Search for H I emission from superdisk candidates associated with radio galaxies

Giant gaseous layers(termed “superdisks”) have been hypothesized in the past to account for the strip-like radio emission gap(or straight-edged central brightness depression) observed between twin radio lobes, in over a dozen relatively nearby powerful Fanaroff-Riley Class II radio galaxies. They could also provide a plausible alternative explanation for a range of observations. Although a number of explanations have been proposed for the origin of the superdisks, little is known about their material content. Some X-ray observations of superdisk candidates indicate the presence of hot gas, but a cool dusty medium also seems to be common. If they are entirely or partly composed of neutral gas, then it may be directly detectable and we report here a first attempt to detect/image any neutral hydrogen gas present in the superdisks that are inferred to be present in four nearby radio galaxies. We have not found a positive H I signal in any of the four sources, resulting in tight upper limits on the H I number density in the postulated superdisks,estimated directly from the central rms noise values of the final radio continuum subtracted image. The estimated ranges of the upper limit on neutral hydrogen number density and column density are 10^-4-10^-3 atoms per cm3 and 10^19-10^20 atoms per cm^2, respectively. No positive H I signal is detected even after combining all the four available H I images(with inverse variance weighting). This clearly rules out an H I dominated superdisk as a viable model to explain these structures, however, the possibility of a superdisk being composed of warm/hot gas still remains open.

A polarization study of the supernova remnant CTB 80

We present a radio polarization study of the supernova remnant CTB 80 based on images at1420 MHz from the Canadian Galactic Plane Survey,at 2695 MHz from the Effelsberg survey of the Galactic plane and at 4800 MHz from the Sino-Germanλ6 cm polarization survey of the Galactic plane.We obtained a rotation measure(RM)map using polarization angles at 2695 MHz and 4800 MHz as the polarization percentages are similar at these two frequencies.RM exhibits a transition from positive values to negative values along one of the shells hosting the pulsar PSR B1951+32 and its pulsar wind nebula.The reason for the change in sign remains unclear.We identified a partial shell structure,which is bright in polarized intensity but weak in total intensity.This structure could be part of CTB 80 or part of a new supernova remnant unrelated to CTB 80.

A new three-stage evolution model for millimeter to centimeter wavelength outbursts in BL Lacertae

The multi-frequency light curves of BL Lacertae during 1997.5 - 1999.5 have been modeled by four outbursts, each having a 3-stage evolution in the （Sm, vm） plane with distinct rising-plateau-decaying phases. It is shown that the observed light curves can be well fitted for the eight frequencies from 350 GHz to 4.8 GHz. The main characteristics of the model-fitting are; （1） the outbursts are found to have very flat spectra with an optically thin spectral index α （defined as Sv α u^-α） of about 0.15. This is consistent with the results previously obtained by Valtaoja et al. （1992）; （2） it is found that there is no spectral flattening between the rising-plateau phase and the decay phase. In other words, the optically thin spectral index does not change from the rising-plateau phase to the decay phase. These features are in contrast to the 3-stage shocked-in-jet model proposed by Marscher ＆ Gear （1985） for submm- IR-optical flares, in which a spectral flattening of △α = 0.5 is predicted when a transition occurs from the Compton/synchrotron phase （or rising-plateau phase） to the adiabatic phase （or decay phase） with α≥ 0.5 for the shock being non-radiative. We propose a new model to interpret the fitting results, suggesting that the 3-stage evolution of the mm-cm outbursts in BL Lacertae may be related to the process of shock formation and propagation in a highly collimated jet （for example, a ＇parabolic＇ jet）. In particular, during the rising phase, the thickness of the synchrotron-radiating region created by the shock may rapidly increase with time （relative to the jet width） due to the rapid injection of relativistic electrons and a magnetic field, and this leads to the observed behavior that the turnover flux density Sm rapidly increases while the turnover frequency um decreases. In the decay phase, the emitting plasma enters into a free expansion regime without further injection of relativistic electrons and a magnetic field （for example, when a transition from a collimated regime into a conical regime occurs）. The plateau phase is a short period between the two regimes with no distinct features determined.

Protruding Structure Buried in Radio Map by Wavelet

An efficient method to identify supernova remnants is provided,in order to iron out the great gap between the predicted number and the observed.We make an attempt to apply D4 wavelet to detect the useful structures buried in radio map,showing that it is an efficient way to separate noises from signals.

