received as a signature of a coronal shock wave at 300 MHz on 9:10:54 UT. It was accompanied with electron beams appearing as type III radio bursts below 80 MHz. Simultaneously, the COSTEP (Comprehensive Suprathermal and Energetic Particle Analyzer) instrument aboard SOHO has measured enhanced electron fluxes in the range 30 keV – 3 MeV. This indicates that a coronal shock wave was able to produce high energetic electrons. A mechanism of electron acceleration up to relativistic velocities is presented and compared with the observations. The electron acceleration takes place at substructures of quasi-parallel collisionless shocks. This revised version was published online in July 2006 with corrections to the Cover Date.     

The radio source Z0254+43: z = 4.067

In January 2005 spectral observations of the radio source Z0254+43 were made on the BTA at the Special Astrophysical Observatory (SAO) of the Russian Academy of Sciences (RAN) and its red shift was found to be z=4.057. The BVRI magnitudes were found to be 22.68, 21.19, 19.94, and 19.23, respectively. Photometric observations in December 2005 on the Zeiss-1000 at the SAO revealed no significant variation in the optical emission from this object over that year. We can discuss its variability on an hourly time scale with some caution. The variability of the flux from Z0254+43 was observed from 1990–2005 on the RATAN-600 over a wide range of frequencies. It turns out that the amplitude of the variability is minimal at a frequency of ∼8 GHz. A model for the variability has been constructed which yields an estimate of ∼28° for the orientation of the jet of Z0254+43 to the line of sight. The luminosity of Z0254+43 in the optical range is ∼2·10²⁶ W/Hz and in the radio frequency range, ∼2·1027 W/Hz. __________ Translated from Astrofizika, Vol. 49, No. 2, pp. 209–220 (May 2006).     

Main results of the global adjustment of VLBI observations

Using a new version of the QUASAR domestic multifunction software package, we have simultaneously processed all of the available VLBI observations performed on global networks of stations over the period 1979–2009. New improved and extended versions of the international reference systems of coordinates of extragalactic radio sources and ground-based VLBI stations and a new independent series of Earth orientation parameters have been obtained. Analysis of the accuracy of these results shows that they are at the level of the best determinations at national and international VLBI data analysis centers.     

VLBI observation of giant radio galaxy J1313+696 at 2.3/8.4 GHz

We report the result of VLBI observation of the giant radio galaxy J1313+696 (4C +69.15) at 2.3/8.4 GHz, only the core component of the giant radio galaxy was detected in the VLBI observation at the dual frequencies. The result shows a steep spectrum core with α=?0.82 (S ∝ ν ^α ) between 2.3 GHz and 8.4 GHz. The steep spectrum core may be a sign of renewed activity. Considering also the upper limit flux density of 2.0 mJy at 0.6 GHz from Konar et al. 2004 (Mon. Not. R. Astron. Soc. 355:845, 2004) the core has a GHz-peaked spectrum, implying that the core is compact and absorbed. Further high resolution VLBI observations are needed to identify if the steep spectrum core is consisting of a core and steep spectrum jet.     

The Double–Double Radio Galaxy 3C293

We present the results of radio continuum observations at frequencies ranging from ~150–5000 MHz of the misaligned double–double radio galaxy (DDRG) 3C293 (J1352+3126) using the GMRT and the VLA, and estimate the time-scale of interruption of jet activity to be less than ~0.1 Myr.     

A new sample of gigahertz-peaked-spectrum, extra-galactic radio sources

A new sample of extra-Galactic radio sources having a peak in the continuous spectrum near 1 GHz has been compiled, using data available in the literature on their radio flux densities, and some characteristic parameters of radio sources in this sample have been calculated. The median value of the calculated spectral indices of radio sources in the sample is -0.95, the median flux density at the peak (S_m) is 465 mJy, and almost half the radio sources (14 out of 30) have ultrasteep spectra, for which the spectral indices in the high-frequency range are α <-1.0 (S ∝ v^α). Translated from Astrofizika, Vol. 41, No. 3, pp. 377–388, July–September, 1998.     