New continuum and polarization observations of the Cygnus Loop with FAST I.Data processing and verification

We report on the continuum and polarization observations of the Cygnus Loop supernova remnant(SNR)conducted by the Five-hundred-meter Aperture Spherical radio Telescope(FAST).FAST observations provide high angular resolution and high sensitivity images of the SNR,which will help to disentangle its nature.We obtained Stokes I,Q and U maps over the frequency range of 1.03-1.46 GHz split into channels of 7.63 kHz.The original angular resolution is in the range of~3’-~3.’8,and we combined all the data at a common resolution of 4’.The temperature scale of the total intensity and the spectral index from the in-band temperature-temperature plot are consistent with previous observations,which validates the data calibration and map-making procedures.The rms sensitivity for the band-averaged total-intensity map is about 20 mK in brightness temperature,which is at the level of confusion limit.For the first time,we apply rotation measure(RM)synthesis to the Cygnus Loop to obtain the polarization intensity and RM maps.The rms sensitivity for polarization is about 5 mK,far below the total-intensity confusion limit.We also obtained RMs of eight extragalactic sources,and demonstrate that the wide-band frequency coverage helps to overcome the ambiguity of RM determinations.

Studying infall in infrared dark clouds with multiple HCO transitions

We investigate the infall properties in a sample of 11 infrared dark clouds(IRDCs) showing blue-asymmetry signatures in HCO^(+)J=1-0 line profiles.We used JCMT to conduct mapping observations in HCO^(+)J=4-3 as well as single-point observations in HCO+J=3-2,towards 23 clumps in these IRDCs.We applied the HILL model to fit these observations and derived infall velocities in the range of 0.5-2.7 km s^(-1),with a median value of 1.0 km s^(-1),and obtained mass accretion rates of 0.5-14 ×10^(-3) Mo yr^(-1).These values are comparable to those found in massive star forming clumps in later evolutionary stages.These IRDC clumps are more likely to form star clusters.HCO^(+)J=3-2 and HCO^(+)J=1-0 were shown to trace infall signatures well in these IRDCs with comparable inferred properties.HCO^(+)J=4-3,on the other hand,exhibits infall signatures only in a few very massive clumps,due to smaller opacities.No obvious correlation for these clumps was found between infall velocity and the NH3/CCS ratio.

Inner jet kinematics and the viewing angle towards the γ-ray narrow-line Seyfert 1 galaxy 1H 0323＋342

Near-Eddington accretion rates onto low-mass black holes are thought to be a prime driver of the multi-wavelength properties of the narrow-line Seyfert 1 （NLS1） population of active galactic nuclei （AGNs）. Orientation effects have repeatedly been considered as another important factor involved, but detailed studies have been hampered by the lack of measured viewing angles towards this type of AGN. Here we present multi-epoch, 15 GHz VLBA images （MOJAVE program） of the radio-loud and Fermi/LAT- detected NLS 1 galaxy 1H 0323＋342. These are combined with single-dish, multi-frequency radio monitoring of the source＇s variability, obtained with the Effelsberg 100-m and IRAM 30-m telescopes, in the course of the F-GAMMA program. The VLBA images reveal six components with apparent speeds of 1 - 7 c, and one quasi-stationary feature. Combining the obtained apparent jet speed （βapp） and variability Doppler factor （Dvar） estimates together with other methods, we constrain the viewing angle 0 towards 1H 0323＋342 to 0 ≤ 4°-13°. Using literature values of βapp and Dvar, we also deduce a viewing angle of 〈8°-9° towards another radio- and γ-ray-loud NLS1, namely SBS 0846＋513.

Refractive Focusing of Interstellar Clouds and Intraday Polarization Angle Swings

Intraday variations of compact extragalactic radio sources in flux density and polarization are generally interpreted in terms of refractive scintillation from the continuous interstellar medium of our Galaxy. However, continuous polarization angle swings of - 180° （for example, the one observed in the QSO 0917＋624） could not be interpreted in this way. Qian et al. have shown that the polarization angle swing observed in the QSO 1150＋812 can be explained in terms of focusing-defocusing effect by an interstellar cloud, which occults two closely-placed polarized components. Here we further show that the polarization angle swing event observed in the QSO 0917＋624 can also be explained in this way. We also found evidence for the cloud eclipsing a non-polarized （core） component during a short period out- side the swing. A particular （and specific） plasma-lens model is proposed to model-fit the polarization swing event of 0917＋624. Some physical parameters related to the plasma-lens and the source components are estimated. The brightness temperatures of the two lensed components are estimated to be -1.6× 10^13 K. Thus bulk relativistic motion with a Lorentz factor less than -20 may be sufficient to avoid the inverse - Compton catastrophe.

视超光速射电源表观速度和谱功率间的关系

本文证明了已知视超光速射电源的表观横向速度β_(app)和其核在10GHz的谱功率P_(10)是相关的.在相对论喷流模型框架下对该结果的解释,表明喷流的Lorentz因子γ,取决于源的固有光度.线性回归分析证明,可能存在两类视超光速源.“小”源,即无延展辐射或其延展瓣的外沿与核的距离小于20kpc的源,比“大”源呈现较低的β_(app),可以用指向效应解释.