The most compact bright radio-loud AGN—I. A new target sample selected for the space VLBI

We investigated the archival ground-based VLBI images of the extragalactic radio sources included in both the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck catalogues, and selected 49 bright and compact sources as potential targets for space Very Long Baseline Interferometry (VLBI) observations at mm wavelengths. These sources have a flat radio continuum spectrum between 33 and 94 GHz. They are identified as core-dominated active galactic nuclei (AGN), located at declinations above ?40^°, and have never been observed with ground-based VLBI at 86 GHz. The radio properties of the 49 new sources are presented. We compare this new sample with similar samples of compact AGN available from earlier studies. The new candidates, together with the existing bright compact AGN sample identified from 86-GHz ground-based VLBI imaging surveys, form a catalogue of more than 160 AGN. These could be primary targets for mm-VLBI observations on the ground, as well as for future mm-wavelength space VLBI missions such as the project with two satellites currently under study in China.     

Periodic oscillation in the long-term radio light curves of Quasar 1156+295

Based on the light curves at 4.8, 8.0 and 14.5 GHz of Quasar 1156+295 from the University of Michigan Radio Astronomy Observatory, we analyze the variability property of total flux at the three radio bands during the time range from 1980 to 2012. With the structure function (SF) analysis we find a similar evolution trend with the characteristic variability timescale of 1.15±0.05 year for all the three radio bands, and a possible quasi-periodic variability period of ～2.3±0.1 year. The Lomb-Scargle periodogram (LSP) analysis implies the existence of multiple periods in the radio light curves, one of them with a period similar to the one found using the SF at around 2.3 year. The structure function analysis of pre- and post-1997 suggests that the characteristic timescales become longer since 1997, confirmed by the result of Lomb-Scargle periodogram and auto-correlation function. In addition, the cross correlation analysis confirms that there is a strong correlation of flux variations at the three radio frequencies with the 14.5 GHz variation leading the other two bands by about 60–120 days.     

Solar Radio Bursts on 16 February 1999

The 2.60–3.80 GHz spectrometer at the Beijing Astronomical Observatory (BAO) recorded a ‘decimetric pulsation’ event (DCIM) around the time 1999 0216 0300. At the beginning and end of this DCIM, two groups of reverse slope type III bursts (RS-III) are also detected; meanwhile, metric type II bursts are recorded by CULG and HIRA during the same time. These solar radio bursts on that day might be caused by the same active region 8458 and a same flare. We present a plausible qualitative model for all of them. This revised version was published online in July 2006 with corrections to the Cover Date.     

Evidence for cospatial optical and radio polarized emission in active galactic nuclei

We investigate the relationship between the optical and radio emission of active galactic nuclei (AGN) by analysing optical and 15+22+43 GHz Very Long Baseline Array (VLBA) polarization observations simultaneous to within a day for 11 BL Lacertae (BL Lac) objects and the blazar 3C279. We have determined and corrected for the Faraday rotation measures in the very long baseline interferometry (VLBI) cores, enabling us to compare the intrinsic (zero-wavelength) VLBI-core polarization angles and the optical polarization angles χ_opt. A clear alignment between these two angles emerges in the transition toward higher radio frequencies, and a prominent peak at 0° is visible in the distribution of |χ_opt−χ_43 GHz|. This correlation implies that the magnetic-field orientations in the regions giving rise to the optical and radio polarization are the same, and can be easily understood if the radio and optical polarization are roughly cospatial. It is difficult to rule out the possibility that they arise in different regions in a straight jet with a uniform magnetic-field structure, but this seems less likely, since the VLBI jets of AGN are often bent on parsec-scales. This may suggest that much or all of the strong optical polarization in these sources arises in the inner radio jets, possibly associated with the formation and emergence of compact new VLBI components.