Peering into the Milky Way by FAST:Ⅳ.Identification of two new Galactic supernova remnants G203.1+6.6and G206.7+5.9

A 5°×7°sky area containing two large radio structures of G203.1+6.6 and G206.7+5.9 with a size of about 2.°5 and 3.°5 respectively is scanned by using the L-band 19-beam receiver of the Five-hundred-meter Aperture Spherical radio Telescope(FAST).The FAST L-band receiver covers a frequency range of 1.0-1.5 GHz.Commissioning of the receiving system,including the measurements of the half-power beam width,gain,and main-beam efficiency is made by observing the calibrators.The multi-channel spectroscopy backend mounted to the receiver allows an in-band spectral-index determination.The brightness-temperature spectral indices of both objects are measured to beβ~-2.6 to-2.7.Polarized emission is detected from the archival Effelsbergλ11 cm data for all the shell structures of G203.1+6.6 and G206.7+5.9.These results clearly indicate a non-thermal synchrotron emitting nature,confirming that G203.1+6.6 and G206.7+5.9 are large shell-type supernova remnants(SNRs).Based on morphological correlation between the radio continuum emission of G206.7+5.9 and the H i structures,the kinematic distance to this new SNR is estimated to be about 440 pc,placing it in the Local Arm.

Tests of gravitational symmetries with radio pulsars

Symmetries play important roles in modern theories of physical laws. In this paper, we review several experimental tests of important symmetries associated with the gravitational interaction, including the universality of free fall for self-gravitating bodies,time-shift symmetry in the gravitational constant, local position invariance and local Lorentz invariance of gravity, and spacetime translational symmetries. Recent experimental explorations for post-Newtonian gravity are discussed, of which, those from pulsar astronomy are highlighted. All of these tests, of very different aspects of gravity theories, at very different length scales, favor to very high precision the predictions of the strong equivalence principle(SEP) and, in particular, general relativity which embodies SEP completely. As the founding principles of gravity, these symmetries are motivated to be promoted to even stricter tests in future.

Progress of research on AGNs at the Urumqi Observatory

We report the progress on Very Long Baseline Interferometry(VLBI) observations of Gigahertz Peaked Spectrum(GPS) radio sources,and single-dish observations of active galactic nuclei(AGNs).The GPS sources are a kind of young AGNs observable in radio.From our VLBI observations at 1.6 and 5 GHz with the European VLBI Network(EVN) including the Urumqi and Shanghai stations,most GPS sources show compact doubles with sizes less than 1 kiloparsec.We have classified the sources into double-lobes,core-jets,and complex structures according to the spectral indices as well as images.We also estimated the values of the jet viewing angle for the symmetric objects.In addition,we are monitoring a few samples of AGNs with the Urumqi 25-meter radio telescope,in order to find flux variability.We detected rapid flux variability in quasar 1156+295,and relatively slow variability in a few of the others.The origin of the rapid variability is discussed.Moreover,we launched a radio-optical monitoring program called Fermi-AGN in 2009.

A RELATION BETWEEN THE APPARENT VELOCITY AND THE SPECTRAL POWER OF SUPERLUMINAL RADIO SOURCES

It is proved that the apparent transverse velocities β_(app) and the spectral powers at 10GHz P_(10) of the cores of known superluminal sources are correlated. An interpretation ofthis result within the framework of the relativistic jet model shows that the jet Lorentzfactor γ depends on the intrinsic luminosity of the sources. The probable existence of twosubpopulations of the superluminal sources is shown by a linear regression analysis. 'Small'sources f i.e. the sources without extended emission or with a distance from the core tothe outer edge of the lobe<20 kpc, show lower β_(app) than the 'large' objects; this can beinterpreted as an orientation effect. Since all BL Lac objects belong to the 'small' class,this result sheds new light on such a question as why these sources have lower apparent ve-locities than the 'classical' superluminal quasars.

FAST ultra-wideband observation of abnormal emission-shift events of PSR B0919+06

PSR B0919+06 is known for its abnormal emission phenomenon, where the pulse emission window occasionally shifts progressively in longitude and returns afterwards. The physical mechanism behind this phenomenon is still under investigation. In this paper, we present our ultra-wideband observation of this pulsar using the Five-hundred-meter Aperture Spherical radio Telescope(FAST), with simultaneous measurements in the frequency ranges 280-780 and 1250-1550 MHz. We have identified three abnormal events, each of which becomes less apparent as the frequency decreases. At 1400 MHz, the averaged profile slightly shifted after the first and third abnormal events, implying a relationship between abnormal event and profile variation. We also found a linear trend in the left-edge position of the averaged profiles between the first and third events as well as after the third event, suggesting the existence of a slow-drifting mode between the two major events. The second event has a comparatively small shift in phase and is thus categorized as a "small flare state". During the third event, a sequence of approximately nine pulses was seen to significantly weaken in all frequency bands, likely associated with the pseudo-nulling observed at 150 MHz.A three-component de-composition analysis of the normal averaged profiles shows that the trailing component is dominant at our observing frequencies, while the centre component has a comparatively steeper spectrum. We found the overall flux density in an abnormal event to slightly differ from that in an ordinary state, and the difference shows a frequency dependence. A comparison of the normal, abnormal and dimmed averaged profile indicates that the leading component is likely to be stable in all states.